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Mars is the Fourth planet from the sun and the second smallest planet in the solar system after Mercury. Mars has a thin atmosphere consisting of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. Martian surface temperatures vary from lows of about -143 °C (−225 °F) at the winter polar caps to highs of up to 35 °C (95 °F) in equatorial summer. Mars has the largest dust storms in the solar system reaching speeds pf over 160 km/h (100 mph). These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature. Mars's average distance from the Sun is roughly 230 million km (143 million mi), and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours. The surface of Mars is primarily composed of tholeiitic basalt although parts of Mars are more silica rich that typical basalt and may be similar to andesitic rocks on Earth or silica glass.

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This true-color image of Mars was taken by the OSIRIS instrument on the ESA

Although Mars has no evidence of a structured global magnetic field observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past. The two polar ice caps appear to be made largely of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters (36 ft). Land forms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water. In 2004, Opportunity detected the mineral jarosite, this mineral forms only in the presence of acidic water, which demonstrates that water once existed on Mars.

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Mars has two relatively small (compared to Earth's) natural moons, Phobos (about 22 km (14 mi) in diameter) and Deimos  (about 12 km (7.5 mi) in diameter), which orbit close to the planet. Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet. 

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Phobos is one of the two moons of Mars. It is the larger of the two moons, and is heavily cratered and appears to have grooves and streaks of material along its sides.

Phobos Moon Profile
Diameter:    22.2 km
Mass:    1.07 × 10^16 kg (0.00001% Moon)
Orbits:
Mars
Orbit Distance:
9,376 km
Orbit Period:
7.7 hours
Surface Temperature:    -40 °C
Discovery Date:    August 17, 1877
Discovered By:    Asaph Hall

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The long, shallow grooves lining the surface of Phobos are likely early signs of the structural failure that will ultimately destroy this moon of Mars.

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Asaph Hall III (October 15, 1829 – November 22, 1907) was an American astronomer who is most famous for having discovered the moons of Deimos and Phobos in 1877.

Phobos’s orbit is so fast it would appear to an observer on the planet to rise in the west and set in the East twice each Martian day.
As Phobos orbits, it is getting closer to the planet as time goes by. Eventually, it will be destroyed by Mars’s tidal forces in several tens of millions of years. It will very likely break up in orbit and its pieces will scatter onto the surface and spread out along the orbit, possibly creating a short-lived ring.
Phobos’s shadow has been photographed on the surface of Mars by several spacecraft.
Phobos is roughly potato-shaped and has a large crater called Stickney. Many of its largest features are named after places in the novel Gulliver’s travels.
No one is quite sure where Phobos formed. It has the same mineralogical characteristics of a C-type asteroid. It’s possible that Phobos (along with Deimos) is a captured asteroid, but there are some problems with that theory.

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Stickney is the largest crater on Phobos, It is 9 km (5.6 mi) in diameter, taking up a substantial proportion of the moon's surface. Stickney has a smaller crater within it, about 2 km (1.2 mi) in diameter, resulting from a later impact.

Phobos has a fine dusty layer on its surface called “regolith” and some astronomers have predicted that this material drifts off of Phobos leaving behind a very faint “tail”.
Phobos has been studied by nearly all of the missions that have successfully explored Mars. The Soviet Phobos 1 and 2 were launched, but only Phobos 2 survived to reach the moon. It sent back small amounts of data before failing and falling silent.
There have been no direct missions to explore Phobos, but the feasibility has be investigated several  times. The most recently funded project is called Phobos and Demos & Mars Environment (PADME) and would launch to Mars for an arrival in 2021.
At least one human mission to Phobos has been suggested, using Phobos as a staging area for missions that would later go to Mars’ surface.

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Phobos 2 was the last space probe designed by the Soviet Union, It was designed to explore Mar's moons Phobos and Deimos and was launched on July 12th 1988 and entered orbit on January 29, 1989.Phobos 2 operated nominally throughout its cruise and Mars orbital insertion phases on January 29, 1989, gathering data on the Sun, interplanetary medium, Mars, and Phobos. Phobos 2 investigated Mars surface and atmosphere and returned 37 images of Phobos with a resolution of up to 40 meters.Shortly before the final phase of the mission, during which the spacecraft was to approach within 50 m of Phobos' surface and release two landers, one a mobile "hopper", the other a stationary platform, contact with Phobos 2 was lost. The mission ended when the spacecraft signal failed to be successfully reacquired on March 27, 1989. The cause of the failure was determined to be a malfunction of the on-board computer.

Mars is small, about half Earth’s diameter, so it cooled off faster than Earth did after their birth in the cloud of dust left over from the sun’s creation. Mars has some of the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular.

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Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.

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Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.

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Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep.

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Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

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Perspective view of crater north of Hellas Planitia (Image ESA/DLR/FU Berlin

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to Earths Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. Because Jupiter is the nect planet from Mars it has the ability to influence the orbit of Mars due to its immense mass, Jupiter brings some change in Mars’ orbit. Mars, in addition to Earth, is the only other planet that has polar ice caps. It’s Northern cap is called – Planum Boreum, and Southern cap is called – Planum Australe. Scientists have had the chance to study the pieces of rocks from Mars that landed on Earth. The rock pieces orbited the solar system for millions of years and finally crash landed on the Earth.

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Polar ice caps exist on other planets. This is Mars' northern polar ice cap, called the Planum Boreum. The deep canyon on the right is called the Chasma Boreale, and is about the same size as the Grand Canyon in Arizona.

How much does a human being weigh on Mar's surface.

Mercury: 0.38
Venus: 0.91
Earth: 1.00
Mars: 0.38
Jupiter: 2.34
Saturn: 1.06
Uranus: 0.92
Neptune: 1.19
Pluto: 0.06
Because weight = mass x surface gravity, multiplying your weight on Earth by the numbers above will give you your weight on the surface of each planet. If you weigh 150 pounds (68 kg.) on Earth, you would weigh 351 lbs. (159 kg.) on Jupiter, 57 lbs. (26 kg.) on Mars and a mere 9 lbs. (4 kg.) on the dwarf planet of Pluto.

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The first photograph of Mars taken from its surface was taken by Viking 1 on July 20 1976.
Currently, two rovers from NASA named, Opportunity and Curiosity, are exploring the surface of Mars.
NASA has plans to create an Earth Independent colony on Mars by the end of 2030.
Presence of Ozone on Mars was first detected in 1971. However, the concentration of Ozone on Mars is 300 thinner than on Earth, and it varies with time and location. At present, scientists have been successfully able to identify three distinct layers of Ozone on the Red Planet.

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On August 20, 1975, NASA’s Viking 1 Orbiter and Lander launched from Cape Canaveral, Florida. Eleven months and half a billion miles later the Viking 1 lander touched down on Mars and sent home the first picture ever taken on the Martian surface. It would be just one of more than 16,000 more taken of the Red Planet by the Viking project.

The amount of land surface available on Mars is almost equal to that available on the Earth.
Duration/length of a day on different planets
Mercury    1408 hours
Venus    5832 hours
Earth    24 hours
Mars    25 hours
Jupiter    10 hours
Saturn    11 hours
Uranus    17 hours

Cartorical Mars Topographic Map

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Mars has all four seasons as that of Earth. However, each season on the Red Planet lasts twice as long as that on Earth.
Due to the close proximity of Mars to Earth and the hope of establishing life on it, Mars has been extensively studied and further investigation and exploration of the planet is going on.
Elon Musk – one of the legendary silicon valley entrepreneur and the man behind “Tesla Motors and Space X” – wants to put man on Mars and make life multi-planetary.
Neptune    16 hours

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In this handout provided by NASA, a SpaceX Falcon 9 rocket launches from Kennedy Space Center on June 3, 2017.

NASA's Opportunity Mars rover has now been exploring the Red Planet for a decade. The golf-cart-size robot landed on the night of Jan. 24, 2004, three weeks after its twin, Spirit. Spirit and Opportunity were originally tasked with 90-day missions that called for them to search for signs of past water activity on the Red Planet. Both rovers far outlasted their warranties; Spirit was declared dead in 2011, and Opportunity continues roving to this day. And they both made big discoveries that have fundamentally reshaped scientists' understanding of Mars and its environmental history. 

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Curiosity is a car sized rover designed to explore Gale Crater on Mars as part of NASA's Mars Science Laboratory mission (MSL), Curiosity was launched from Cape Canaveral on November 26, 2011, at 15:02 UTC aboard the MSL spacecraft and landed on Aeolis Palus in Gale Crater on Mars on August 6, 2012, 05:17 UTC. The Bradbury Landing site was less than 2.4 km (1.5 mi) from the center of the rover's touchdown target after a 560 million km (350 million mi) journey. The rovers goals include an investigation of the Martian climate and geology assessment of whether the selected field site inside Gale Crater has ever offered environmental conditions favorable for microbial life, including investigation of the role of Water and planetary habitability studies in preparation for human exploration.

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MRO image of Gale Crater illustrating the landing location and trek of the Rover Curiosity. In 2 years, Curiosity traversed 3 miles to reach the base of Mount Sharp. The few years of trekking are likely to be at least as challenging. (Credits: NASA/JPL, illustration, T.Reyes)

In December 2012, Curiosity's two-year mission was extended indefinitely. On August 5, 2017, NASA celebrated the fifth anniversary of the Curiosity rover landing and related exploratory accomplishments on the planet Mars. The rover is still operational, and as of August 26, 2018, Curiosity has been on Mars for 2152 sols 2211 total days since landing on August 6, 2012.Curiosity has a mass of 899 kg (1,982 lb) including 80 kg (180 lb) of scientific instruments. The rover is 2.9 m (9.5 ft) long by 2.7 m (8.9 ft) wide by 2.2 m (7.2 ft) in height.

Curiosity is powered by a radiosotope thermoelectric generator (RTG) 

What Has The Curiosity Rover Discovered? A Collaboration With Joe Scott

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Curiosity is powered by a radiosotope thermoelectric generator (RTG) 
The multi-mission radioisotope thermoelectric generator (MMRTG) is a type of radiosotope thermoelectric generator developed for NASA space missions such as the Mars Science Laboratory (MSL) under the jurisdiction of the United States Department of Energy's Office of Space and Defense Power Systems within the Office of Nuclear Energy.

VENUS

Venus and Earth are often called twins because they are similar in size mass density composition and gravity.Venus is the hottest world in the solar system. Although Venus is not the planet closest to the sun, its dense atmosphere traps heat in a runaway version of the greenhouse effect that warms Earth. As a result, temperatures on Venus reach 870 degrees Fahrenheit (465 degrees Celsius), more than hot enough to melt lead. Probes that scientists have landed there have survived only a few hours before being destroyed by the intense heat and extremely acidic environment. Venus has a hellish atmosphere consisting mainly of carbon dioxide with clouds of sulfuric acid, and scientists have only detected trace amounts of water in the atmosphere. 

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The atmosphere is heavier than that of any other planet, leading to a surface pressure 90 times that of Earth. Incredibly, however, early in Venus' history the planet may have been habitable according to models from NASA researchers at the Goddard Institute for Space Studies. The surface of Venus is extremely dry. During its evolution, ultraviolet rays from the sun evaporated water quickly, keeping it in a prolonged molten state. There is no liquid water on its surface today because the scorching heat created by its ozone-filled atmosphere would cause any to boil away. Roughly two-thirds of the Venusian surface is covered by flat, smooth plains that are marred by thousands of volcanoes, some which are still active today, ranging from about 0.5 to 150 miles (0.8 to 240 kilometers) wide, with lava flows carving long, winding canals up to more than 3,000 miles (5,000 km) in length, longer than on any other planet.

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Picture taken by Venera

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The hellish surface of Venus. The apparent distortion on the horizon is due to atmospheric refraction from the super-dense air.

Six mountainous regions make up about one-third of the Venusian surface. One mountain range, called Maxwell, is about 540 miles (870 km) long and reaches up to some 7 miles (11.3 km) high, making it the highest feature on the planet.

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Maat Mons is displayed in this 3-dimensional perspective view of the surface of Venus taken by NASA Magellan. The viewpoint is located north of Maat Mons. Maat Mons is a massive shield volcano, It is the second-highest mountain, and the highest volcano, on the planet Venus, It rises 8 kilometres (5.0 mi) above the mean planetary radius at 0.5N 194.6 E and nearly 5 km above the surrounding plains. Maat Mons has a large summit caldera 28×31 km in size. Within the large caldera there are at least five smaller collapsed craters up to 10km in diameter.

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Surface features on Venus

Venus also possesses a number of surface features unlike anything on Earth. For example, Venus has coronae, or crowns — ring-like structures that range from roughly 95 to 360 miles (155 to 580 km) wide. Scientists believe these formed when hot material beneath the crust rises up, warping the planet's surface. Venus also has tesserae, or tiles — raised areas in which many ridges and valleys have formed in different directions.

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With conditions on Venus that could be described as infernal, the ancient name for Venus — Lucifer — seems to fit. However, this name did not carry any fiendish connotations; Lucifer means "light-bringer," and when seen from Earth, Venus is brighter than any other planet or even any star in the night sky because of its highly reflective clouds and its closeness to our planet.

Venus takes 243 Earth-days to rotate on its axis, by far the slowest of any of the major planets, and because of this sluggish spin, its metal core cannot generate a magnetic field similar to Earth's.
If viewed from above, Venus rotates on its axis the opposite way that most planets rotate. That means on Venus, the sun would appear to rise in the west and set in the east. On Earth, the sun appears to rise in the east and set in the west.

The Venusian year — the time it takes to orbit the sun — is about 225 Earth-days long. Normally, that would mean that days on Venus would be longer than years. However, because of Venus' curious retrograde rotation, the time from one sunrise to the next is only about 117 Earth-days long.

VENUS & MERCURY - A Traveler's Guide to the Planets | Full Documentary

Orbit and rotation.
Average distance from the sun: 67,237,910 miles (108,208,930 km). By comparison: 0.723 times that of Earth

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Venus has a very complicated rotation which makes a year 224 Venus days. It's rotation is different then any other planet in the solar system.

Perihelion (closest approach to sun): 66,782,000 miles (107,476,000 km). By comparison: 0.730 times that of Earth

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Aphelion (farthest distance from sun): 67,693,000 miles (108,942,000 km). By comparison: 0.716 times that of Earth.

Atmospheric composition (by volume) 96.5 percent carbon dioxide, 3.5 percent nitrogen, with minor amounts of sulfur dioxide, argon, water, carbon monoxide, helium and neon.

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Magnetic field: 0.000015 times that of Earth's field.
Internal structure: Venus's metallic iron core is roughly 2,400 miles (6,000 km) wide. Venus' molten rocky mantle is roughly 1,200 miles (3,000 km) thick. Venus' crust is mostly basalt, and is estimated to be six to 12 miles (10 to 20 km) thick on average.Climate
The very top layer of Venus' clouds zip around the planet every four Earth-days, propelled by hurricane-force winds traveling roughly 224 mph (360 kph). This super-rotation of the planet's atmosphere, some 60 times faster than Venus itself rotates, may be one of Venus' biggest mysteries. The winds at the planet's surface are much slower, estimated to be just a few miles per hour.

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volcanoes on the surface of Venus imaged by the Magellan spacecraft.

The Venus Express spacecraft, a European Space Agency mission that operated between 2005 and 2014, intriguingly found evidence of lightning on the planet. This lightning is unique from that found on the other planets in the solar system, in that it is not associated with water clouds. Instead, on Venus, the lightning is associated with clouds of sulfuric acid. Scientists are excited by these electrical discharges, because they can break molecules into fragments that can then combine with other fragments in unexpected ways.

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The European Space Agency’s Venus Express spacecraft has discovered an ozone layer high in the atmosphere of Venus. Comparing its properties with those of the equivalent layers on Earth and Mars will help astronomers refine their searches for life on other planets.

Unusual stripes in the upper clouds of Venus are dubbed "blue absorbers" or "ultraviolet absorbers" because they strongly absorb light in the blue and ultraviolet wavelengths. These are soaking up a huge amounts of energy — nearly half of the total solar energy the planet absorbs. As such, they seem to play a major role in keeping Venus as hellish as it is. Their exact composition remains uncertain;

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This is a false-colour image taken with the Venus Monitoring Camera (VMC) on board ESA’s Venus Express. It shows the full view of the southern hemisphere from equator (right) to the pole. The south pole is surrounded by a dark oval feature. Moving to the right, away from the pole and towards the equator, we see streaky clouds, a bright mid-latitude band and mottled clouds in the convective sub-solar region.

This image was taken in the ultraviolet at 365 nanometres on 23 July 2007 as Venus Express was 35 000 km from the surface of the planet.

Research & exploration
The United States, Soviet Union, European Space Agency and Japanese Aerospace Exploration Agency have deployed many spacecraft to Venus, more than 20 in all so far. NASA's Mariner 2 came within 21,600 miles (34,760 km) of Venus in 1962, making it the first planet to be observed by a passing spacecraft. The Soviet Union's Venera 7 was the first spacecraft to land on another planet, and Venera 9 returned the first photographs of the Venusian surface. The first Venusian orbiter, NASA's Magellan, generated maps of 98 percent of the planet's surface using radar, showing details of features as small as 330 feet (100 meters) across.

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Vanera 9 manufacturer's designation: 4V-1 No. 660, was a Soviet unmanned space mission to Venus, It consisted of an orbiter and a lander. It was launched on June 8, 1975, at 02:38:00 UTC and had a mass of 4,936 kilograms (10,882 lb).The orbiter was the first spacecraft to orbit Venus, while the lander was the first to return images from the surface of another planet. It was the first spacecraft to return an image from the surface of another planet. Many of the instruments began working immediately after touchdown and the cameras were operational 2 minutes later. These instruments revealed a smooth surface with numerous stones. The lander measured a light level of 14,000 lux, similar to that of Earth in full daylight but no direct sunshine.Venera 9 measured clouds that were 30–40 km (19–25 mi) thick with bases at 30–35 km (19–22 mi) altitude. It also measured atmospheric chemicals including hydrochloric acid, bromine, and iodine. Other measurements included surface pressure of about 9,100 kilopascals (90 atm) temperature of 485 °C (905 °F), and surface light levels comparable to those at Earth mid-latitudes on a cloudy summer day. Venera 9 was the first probe to send back black and white television pictures from the Venusian surface showing no shadows, no apparent dust in the air, and a variety of 30 to 40 cm (12 to 16 in) rocks which were not eroded. Planned 360-degree panoramic pictures could not be taken because one of two camera lens covers failed to come off, limiting pictures to 180 degrees. This failure recurred with Vanera 10.

Japan's Akatsuki launched to Venus in 2010, but its main engine died during a pivotal orbit-insertion burn, sending the craft hurling into space. Using smaller thrusters, the Japanese team successfully performed a burn to correct the spacecraft's course. A subsequent burn in November 2015 successfully put Akatsuki into orbit around the planet. In 2017, having successfully achieved a modified science orbit around Venus, Akatsuki spotted another huge gravity wave in Venus's atmosphere. Venus' gravity is almost 91 percent of Earth's, so you could jump a little higher and objects would feel a bit lighter on Venus, compared with Earth. "You probably wouldn't notice the difference in gravity so much, but what you would notice is the dense atmosphere," Svedhem said. "The air is so thick that if try to move your arm quickly, you would feel resistance. It would almost be like being in water."

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An artist rendering of the Akatsuki spacecraft over Venus. Image Credit: JAXA

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Likewise, it'd be hard to miss the change in atmospheric pressure. At sea level on Earth, the air presses down on our bodies at 14.5 pounds per square inch, or 1 bar; the surface pressure on Venus is 92 bar. To experience that pressure on Earth, you'd have to travel more than 3,000 feet (914 m) down into the ocean.
In Venus's atmosphere, winds travel up to 249 mph (400 km/h) — faster than tornado and hurricane winds on Earth. But on the planet's surface, the wind only travels at about 2 mph (3 km/h). And though the planet does have lightning, the blinding flashes never reach the surface. Additionally, the blistering heat prevents any rainstorms from touching ground on Venus.

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Venus likely doesn't have earthquakes because it lacks tectonic plate activity that releases heat from its interior. Instead, what may happen is that the heat builds to a critical point over millions of years, and then suddenly gets released from some kind of mechanism, such as large-scale volcanic activity that remolds the surface of the planet.The atmosphere of Venus is very thick and is about 90 times more massive than Earth's atmosphere. It is mostly carbon dioxide gas (about 96%), with some nitrogen (about 3%) and a very small amount of water vapor (0.003%).Air on Venus
The atmosphere of Venus is very hot and thick. You would not survive a visit to the surface of the planet - you couldn't breathe the air, you would be crushed by the enormous weight of the atmosphere, and you would burn up in surface temperatures high enough to melt lead.

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Neptune compared to Earth. Source: Wikipedia

Neptune is the eighth and farthest away known planet from the sun in our Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than it's near twin Uranus which is 15 times the mass of Earth and slightly larger than Neptune. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 Billion km). Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium along with traces of hydrocarbons and possibly nitrogen. but it contains a higher proportion of "ices" such as water, ammonia and methane.  However, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasize this distinction.Traces of methane in the outermost regions in part account for the planet's blue appearance. The cause of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (−218 °C; −361 °F). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C; 9,300 °F) Neptune has a faint and fragmented ring system  (labelled "arcs"), which was discovered in 1982, then later confirmed by Voyager 2.

Triton is Neptune’s largest moon and is the only large moon in the solar system to orbit in the opposite direction to its planet’s rotation, this is known as a retrograde orbit.

Moon Profile
Diameter:    2,706.8 km
Mass:    2.14 × 10^22 kg (29.1% Moon)
Orbits:
Neptune
Orbit Distance:
354,759 km
Orbit Period:
5.9 days (retrograde)
Surface Temperature:    -135 °C
Discovery Date:    October 10, 1846
Discovered By:    William Lassell

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William Lassell,(18 June 1799 – 5 October 1880) was an English merchant and astronomer, He is remembered for his improvements to the reflecting telescope and his ensuing discoveries of four planetary satellites.

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Global orthographic view of Triton

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The Geysers of Triton.

Triton is a frozen wonderland, exhibiting a strange array of terrain types. Its icy surface has craters, geysers, and rugged landscape called “cantaloupe terrain”. These all indicate some sort of activity going on inside, and cryovolcanism spouting material to the surface.
The geysers of Triton spewing nitrogen gas out from beneath the surface into long plumes that rise as high as 8 kilometres. As a result, Triton has a very thin nitrogen atmosphere.
The southern polar cap of Triton is covered with frozen nitrogen and methane. There may be a north polar cap as well.

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Map of Triton Moon of Neptune

Triton could be divided into layers of ice around a rocky core. The crust is largely water ice. There could be subsurface ocean of slushy or liquid water.
Triton orbits Neptune in retrograde — that is, opposite to the direction of Neptune’s rotation on its axis. This may imply that Triton was captured by Neptune’s gravity into its inclined orbit.
Triton rotates once on its axis as it orbits the planet. It keeps the same face toward Neptune at all times.
Triton will wander too close to Neptune in its orbit in about 3.5 billion years, and the gravitational pull of the planet will break Triton up. The result will be a ring system.
Voyager 2 was the only spacecraft to visit and map Triton. It flew by in 1989. There are no other missions planned to Neptune or Triton for the foreseeable future.

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Titan is the Saturn’s largest moon and is the second largest moon in our solar system. If it were not orbiting Saturn, Titan could be considered a planet as it is larger than Mercury. Titan is covered with a thick atmosphere that some consider to be similar to that of early Earth.

Titan Moon Profile
Diameter:    5,149.4 km
Mass:    1.35 × 10^23 kg (1.8 Moons)
Orbits:
Saturn
Orbit Distance:
1,221,865 km
Orbit Length:
15.9 days
Surface Temperature:    -179 °C
Discovery Date:    March 25, 1655
Discovered By:    Christiaan Huygens

 

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Christiaan Huygens
14 April 1629 – 8 July 1695) was a Dutch physicist, mathematician astronomer and inventor.  who is widely regarded as one of the greatest scientists of all time and a major figure in the scientific revolution. His most famous invention was that of the pendulum clock in 1956 he also invented the Huygenian eyepiece which improved the design of the telescope. In 1655, Huygens proposed that Saturn was surrounded by a solid ring, "a thin, flat ring, nowhere touching, and inclined to the ecliptic." Using a 50 power refracting telescope that he designed himself, Huygens also discovered the first of Saturn's moons, Titan. In the same year he observed and sketched the Orion Nebula, Using his modern telescope he succeeded in subdividing the nebula into different stars. The brighter interior now bears the name of the Huygenian region in his honour. He also discovered several interstellar nebula into different stars.In 1659, Huygens was the first to observe a surface feature on another planet, Syrtis Major a volcanic plain on Mars.He used repeated observations of the movement of this feature over the course of a number of days to estimate the length of day on Mars, which he did quite accurately to 24 1/2 hours. This figure is only a few minutes off of the actual length of the Martian day of 24 hours, 37 minutes.

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Titan's methane cycle is strikingly similar to Earth’s hydrologic cycle and the only other one known to include stable bodies of surface liquids, such as this north polar sea Ligeia Mare. The Cassini mission has characterized Titan’s surface liquid inventory and Ligeia Mare is now known to have a mixed composition of methane, ethane, and dissolved nitrogen. The sea appeared quiescent throughout the 90 Kelvin north polar winter, but on July 10th, 2013 transient features were discovered. Dynamic phenomena are expected to occur with increased frequency and intensity as the 2017 northern summer solstice approaches and will afford Cassini the opportunity to begin characterizing the nature of energetic processes in these alien seas. This image has been modified for aesthetic appeal and is shown in false colour. Credit: NASA/JPL-Caltech/ASI/Cornell
 

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Titan’s diameter is 50 percent larger than Earth’s Moon, making it among the largest natural satellites in the solar system.
Titan’s most obvious feature is its heavy, hazy atmosphere. The most abundant gas is nitrogen, with methane and ethane clouds and a thick organic smog.
It was discovered in 1655 by Dutch astronomer Christiaan Huygens. It is named for mythological Titans, the brothers and sisters of the Greek god Cronus.
The composition of Titan is known to be water ice over a rocky interior. Its surface has liquid hydrocarbon lakes and the vents of cryovolcanoes, distributed among areas of bright and dark terrain that show evidence of some impact cratering.

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In 2005 the robotic Huygens probe landed on Titan, Saturn's enigmatic moon, and sent back the first ever images from beneath Titan's thick cloud layers. This artist's impression is based on those images. In the foreground, sits the car-sized lander that sent back images for more than 90 minutes before running out of battery power. The parachute that slowed Huygen's re-entry is seen in the background, still attached to the lander. Smooth stones, possibly containing water-ice, are strewn about the landscape. Analyses of Huygen's images and data show that Titan's surface today has intriguing similarities to the surface of the early Earth.

Image Credit: ESA

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Descent to Titan

Titan is thought to have several layers: a rocky core, surrounded by layers of crystalline ice. It is likely that the core is still hot, with a layer of liquid water and ammonia.
Like other moons around their primary planets, Titan has a rotation period that is the same as its orbital period. That means it turns on its axis in the same length of time as it takes to orbit Saturn.
Titan may have formed as material in orbit around early Saturn began to accrete. Giant impacts and collisions may have disturbed the orbits of Titan and other moons into their current positions.
Several probes have imaged Titan, but only one has visited the surface — the Huygens lander. It arrived on January 14, 2005, and sent data for about an hour and a half, making it the most distant landing of any mission in the solar system.

For the probe landing’s 10th anniversary, a new sequence has been rendered from Huygens’ Descent Imager/Spectral Radiometer (DISR) data. The craft landed on Saturn’s largest moon on 14 Jan 2005.

In this video, we will visit Titan, and see whether or not it is a habitable planet...

Io is the innermost and the second smallest of the four Galilean moons. It was discovered, along with Europa, Ganymede and Callisto by Galileo Galilei in 1610.

Io Moon Profile
Diameter:    3,643.2 km
Mass:    8.93 x 10^22 kg (1.2 Moons)
Orbits:
Jupiter
Orbit Distance:
421,800 km
Orbit Period:
1.77 days
Surface Temperature:    -163 °C
Discovery Date:    January 8, 1610
Discovered By:    Galileo Galilei

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Portrait of Galileo Galilei by Giusto Sustermans (1636). Credit: nmm.ac.uk

When it comes to scientists who revolutionized the way we think of the universe, few names stand out like Galileo Galilei. A noted inventor, physicist, engineer and astronomer, Galileo was one of the greatest contributors to the Scientific Revolution. He build telescopes, designed a compass for surveying and military use, created a revolutionary pumping system, and developed physical laws that were the precursors of Newton's law of Universal Gravitation and Einstein's theory of Relativity. Galileo was born in Pisa, Italy, in 1564, into a noble but poor family. He was the first of six children of Vincenzo Galilei and Giulia Ammannati, who’s father also had three children out of wedlock. Galileo was named after an ancestor, Galileo Bonaiuti (1370 – 1450), a noted physician, university teacher and politician who lived in Florence.

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Io has more than 400 active volcanoes on its surface. They make this little moon the most actively volcanic world in the solar system.
The volcanism on Io is due to tidal heating, as the moon is stretched by Jupiter’s strong gravitational pull and by the lesser gravitational effects of the other satellites.
The volcanoes of Io are constantly erupting, creating plumes that rise above the surface and lakes that cover vast areas of the landscape.
Io has a very thin atmosphere that contains mostly sulfur dioxide (emitted from its volcanoes). Gases from the atmosphere escape to space at the rate of about a ton per second. Some of the material becomes part of a ring of charged particles around Jupiter called the Io plasma torus.

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Interactive Sphere

Io has often been described as looking like a pizza covered with melted cheese, tomato sauce and olives. The reason for this distinct surface is its vast number of active volcanoes. There are hundreds of volcanoes scattered over the surface of the moon, which is a bit larger than Earth’s Moon. Many of the volcanoes are still active and Voyager 1 and 2 were able to capture pictures of erupting volcanoes with plumes as tall as 190 miles.

The path of Io around Jupiter is highly elliptical causing the tidal forces exerted on the moon to be immense. The effect of this is that the solid body of the moon can bulge out to almost 330 feet. This movement makes the moon incredibly hot, keeping the subsurface crust in a liquid state. This liquid sub-layer is one of the reasons for the high volcanic activity. One result of the volcanic activity is that there are very few crater marks as new lava is constantly filling in any craters that are created. Because of this, Io has a very young surface.

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Io is the most volcanic world in the solar system. Slightly larger than our Moon, the world fosters multiple erupting volcanoes on a daily basis, some of which shoot plumes of lava 250 miles (400 kilometers) above the surface. Vast lakes and rivers of dark magma flow while a caldera named Loki emits more heat than all the volcanoes on Earth combined.

Why so hot?Io is caught in a cosmic tug-of-war between gigantic Jupiter on one side and large jovian moons (Europa and Ganymede) on the other. The varying gravitational pulls stretch Io like a rubber band. On Earth, we experience ocean tides from the Moon’s gravity. On Io, there is a ground swell.

The stretching of all that rock produces heat, which melts a layer not far below the moon’s crust that then bursts out of volcanoes that pepper its surface. Io is literally turning itself inside out.

The satellite’s rapidly changing terrain presents a challenge for mapmakers. In April 1997, the Galileo spacecraft imaged Pele, Io’s most distinctive volcano, and its ring of bright orange sulfur deposits 870 miles (1,400km) in diameter. On a subsequent flyby five months later, a mountain the size of Arizona had materialized on Pele’s flank.

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Pillan Patera is a patera or a complex crater with scalloped edges, on Jupiters moon IO, It is located at 12.34S 243.25W south of Pillam Mons and West of Reiden Patera, It is named after the Araucanian thunder, fire and volcano god, Pillan Patera is approximately 70 kilometers in diameter.[1] In the summer of 1997, it erupted in an event now defined as "Pillanian" eruption style. At temperatures higher than 1,600 °C, (2912 °F) a 140 kilometer high plume eruption deposited dark pyroclastic materials rich in orthopyroxene over an area greater than 125,000 km2. This was followed by the emplacement of over 3,100 km2 in dark flow-like material north of the caldera. The high temperature part of the eruption lasted from 52 to 167 days and between May and September 1997, with peak eruption temperatures around June 28, 1997.

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We identify nine new faint thermal sources on Io via color ratio images constructed from relatively high spatial resolution Galileo NIMS data acquired late in the mission. All of these identifications are associated with small dark paterae. We utilize NIMS data to quantify their volcanic thermal emission as ∼0.53 × 1012 W (or ∼0.5% of Io’s total heat flow). In addition, we refine our previous estimates of the thermal emission from 47 hot spots and highlight several hot spots within the Amirani flow field.

Small dark paterae still out-number faint (close to the limit of detection) hot spots identified in high spatial resolution multi-wavelength NIMS data. In particular, we point out 24 small dark paterae that were scanned by NIMS (at resolutions down to ∼17 km/pixel) but had no detectable volcanic thermal emission. All dark paterae are expected to have some volcanic thermal emission, but the small size and finite number of detectable faint sources limit their contribution to the total heat flow on Io. Compared to small paterae, small dark flows are more numerous but must have significantly lower surface temperatures.

Finally, we update and summarize our results for the global heat flow on Io due to 242 recently active volcanic features including other dark paterae as well as large dark flows. The volcanic thermal emission from known hot spots, undetected (scanned) dark patera and outbursts can account for only ∼56.2 × 1012 W (or ∼54%) of Io’s total heat flow. Approximately 49 × 1012 W (or ∼46%) of Io’s heat flow remains an enigma.

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Model of the possible interior composition of Io with various features labelled. Credit: Wikipedia

The volcanic plumes of Io rise up as high as 200 km, showering the terrain with sulfur, sulfur dioxide particles, and rocky ash.
Io has a number of mountains, some of which rise up as high as Mount Everest on Earth. The average height of Io’s peaks are around 6 km.
Io is made mostly of silicate rocks, and its surface is painted with sulfur particles from the volcanoes and frosts that are created as the atmospheric gases freeze out and fall to the ground.
Robotic missions to Io could study its volcanism in closer detail. No human missions are planned as yet, due to the extreme radiation environment and highly toxic atmosphere and surface.

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Tupan Patera, seen here, expels both hot black lava and red sulfur. The rim is around 3,000 feet high, and the lake itself is about 50 miles across.

The 1997 eruption was the largest effusive eruption ever witnessed. During a 100-day period, at least 31 km3 of lava were erupted, with 25 km3 shortly afterward. The eruption sheds light on emplacement of very large, voluminous flows millions of years ago on Mars and Earth. The highest effusion rates exceeded 10,000 cubic meters per second.[3] The eruption produced a large, dark, deposit, 400 kilometers in diameter, which surrounds Pillan and partially covers a bright red ring left by the volcano Pele's plume. Since the eruption, the Pillan plume deposit has faded, coated by material from Pele and Kami-Nari Patera a a small volcano to the east of Pillan Patera.

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Ganymede is Jupiter’s largest moon and also the largest moon in the solar system. Ganymede was discovered in 1610 by astronomer Galileo Galilei and is named for the mythical Greek son of a King who was carried to the sky by Zeus posing as an eagle.

Ganymede Moon Profile
Diameter:    5,262.4 km
Mass:    1.48 x 10^23 kg (2.0 Moons)
Orbits:
Jupiter
Orbit Distance:
1,070,400 km
Orbit Period:
7.16 days
Surface Temperature:    -163 °C
Discovery Date:    January 7, 1610
Discovered By:    Galileo Galilei

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Detail of Galileo Galilei and his Telescope - engraving 1864 by Corbis

Ganymede is the largest moon in the solar system, and is larger than the planet Mercury. If it were not orbiting as Jupiter’s second-largest moon, it could be considered a dwarf planet.
Ganymede is the only moon in the solar system known to have a substantial magnetosphere. That implies there is something inside helping to generate a strong magnetic field.
Like Europa, Ganymede is thought to have a subsurface ocean, overlying a liquid iron and nickel core. That core is what helps generate the magnetic field.
The surface of Ganymede is icy and covered with two main types of landscape: young, lighter regions and darker, older and cratered terrain. The dark areas appear to contain clays and organic materials.

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Artist's concept for a future settlement on Ganymede. Credit: futuretimeline.net

Ganymede was likely formed in place around the infant Jupiter in the early solar system. Several smaller worlds likely accreted together to make this moon.
Ganymede has a thin atmosphere that appears to contain oxygen. This was confirmed by Hubble Space Telescope observations. The oxygen is likely freed as water ice on the surface is broken apart into hydrogen and oxygen by solar radiation.
The first mission to explore Ganymede up close was Pioneer 10, followed by the Voyager missions, Galileo, and New Horizons.
Several missions to explore Ganymede in more detail have been suggested, but most have been cancelled or are still on the drawing boards.

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Enceladus is the sixth-largest moon of Saturn and, after Titan, one of the most-studied worlds in the system. It was discovered in 1789 by William Herschel and named after the Greek mythological giant Enceladus.

Enceladus Moon Profile
Diameter:    504.2 km
Mass:    1.08 × 10^20 kg (0.1% Moon)
Orbits:
Saturn
Orbit Distance:
238,037 km
Orbit Length:
1.4 days
Surface Temperature:    -198 °C
Discovery Date:    August 28, 1789
Discovered By:    William Herschel

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William Herschel Telescope. WHT

Enceladus was first studied in detail by the Voyager spacecraft. The Cassini mission did close flybys of this moon, to map its surface in high resolution.
Enceladus is a largely icy world with some percentage of its mass being silicates. It appears to have a rocky core mixed with with water ice, and a frozen mantle.
Cryovolcanic activity in Enceladus is sending geysers of water ice particles out from underneath the surface. The Cassini spacecraft has imaged these geysers spouting from so-called “tiger stripes” vent areas on this moon.
The icy particles from Enceladus spread out to space and feed the nearby E-ring with material.

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CGI rendition of Cassini's flyby of Enceladus with Saturn in the background.

The volcanic action on Enceladus led scientists to suggest that a liquid water ocean lies under the surface of this moon, and is feeding the geysers seen by Cassini.
Enceladus is thought to be heated from within by either radioactive heating (the decay of radioactive elements in the core) or tidal flexing as Saturn’s immense gravity pulls on the moon.
Future missions have been proposed to explore Enceladus and perhaps bring back samples of its icy plume material. These would also study the other moons of Saturn, plus the ring system.
As with Europa at Jupiter, scientists suspect that Enceladus could be a habitable world to some forms of life. There is no proof of life there, but future missions could test for life signs.
Enceladus is now known to have a subsurface ocean made of liquid water. Images from the Cassini spacecraft helped mission scientists deduce and prove the existence of that ocean.

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Image Spectra of Mercury

Mercury is the smallest and innermost planet in the Solar System and it's orbital period around the sun lasts 87.97 days being the shortest of all the planets in the Solar System. Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet and never exceeds 28° away from the sun. 
 

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Mercury Planet Map

For every two orbits of the Sun, Mercury completes three rotations about its axis and up until 1965 it was thought that the same side of Mercury constantly faced the Sun. Thirteen times a century Mercury can be observed from the Earth passing across the face of the Sun in an event called a transit, the next will occur on the 9th May 2016.
Mercury Planet Profile
Diameter:    4,879 km
Mass:    3.30 x 10^23 kg (5.5% Earth)
Moons:    None
Orbit Distance:    57,909,227 km (0.39 AU)
Orbit Period:    88 days
Surface Temperature:    -173 to 427°C
First Record:    14th century BC
Recorded By:    Assyrian astronomers

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Image Right - Composite image of the north pole of Mercury. Red are the areas of permanent shadow; yellow delineates radar bright deposits mapped from Earth. Data are plotted on a photomosaic of MESSENGER images. NASA

Mercury does not have any moons or rings.
Your weight on Mercury would be 38% of your weight on Earth.
A day on the surface of Mercury lasts 176 Earth days.
A year on Mercury takes 88 Earth days.
Mercury has a diameter of 4,879 km, making it the smallest planet.
It’s not known who discovered Mercury.

Unlike many other planets which “self-heal” through natural geological processes, the surface of Mercury is covered in craters. These are caused by numerous encounters with asteroids and comets. Most Mercurian craters are named after famous writers and artists. Any crater larger than 250 kilometres in diameter is referred to as a Basin. The Caloris Basin is the largest impact crater on Mercury covering approximately 1,550 km in diameter and was discovered in 1974 by the Mariner 10 probe.

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Craters on Mercury appear to form the image of Mickey Mouse. This scene lies to the northwest of the recently named crater Magritte, in Mercury's south. The image is not map projected; the larger crater actually sits to the north of the two smaller ones. Date acquired: June 3, 2012.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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The Mickey Mouse on Mercury is formed by a huge crater about 65 miles (105 kilometers) wide that was later peppered by other impacts to create the "ears." The scene is located to the northwest of another crater that Messenger scientists recently dubbed "Magritte" in Mercury's southern region. Despite being further from the Sun, Venus experiences higher temperatures. The surface of Mercury which faces the Sun sees temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. This is due to the planet having no atmosphere to help regulate the temperature.
Even though the planet is small, Mercury is very dense. Each cubic centimetre has a density of 5.4 grams, with only the Earth having a higher density. This is largely due to Mercury being composed mainly of heavy metals and rock.
One of five planets visible with the naked eye a, Mercury is just 4,879 Kilometres across its equator, compared with 12,742 Kilometres for the Earth.

Despite being further from the Sun, Venus experiences higher temperatures. The surface of Mercury which faces the Sun sees temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. This is due to the planet having no atmosphere to help regulate the temperature.
Even though the planet is small, Mercury is very dense. Each cubic centimetre has a density of 5.4 grams, with only the Earth having a higher density. This is largely due to Mercury being composed mainly of heavy metals and rock.
One of five planets visible with the naked eye a, Mercury is just 4,879 Kilometres across its equator, compared with 12,742 Kilometres for the Earth

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The Rembrandt impact basin was discovered by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during its second flyby of Mercury in October 2008.  Images show that the Rembrandt basin is remarkably well preserved. Most large impact basins on Mercury, the Moon, and other inner planets are flooded by volcanic flows that cover their entire floor.  The number per area and size distribution of impact craters superposed on Rembrandt’s rim indicates that it is one of the youngest impact basins on Mercury. 

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Ten years ago, on August 3, 2004, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft blasted off from Cape Canaveral, Florida, for a risky mission that would take the small satellite dangerously close to Mercury’s surface, paving the way for an ambitious study of the planet closest to the Sun. The spacecraft traveled 4.9 billion miles (7.9 billion kilometers) — a journey that included 15 trips around the Sun and flybys of Earth once, Venus twice, and Mercury three times — before it was inserted into orbit around its target planet in 2011.

 

“We have operated successfully in orbit for more than three Earth years and more than 14 Mercury years as we celebrate this amazing 10th anniversary milestone,” said MESSENGER Mission Operations Manager Andy Calloway, of the Johns Hopkins University Applied Physics Laboratory (APL). “The MESSENGER spacecraft operates in one of the most challenging and demanding space environments in our Solar System, and we have met that challenge directly through innovation and hard work, as exemplified by the stunning discoveries and data return achievements. Our only regret is that we have insufficient propellant to operate another 10 years, but we look forward to the incredible science returns planned for the final eight months of the mission.”

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MESSENGER is only the second spacecraft sent to Mercury. Mariner 10 flew past it three times in 1974 and 1975 and gathered detailed data on less than half the surface. MESSENGER took advantage of an ingenious trajectory design, lightweight materials, and miniaturization of electronics, all developed in the three decades since Mariner 10 flew past Mercury.

"It was quite challenging to design and execute a trajectory that could culminate in Mercury orbit," said Mission and Spacecraft Systems Engineer Dan O'Shaughnessy, of APL. "Designing an attendant spacecraft that was light enough to carry the necessary propellant to execute such a trajectory with enough room left over for a payload capable of global characterization of the planet is an impressive accomplishment."

Additionally, he said, "the team's concept of operations that streamlines planning while optimizing the use of our payload -- despite substantial thermal and power constraints -- is an amazing feat." 

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Composition and Surface Features:
As one of the four terrestrial planets of the Solar System, Mercury is composed of approximately 70% metallic and 30% silicate material. Based on its density and size, a number of inferences can be made about its internal structure. For example, geologists estimate that Mercury’s core occupies about 42% of its volume, compared to Earth’s 17%.

The surface of Mercury, as photographed by the Messenger spacecraft on March 29, 2011

The interior is believed to be composed of a molten iron which is surrounded by a 500 – 700 km mantle of silicate material. At the outermost layer is Mercury’s crust, which is believed to be 100 – 300 km thick. The surface is also marked by numerous narrow ridges that extend up to hundreds of kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.
Another theory is that Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. In this scenario, Mercury would have originally been twice its present mass, but would have been subjected to temperatures of 25,000 to 35,000 K (or as high as 10,000 K) as the protosun contracted. This process would have vaporized much of Mercury’s surface rock, reducing it to its current size and composition.

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Mercury’s abnormally dark coloring has puzzled scientists for years — but a new study using NASA data has revealed the origins of the planet’s unique look. Patches of a carbon-rich material called graphite — the same stuff that’s in a pencil — cover Mercury’s surface, tinting it dark gray.
These patches are thought to come from an ancient carbon crust that's been hiding underneath Mercury's surface, a study published in Nature Geoscience say's. The carbon comes up to the surface when asteroids or other objects hit the planet, leaving large impact craters that expose the ancient materials underneath.

Exploring Mercury by Spacecraft: The MESSENGER Mission

The third lecture in the 2011 Exploring Space Lecture Series featured Sean C. Solomon, the Principal Investigator for the MESSENGER mission and the Director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. Until recently, Mercury was the least explored of the terrestrial planets, visited only by Mariner 10 in the 1970s. MESSENGER flybys in 2008 and 2009 revealed terrain seen by spacecraft for the very first time. In March 2011, as MESSENGER went into orbit, it opened a new era of comprehensive observation and study of the innermost planet, and continues to contribute to our understanding of the nature of Mercury and why it is different from its planetary neighbors. See Mercury in a new light as Sean Solomon guides us through the latest images and results. Presented as a live webcast on Thursday, May 12, 2011 at 8pm ET at the National Air and Space Museum in Washington, DC.

This could finally explain the origins of Mercury's dark surface. Experts originally thought the planet's dark patches could be made of iron, since iron makes up similar dark patches on the Moon, according to NASA. But Messenger data indicated that Mercury's surface is very iron-poor, leaving astronomers stumped about where the planet's dark coloring comes from. "There are only a few things that can darken a surface," said Francis McCubbin, an astromaterials curator at NASA Johnson Space Center, who was not involved in the study. Some experts offered the idea that carbon could explain the color, but astronomers weren't sure if the carbon came from within the planet itself or if it was brought to Mercury by asteroids.In this study, scientists used data from the final orbits of NASA's now defunct MESSENGER spacecraft, which orbited Mercury for four years. During MESSENGER's last few months in operation, 

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The MESSENGER spacecraft is shown in the deployed configuration. The sunshade protects the spacecraft from the direct solar illumination. 

NASA brought the probe closer and closer to Mercury and eventually let the spacecraft crash into the planet's surface. As MESSENGER got to lower altitudes, the spacecraft was able to analyze Mercury's surface more in-depth, allowing researchers to understand what the planet's dark patches are made of. "The investigation really happened with those last sets of measurements, on its death dive," said study author Patrick Peplowski, a research scientist at the Johns Hopkins University Applied Physics Laboratory. "It’s the spacecraft’s parting gift."

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Patrick Peplowski, a research scientist at the Johns Hopkins University Applied Physics Laboratory.

Image to the left - I am an experimental nuclear physicist specializing in nuclear (gamma-ray and neutron) spectroscopy of solar system objects, nuclear astrophysics, and radiation transport modeling. My spaceflight mission experience includes MESSENGER (GRS instrument scientist) and Dawn at Vesta (science team associate). My instrument development experience includes the Engineering Radiation Monitor on the Van Allen Probes, the Radiation Monitor Subystem on the Europa mission, and the GeMini gamma-ray spectrometer being developed under NASA's Maturation of Instrumentation for Solar System Exploration (MatISSE) program. I'm the principal investigator for grants awarded under NASA's Planetary Mission Data Analysis Program (PMDAP), Mars Data Analysis Program (MDAP), and Discovery Data Analysis Program (DDAP). My primary research focus is the elemental composition of Mercury, the Moon, and small solar-system objects (asteroids 433 Eros, 4 Vesta, 16 Psyche, Mars moons).

The researchers believe this carbon material must have formed on Mercury during its early days. Carbon minerals floated to the top of a global magma ocean that once covered the planet's entire surface, the study claims. There, the carbon hardened into a crust, and was eventually covered up by massive amounts of lava that spewed from Mercury's early volcanoes. The lava then cooled on top of the carbon, forming a secondary crust. But multiple exposures of the carbon crust mixed with the top layer, giving Mercury its dark shade.

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A number of comets and asteroids shown to scale. The smallest of these is still several times larger than a football field. 

In other words: the carbon is a very old material that's been part of Mercury since the planet first formed. "It offers us the opportunity to see an ancient surface," said Peplowski. "Studying this, we can try to better understand how terrestrial planets form."

A third hypothesis is that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost and not gathered to form Mercury. Naturally, further analysis is needed before any of these theories can be confirmed or ruled out.

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At a glance, Mercury looks similar to the Earth’s moon. It has a dry landscape pockmarked by asteroid impact craters and ancient lava flows. Combined with extensive plains, these indicate that the planet has been geologically inactive for billions of years. However, unlike the Moon and Mars, which have significant stretches of similar geology, Mercury’s surface appears much more jumbled. Other common features include dorsa (aka. “wrinkle-ridges”), Moon-like highlands, montes (mountains), planitiae (plains), rupes (escarpments) and valles (valleys).

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Mercury is too hot and too small to retain an atmosphere. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.

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Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. As a result of this and its high eccentricity, the planet experiences considerable variations in temperature. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C), while the side in shadow dips down to 100 K (-173° C).
Yes, Mercury is a planet of extremes and is riddled with contradictions. It ranges from extreme hot to extreme cold; it has a molten surface but also has water ice and organic molecules on its surface; and it has no discernible atmosphere but possessing an exosphere and magnetosphere. Combined with its proximity to the Sun, it is little wonder why we don’t know much about this terrestrial world.

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SATURN

Saturn is the sixth planet from the sun and the second largest in our solar system, Saturn lies between Jupiter and Uranus and is categorized as a giant gas planet with an average radius about nine times larger than Earth. It has only one-eighth the average density of Earth,  but with its larger volume Saturn is over 95 times more massive. Saturn's makeup is probably composed of iron-nickel and rock (Silicon and oxygen compounds). This core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally a gaseous outer layer. 
 

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Saturn has a pale yellow hue due to ammonia crystals in it's upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's, but has a magnetic moment 580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's. The outer atmosphere is generally bland and lacking in contrast, although long lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph; 500 m/s), higher than on Jupiter, but not as high as those on Neptune.

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The planets most famous feature is it's prominent ring system that is composed mostly of ice particles, with a smaller amount of rocky debris and dust, At least 62 moons are known to orbit Saturn, of which 53 are officially named. This does not include the hundreds of moonlets in the rings, Titan, Saturns's largest moon and the second largest in the solar system is larger than the planet Mercury, although less massive and is the only moon in the solar system to have a substantial atmosphere.

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Some of Saturn's Moons as Seen by Cassini

Saturn has the second-shortest day in the solar system. One day on Saturn takes only 10.7 hours (the time it takes for Saturn to rotate or spin around once), and Saturn makes a complete orbit around the Sun (a year in Saturnian time) in about 29.4 Earth years (10,756 Earth days). Its axis is tilted by 26.73 degrees with respect to its orbit around the Sun, which is similar to Earth's 23.5-degree tilt. This means that, like Earth, Saturn experiences seasons.

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Formation
Saturn took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this gas giant. About 4 billion years ago, Saturn settled into its current position in the outer solar system, where it is the sixth planet from the Sun. Like Jupiter, Saturn is mostly made of hydrogen and helium, the same two main components that make up the Sun.

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Images of Saturn’s main rings, the F Ring, and its shepherd moons obtained by NASA’s Cassini spacecraft. The narrow F Ring is located just outside of the outer edge of the main rings. Two satellites sandwiching the F Ring slightly above and to the left of the centre of the image are the shepherd satellites Prometheus (inner orbit) and Pandora (outer orbit). Image credit: NASA / Kobe University.

The rings of Saturn are the most extensive ring system of any planet in our solar system, They consist of countless particles, ranging from microns to meters in size. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation, some features of the rings suggest a relatively recent origin, but theoretical models indicate they are likely to have formed early in the Solar System's history.

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The rings have numerous gaps where particle density drops sharply,  two opened by known moons embedded within them, and many others at locations of known destabilizing orbital resonances with the moons of Saturn, other gaps remained unexplained stabilizing resonances on the other hand are responsible for the longevity of several rings such as the Titan Ringlet and the G Ring. Well beyond the main rings is the Phoebe ring which is presumed to originate from Phoebe and thus to share it's retrograde orbital motion. It is aligned with the plane of Saturn's orbit. Saturn has an axial tilt of 27 degrees, so this ring is tilted at an angle of 27 degrees to the more visible rings orbiting above Saturn's equator.

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The rings are named alphabetically in the order they were discovered. The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently. The D Ring is exceedingly faint and closest to the planet. The narrow F Ring is just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained. The dense main rings extend from 7,000 km (4,300 mi) to 80,000 km (50,000 mi) away from Saturn's equator, whose radius is 60,300 km (37,500 mi). With an estimated local thickness of as little as 10 m and as much as 1 km, they are composed of 99.9% pure water ice with a smattering of impurities that may include tholins or silicates, The main rings are primarily composed of particles ranging in size from 1 cm to 10 m.

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Cassini spacecraft picture of Saturn's north polar clouds

Based on Voyager observations, the total mass of the rings was estimated to be about 3 × 1019 kg. This is a small fraction of the total mass of Saturn (about 50 ppb) and is just a little less than the moon Mimas.More recent observations and computer modeling based on Cassini observations show that this may be an underestimate due to clumping in the rings and the mass may be three times this figure. Although the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as the innermost moon Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites (Similar to Prometheus and Pandora's maintenance of the F ring.In 1980, Voyager 1 made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.

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Mimas looks like the Death Star in this image from Cassini in 2010.

URANUS

Uranus is the seventh planet from the Sun. While being visible to the naked eye, it was not recognized as a planet due to its dimness and slow orbit. Uranus became the first planet discovered with the use of a telescope. Uranus is tipped over on its side with an axial tilt of 98 degrees. It is often described as “rolling around the Sun on its side.”
Uranus Planet Profile
Equatorial Diameter:    51,118 km
Polar Diameter:    49,946 km
Mass:    8.68 × 10^25 kg (15 Earths)
Moons:    27 (Miranda, Titania, Ariel, Umbriel & Oberon)
Rings:    13
Orbit Distance:    2,870,658,186 km (19.19 AU)
Orbit Period:    30,687 days (84.0 years)
Effective Temperature:    -216 °C
Discovery Date:    March 13th 1781
Discovered By:    William Herschel

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Planet Uranus by Voyager 2 in 1986

Uranus was officially discovered by Sir William Herschel in 1781.
It is too dim to have been seen by the ancients. At first Herschel thought it was a comet, but several years later it was confirmed as a planet. Herscal tried to have his discovery named “Georgian Sidus” after King George III. The name Uranus was suggested by astronomer Johann Bode. The name comes from the ancient Greek deity Ouranos.
Uranus turns on its axis once every 17 hours, 14 minutes.
The planet rotates in a retrograde direction, opposite to the way Earth and most other planets turn.
Uranus makes one trip around the Sun every 84 Earth years.
During some parts of its orbit one or the other of its poles point directly at the Sun and get about 42 years of direct sunlight. The rest of the time they are in darkness.

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Artist's impression of Uranus' atmosphere

Uranus is often referred to as an “ice giant” planet.
Like the other gas giants, it has a hydrogen upper layer, which has helium mixed in. Below that is an icy “mantle, which surrounds a rock and ice core. The upper atmosphere is made of water, ammonia and the methane ice crystals that give the planet its pale blue colour.
Uranus hits the coldest temperatures of any planet.
With minimum atmospheric temperature of -224°C Uranus is nearly the coldest planet in the solar system. While Neptune doesn’t get as cold as Uranus it is on average colder. The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks.
Uranus has two sets of very thin dark colored rings.

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The ring particles are small, ranging from a dust-sized particles to small boulders. There are eleven inner rings and two outer rings. They probably formed when one or more of Uranus’s moons were broken up in an impact. The first rings were discovered in 1977 with the two outer rings being discovered in Hubble Space Telescope images between 2003 and 2005.
Uranus’ moons are named after characters created by William Shakespeare and Alexander Pope.
These include Oberon, Titania and Miranda.  All are frozen worlds with dark surfaces. Some are ice and rock mixtures.  The most interesting Uranian moon is Miranda; it has ice canyons, terraces, and other strange-looking surface areas.
Only one spacecraft has flown by Uranus.
In 1986, the Voyager 2 spacecraft swept past the planet at a distance of 81,500 km. It returned the first close-up images of the planet, its moons, and rings.

Voyager 2 has discovered two "shepherd" satellites associated with the rings of Uranus. The two moons - designated 1986U7 and 1986U8 - are seen here on either side of the bright epsilon ring; all nine of the known Uranian rings are visible. 

The image was taken Jan. 21, 1986, at a distance of 4.1 million kilometers (2.5 million miles) and resolution of about 36 km (22 mi). The image was processed to enhance narrow features. The epsilon ring appears surrounded by a dark halo as a result of this processing; occasional blips seen on the ring are also artifacts. Lying inward from the epsilon ring are the delta, gamma and eta rings; then the beta and alpha rings; and finally the barely visible 4, 5 and 6 rings. The rings have been studied since their discovery in 1977, through observations of how they diminish the light of stars they pass in front of. This image is the first direct observation of all nine rings in reflected sunlight. They range in width from about 100 km (60 mi) at the widest part of the epsilon ring to only a few kilometers for most of the others. 

The discovery of the two ring moons 1986U7 and 1986U8 is a major advance in our understanding of the structure of the Uranian rings and is in good agreement with theoretical predictions of how these narrow rings are kept from spreading out. Based on likely surface brightness properties, the moons are of roughly 2O- and 3O-km diameter, respectively. 

The Voyager project is managed for NASA by the Jet Propulsion Laboratory. 

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Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL

Uranus's mass is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27 g/cm3 makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, because different figures emerge depending on the model chosen; it must be between 9.3 and 13.5 Earth masses.

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Voyager 2 Photograph - Artists Impression Of Voyager 2 At Uranus by Julian Baum

The standard model of Uranus's structure is that it consists of three layers: a rocky silicate iron nickel core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20% of Uranus's; the mantle comprises its bulk, with around 13.4 Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus's radius. The rotational period of the interior of Uranus is 17 hours, 14 minutes. As on all the giant planets  its upper atmosphere experiences strong winds in the direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours. Uranus has a ring system a magnetosphere and numerous moons.

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Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of water ice as well as frozen carbon dioxide on the surface of Titania, which in turn suggested that the moon may have a tenuous carbon dioxide atmosphere with a surface pressure of about 10 nanopascals (10−13 bar). Measurements during Titania's occultation of a star  put an upper limit on the surface pressure of any possible atmosphere at 1–2 mPa (10–20 nbar).

Titania is the largest moon that travels around Uranus and is also the eighth largest sattelite in the solar system at a diameter of 1,578 kilometres (981 mi).
Titania consists of approximately equal amounts of ice and rock and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core mantle boundary. The surface of Totania which is relatively dark and slightly red in color appears to heave been shaped by both impacts and endogenic processes. It is covered with numerous impact craters reaching up to 326 kilometers (203 mi) in diameter, but is less heavily cratered than Oberon which is the moon outermost of the five largest moons of Uranus. Titania probably underwent an early endogenic resurfacing event which obliterated its older, heavily cratered surface. Titania's surface is cut by a system of enormous canyons and scarps,  the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from an accretion disk which surrounded the planet just after its formation.

The Uranian system has a unique configuration among those of the planets because its axis of rotation is tilted sideways, nearly into the plane of its solar orbit. Its north and south poles, therefore, lie where most other planets have their equators. 

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ERIS

Eris is the most distant dwarf planet from the Sun and has the greatest mass. Eris is the second largest dwarf planet (very a close second to Pluto) and at one point was considered for the position of the 10th planet. Eris’ discovery promoted discussion that eventually lead to the classification of ‘Dwarf Planets’.

Eris Dwarf Planet Profile
Diameter:    2,326 km
Mass:    1.66 × 10^22 kg (0.23 Moons)
Orbit Distance:    10,120,000,000 km (68.01 AU)
Orbit Period:    560.9 years
Surface Temperature:    -231°C
Moons:    1 (Dysnomia)
Discovery Date:    January 5th 2005
Discovered By:    M.E. Brown C.A. Trujillo & D.L. Rabinowitz

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(M. E. Brown)

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C.A. Trujillo &         D.L. Rabinowitz

It takes icy Eris 557 Earth years to complete a single orbit around our sun. The plane of Eris' orbit is well out of the plane of the solar system's planets and extends far beyond the Kuiper Belt, a zone of icy debris beyond the orbit of Neptune.The dwarf planet is often so far from the sun that its atmosphere collapses and freezes on the surface in an icy glaze. The coating gleams brightly, reflecting as much sunlight as freshly fallen snow. Scientist's believe surface temperatures to vary from about -359 degrees Fahrenheit (-217 degrees Celsius) to -405 degrees Fahrenheit (-243 degrees Celsius). The thin atmosphere will thaw in hundreds of years as Eris gets closer to the sun, revealing a rocky surface scientists believe is similar to Pluto.

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The Kuiper belt occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer solar system extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt but is far larger - 20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed ices) such as methane ammonia and water. The Kuiper belt is home to three officially recognised dwarf planets, Pluto, Haumea, and Makemake. Some of the solar systems moons such as Neptunes Triton and Saturns Phoebe may have originated from this region.

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Much like its atmosphere, because no probe has yet to visit Eris, not much is known about its geography except speculations. Because of its distance, no images of its surface is available at this time and it is not due to be within its perihelion until approximately the 23 century. During that time, we would have a better ability to possibly be able to gain images of the surface to get a more accurate description. Until then, it is believed that the planet retains a temperature of approximately -238°C (-396°F) which keeps its methane and nitrogen in frozen form for most of its rotation. Only during its perihelion does it turn gaseous and may create a form of atmosphere. However, during its aphelion, these gases become heavy again and fall frozen back to the surface of the planet becoming the equivalent of snow and ice on Earth.

Eris is the brightest planet, 2nd most reflective body, in our solar system. The only "body" in our solar system that is brighter is Saturn's satellite Enceladus. It appears a bright white due to the methane-nitrogen rich "snow" that covers its surface. It reflects approximately 96% of all light that reaches it which is brighter than fresh snow on Earth.

Called Pluto's twin in size, it was once believed that Eris was much larger than Pluto. Recent discovery's, however, proved that Eris is in fact almost the same size. Eris is approximately 2,326 km (1,445 miles) in diameter whereas Pluto is 2,374 km (1,475 miles) in diameter. That is only a 48 km (30 mile) difference. Of course, it is not known as to how accurate their calculations of either planets size, they do strongly believe that they have Eris' size correct within 11 km (7 miles). There is also strong belief that while Eris is close in size to Pluto, it is much denser, it is estimated that Eris is 27% heavier than Pluto.

This video describes how astronomers have accurately measured the diameter of the faraway dwarf planet Eris for the first time by catching it as it passed in front of a faint star. This event was seen at the end of 2010 by telescopes in Chile, including the TRAPPIST telescope at ESO's La Silla Observatory. The observations show that Eris is an almost perfect twin of Pluto in size. Eris seems to have a very reflective surface, suggesting that it is covered in ice, probably a frozen atmosphere.

Because of its distance, information on its climate and atmosphere is currently unknown. We currently do not have a means to fully observe the planet and no probes have yet to visit this tiny world. There are, however, speculations that during its perihelion the ices on the surface turn gaseous from the warming and may create a sort of atmosphere but whether that atmosphere is held by its gravity which is very weak, 45 kg (100 lbs) on Earth is 4 kg (9 lbs) on Eris, is still unknown.

Because of the amount of methane and nitrogen that is retained by the planet, though, there is strong belief that either the planet has an internal method of renewing its methane and nitrogen or that its atmosphere does not expel enough (meaning the planet maintains it somehow) for it to lose its content. This also indicates there may be snow and dew on the surface as its weather patterns as it goes from perihelion to aphelion in its rotation.

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Only one moon has been identified thus far, Dysnomia. It orbits in a nearly circular orbit around Eris and aided in helping astronomers to calculate Eris' size and mass.

Exploration

There have been no missions to date to Eris or any missions planned for the future at this time. All observations of the dwarf planet are done through near-Earth orbiting or Earthbound telescopes. Eris' distance from Earth makes it difficult for exploration of this planet. Even traveling at the speed of light, it would take 12 hours to reach the planet which is an equivalent of 18 years at our current spacecraft velocity.

MAKEMAKE

Makemake is the second furthest dwarf planet from the Sun and is the third largest dwarf planet in the solar system. Makemake was discovered on March 31st 2005 and was recognized as a dwarf planet by the International Astronomical Union (IAU) in July 2008. Until April 2016 Makemake was thought to be the only one of the four outer dwarf planets to not have any moons.

Makemake Dwarf Planet Profile
Equatorial Diameter:    1,434 km
Polar Diameter:    1,422 km
Mass:    2-5 × 10^21 kg (0.04 Moons)
Orbit Distance:    6,850,000,000 km (45.79 AU)
Orbit Period:    309.9 years
Surface Temperature:    -239°C
Moons:    1 (MK 2 - S/2015 (136472) 1)
Discovery Date:    March 31st 2005
Discovered By:    Michael E. Brown, Chad Trujillo & David Rabinowitz

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Makemake could have been discovered earlier.
Makemake is the second brightest Kuiper Belt object after Pluto, theoretically Clyde Tombaugh (discover of Pluto) could have detected it during his search for trans-Neptunian planets around 1930. However Makemake would have been almost impossible to find against the dense background of stars of the Milky Way.
Makemake has one moon.
Discovered in April 2016 and nicknamed MK 2 (designation S/2015 (136472) 1) it is estimated to be 160 km in diameter. The moon was spotted about 20,000 km from Makemake in observations made by the Hubble Space Telescope. Satellites offer an easy method to measure an object mass, so before the moon’s discovery Makemake’s mass could only be estimated.

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Makemake is shown in this Hubble Space Telescope image. Note: The cross-shaped spikes extending from Makemake are not part of the dwarf planet, ...

Makemake lacks its expected atmosphere.
Astronomers thought Makemake would have developed an atmosphere similar to Pluto’s, its chance passing in front of a bright star in 2011 revealed it mostly lacks a gas envelope. If present, Makemake’s atmosphere would likley be methane and nitrogen-based.
Makemake is a classical Kuiper belt object.
This means its orbit lies far enough from Neptune to not be significantly affected by Neptune’s gravity (unlike Pluto) and will remain stable over the age of the Solar System,
Makemake was named three years after its discovery in 2008.
The name comes from the the creator of humanity and god of fertility in the mythos of the Rapa Nui (the native people of Easter Island). The name was partly chosen due to Makemake’s discovery close to Easter. Makemake was discovered in March 2005 and is classified as a dwarf planet, the fourth to attain this designation. Its color is reddish-brown, and it has no atmosphere. The dwarf planet's surface may be covered in frozen methane. It is named after the god of fertility in Rapanui (Easter Island) mythology.

The Solar System - Part Two