Some scientists believe Titan's current environmental status may be similar to that of Earth's before life formed. During the formation of the solar system, Titan formed past the frost line where temperatures in the solar nebula were cold enough for methane, ammonia and water to condense into ice. Titan is composed mostly of ices , including methane and ammonia ice. Some of this ice sublimated long ago to form an atmosphere on Titan.
Over billions of years, solar ultraviolet light broke apart the ammonia molecules into nitrogen and hydrogen. The lighter hydrogen escaped from the atmosphere but the heavier nitrogen molecules remained and accumulated to give Titan its nitrogen atmosphere. Titan was recently visited by the Huygens probe , which landed successfully on the moon's mountainous surface after being released from the Cassini spacecraft in The Huygens probe sent back the first real images of Titan's mysterious surface, and scientists are still analyzing the data.
All are heavily cratered , indicating generally ancient surfaces, but each shows evidence of past geological activity. The geological activity may seem surprising when we consider how small these moons are; all are considerably smaller than the Galilean moons. Enceladus, with its grooved terrain, is barely km across small enough to fit inside Colorado. In fact, the geological activity is probably a demonstration of the effectiveness of low temperature icy geology.
Icy combinations of water, ammonia and methane can melt, deform and flow at remarkably low temperatures, producing a variety of volcanic and tectonic features. The surface of Enceladus shows signs of intense recent geologic activity. The Cassini spacecraft passed by the moon in and returned images showing ice geysers erupting from the surface.
The material spewed out from these eruptions is a primary cause of Saturn's E-ring. Uranus' medium-size moons are made of ice and rock.
Like other objects that formed far from the Sun, the ice is made up of ammonia, methane and water. Close-up pictures two centuries later showed that Oberon is about half ice and half rock , with craters peppering its surface.
Titania has fault lines running across its surface , hinting at past or present seismic activity. English astronomer William Lassell spotted Umbriel , the darkest of Uranus' big moons, in In Voyager 2's quick fly-by, scientists were unable to figure out how the surface got so dark, or why there is a bright ring on Umbriel's surface that is 90 miles kilometers in diameter.
Ariel , another find from Lassell in , has the youngest and brightest known surface among the moons. This could be due to meteorite hits, as well as possible geologic activity — among its features are grabens, or valleys surrounded by faults. Voyager 2 detected a silicate rock and water ice composition on this moon, with hints of carbon dioxide. The last find before the space age was perhaps the most bizarre moon of all — Miranda.
Found by Gerard P. Kuiper at the McDonald Observatory in , the moon appears to be a jumble of disconnected features. It's unclear what made the mashed-up surface. Perhaps a huge meteorite blew the moon apart and it reassembled, or smaller meteorites melted the surface and caused slush to flow and freeze again. When Voyager 2 skimmed closest to Uranus' system in January , its view turned up 10 new satellites within a month.
While most of NASA's pictures of these moons show them as distant dots, if not for the spacecraft the moons probably would have gone unseen for decades if not longer. Ophelia and Cordelia are sometimes called shepherd moons. They are close to Uranus' Epsilon ring, with their gravitational influence essentially keeping the ring from falling apart.
No one knows how big Cordelia is, or what its composition might be. Ophelia is believed to be 12 miles 20 kilometers in diameter. Voyager 2 captured a picture of Puck that showed a mottled, dark surface. Astronomers believe it is made of carbonaceous material, and assume that much of the rest of the moons are of the same material. A study of the moon suggested water ice is on its surface.
New experiments re-create the environment of Europa and find that the icy moon shines, even on its nightside. The effect is more than just a cool visual.
Made of a pair of two-wheeled vehicles, DuAxel is designed to descend craters and near-vertical cliffs on the Moon, Mars, and beyond. The next full Moon will peak after midnight on Wednesday morning, Sept. Candy-Colored Phobos. NASA's next giant leap may be aided by tiny lunar robots that would help scout the lunar surface.
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