Saturn's moon Titan may be worlds away from Earth, but the two bodies have some characteristics in common: Wind, rain, volcanoes, tectonics and other Earth-like processes all sculpt features on Titan, but act in an environment more frigid than Antarctica.
"It is really surprising how closely Titan's surface resembles Earth's," said Rosaly Lopes, a planetary geologist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., who is presenting the results of two new studies at the annual meeting of the of the International Astronomical Union (IAU) in Rio de Janeiro, Brazil on Friday. "In fact, Titan looks more like the Earth than any other body in the solar system, despite the huge differences in temperature and other environmental conditions."
This view of Titan comes from observations made by the Cassini-Huygens mission, which has revealed details of Titan's geologically young surface, showing few impact craters, and featuring mountain chains, dunes and even "lakes."
The RADAR instrument on the Cassini orbiter has now allowed scientists to image a third of Titan's surface using radar beams that pierce the giant moon's thick, smoggy atmosphere. As its name implies, Titan is no small moon, with a size approaching that of Mars.
Titan gets about 1 percent the amount of sunlight Earth receives.
Titan is the only moon in the solar system known to possess a thick atmosphere, and it is the only celestial body other than Earth to have stable pools of liquid on its surface. Lakes that pool on Titan's surface are thought to be filled not with water, but with liquid hydrocarbons, such as methane and ethane.
"With an average surface temperature hovering around -180 C [-292 degrees Fahrenheit], water cannot exist on Titan except as deep-frozen ice as strong as rock," Lopes said.
On Titan, methane takes water's place in the hydrological cycle of evaporation and precipitation (rain or snow) and can appear as a gas, a liquid and a solid. Methane rain cuts channels and forms lakes on the surface and causes erosion, helping to erase the meteorite impact craters that pockmark most other rocky worlds, such as our own moon and the planet Mercury.
Other new research presented at the IAU General Assembly points to current volcanic activity on Titan, but instead of scorching hot magma, scientists think these "cryovolcanoes" eject cold slurries of water-ice and ammonia.
The ammonia signature seems to vary, which suggests that ammonia frosts are ejected and then subsequently dissipate or are covered over. Although the ammonia does not stay exposed for long, models show that it exists in Titan's interior, indicating that a process is at work delivering ammonia to the surface. RADAR imaging has indeed found structures that resemble terrestrial volcanoes near the site of suspected ammonia deposition.
New infrared images of this region, with ten times the resolution of prior mappings, will be unveiled at the IAU meeting.
"The images provide further evidence suggesting that cryovolcanism has deposited ammonia onto Titan's surface," said Robert M. Nelson, a senior research scientist, also at JPL, who presented results on Wednesday.
The presence of ammonia and hydrocarbons could have interesting implications for the possibility of life existing on Titan.
"It has not escaped our attention that ammonia, in association with methane and nitrogen, the principal species of Titan's atmosphere, closely replicates the environment at the time that life first emerged on Earth," Nelson said. "One exciting question is whether Titan's chemical processes today support a prebiotic chemistry similar to that under which life evolved on Earth?"
Yet more terrestrial-type features on Titan include dunes formed by cold winds, and mountain ranges. These mountains might have formed tectonically when Titan's crust compressed as it went into a deep freeze, in contrast to the Earth's crust, which continues to move today, producing earthquakes and rift valleys on our planet.
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