Recent pictures from the Cassini spacecraft of Titan’s south polar region reveal new lake features not seen in images of the same region taken a year earlier. Extensive cloud systems seen in images covering the area during past year suggest that the new lakes could be the result of a large rainstorm and that lakes on Titan owe their presence, size and distribution to the moon’s weather and changing seasons. But there must also be large underground reservoirs as well of the liquids on Titan. Those liquids aren’t water, of course, but super-cold hydrocarbons like methane.
For several years, Cassini scientists have suspected that dark areas near the north and south poles of Saturn’s largest satellite might be liquid-filled lakes. Cassini’s Imaging Science Subsystem (ISS) have now surveyed nearly all of Titan’s surface, helping to create an updated global map.
Observations have documented greater stores of liquid methane in the northern hemisphere than in the southern hemisphere. And, as the northern hemisphere moves toward summer, Cassini scientists predict large convective cloud systems will form there and precipitation greater than that inferred in the south could further fill the northern lakes with hydrocarbons.
Some of the north polar lakes are large. If full, Kraken Mare — at 400,000 square kilometers — would be almost five times the size of North America’s Lake Superior. All the north polar dark ‘lake’ areas observed by ISS total more than 510,000 square kilometers — almost 40 percent larger than Earth’s largest “lake,” the Caspian Sea.
However, evaporation from these large surface reservoirs is not great enough to replenish the methane lost from the atmosphere by rainfall and by the formation and eventual deposition on the surface of methane-derived haze particles.
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“A recent study suggested that there’s not enough liquid methane on Titan’s surface to resupply the atmosphere over long geologic timescales,” said Dr. Elizabeth Turtle, Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab in Laurel, Md., and lead author of a paper published today in the journal Geophysical Research Letters. “Our new map provides more coverage of Titan’s poles, but even if all of the features we see there were filled with liquid methane, there’s still not enough to sustain the atmosphere for more than 10 million years.”
Combined with previous analyses, the new observations suggest that underground methane reservoirs must exist.
Titan is the only satellite in the solar system with a thick atmosphere in which a complex organic chemistry occurs. “It’s unique,” Turtle said. “How long Titan’s atmosphere has existed or can continue to exist is still an open question.”
That question and others related to the moon’s meteorology and its seasonal cycles may be better explained by the distribution of liquids on the surface. Scientists also are investigating why liquids collect at the poles rather than low latitudes, where dunes are common instead.
“Titan’s tropics may be fairly dry because they only experience brief episodes of rainfall in the spring and fall as peak sunlight shifts between the hemispheres,” said Dr. Tony DelGenio of NASA’s Goddard Institute for Space Studies in New York, a co-author and a member of the Cassini imaging team. “It will be interesting to find out whether or not clouds and temporary lakes form near the equator in the next few years.”
Titan and the transformations on its surface brought about by the changing seasons will continue to be a major target of investigation throughout Cassini’s Equinox mission.