NASA is releasing more information gleaned from the Cassini spacecraft's final mission, focusing on Saturn's moon Titan. Long considered one of the solar system's possibilities for life, Cassini has found that the small liquid lakes on Titan's northern hemisphere are deep, resting upon hills and filled with methane. The discoveries took scientists by surprise.
Cassini was studying Saturn from 1997 through 2017. On the spacecraft's last mission, known as the Grand Finale, it was able to get a closer look at the Titan by using the moon's gravity to alter its trajectory. Having already gathered information about the moon's vast methane seas, the Grand Finale offered a chance to better understand the smaller elements of Titan's surface.
Published in Nature Astronomy, the study showed that the lakes run deeper than 300 feet (100 meters). Cassini also offered detailed information on Titan's equivalent of Earth's hydrological cycle-the patterns of the liquid methane on the moon's surface. Considering that Titan is the only body in the solar system besides Earth known to have stable liquid on its surface, the chance to study it is a rare opportunity.
Titan, as far as scientists know, does not have water. Instead, its temperatures are so cold that the plentiful methane and ethane on its surface are liquid instead of gaseous as they are on Earth. While the northern seas were known to be filled with methane, they didn't expect northern lakes to be as well. Why? Because lakes on Titan's southern hemisphere, like Ontario Lacus, are a roughly equal mix of methane and ethane. While similar, ethane is heavier than methane and has more carbon and hydrogen atoms.
"Every time we make discoveries on Titan, Titan becomes more and more mysterious," said lead author Marco Mastrogiuseppe, Cassini radar scientist at Caltech in Pasadena, California in a press statement. "But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan."
Further complicating the moon's environment is that there appears to be no northern unity either. The small lakes exist on Titan's northwest, seemingly a world apart from the vast oceans in the northeast, filled with liquid methane and dotted with tiny islands.
"It is as if you looked down on the Earth's North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does," says Cassini scientist and co-author Jonathan Lunine of Cornell University.
Using Cassini's radar instrument, scientists were able to do more than just confirm earlier findings that the lakes are far above sea level. They found that these lakes rest on top out landmasses of their own, similar to flat-topped hills like the mesas or buttes that dot the American southwest. These Titanian objects stand out in their environs, sticking hundreds of feet above the surrounding landscape.
Both the location of the lakes and their small size-just tens of miles long-makes NASA scientists believe that they formed "when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed," according to the agency's press statement. That reminded scientists of a similar process on Earth, which creates what are known as karstic lake.
The karstic process, which is often associated with sinkholes, is a well-known phenomena on Earth. Around 20% of the United States is underlain by karst landscapes, according to the United States Geologic Survey, with 40% of groundwater used for drinking coming from karst aquifers. The process, which typically involves limestone, has created bodies of waters often associated with legends on Earth, like Germany's Eichener See.
With lakes of methane standing atop mesa-like structures, Titan's own mysteries are only growing. For scientists who worked with Cassini, there's a clear sense of pride that its Grand Finale appears to be living up to the name.
"This was Cassini's last hurrah at Titan, and it really was a feat," Lunine says.
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