Radar data from NASA’s Cassini spacecraft have revealed that small lakes in the northern hemisphere of Saturn’s hazy moon Titan are surprisingly deep (approximately 330 feet, or 100 m), perched atop hills and filled with methane. The findings, reported in two papers in the journal Nature Astronomy, also provide new information about the way liquid methane rains on, evaporates from and seeps into Titan.
Titan’s hydrologic cycle works similarly to Earth’s — with one major difference. Instead of water evaporating from seas, forming clouds and rain, Titan does it all with methane and ethane.
The much larger northern seas are filled with methane, but finding the smaller northern lakes filled mostly with methane is a surprise.
“Every time we make discoveries on Titan, Titan becomes more and more mysterious,” said Cassini radar scientist Dr. Marco Mastrogiuseppe, of Caltech.
“But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan.”
“Adding to the oddities of Titan, with its Earth-like features carved by exotic materials, is the fact that the hydrology on one side of the northern hemisphere is completely different than the that of other side,” said Cassini scientist Dr. Jonathan Lunine, Cornell University.
“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.”
On the eastern side of Titan, there are big seas with low elevation, canyons and islands. On the western side: small lakes.
And the new measurements show the lakes perched atop big hills and plateaus.
The new radar measurements confirm earlier findings that the lakes are far above sea level, but they conjure a new image of landforms — like mesas or buttes — sticking hundreds of feet above the surrounding landscape, with deep liquid lakes on top.
The fact that these western lakes are small — just tens of miles across — but very deep also tells planetary scientists something new about their geology: it’s the best evidence yet that they likely formed when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed. On Earth, similar water lakes are known as karstic lakes.
The researchers also used Cassini data to reveal transient lakes on Titan. Different sets of observations — from radar and infrared data — seem to show liquid levels significantly changed.
“The best explanation is that there was some seasonally driven change in the surface liquids,” said Dr. Shannon MacKenzie, a planetary scientist at the Johns Hopkins Applied Physics Laboratory.
“One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface.”
The results support the idea that hydrocarbon rain feeds the lakes, which then can evaporate back into the atmosphere or drain into the subsurface, leaving reservoirs of liquid stored below.
M. Mastrogiuseppe et al. Deep and methane-rich lakes on Titan. Nature Astronomy, published online April 15, 2019; doi: 10.1038/s41550-019-0714-2
Shannon M. MacKenzie et al. The case for seasonal surface changes at Titan’s lake district. Nature Astronomy, published online April 15, 2019; doi: 10.1038/s41550-018-0687-6