A Methane Sea on Saturn’s Moon Titan Could Be Over 300 Metres Deep

A Methane Sea on Saturn’s Moon Titan Could Be Over 300 Metres Deep
A false-colour mosaic Titan's polar regions. Kraken Mare is the dim splotch to the right of centre. (Image: NASA/JPL-Caltech/University of Arizona/University of Idaho)

Data gathered by NASA’s Cassini probe has allowed scientists to estimate the depth of Kraken Mare — the biggest methane sea on Saturn’s moon Titan.

New research published in the Journal of Geophysical Research is expanding our knowledge of Titan’s hydrocarbon seas, specifically Kraken Mare. This sea, approximately 1,000 km long, is larger than all five of North America’s Great Lakes combined and holds around 80% of the moon’s surface liquids. The seas on Titan contain lots of methane and ethane and are comparable to liquified natural gas on Earth.

Titan is the only moon in the solar system known to host an atmosphere. The thick, nitrogen-rich blanket that covers the moon hides a complex hydraulic system on the surface, but instead of liquid water, the rivers, lakes, and seas on Titan consist of oily black methane. Titan features other curiosities as well, such as gigantic dust storms, ice volcanoes, and enormous sand dunes.

As the new research shows, the deepest parts of Kraken Mare could be more than 300 metres deep. The team, led by Valerio Poggiali, a research associate at the Cornell Centre for Astrophysics and Planetary Science, can’t actually be sure of that figure, because the radar pings used to determine sea depth never actually reached the seafloor.

False-colour image of  Kraken Mare.  (Image: NASA/JPL-Caltech/Agenzia Spaziale Italiana/USGS) False-colour image of Kraken Mare. (Image: NASA/JPL-Caltech/Agenzia Spaziale Italiana/USGS)

NASA’s Cassini spacecraft orbited Saturn from 2004 to 2017, and scientists have already studied some of the smaller seas on Titan using Cassini’s onboard altimeter. On August 21, 2014, Cassini flew to within 970 km of Titan’s surface and was able to send radar pings into Kraken Mare. Interestingly, this was the same flyby that resulted in the discovery of Ligeia Mare — a “magic” vanishing island on Titan.

Researchers at Cornell and NASA’s Jet Propulsion Laboratory devised a neat technique for determining the depth of Titan’s seas, which involves measuring differences between the time it takes radar to bounce back from the surface of the sea as opposed to the sea bottom. This technique helps to estimate sea depth, but the researchers have to make certain assumptions about the density of fluids on Titan and how quickly radio waves pass through them.

Using this technique, the team measured the depth of Moray Sinus, a northern estuary on Kraken Mare, which they found to be 85 metres deep. The absorption rate of the radar waves suggests the liquid in this part of the sea consists of 70% methane, 16% nitrogen, and 14% ethane. The scientists were expecting more methane than this due to the size and location of the sea, but this discovery suggests a more uniform distribution of chemicals across the moon’s various bodies of water.

Altimeter scans done across the main portion of Kraken Mare were less conclusive. As the authors write in the study, the NASA probe found “no evidence for signal returns from the sea floor, suggesting the liquid is either too deep or too absorptive for Cassini’s radio waves to penetrate.” That said, if the liquid in this part of the sea is similar in composition to the liquid found at Moray Sinus, then it must be deeper than 100 metres and possibly as deep as 300 metres, according to the study.

Poggiali is hopeful that a robotic submarine might be sent to Titan one day to explore Kraken Mare or some other body of water. And in fact, he sees the new research as a step in that direction.

“Thanks to our measurements, scientists can now infer the density of the liquid with higher precision, and consequently better calibrate the sonar aboard the [future robotic submarine] and understand the sea’s directional flows,” explained Poggiali in a Cornell University statement.

A conceptual plan from 2015 showed how such a mission might look, but nothing has actually been approved in this regard. That said, NASA will be sending an aerial drone, called Dragonfly, to Titan, which should arrive at the moon at some point in the mid-2030s.