Ligeia Mare, one of the largest seas on Saturn’s moon Titan, consists of pure methane and likely has a seabed covered by a sludge of organic-rich material, according to a new analysis of radiometry data from NASA’s Cassini mission.
This image from Cassini shows Ligeia Mare, the second largest known body of liquid on Titan. The image is a false-color mosaic of synthetic aperture radar images obtained by Cassini between February 2006 and April 2007. Dark areas (low radar return) are colored black while bright regions (high radar return) are colored yellow to white. In this color scheme, liquids, which are dark to the radar, end up appearing black and the solid surface of Titan, which appears bright to the radar, ends up appearing yellow. Image credit: NASA / JPL-Caltech / ASI / Cornell.
Of the hundreds of moons in the Solar System, Titan is the only one with a thick atmosphere and large liquid reservoirs on its surface, making it in some ways more like a terrestrial planet.
Both Earth and Titan have nitrogen-dominated atmospheres – over 95% nitrogen in Titan’s case.
However, unlike our planet, Titan has very little oxygen; the rest of the atmosphere is mostly methane and trace amounts of other gases, including ethane. And at the low temperatures found at Saturn’s great distance from the Sun, the methane and ethane can exist on the surface in liquid form.
For this reason, planetary researchers had long speculated about the possible existence of hydrocarbon lakes and seas on Titan, and data from NASA’s Cassini mission does not disappoint.
There are three large seas on Titan — Kraken Mare, Punga Mare, and Ligeia Mare — all close to the north pole, surrounded by dozens of smaller lakes in the northern hemisphere. The exact make-up of these liquid reservoirs remained elusive until recently.
A new study using scans from Cassini’s radar during flybys of Titan from February 2007 to January 2015 confirms that Ligeia Mare is mostly liquid methane.
“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart,” said lead author Dr. Alice Le Gall, from the Laboratoire Atmosphères, Milieux, Observations Spatiales and Université Versailles Saint-Quentin in France.
“Instead, this sea is predominantly made of pure methane.”
According to Dr. Le Gall and co-authors, there are a number of possible explanations to account for the unexpected composition confirmed by this study.
“Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it,” Dr. Le Gall said.
“It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation.”
This labeled graphic illustrates how different organic compounds make their way to the seas and lakes on Titan. In the atmosphere of Titan, nitrogen and methane react to produce a wealth of organic molecules. Scientists believe that the heaviest of these molecules fall to the surface. When they reach the sea, either by directly falling from the air, via rain or through rivers, some (shown in yellow) become dissolved in the liquid methane, while the insoluble ones — such as nitriles and benzene (shown in brown) — tend to sink to the sea floor. Image credit: ESA.
In their research, the team combined observations of thermal emission from Ligeia Mare at one microwave wavelength, and also used data from a radio sounding experiment performed in May 2013.
During the radio sounding, Cassini’s instrument detected echoes from the seafloor and inferred the depth of Ligeia Mare along the orbiter’s track over the sea — the first-ever detection of the bottom of an extraterrestrial sea.
The team was surprised to find depths in Ligeia Mare as great as 525 feet (160 m) at the deepest point along the radar track.
With this depth information, they were able to separate the contributions made to the sea’s observed thermal emission by the liquid sea and the seabed.
“This revealed that the seabed of Ligeia Mare is likely covered by a sludge layer of organic-rich compounds,” Dr. Le Gall said.
The researchers also looked at the temperature of Ligeia Mare from winter to spring.
They expected that, like the seaside on Earth, the surrounding solid terrain would warm more rapidly than the sea.
Interestingly, the measurements showed no significant difference between the sea’s temperature and that of the shore, but they did reveal a general lag in warming in Titan’s north polar region as summer approaches.
This suggests that the terrains surrounding the lakes and seas are flooded with liquid hydrocarbons, which would alter their thermal characteristics.
The findings were published online February 25, 2016 in the Journal of Geophysical Research: Planets.
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A. Le Gall et al. Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission. Journal of Geophysical Research: Planets, published online February 25, 2016; doi: 10.1002/2015JE004920
