Cassini Mission Tracks Vanishing Methane On Saturn's Moon Titan

Lawrence LeBlond for redOrbit.com – Your Universe Online

Part of the surface of Saturn’s moon Titan has been actively tracked by NASA’s Cassini mission for the past several years. During that time, the mission has found a remarkable presence of hydrocarbon methane lakes and seas dotting the surface of the moon. But a model developed by mission leaders at the Jet Propulsion Laboratory (JPL) in Pasadena, California, suggests that the supply of these methane lakes will soon come to end.

Of course, the researchers use the term soon on a geological time scale. “We are seeing an active Titan whose active chemistry may come to an end in some tens of million years,” said JPL scientist Christophe Sotin, who has been analyzing Cassini measurements of Titan’s lakes and seas.

Sotin and colleagues’ model points to a strong possibility that a gigantic outburst of methane was released eons ago, leading to a strong orange global smog haze and massive hydrocarbon sand dunes. This methane outburst may have been the result of a huge impact or volcanic eruption, the team surmises.

The methane cycle on Titan is somewhat similar to our own water cycle here on Earth. The gas exists as solid ice at the poles, vapor in the atmosphere and liquid in its lakes and seas. The methane and its derivatives are the driving forces behind the climate and weather of the moon. Titan is the only moon in the Solar System that has stable liquid on its surface.

But if the huge impact/volcanic eruption theories hold any water, then it is likely that Titan at one time was just as arid as every other moon in the Solar System. And it could be going that way again.

Over several years of investigative work, Sotin and his colleagues compared observations of the lakes then and now to understand the evolution of the methane-based hydrocarbon reservoirs. By measuring the volumes of the different reservoirs in the atmosphere, subsurface and surface, the team estimated the exchange rates of hydrocarbons between those reservoirs. What they discovered is that the moon’s methane is not being replaced quickly enough to sustain the methane cycle.

The measurements have shown that Titan’s lakes have not appeared to change in size, meaning their evaporation rate is small or that rainfall is making up for any evaporation. But Sotin noted that precipitation levels on Titan are low, which suggest the methane lakes are not very volatile. This knowledge would indicate that the liquid may be mostly ethane, which does not evaporate as quickly as methane.

During the eight-plus years that Cassini has been in the Saturnian system, scientists have not seen anything more than an occasional outburst of hydrocarbon rain on Titan. This indicates that methane is constantly being lost by breaking down in the form of ethane and other heavier molecules. Furthermore, there does not appear to be any fresh methane release from the interior.

Methane is made up of atoms of carbon and hydrogen. Methane was part of Titan’s original composition when the body was formed some 4.6 billion years ago along with the rest of the Solar System. Methane moves from the interior to the moon’s rich nitrogen atmosphere, eventually pushing through the upper atmosphere and being ripped apart by sunlight. The remaining particles go on to make more carbon-rich elements such as ethane and other aerosols that have been identified in the moon’s surface and atmosphere by NASA’s Cassini spacecraft and ESA’s Huygens probe.

Researchers have taken the evidence found on Titan and looked back into the evolution of our own rock: Earth. The conditions that exist on Titan are similar to the conditions that existed here long before life began to take shape.

Back on Titan, methane destruction will continue and eventually the moon will be devoid of its precious methane environment. All that will remain is a fossil record of the world it once was.

“The discoveries made by Cassini have revolutionized our understanding of Titan,” said Sotin in a statement. “They open new avenues for seeking habitable worlds around exoplanets. They also trigger new questions about the exchange processes between the interior and the atmosphere — and about the composition of these organic particles — that only future missions to Titan will be able to answer.”

The Cassini-Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency (ASI). The Cassini mission is managed by the JPL, a division of Caltech. The visual and infrared mapping spectrometer team is based at the University of Arizona-Tucson.

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