"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. Titan, it appears, will eventually run out of methane.

Sotin and other researchers present a new model that points to the strong possibility that a gigantic outburst of methane was released eons ago, maybe after a huge impact, leading to both Titan's global orange smog haze and its continent-size hydrocarbon sand dunes.

The cycle of methane on Titan bears some resemblance to Earth's water cycle. Methane exists as solid ice, vapor in the atmosphere and liquid as rain and in Titan's lakes and seas. Methane and its byproducts are at the heart of the photochemistry, climate and weather that create changes that Cassini's instruments are observing on Titan today.

But the researchers find that Titan's impressive methane-based hydrocarbon features may be passing phenomena, the after-effects of a gigantic outburst of methane released from Titan's interior eons ago. A big impact could have been the culprit that drenched Titan in methane. Another possibility is volcanic activity that oozed methane slush, resurfacing the moon.

But the methane is not being replaced, so Titan's "methane era" may one day draw to a close.

Sotin and his colleagues compared observations of the lakes, obtained several years apart, to understand the evolution of Titan's methane-based hydrocarbon reservoirs. They were able to estimate the volumes of the different reservoirs in the atmosphere, subsurface and surface to estimate the exchange rates of hydrocarbons between those reservoirs. Titan's methane, it turns out, is not being replaced fast enough to sustain the methane cycle, according to the new model Sotin and his colleagues have published.

Cassini measurements show that the lakes don't appear to be changing in size. This means their evaporation rate is small or that rainfall is making up for any evaporation. But precipitation rates on Titan are low, said Sotin, so the liquid in the lakes must not be very volatile. This indicates the liquid may be mostly ethane, which doesn't evaporate as quickly as methane.

Methane is made up of atoms of carbon and hydrogen. Methane was part of Titan's original makeup when it formed along with the rest of the solar system 4.6 billion years ago. Methane moves from the interior to Titan's nitrogen atmosphere, eventually finding its way to Titan's upper atmosphere where it is broken apart by sunlight. Its hydrogen atoms rise to the top of the atmosphere and are lost to space. The remaining ingredients go on to make more carbon-rich products such as ethane, acetylene and aerosols that have been identified by Cassini and ESA's Huygens probe in Titan's atmosphere and surface.

Dynamic, methane-driven photochemistry at Titan produces prebiotic molecules similar to those that must have existed on an early Earth. Though it may be shorter-lived than previously believed, details about Titan's methane cycle provide a glimpse into the kinds of conditions that existed on our planet before life evolved.

Over time, the destruction of methane by sunlight will reduce the overall amount of methane in Titan's environment. Meanwhile, products derived from methane's destruction continue to accumulate on the surface, leaving a fossil record of the world that Titan once was.

"The discoveries made by Cassini have revolutionized our understanding of Titan," said Sotin. "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."

An array of other Visual and Infrared Mapping Spectrometer (VIMS) findings about Titan's lakes and seas were also reported in the paper:

New Placid Lakes Discovered: Two new dark features appeared in the VIMS data that are interpreted as knee-deep lakes, and another three new, mirror-smooth lakes were found in previously unexplored northern terrain. They are named Freeman, Cardiel and Towada.

Seas Connected by a Swamp? The infrared spectrometer measurements are helping to characterize the seas Ligeia Mare and Kraken Mare in more detail. The seas, which were also partially viewed by the Titan imaging radar, may be connected by a swampy area or by small rivers.

Evidence of a Mostly Ethane Lake: The scientists report that Ligeia Mare is probably composed mostly of ethane. Comparisons with earlier Cassini radar images of the same area showed the lakes did not change in size between 2007 and 2010 despite the emergence of Titan's northern spring in August 2009. This lack of change is an indication that the lakes are mostly ethane, which wouldn't evaporate as quickly as methane in the relative warmth of Titan's springtime.

The Question of a Titan Ocean Gets More Complicated: The strong correlation between small lakes seen by Titan radar and similar small dark patches seen by VIMS supports recent radar observations that question whether Titan's rotation is non-synchronous. If Titan's rotation is synchronous, it could complicate theories that argue for a global subsurface ocean on Titan that decouples the moon's outer ice shell from the inner core.

This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from the following publication:

"Observations of Titan's Northern Lakes at 5 Microns: Implications for the Organic Cycle and Geology," Christophe Sotin et al., Icarus, December 2012.

-- Mary Beth Murrill, Cassini science communication coordinator

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