Cassini observations of Titan's atmosphere are revealing the mechanisms by which Titan's atmosphere is being lost to space. Titan has a weak cold trap which hangs onto only some of the atmospheric methane. Thus, most methane is not trapped through freezing in the atmosphere. Instead, it leaks out to higher altitudes where it is chemically changed into more complex substances by incoming ultraviolet light or charged particles. In this form, it is lost to space.

On Earth, water evaporates into the atmosphere and travels no higher than a zone called the tropopause, condensing as liquid or ice at or below that level. Returning eventually to the ground as rain or snow, water remains in a lower level of Earth's atmosphere called the hydrosphere. But, in the future, that is likely to change as the sun heats up, and Earth's atmosphere begins to fritter away like Titan's is dissipating today.

As described by Lunine, here is how the end might come:

The sun will evolve into a red giant star sometime within the remaining 4 billion years of its lifetime. In a billion years or so from now, the sun's growing brightness could tilt the balance of Earth's hydrosphere and trigger the start of Earth's water loss – akin to Titan's current loss of methane.

The Cassini spacecraft looks down on the north pole of Titan, showing night and day in the northern hemisphere of Saturn's largest moon.
The Cassini spacecraft looks down on the north pole of Titan, showing night and day in the northern hemisphere of Saturn's largest moon.

The increase in the sun's luminosity makes it almost unavoidable that within the next few billion years, the tropopause temperature will be elevated to the point that loss from the Earth's oceans by evaporation and then sunlight will break down the water vapor into hydroxyl radicals and hydrogen. "From that point on," wrote Lunine, "surface liquid water will be limited to the poles, will have a short photochemical lifetime, and would persist on the surface only if resupplied by mantle/deep crustal water. At the equator, occasional intense rainstorms might scour out fluvial features, but the principal sedimentary deposits will be sand dunes."

With most of Earth's carbon dioxide locked up as carbonates, Earth will look somewhat like Titan, with dry dunes in the equatorial regions and remnant water at the poles.

"In the absence of water, plate tectonics will likely cease and prevent carbonate deposits from recycling into the deeper crust to reform carbon dioxide. Earth might thus retain a dry climate, but one in which small amounts of water were stable at the poles, for some significant length of time."

Titan remains a main target for study by Cassini, with 10 more close flybys scheduled in the current Equinox phase of the mission through September 2010.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Cassini-Huygens mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries.

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from the following publication: “Titan as an analog of Earth's past and future,” J. I. Lunine (University of Arizona), European Physical Journal Conferences, Volume 1, p.267-274, 2009.

-- Mary Beth Murrill, Cassini science communication coordinator

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