mosaic of image swaths from Cassini's Titan Radar Mapper
This mosaic of image swaths from Cassini's Titan Radar Mapper, taken with the synthetic-aperture radar (SAR), features a large dark region several hundred kilometers across that differs in several significant ways from potential lakes observed on Titan.
Cassini spacecraft measurements of Saturn's moon Titan are shedding light on the grimly fascinating topic of Earth's final days.

The beginning of the end of the world might look a lot like Titan does now, with Earth losing its water in the same way Titan is losing its methane.

"From the point of view of climate, the methane cycle perhaps gives us a hint of what Earth's water cycle will look like in the far future as increasing luminosity of the sun changes the stability of water in the oceans and the atmosphere," wrote Cassini interdisciplinary scientist Jonathan Lunine, a professor of planetary science at the University of Arizona, in a recent paper published in the European Physical Journal Conferences. Frigid Titan has methane rainfall and methane lakes. Methane behaves on Titan the way water does on Earth -- it exists as a vapor in the atmosphere.

For the present, Earth retains and recycles water in its atmosphere. But eventually, said Lunine, Earth will lose its water the way Titan is losing its methane. A growing 'dunescape' may result on Earth in the same way that Titan's equatorial regions are dominated by arid dunes of organic material.

"Substitute water for methane, the organic dunes with silicate dunes, and the hydrocarbon products of photochemistry with oxygen and ozone, and one has a possible -- admittedly speculative -- model for our planet's future hydrological cycle," wrote Lunine. "Thus by studying how the various geologic and meteorological processes interact on Titan, we have a peek at the Earth's future "simulated" on a planetary scale with different materials, but operating actively in the present day."

An intricate, fingerprint-like pattern of dunes is seen in this dramatic radar image of Saturn's moon Titan.
An intricate, fingerprint-like pattern of dunes is seen in this dramatic radar image of Saturn's moon Titan captured by NASA's Cassini spacecraft on May 21, 2009 from an altitude of 965 kilometers (about 600 miles).
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 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