The Berkeley Science Review: Articles

Methane Rain
Seeing through the haze of Titan, Saturn's largest moon (view PDF)
by Katie Hart

With skies obscured by a thick hydrocarbon miasma, early telescopic images of Titan, Saturn's largest moon, revealed an object shrouded in mystery. But astronomers, excited by what appeared to be an Earth-like atmosphere, were determined to probe further. Only in the last few years have they been able to start piecing together what truly lies beneath the haze.

Titan is covered by a dense atmosphere composed largely of nitrogen. Such a substantial atmosphere is a rarity among solid surface satellites in our solar system, and Titan's is particularly intriguing because its composition closely resembles Earth's—that is, Earth's atmosphere over a billion years ago. "Some people argue about this," says Berkeley astronomer Máté Ádámkovics, "but there is pretty good consensus in the field that the atmosphere of Titan can be an analog for what Earth's atmosphere was like before the rise of oxygen."

The mysteries of Titan's atmosphere were an object of interest for NASA and the European Space Agency, who jointly launched the Cassini-Huygens satellite in 1997 to study Saturn and its moons. The Cassini satellite has provided spectacular images and information concerning the planet's rings and moons, and in late 2004, the Huygens probe was released from Cassini and descended into Titan's atmosphere to reveal for the first time what exactly was lurking under all the haze.

The probe touched down on the mainland continent of Xanadu, and for the first time it was clear that methane in the atmosphere was condensing on organic particulates to form clouds. But that was not the only discovery. "When the probe landed on the surface, it saw channels carved by flowing liquid. The most likely liquid was methane," says Ádámkovics.

Methane exists largely as a gas on Earth, but on Titan, where surface temperatures reach -180 C (-292 F) and surface pressures are 1.5 times greater than Earth's, methane can coexist as a gas, liquid, and solid, just as water does on our planet. "Essentially, you have an analogous planetary system that has all the complexity of processes, but has a different material, methane, driving it instead of water," Ádámkovics explains.

Ádámkovics, a postdoctoral researcher, and Professor of Astronomy and Earth and Planetary Science Imke de Pater wanted to know more about what was going on in Titan's atmosphere. "We know there are clouds and that channels were carved by flowing liquid," says Ádámkovics. "But when did it happen? Is it happening right now, or is this a remnant of ancient geology? So the real question is, 'Is it raining?'"

The short answer is yes. De Pater's group imaged Titan's atmosphere using infrared data collected by the W. M. Keck Observatories on Hawaii's dormant Mauna Kea volcano and the Very Large Telescope atop Paranal Mountain in Chile to reveal methane drizzle over regions of Xanadu. The major spectroscopic challenge was penetrating the thick fog, but ultimately, the group was able to indirectly measure atmospheric opacity close to the surface. Their measurements were best fit by models including liquid methane—in other words, by invoking rain. The presence of methane rain offers confirmation of a self-contained methane cycle on Titan comparable to Earth's water cycle.

More recent analysis of the data collected by the Huygens probe corroborates their evidence. According to Ádámkovics, some research groups have identified intermittent bright flashes on the surface of Titan and proposed that they may be raindrops reflecting light. The data remain unpublished, however, and their interpretation is somewhat controversial.

Although the existence of rain had been postulated previously, the discovery served to definitively answer one of the questions that had baffled astronomers. "When the methane was discovered in the atmosphere," says de Pater, "people also realized that the methane should photodissociate. Basically, all the methane should be gone within about 10 million years. So that was a big puzzle. One way of solving that puzzle is if you can have a cycle like the [water] cycle on Earth, where you have a continuous recycling. So, indeed, if it does rain and then evaporate or sublimate from the surface, then you can actually close the methane cycle."

Like many significant discoveries, the existence of a closed methane cycle on Titan raises more questions than it answers. "There are still many things that are unknown," de Pater is quick to admit. "We see all of these channel-like features near the equatorial regions, but we still haven't seen big rainstorms that cause them. Maybe it happened thousands of years ago. We just don't know." She and Ádámkovics are both anxious to know more about the surface composition of Titan. For instance, it is hypothesized that Titan's bulk composition is water ice, and the existence of ammonia, a kind of antifreeze, suggests that water might be able to exist in liquid form on the surface. The potential presence of liquid water is of obvious interest, especially in light of the similarities between the atmospheric compositions of Titan and Earth. As de Pater says, "People immediately start thinking about the potential for prebiotic molecules."

Titan represents a kind of laboratory for understanding processes that could have occurred on our own planet, and may, in fact, be undergoing the same kinds of chemical evolutions that ultimately led to life here on Earth. It remains an object of curiosity, and as such, telescopes will continue to be fixed on the hazy orange orb until closer contact can be made again. "There are still many puzzles," de Pater says with a smile. "And that's why people want to go back to Titan."

Katie Hart is a graduate student in chemistry.

Want to know more? Check out:
astro.berkeley.edu/~imke

Comments on this article? Drop us a line at with 'letter to the editor' in the subject!