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In Depth

According to Dr. Roe, Titan is a bizarrely familiar place for inhabitants of the U.S. desert southwest, where precipitation is intermittent but leaves indelible marks in the terrain. Initial flybys of Titan by the Voyager spacecrafts in the early 1980s showed Titan as a fuzzy orange ball with no evidence of clouds.

Researchers now know that the most abundant element in Titan?s atmosphere is nitrogen, the same as in Earth?s atmosphere. But on Titan, methane, not oxygen, is the second most abundant element and plays a critical role in the moon?s weather. And whereas Earth is near the ?triple point? of water, where water can exist simultaneously in gaseous, liquid, and solid states, Titan is near the triple point of methane.

In the 1990?s, researchers realized that by looking at different wavelengths, they could see all the way to the surface of Titan. Observations made with the Hubble Space Telescope revealed dark and bright surface features, and suggested that clouds might be present. Spectral techniques taking advantage of the variation in methane?s opacity with wavelength found that variations in the troposphere (or lowest portion of the atmosphere) were best explained by clouds forming and dissipating but researchers still did not know where on Titan the clouds were located or what they looked like.

Starting in 1999, Dr. Roe and his cohorts utilized the 10-meter Keck telescopes and their adaptive optics (which help correct Earth?s atmospheric distortions) to more closely study Titan. They found ever-changing clouds seasonally clustered near Titan?s south pole, the warmest place on Titan around the time of the southern summer solstice (which lasts a long time since Saturn and its moons take 30 years to go around the Sun).

Dr. Roe?s research points to a pole-to-pole circulation cell driven by the heat of the southern summer, causing upwelling at the south pole and downwelling at the north pole. Within the upwelling zone, methane condenses and forms clouds. As such, rain is concentrated near the south pole; in less than a decade, Dr. Roe expects to see storms and rainfall at the north pole during the northern summer solstice.

Since 2004, the Cassini spacecraft and its Huygens probe have produced high-resolution images of Titan?s surface that suggest significant fluid flows and extreme but infrequent rainfall that causes massive flash flooding and carves channels. Cassini imaging of Titan's surface found dark patches of liquid methane pooled on Titan's surface in the aftermath of one of the large storms observed by Dr. Roe and his colleagues. Those dark wet patches then disappeared over months as the methane evaporated back into the atmosphere or seeped into the surface.

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