In image above, NASAs Cassini spacecraft chronicled the change of seasons as it captured clouds concentrated near the equator of Saturns largest moon, Titan, on Oct. 18, 2010. Dr. Ralph Lorenz of Johns Hopkins Applied Physics Laboratory, gave a talk this week at the Lunar and Planetary Science Conference (LPSC) in Texas summarizing what NASA has learned about rainfall on Titan, one of Saturn’s moons.
In his talk, Lorenz proposed sending a probe to its surface to study the rainfall in more detail. Titan and Earth are the only “worlds” in our solar system that have rainfall, except that on Titan, it rains methane, not water. Lorenz’s probe mission is one of three currently being considered by NASA, with the winner to be announced this April.
Lorenz pointed out that the Cassini mission recoreded what is believed to have been rainfall on Titan has been observed in the past; once in 2004 and then again in 2007, which has led to the formation of rivers, lakes and streams despite an annual surface temperature of just -179°C.
Titan receives far less rain than Earth, and has far less clouds. On Earth, we typically get 50-65% cloud cover at any given time, whereas on Titan, it’s more like 0.2-0.6%, which results in centuries passing between rain showers, which as expected dump a lot of rain when they do occur.
When observing Titan's poles, the rainfall appears to occur far more often. In fact, Lorenz points out, if a probe were to sit on the surface near the northern pole, it would have a fifty-fifty chance of being rained on if it sat there for 2500 hours (96 Earth days), which he is suggesting would be the duration of his proposed Titan Mare Explorer (TiME), which would land in Ligeia Mare, one of Titan’s lakes sometime in 2023.
In the earlier rain showers recorded by the Cassini probe, dark patches appeared on Titan’s surface, in sync with cloud cover. Researchers observed that the dark patches slowly disappeared, consistent with evaporation. These conclusions were supported by Dr Elizabeth Turtle, with Planetary Image Research Lab. at the University of Arizona, who spoke at the conference in support of the Lorenz mission, saying that her studies of the more recent rainfall, showed that the texture of the surface had changed significantly with dark patches on the surface covering about 500,000 sq km which could be clearly seen after the passing of the cloud cover.
In a 2009 paper published in the journal Geophysical Research Letters, Cassini Imaging Team scientists presented evidence, in the form of images taken a year apart, of changes in the appearance and presence of Titan's akes, apparently due to extensive cloud systems covering the area in the intervening year and producing methane rainfall.
And in concert with the paper, having surveyed nearly all of the satellite’s surface at a global scale, the Cassini Imaging Team released maps of Titan, including the first near-infrared images of the leading hemisphere of Titan's northern "lake district" captured in August 2008, which complement existing high-resolution data from Cassini's Visible and Infrared Mapping Spectrometer (VIMS) and RADAR instruments.
The bservations show that there are greater stores of liquid methane in the northern hemisphere than in the southern hemisphere. But as the northern hemisphere moves toward summer, Cassini scientists predict large convective cloud systems will form there and precipitation greater than that inferred in the south could further fill the northern lakes with hydrocarbons. Lakes may also begin to appear in the equatorial regions where they are currently absent.
But there is still a missing link in the way in which methane is cycled through the atmosphere. Even evaporation from the surface area of all the lakes combined - some 510,000 square kilometres - is not great enough to replenish the methane lost from the atmosphere by rainfall and by the formation and eventual deposition on the surface of methane-derived haze particles.
"A recent study suggested that there's not enough liquid methane on Titan's surface to resupply the atmosphere over long geologic timescales," said Elizabeth Turtle, Cassini Imaging Team associate and lead author of the paper. "Our new map provides more coverage of Titan's poles, but even if all of the features we see there were filled with liquid methane, there's still not enough to sustain the atmosphere for more than 10 million years."
Combined with previous analyses, the observations suggest that underground methane reservoirs must exist. But as to why the liquids apparently collect at the poles rather than low latitudes, where dunes are common instead, is another mystery, but one that could also be found in seasonal changes of the moon's meteorology.
"Titan's tropics may be fairly dry because they only experience brief episodes of rainfall in the spring and fall as peak sunlight shifts between the hemispheres," says Dr. Tony DelGenio of NASA's Goddard Institute for Space Studies, and a member of the Cassini imaging team. "It will be interesting to find out whether or not clouds and temporary lakes form near the equator in the next few years."
The image below shows a region on Titan's southern latitudes that appears to have been flooded by a summer cloudburst of hydrocarbon rain. The images from NASA's Cassini spacecraft were taken before and after a large storm system was observed.
Image credit: NASA/JPL/SPACE SCIENCE INSTITUTE