March 24, 2009 at 10:25 PM ET
NASA / JPL
Click for video: A short but fierce "gullywasher" of methane rain falls on the
mountains surrounding Titan's Hotei Arcus in this artist's view, based on mapping
data from the Cassini spacecraft. Click on the image to see a virtual flyover.
Pictures from the Cassini orbiter have been processed to provide a psychedelic flyover of Titan, Saturn's largest and most mysterious moon. But wait ... there's more: You also can watch moon shadows dance over Saturn's rings, a phenomenon that occurs during a season that comes only once every 15 years.
The virtual flight comes courtesy of Cassini's radar-mapping instrument, which can see places hidden from human eyes. Titan is shrouded by a thick layer of orangish smog, but the radio waves cut right through the atmosphere to map the mountains, valleys and dune fields below.
Randy Kirk, a member of the radar team from the Astrogeology Science Center at the U.S. Geological Survey in Flagstaff, Ariz., used overlapping radar data from Cassini's 19 flybys to create 3-D maps for about 20 areas of Titan, covering close to 2 percent of the moon's surface.
"These flyovers let you take in the bird's-eye sweeping views of Titan, the next best thing to being there," he said in a news release issued today by NASA's Jet Propulsion Laboratory. "We've mapped many kinds of features, and some of them remind me of Earth. Big seas, small lakes, rivers, dry river channels, mountains and sand dunes with hills poking out of them, lava flows."
Titan's rivers and lakes, however, are filled with liquid hydrocarbons rather than water. That's due to the moon's composition as well as its cold temperatures. The methane cycle on Titan is much like the water cycle on Earth, driving the weather on Titan (including methane rainstorms) as well as surface flows and changes in the terrain. In fact, some have compared Titan's organic-rich atmosphere to what Earth might have had in its history, only chillier.
Among the areas mapped in 3-D is Ganesa Macula, which was thought to be an ice volcano. The radar readings reveal no volcanic dome, however, which is keeping scientists guessing about Ganesa Macula's true nature. "It could be a volcanic feature, a crater, or something else that has just been heavily eroded," Kirk said.
The maps trace Titan's north polar lakes of methane and ethane, as well as the mountains that rise up to 4,000 feet (1,200 meters) above the shoreline. If Titan's terrain follows Earth's model, those lakes should be no deeper than about 300 feet (100 meters), NASA reported. Elsewhere, the radar imagery has measured vast fields of sand dunes measuring at least as high as the lakes are deep.
Kirk, who presented his first batch of 3-D maps today at the Lunar and Planetary Science Conference in Texas, expects that there will be more such maps to come. Such maps will give scientists a better sense of Titan's hidden topography - and a fuller understanding of how the moon's weather works.
Leaping and hopping on a moonshadow
Meanwhile, Cassini's cameras are tracking the long shadows of Saturn's moons as they pass over the planet's rings. This is the prime season for such sights, thanks to the approach of Saturn's equinox.
Like Earth and most of the other planets, Saturn is tilted with respect to the plane of its orbit around the sun. And so are its rings. But twice during Saturn's 29.5-Earth-year orbit, the planet's rings are aligned so that the sun's rays pass straight across. Think of it as the Saturnian equivalent to the spring and fall equinoxes on Earth. Here's how the Cassini imaging team at the Colorado-based Space Science Institute explains the effect:
"During these times the shadows of the planet's rings fall in the equatorial region on the planet, and the shadows of Saturn's moons external to the rings, especially those whose rings are inclined with respect to the equator, begin to intersect the planet's rings. When this occurs, the equinox period has essentially begun, and any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings, will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn's equator and the rings themselves will go dark, being illuminated only on their edge. The next equinox on Saturn, when the sun will pass from south to north, is Aug. 11, 2009."
Got all that? The members of Cassini's imaging team have been anxiously awaiting the show, and one of the videos released this week shows the shadow of a 70-mile-wide (113-kilometer-wide) moon called Epimetheus slicing across the rings in January. An even smaller moon, Pan, was caught casting a shadow last month - and more moons will follow. Such observations could reveal ever-so-slight warps in Saturn's icy rings, providing fresh clues about their structure.
"One of the best things about being in orbit around Saturn are those mind-expanding opportunities that arise every now and again to see some celestial phenomenon you couldn't possibly see here on Earth," Carolyn Porco, leader of the Cassini imaging team, said in Monday's image advisory. "It's at those times you feel a real sense of privilege to be alive ... now... to witness such remarkable sights. And from the looks of it, the next year is going to be one remarkable sight after another."