Titan forecast calls for rain, Huygens data shows
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 21, 2005
Liquid methane apparently falls like rain on Saturn's smog-shrouded moon Titan, washing down icy channels that ultimately spill into broad lakebeds dotted with ice islands and shoals, according to the latest data from Europe's Huygens probe. While the spacecraft did not detect any standing pools of liquefied natural gas in its immediate area, the data indicate rainfall is common on Titan and that liquid methane is present within a few inches of its surface.
Said Martin Tomasko, principal investigator with the Huygens descent imager instrument: "What we know is the place we landed is dry at the moment. But the liquid is not 200 meters underground, the liquid was within a few centimeters of the surface, indicating that it must have rained not very long ago. Does that mean yesterday or the day before, the week before? We don't really know. But the feeling is, in the place we landed, it must rain fairly frequently. But we can't be more precise than that."
NASA's Cassini Saturn orbiter released the Huygens probe on Christmas Eve. The small spacecraft, built by the European Space Agency, slammed into Titan's thick nitrogen atmosphere Jan. 14 and descended by parachute to the moon's frozen surface, snapping pictures and sampling the atmosphere as it fell.
Hitting the surface at about 11 mph, Huygens broke through a thin crust-like material and settled several inches into a spongy hydrocarbon "soil" with the consistency of loose sand. Nearby chunks of dirty water ice show clear signs of fluid erosion, indicating the spacecraft landed in a zone that at least occasionally experiences flowing liquids.
At a news conference early today to unveil the latest data from Huygens, Owen said pre-landing predictions that liquid methane should exist on the surface of the ultra-cold world - minus 290 degrees Fahrenheit at the surface - were pretty much correct.
"What we've learned is that our speculation is really pretty good," Owen said. "The main difference, the main new thing that we have is that indeed, we can detect liquid methane on the surface. It's not seas of liquid ethane, it's really liquid methane, liquid natural gas."
Stereo images show a ridge system in the image is more than 300 feet above the surrounding terrain. Dark channels can be seen in the light-colored elevated terrain leading to larger river bed-like features that empty into a basin.
After studying the images for a week, Tomasko said his team believes the channels "really are evidence of rain."
"These branching, dendritic channels are evidence of rain and the dark material in the bottoms of the channels is very likely this photochemical smog that falls out of the atmosphere, coats the whole terrain and gets preferentially washed off the top of the ridges," he said.
"The top of the ridges are ... not really very bright, they're relatively dark, but the dark material is definitely concentrated in the bottom of these drainage channels. And these ridges, we think, are made not of silicate rocks as on the Earth, but frozen, hard water ice. So we think we're seeing water ice ridges washed off by rainfall with a liquid and a concentration of these organic materials in the bottom of the (channels)."
Looking at a larger mosaic that included the original picture, Tomasko said "we see this river system which flows down into this delta, into this low-lying terrain. We see this ridge draining from the back and these dendritic structures and then coming down from the front draining also into this broad, low-lying terrain."
In a new picture released today, Tomasko described additional features that indicate flowing liquids, including short, stubby channels that could indicate methane springs and areas that might be extrusions of water ice. Another photo showed a thin ridge in one of the pool-like basins that had multiple channels cut through it, presumably from erosion, giving the appearance of a chain of islands.
For methane be present in the atmosphere today, it must be constantly replenished. Owen said the source of the methane was Titan itself.
"The photochemistry is happening up above, breaking methane apart, fragments are combining, making more complex things, making these smog particles and they're precipitating down to the surface," he said. "The methane, as we expected from the beginning, must condense because it's so cold on the surface of Titan that we would expect liquid natural gas to be present there. And now the question is, is it really there?"
Data from Huygens shows nitrogen is the dominant gas in the upper atmosphere of Titan. But as the probe descended, methane concentrations shot up.
"This is just like what happens on the Earth with water vapor," Owen said. "Water on Earth is confined to the lower atmosphere. ... The reason is, there's a very low temperature point in the atmosphere and there's the same thing on Titan, there's a kind of cold trap that forces the methane to be down below so that the methane increases more rapidly than the nitrogen as you go down into the lower atmosphere. That's where it is.
"Now, when you come to the surface, you would expect everything to be stable and that's what the nitrogen indeed does. ... However, the methane suddenly jumps up by about 30 percent. Boom, in three minutes, up it goes. That methane must be coming out of the ground and that's the exciting part. It means there's liquid methane very near the surface, maybe right on the surface."
Tomasko said it's possible Huygens landed in Titan's equivalent of an arid region on Earth.
"We don't think we have open pools of liquid methane, but the methane kind of sinks down into the surface material," he said. "It's more like Arizona or someplace like that where the river beds are dry most of the time but after rain, you might have open flowing liquids and pools. These pools gradually dry out, the liquid sinks down into the surface. Perhaps it's very seasonal."
No one yet knows. But Jean-Pierre Lebreton, the Huygens mission scientist for the European Space Agency, said Titan would make an ideal target for some future robot lander.