Christiaan Huygens, a Dutch astronomer, discovered Titan in the 17th century. But scientists got a glimpse of what the moon's surface may look like only a few years ago. Enshrouded by a cocoon of diffuse and obscuring haze, Saturn's biggest moon remained hidden until October 1994, when University of Arizona scientists and colleagues observing with the Hubble Space Telescope discovered a mysterious, bright feature near its equator.
The discovery team, headed by Peter H. Smith of the University of Arizona Lunar and Planetary Laboratory (LPL) in Tucson, mapped a bright region the size of Australia.
That region may be a very large range of ice mountains continually eroding under methane rain, Smith suggests. But we won't know for sure until the Cassini Mission reaches Saturn and releases the Huygens Probe for its descent through Titan's atmosphere in 2004, he adds.
Smith, a senior research scientist at LPL and deputy investigator of the Descent Imager Spectral Radiometer aboard the European Space Agency's Huygens Probe, is talking about it today (Aug. 16) at the 24th General Assembly of the International Astronomical Union in Manchester, England.
Joining Smith in the 1994 Titan observations were the LPL's Mark Lemmon and Ralph D. Lorenz, Larry A. Sromovsky of the University of Wisconsin - Madison, John J. Caldwell of the Institute for Space Science and Terrestrial Science in Concord, Ontario, and Michael D. Allison of the NASA Goddard Institute for Space Studies in New York.
Bigger than Earth's moon, Titan is one of the largest satellites in the solar system. It is not a friendly place by human standards. Saturn is 10 times farther away from the sun than is the Earth, and Titan receives only one percent of the Earth's sunlight. Daylight on Titan looks like twilight on Earth.
If a person could survive the frigid temperatures that drop to mere 90 degrees Kelvin, he or she would be able to read a book and see colors while standing on Titan's surface. (90 degrees Kelvin is minus 180 degrees Celsius or minus 356 degrees Fahrenheit.)
Titan's atmosphere is five times denser than the Earth's and is filled with organic haze. Titan's atmosphere resembles oxygen-free, primordial atmosphere on young Earth, in which the first living organisms breathed. Titan is tidally locked to Saturn and the moon's day lasts 15.9 Earth-days.
Octover 1994 Discovery
Smith and his colleagues used the Hubble Space Telescope Wide Field Planetary Camera to observe Titan at near-infrared wavelengths during two weeks in 1994. They hoped that for the first time they would be able to see through dense haze hanging in the moon's atmosphere down to the surface and discover motions of the clouds.
"Try as we might, looking at the data we couldn't see anything that was a significant brightening on the surface and moved. It looked like there were no clouds, or, if clouds were there, they were right at the noise level and we couldn't tell the difference," Smith recalls.
He and his team looked for clouds as there was no guarantee that they would see the surface. "When I wrote my proposal, I said that we intended to map the surface features. Proprosal reviewers replied: 'That's impossible.' Of course, we saw no clouds and we mapped the surface," says Smith.
Looking at an object so far away from the Sun, the best they could get was the image in which the entire Titan's disc was barely 20 pixels across. "That gave us a surface resolution of about 300 kilometers (180 miles) per pixel, Smith says.
Still, the resulting map showed a very bright region at the leading face of Titan (the side pointing in the direction of its motion). The mysterious feature was about the size of Australia, but even today nobody knows whether it is a continent or not. If it were a continent, it would have to be in the ocean.
Follow-up Observations
Smith and his collaborators, and other teams of scientists, since have made follow-up observations and speculated about what the feature can be. Smith's group suggests that the feature is a great range of ice mountains.
"There is a lot of water ice on Titan, and at 90 degrees Kelvin ice is as strong as granite, so you can make big mountains out of it," Smith said.
"I think what we see as the bright region is a very large range of ice mountains. You've got a constant wind that blows up the wet air from the methane ocean. The air freezes out, and clouds form on the top of these mountains. The rain is methane rain that erodes these hills and exposes fresh ice, which is very bright," he says.
"There has to be something different about this place to keep it bright for a long period, because you have all the dark and condensed haze raining from the sky. People have suggested that Titan was hit by a gigantic asteroid, exposing fresh ice.
"But the rate at which the haze falls out from the atmosphere, that would have had to have happened very recently. And presently an impact by a body large enough to leave a crater the size of Australia is very unlikely."
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