16 AUGUST 2000
MANCHESTER, England -- 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.
"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.
"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."
"There has to be a source of methane on the surface, otherwise all the methane in the atmosphere would be destroyed in about 10 thousand years. It cannot be solid methane ice, because it would not give off much methane gas. So it is probably some source of liquid methane, and liquid methane is stable on the surface if it's mixed with ethene, which also exists on Titan. The boiling point of the combination of the two is very close to the surface temperature. You would then have a source of liquid, although it would be probably mushy, gunky, and very dark," Smith says. Indeed, the map shows a very large, dark feature on the opposite side to the bright feature on Titan.
Lunine and Stevenson suggested why Saturn's moon must have a methane ocean. Methane would rise from the ground and diffuse in the atmosphere, where the sun's ultraviolet radiation would photochemically destroy it, creating organic haze that resembles the notorious smog over Los Angeles.
"These big, organic molecules stick together like tar and slowly sink back to the surface, continually raining down from the moon's atmosphere," Smith says. Titan's atmosphere is much more diffuse than the Earth's and stretches several hundred kilometers above the surface."When the haze gets to about 80 kilometers (48 miles) above the surface, it rains out, and the atmosphere is crystal-clear with the visibility of hundreds of miles." About 40 kilometers (24 miles) above the surface, temperatures plummet to a level cold enough to freeze nitrogen. Heavy, millimeter-sized droplets fall very rapidly to the surface and may pond in lakes or an ocean.
"We're hoping that the wind will blow the probe east, as close to that region as possible," Smith says. But first, Cassini instruments will see the moon's atmosphere. Obliquity causes a significant difference from summer to winter in the sun's position in the sky, which drives atmospheric dynamics and changes the density of the haze particles on Titan.
The Huygens Probe's Descent Imager Spectral Radiometer (DISR), conceived by LPL Research Professor Martin G. Tomasko and Smith, consists of 13 separate instruments. Three instruments are imagers of low, medium, and high resolution. "With these 3 cameras and a spinning spacecraft, we'll time our pictures in such a way that we'll take an entire hemispheric panorama from above the horizon down to the surface. We'll take about 50 such panoramas during the descent through the atmosphere. The information we'll get out of these images will be absolutely stupendous," Smith anticipates.
The DISR spectrometers will measure the absorption of various gases and hazes in the atmosphere at near infrared wavelengths, which are absorption bands of organic molecules. "By watching where the light gets absorbed we will know where methane is," Smith explains. As DISR approaches the surface, it turns on a special lamp. "Although in many wavelengths it is quite bright on Titan, in the methane bands it is pitch-black," he adds.
The LPL scientists tested the landing light on the black asphalt street and took measurements.Then they hosed the street with water and took measurements again. "The difference between dry asphalt and asphalt with water was obvious, so we could measure the composition of the street, which after all isn't too different from Titan's tar," Smith concludes.
Related Link:br> Surface map of Titan
INTERNATIONAL ASTRONOMICAL UNION
XXIVth GENERAL ASSEMBLY, MANCHESTER, UK
7 - 18 August 2000
August 8, 2000
Dr Coustenis and her colleagues say that frozen methane could account for the observed bright region. One possible explanation is an ice-covered mountainous plateau. They are investigating theoretically whether methane ice could exist at high altitudes in Titan's equatorial regions, or at higher latitudes, where the temperature is lower than near the equator.
Titan's surface cannot be seen in the normal way, because of chemical haze in its thick nitrogen atmosphere. Although ordinary light cannot penetrate the haze, infrared radiation can. Measurements at different infrared wavelengths reveal information about various levels in Titan's atmosphere and its surface below. In computer processing, the contribution from the atmosphere is assessed and subtracted to leave an image of the surface alone.
These new images of Titan's surface show its appearance at wavelengths of 1.3 and 1.6 microns. Their exceptional quality is due to excellent observing conditions and a new adaptive optics system on the CFHT, which compensates for the blurring effect of moving air in Earth's atmosphere.
Titan is of special interest because it is the only moon in the solar system with a thick atmosphere. A little larger than the planet Mercury, it is the second largest moon in the solar system. The atmosphere is almost entirely nitrogen and about half as thick again as Earth's atmosphere. Astronomers have suspected that Titan's surface may have lakes or oceans of liquid hydrocarbons, but the new insights offer even more exotic and complex possibilities: ices, rocks, organics, etc. Titan is the target of the Huygens probe due to arrive in 2004 as part of the NASA/ESA Cassini-Huygens mission to Saturn and its moons.