Rendezvous with Titan
Rendezvous with Titan
Thursday, January 13, 2005
For nearly a decade, scientists around the world have been waiting patiently for the European Space Agency's Huygens probe to arrive at its destination: Saturn's giant moon Titan. Now, as the Huygens science team gathers at ESA's control center in Darmstadt, Germany, that wait is almost over. In less than 24 hours, Huygens will descend down through Titan's thick shroud of fog, taking a host of measurements along the way. The data the probe sends back will reveal Titan in far more detail than any previous mission has offered. Results from Hugyens may also provide a key to the origin of life on Earth.
Like Earth, Titan has a thick atmosphere rich in nitrogen. Previous missions to the outer solar system have also detected methane in its atmosphere. Methane is composed of carbon and hydrogen. This has led scientists to speculate that Titan is a prebiotic laboratory of sorts. If methane and nitrogen are present, they say, then perhaps in the 4.5 billion years since the solar system formed, more complex organic molecules have had time to form.
Investigators doubt that life ever took hold on Titan. It's much too cold there: the average temperature on the moon's surface is about minus 180 Celsius, or minus 292 Fahrenheit. But even at these frigid temperatures, chemical reactions still take place. So investigators are keen on learning just how far organic chemistry has progressed there.
One of the mysteries researchers hope will get resolved by Huygens is whether or not there are lakes of liquid methane on Titan's surface. On Earth, methane is present in its gaseous form. But because of the freezing temperatures and high atmospheric pressure on Titan, it could exist there as a liquid.
"Methane can exist as a solid or a liquid or a gas on Titan. And you could have methane rain, in principle. The organic smog is settling out of the atmosphere," and accumulating on the ground." Over the age of the solar system, there's enough to produce an ocean of liquid methane-ethane on the surface," said Marty Tomasko of the Universit of Arizona in Tucson, Arizona. Tomasko heads Huygens' DISR (Descent Imager/Spectral Radiometer) team.
Scientists expected to find bodies of liquid methane on the surface, perhaps even an ocean. These could serve as a source for the methane that has been detected in Titan's atmosphere. Methane gas in the atmosphere breaks down and dissipates quickly. For Titan to have so much of it in its atmosphere, it must have a source that constantly replenishes it.
Huygens science team members expected, by this point in the Cassini-Huygens mission, to have detected liquid on Titan's surface through a phenomenon known as "specular reflection." If you've ever flown over a smooth body of water, such as a lake or a river, on a sunny day, you may have noticed that when the sun strikes its surface at just the right angle, the surface acts like a mirror, producing a very bright reflection. This is known as a specular reflection.
If large bodies of liquid were present on Titan's surface, their specular reflections should have shown up in images captured as Cassini flew by Titan earlier in the Cassini-Huygens mission. But no such bright spots have been seen, leaving scientists wondering whether they need to revise their ideas about the source of the methane in Titan's atmosphere.
The problem, says Tomasko, could be that "the resolution from the orbiter [Cassini] is limited to about one kilometer. There could be smaller pools. There could be small streams and lakes. They could be covered with some kind of organic sludge. There are lots of things that could be there." Morover, during each of Cassini's flybys of Titan, the spacecraft was able to search for specular reflections on only a narrow swath of Titan's surface. Huygens, which during its descent will take hundreds of more-detailed images, over a wide area of Titan's surface, should be able to provide a definitive answer to this question.
The probe's descent through Titan's atmosphere will begin shortly after 11:00 AM GMT (5:00 AM EST). Two and a half hours later, Huygens will reach the moon's surface. If it is not damaged by the impact, it may continue to make measurements for as much as two hours longer. The first data are expected to arrive back on Earth shortly after 5:00 PM GMT (11:00 AM EST).
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