January 14, 2005

Huygens Probe Successfully Lands on Titan

Astrobiology Magazine -- 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. Huygens has finally descended down through Titan's thick shroud of fog, taking a host of measurements along the way, and has successfully landed on Titan.

Scientists Confirm Huygens Landing on Titan

Mission controllers were confident the Huygens probe made a soft landing by parachute because it was transmitting steadily long after it was to have landed, said David Southwood, the European Space Agency's science director.

"We know that it has landed based on the laws of gravity," Southwood said. "It simply cannot still be flying. It's got to be on a solid surface, and it must be soft."

Southwood said the early signal showed little more than that Huygens was still alive. He said the mission wouldn't be a success until a full set of data could be sent back via the Cassini mother ship orbiting Saturn.

"We still can't fully celebrate - we need to wait for the data to come from Cassini but we have enormous faith in this mission," Southwood said.

Southwood later said the probe had relayed scientific data - expected to include pictures and atmospheric measurements - to the mother ship and the information had transmitted the information back to Earth.

The heart of the mission was its 2 1/2-hour parachute descent, during which it was to take pictures and sample the atmosphere, believed to resemble that of the Earth when it was young.

Early signals confirmed it had powered up for entry and deployed the parachute, and officials were optimistic it had made a safe landing because Huygens was designed to go on transmitting from the surface for at least three minutes before its batteries died - a total transmission of less than three hours. But the signal had kept coming for more than five hours.

"It's lasted much longer than we ever dreamed," Southwood said.

Mission officials - who have waited since 1997 for Huygens to reach its destination - had tears in their eyes as the first signal was picked up, indicating that the probe was transmitting to its mother ship, the international Cassini spacecraft.

Huygens was spun off from Cassini on Dec. 24 to begin its free-fall toward Titan, the first moon other than the Earth's to be explored by spacecraft.

Named after Titan's discoverer, the 17th century Dutch astronomer Christiaan Huygens, the probe carries instruments to explore Titan's atmosphere and find out whether it has the cold seas of liquid methane and ethane that have been theorized by scientists.

Timers inside the 705-pound probe awakened it just before it entered Titan's atmosphere. Huygens is shaped like a wok and covered with a heat shield to survive the intense heat of entry.

On the way down, it was to shed its shield and use a special camera and instruments to collect information on wind speeds and the makeup of Titan's atmosphere. The data is transmitted back to Cassini for relay to NASA's Deep Space Network in California and on to ESA controllers in Darmstadt, Germany.

Saturn's Mysterious Moon

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 began shortly after 11:00 AM GMT (5:00 AM EST). Two and a half hours later, Huygens reached the moon's surface. If it was 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).


On the Net:


European Space Agency

Cassini-Huygens Mission

Saturn-- JPL Cassini Main Page

Space Science Institute

Where is Cassini Now?