A World Unveiled: Crème brûlée on Titan

Penetrating the orange haze of a frigid, alien world, a space probe parachuted onto Saturn’s moon Titan late last week and came face-to-face with an unexpected landscape—a terrain that looks a lot like Earth.

ORANGE VIEW. Grapefruit-size chunks on Titan. ESA, NASA, JPL, Univ. Arizona
TITAN PANORAMA. A 360° view of the terrain around Huygens’ landing site (circle). White streaks may be methane fog. The mosaic resolves features 40 meters across. ESA, NASA, JPL, Univ. Arizona

Sinuous drainage channels appear to lead to a shoreline limned by ground fog and structures that resemble sandbars. Littering the landing site of the European Space Agency’s (ESA) Huygens probe are fist-shaped rocks reminiscent of river rocks on our planet.

“I was stunned at how Earthlike Titan ended up looking,” says Huygens researcher Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz.

The probe glided through Titan’s atmosphere for just under 2.5 hours on Jan. 14 and survived for an additional 70 minutes on the moon’s surface.

The landing represents an achievement “not likely to be repeated in the lifetime of anyone alive today,” said David Southwood, ESA science director, during a news briefing at the agency’s mission operations center in Darmstadt, Germany. The images drew the biggest wows.

The saucer-shaped Huygens also gathered data on the weather and composition of Titan’s atmosphere and surface. Scientists are interested in Titan’s chemistry because it may resemble that of early Earth (SN: 11/20/04, p. 328: A Titan of a Mission).

The mix of materials that Huygens encountered stands in stark contrast to that on Earth. Unlike the water-carved riverbeds on Earth, the channels on Titan were probably sculpted by liquid methane and ethane. The probe provided the first direct evidence for methane at Titan’s surface. Just as Huygens landed, it recorded a sharp increase in methane gas. Heat from the probe may have evaporated liquid methane at or just beneath the surface, suggests Jonathan Lunine of the University of Arizona in Tucson.

The rocklike objects imaged on Titan are most likely made of water-ice. They were probably rounded by rolling and tumbling around in liquid. The same process sculpts river rocks on Earth and shaped rocks imaged on Mars just a year ago by the rover Opportunity.

Whether the shoreline surrounding a flat, dark area at the Titan landing site is really a hydrocarbon sea remains unclear. The Huygens probe certainly didn’t land in liquid, said John Zarnecki of the Open University in Milton Keynes, England, at the mission operations center. By measuring the force of impact on a small device protruding from Huygens and comparing it to the force produced by various materials in a laboratory, Zarnecki’s team discerned that the probe probably landed on something with a hard skin but a mushy interior. His team likened the texture to that of wet sand or crème brûlée.

A previous suggestion that the hydrocarbons methane and ethane on Titan collect in puddles and lakes “may be an oversimplification,” cautions Soderblom. “Maybe you have a really, really loose surface . . . and the methane is just a meter below, flowing all over the place.”

Titan resembles Earth not only in its topography but also because it is dynamic, he notes. The lack of even a single large crater in the Huygens images and earlier radar pictures indicates a young, rugged landscape, as do the orb’s river channels and hills. Icy eruptions of ammonia and water from the surface may churn up rocks and create hills, which might then be continually eroded by liquids raining from the atmosphere or seeping from near-surface reservoirs, Soderblom suggests.

The probe’s descent was the culmination of a 7-year journey. Until Dec. 25, Huygens had piggybacked on NASA’s Cassini spacecraft, which settled into orbit around Saturn last July.

Amidst the great success of the mission, there was one disappointment. Nobody had turned on one of two receivers on Cassini, which acted as a relay for Huygens, so about half the 700 images taken by the probe, as well as some data on wind speeds, were lost.

However, Marty Tomasko of the University of Arizona, whose team built Huygens’ imaging system, says that scientists can more than adequately piece together Titan’s landscape from the myriad images that they have.

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