Maan Titan is “een ijssorbet”

By precisely tracking NASA’s Cassini spacecraft on its lowswoops over Saturn’s moon Titan, scientists have determined thedistribution of materials in the moon’s interior. The subtlegravitational tugs they measured suggest the interior has been toocold and sluggish to split completely into separate layers of iceand rock.

The finding, to be published in the March 12 issue of thejournal Science, shows how Titan evolved in a different fashionfrom inner planets such as Earth, or icy moons such as Jupiter’sGanymede, whose interiors have split into distinctive layers.

“These results are fundamental to understanding the history ofmoons of the outer solar system,” said Cassini Project ScientistBob Pappalardo, commenting on his colleagues’ research. Pappalardois with NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Wecan now better understand Titan’s place among the range of icysatellites in our solar system.”

Scientists have known that Titan, Saturn’s largest moon, isabout half ice and half rock, but they needed the gravity data tofigure out how the materials were distributed. It turns out Titan’sinterior is a sorbet of ice studded with rocks that probably neverheated up beyond a relatively lukewarm temperature. Only in theoutermost 500 kilometers (300 miles) is Titan’s ice devoid of anyrock, while ice and rock are mixed to various extents at greaterdepth.

“To avoid separating the ice and the rock, you must avoidheating the ice too much,” said David J. Stevenson, one of thepaper’s co-authors and a professor of planetary science at theCalifornia Institute of Technology in Pasadena. “This means thatTitan was built rather slowly for a moon, in perhaps around amillion years or so, back soon after the formation of the solarsystem.”

This incomplete separation of ice and rock makes Titan less likeJupiter’s moon Ganymede, where ice and rock have fully separated,and perhaps more like another Jovian moon, Callisto, which isbelieved to have a mixed ice and rock interior. Though the moonsare all about the same size, they clearly have diversehistories.

The Cassini measurements help construct a gravity map, which mayhelp explain why Titan has a stunted topography, since interior icemust be warm enough to flow slowly in response to the weight ofheavy geologic structures, such as mountains.

Creating the gravity map required tracking minute changes inCassini’s speed along a line of sight from Earth to the spacecraftas it flew four close flybys of Titan between February 2006 andJuly 2008. The spacecraft took paths between about 1,300 to 1,900kilometers (800 to 1,200 miles) above Titan.

“The ripples of Titan’s gravity gently push and pull Cassinialong its orbit as it passes by the moon and all these changes wereaccurately recorded by the ground antennas of the Deep SpaceNetwork within 5 thousandths of a millimeter per second [0.2thousandths of an inch per second] even as the spacecraft was overa billion kilometers [more than 600 million miles] away,” saidLuciano Iess, a Cassini radio science team member at SapienzaUniversity of Rome in Italy, and the paper’s lead author. “It was atricky experiment.”

The results don’t speak to whether Titan has an ocean beneaththe surface, but scientists say this hypothesis is very plausibleand they intend to keep investigating. Detecting tides induced bySaturn, a goal of the radio science team, would provide theclearest evidence for such a hidden water layer.

A Cassini interdisciplinary investigator, Jonathan Lunine, saidof his colleagues’ findings, “Additional flybys may tell us whetherthe crust is thick or thin today.” Lunine is with the University ofRome, Tor Vergata, Italy, and the University of Arizona, Tucson.”With that information we may have a better understanding of howmethane, the ephemeral working fluid of Titan’s rivers, lakes andclouds, has been resupplied over geologic time. Like the history ofwater on Earth, this is fundamental to a deep picture of the natureof Titan through time.”