jeudi 29 août 2013

Cassini Data: Saturn Moon May Have Rigid Ice Shell












NASA / ESA - Cassini-Huygens Mission to Saturn & Titan patch.

Aug. 29, 2013

 The Cassini probe jettison the Huygens capsule on Titan. Credit: ESA/NASA/JPL

An analysis of gravity and topography data from the Saturnian moon Titan obtained by NASA's Cassini spacecraft suggests there could be something unexpected about the moon's outer ice shell. The findings, published on Aug. 28 in the journal Nature, suggest that Titan's ice shell could be rigid, and that relatively small topographic features on the surface could be associated with large ice "roots" extending into the underlying ocean.

The study was led by planetary scientists Douglas Hemingway and Francis Nimmo at the University of California, Santa Cruz, who used data from Cassini. The researchers were surprised to find a counterintuitive relationship between gravity and topography.

"Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain," said Nimmo, a Cassini participating scientist. "On Titan, when you fly over a mountain, the gravity gets lower. That's a very odd observation."


This image of the surface of Saturn's moon Titan was obtained by the European Space Agency's Huygens Probe on Jan. 14, 2005, after it was delivered to Titan by NASA's Cassini spacecraft. Image Credit: ESA/NASA/JPL/University of Arizona.

One potential explanation is that each bump in the topography on the surface of Titan is offset by a deeper "root" that is big enough to overwhelm the gravitational effect of the bump on the surface. The root could act like an iceberg extending below the ice shell into the ocean underneath it. In this model, Cassini would detect less gravity wherever there is a big chunk of ice rather than water because ice is less dense than water.

"It's like a big beach ball under the ice sheet pushing up on it, and the only way to keep it submerged is if the ice sheet is strong," said Hemingway, the paper's lead author and a Cassini team associate. "If large roots under the ice shell are the explanation, this means that Titan's ice shell must have a very thick rigid layer."

If these findings are correct, a thick rigid ice shell makes it very difficult to have ice volcanoes, which some scientists have proposed to explain other features seen on the surface. They also suggest that convection or plate tectonics are not recycling Titan's ice shell, as they do with Earth's geologically active crust.

A View from Huygens

This movie was built with data collected during the 147-minute plunge through Titan's thick orange-brown atmosphere to a soft sandy riverbed by the European Space Agency’s Huygens Descent Imager/Spectral Radiometer on Jan. 14, 2005. Video Credit: ESA/NASA/JPL/University of Arizona.

In 4 minutes and 40 seconds, the movie shows what the probe “saw” within the few hours of the descent and the landing. On approach, Titan appeared as just a little disk in the sky among the stars, but after landing, the probe's camera resolved little grains of sand millions of times smaller than Titan.

At first, the Huygens camera just saw fog over the distant surface. The fog started to clear only at about 60 kilometers (37 miles) altitude, making it possible to resolve surface features as large as 100 meters (328 feet). Only after landing could the probe's camera resolve the little grains of sand. The movie provides a glimpse of such a huge change of scale.

The Huygens probe was delivered to Saturn's moon Titan by the Cassini spacecraft, which is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. NASA supplied two instruments on the probe, the descent imager/spectral radiometer and the gas chromatograph mass spectrometer.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. The mission is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate, Washington.

More information on Cassini can be found at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov and http://www.esa.int/esaMI/Cassini-Huygens/

Images (mentioned), Video (mentioned), Text, Credits: NASA / JPL / Jia-Rui Cook / University of California / Tim Stephens.

Best regards, Orbiter.ch