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Cassini unravels secrets of Titan's cryovolcanism
by Tushna R. Commissariat
Posted: 2 March 2010

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Scientists have long believed that the Hotei Regio area on Saturn's murky moon Titan shows signs of ancient cryovolcanism, but have debated over whether there is sufficient evidence to show there is any current activity. Now, data from two sets of instruments onboard NASA’s Cassini spacecraft have been combined to reveal the latest findings.

The Visual and Infrared Mapping Spectrometer (VIMS) is a composite of two cameras: one that measures visible wavelengths, while the other measures infrared, giving details about the composition of the surface and atmosphere. The Cassini Radar (RADAR) takes pictures using microwaves and can pierce the foggy depths of Titan's atmosphere. Both instruments are now in agreement that a variety of geological processes have taken place at the 700 kilometre wide Hotei Regio region; including large impacts, floods, debris flows and ice volcanoes.

The radar mapper sees through Titan’s haze to reveal the highs and lows of the terrain in the topography map (top). In the second image down, arrows point to corresponding features seen in the mineral and chemical make-up of the region produced from data provided by Cassini’s visual and infrared spectrometer. Titan radar data (third image down) reveal the surface roughness of the area. Data from the two instruments were combined (bottom) to show areas that may have been dominated by past ice volcano flows, along with drainage channels and possible mountainous terrain. The “VIMS dark blue units” are areas thought to be rich in water ice. Image: NASA/JPL/USGS/UA.

Authors of a new paper say that while the area, located south of the equator, shows distinct signs of being shaped by cryovolcanism, no current signs of activity have been found. The area was probably active in the recent past, somewhere in the last ten thousand years and was mostly kick-started by a large impact in that area. Data of surface details, taken in the visible and infrared wavelengths, show circular structures across a basin formed by the impact. The impact would have created zones of weakness within the tectonic plates, setting the stage for slushy ice volcanoes of water, methane or ammonia.

Radar images show a kilometre deep depression at the center of Hotei Regio filled with cryovolcanic flows that are 100 to 200 metres thick. The terrain surrounding the depression is elevated with both rough and smooth areas and is thought to represent channels carved by flows of liquid methane streaming down from the mountains to the outlying margins of the depression. The dark blue patches, seen in VIMS images, shows the infrared colour is consistent with enrichment in water ice.

In December of 2008, Cassini scientists reported variations in brightness and reflectance of Hotei Regio and thought new matter had been ejected to the surface during a new eruption. But the findings of the new paper conclude there is not substantial evidence to show ongoing eruptions on the surface. Alternatively, they found that using low-resolution visible and infrared spectrometric techniques while looking through Titan's aerosol-laden atmosphere could produce effects that were confused with active cryovolcanism. The scientists conclude that while evidence points to geologically recent volcanic activity at Hotei Regio, "the VIMS observations to date do not provide compelling evidence for Cassini actually having witnessed ongoing volcanic activity in Hotei Regio."

The Cassini team continues on its endeavour to accurately detail Titan, providing new insights on the evolution of Earth-like bodies.