Titan's Icy Volcanoes Erupting Methane?
|Click images for large view. Details of the circular feature show up at wavelengths larger than 1.3 micron. The first six panels are images taken in six infrared windows, and are georeferenced. Ratio images are represented in order to visualize compositional variations. The last panel is a colour composite image (red,2.75 micron; green, 2.0; blue, 1.6).
Image Credit: NASA/ESA
Titan's icy volcanoes may be releasing methane, scientists report this week in the journal Nature.
Looking at infrared data gathered by the Cassini spacecraft as it flew by Titan on October 26, 2004, Christophe Sotin and colleagues saw a 30-kilometer-wide feature that they believe is an ice volcano.
"We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere," the scientists write.
The surface temperature of Titan is about 94 degrees Kelvin (minus 291 Fahrenheit), so for a volcano to erupt, heat would be needed to melt Titan's icy crust. The scientists say that Titan's eccentric orbit around Saturn should tidally compress the moon, providing enough heat for such cryovolcanic eruptions.
The infrared data analyzed by Sotin's team came from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). VIMS measures radiation from atmospheres and surfaces to determine structures, chemical compositions, and temperatures. In the VIMS data, the dome appears as a bright circular feature, with two elongated wings extending westward.
"Such a structure resembles volcanic edifices with lobate flows, such as those observed on Earth or Venus for example, although the volcanic material is different," the scientists write.
A volcano on Titan wouldn't expel hot-rock lava like on Earth. Instead, liquid water mixed with ammonia or other chemicals would probably ooze out of the volcano, and then freeze solid on the surface. However, the scientists say they saw no evidence for water ice in the VIMS images of the dome.
|Titan's changing face as dark and light patches rotate in circulation. Image Credit: JPL/Space Science Institute
They did see curved black linear features reminiscent of the dark river-like channels glimpsed by the Huygens probe.
"Such structures could have been formed by the large release of methane-producing rains following the eruptions," the scientists write. "If these structures are channels, they would have dried out due to the short timescale for photo-dissociation of methane in the atmosphere."
Methane - CH4 - is the second-most abundant gas on Titan, and it can be broken apart in the upper atmosphere by ultraviolet light. Due to Titan's low gravity, some of the hydrogen from this photo-dissociation escapes out into space. The remaining fragments become carbon-rich products like acetylene - C2H2 - and ethane - C2H6.
Since methane is not recycled in the atmosphere, something must be continually producing it. Scientists had speculated that the methane source could be liquid hydrocarbon seas, but so far there is no evidence for such large bodies of liquid on Titan's surface. Some scientists now suggest there could be reservoirs of liquid hydrocarbons just beneath the surface, but ultimately the source of Titan's methane is still a mystery.
|Shorelines may be dry but intermittently defined by drainage channels of methane rain. Click image for larger view. Credit: ESA
No impact craters were seen on the October flyby. This could be due to material from volcanic eruptions covering the surface and burying any craters.
Cassini RADAR team member Ralph Lorenz of the University of Arizona says that while the feature found by VIMS is interesting, there is no evidence it is a source of methane. He also notes that RADAR data would be needed to confirm that the VIMS feature is volcanic.
"It might well be a volcano, but it would be hard to say for sure without RADAR data," says Lorenz. "It looks as much like a giant cat poo as it does a volcano."
Cassini has performed six flybys of Titan so far, and nearly 40 flybys are scheduled over the next few years. The next flyby of Titan will be on August 22.
Related Web Pages
Titan's Face Lifted
RADAR surprises from Titan
Huygens, Phone Home
Saturn-- JPL Cassini Main Page
Space Science Institute
Where is Cassini Now?
Did Fluid Once Flow on Titan?