* Astronomy

A giant methane-spewing volcano has been spotted in close-up images of Saturn's giant moon Titan.
But the Cassini and Huygens spacecraft failed to find any signs of the expected oceans or lakes of liquid methane.


Expand

On Oct. 26, 2004, the Cassini spacecraft flew over Saturn's moon Titan at less than 1,200 kilometres at closest approach. Cassini acquired several infrared images with spatial resolution ranging from a few tens of kilometres to 2 kilometres per pixel.

The visual and infrared mapping spectrometer instrument took images from visible wavelengths to the 5.1 micron wavelength. This figure shows the mosaic obtained at the 2.03 micron wavelength. Observations are centred on the hemisphere of Titan that points away from Saturn.
The high-resolution image is 30 kilometres per pixel. It shows the site where the European Space Agency's Huygens probe successfully landed on Jan. 14, 2005. The right inset shows a circular feature that scientists think is a volcano.

---

The infrared images, which penetrate the perpetual deep smog of Titan, reveal the first clear signs of an unusual kind of ice volcanism.
A large dome-like structure 30 kilometres across may be a “cryo-volcano” produced by an up welling plume of hydrocarbons ices.
The energy source powering such a volcano would be tidal heating of material on the moon, produced by gravitational interaction with Saturn.
Such low-temperature volcanism could be a significant source of methane in the atmosphere of Titan, according to Christophe Sotin, at the University of Nantes in France, and his colleagues.
Before the Cassini mission, the presence of methane in Titan's thick atmosphere was taken as evidence that there must be oceans of liquid methane. That is because methane breaks down within 10 million years on Titan and so a source is needed to replenish it.

Cassini flyby images and those taken during the Huygens Probe's descent to the surface do show low, dark plains and apparently raised, light regions. They also show some river-like, dark, linear features connecting to the dark regions.
But there are no reflections of the kind that would be expected from liquid surfaces, so oceans or even large lakes of methane are now virtually ruled out.

There are also dark river-like channels on the flanks of the newly found dome structure. The team suggests that while it is clear there are no large bodies of liquid on Titan today, there may have been episodes of methane rain following major volcanic eruptions of methane into the air.
But because of methane's short lifetime, they say, these channels would have quickly dried out.

---

From infrared images that show variations in brightness and texture, a geological map of the circular feature, thought to be a volcano, has been obtained using Cassini's visual and infrared mapping spectrometer.
The geologic map shows that the circular feature has what appear to be several series of flows, as shown by the black lines.
The flows represent episodes of activity on the volcano. A dark central pit, called a caldera, is similar to vents that appear above reservoirs of molten material on Earth's volcanoes.
The colours on the map represent the brightness of features.
Yellow and light green represent bright patches.
Blue represents dark patches.
Red represents mottled material.
The yellow area is where the volcano lies.
The images were taken during Cassini's Oct. 26, 2004, flyby of Titan.

---

Details of the circular feature, which scientists think is an ice volcano, which could be a source of methane in Titan's atmosphere, show up at wavelengths larger than 1.3 microns (a micron is one-millionth of a meter).
The first six panels are images of the feature taken in six infrared windows. Images made up of two colours (ratio images) are represented in order to visualize compositional variations, which appear to be slight. The last panel is a colour composite image (red, 2.75 micron; green, 2.0 micron; blue, 1.6 micron).

These images were acquired with Cassini's visual and infrared mapping spectrometer, designed to peer through Titan's thick haze to the surface.
Titan, Saturn's largest moon, is the only known moon to have a significant atmosphere, composed primarily of nitrogen, with methane (methane is about 2 to 3 percent) as the largest remaining component.
One goal of the Cassini mission is to find an explanation for what is replenishing and maintaining this atmosphere.
This dense atmosphere makes the surface very difficult to study with visible-light cameras, but infrared instruments like the visual and infrared mapping spectrometer can peer through the haze. Infrared images provide information about both the composition and the shape of the area studied.
These images were taken during Cassini's Oct. 26, 2004, flyby of Titan.