The Why Files The Why Files --

Star struck! The International Year of Astronomy

Moony tunes. A ringing endorsement

From the bleak landscape of our lifeless, unchanging moon, you'd be hard-pressed to imagine that moons are some of the most dynamic denizens of the solar system. The latest support for this idea comes from Saturn's satellite Titan, where the international spaceship Cassini is seeing more signs of active ice volcanoes in the outer solar system.

These "cryovolcanoes" (pardon our astro-jargon!) are eruptions of a frigid soup containing simple molecules like ammonia and water. On some moons, ice volcanoes are spewing gas into the atmosphere, but Titan's seem to be flowing onto the surface.

Photo shows lobes of different colors
Image: Feb. 22, 2008, JPL/NASA
The Cassini Radar Mapper imaged a part of Titan where surface changes were previously seen. These lobed features could have been formed by an ice volcano that oozed a frigid mix of water and ammonia, which would be ultra-handy for cleaning the windows on your spaceship... for a larger image, showing details 300 to 500 meters across.

Titan is shrouded beneath a hydrocarbon smog, which blocks visible light but not infrared radiation, and early evidence for cryovolcanoes came from Robert Nelson, who used Cassini's visual and infrared mapping spectrometer to detect variations over time on two regions of Titan.

More recently, data from Cassini's radar instrument have supported this idea, says Rosaly Lopes, a radar scientist at the Jet Propulsion Laboratory. "We went back to some data we had acquired a couple of years back, and sure enough, we saw patterns that looked like interleaving flows that could also be cryovolcanic."

Although the new images do not prove the cryovolcano hypothesis, they support it, Lopes says.

The existence of active cryovolcanoes would instantly make Titan more interesting than Mr. Green Cheese orbiting Earth. "Cryovolcanoes are some of the most intriguing features in the solar system," says Lopes. "To put them in perspective -- if Mount Vesuvius had been a cryovolcano, its lava would have frozen the residents of Pompeii."

Titan's rocky surface appears to have features that resemble a moiré pattern in this photo collage
Photo collage: ESA/NASA/JPL/University of Arizona
Before the European Space Agency's Huygens spaceship crashed into Titan, it took a bunch of pix. The dark channels here, each about 30 to 40 meters across, were apparently eroded by liquid methane.

Life at the end of the road?

On Earth, frigid volcanoes occur only when we forget a bottle of bubbly in the freezer, but "Cryovolcanism is a fairly common process in satellites of the outer solar system," says Lopes. On Saturn's moon Enceladus, plumes of gas are entering the atmosphere from ice volcanoes, and signs of cryovolcanism have appeared on Neptune's Triton, and Jupiter's Io, Europa and Ganymede.

Scientists think the eruptions on these moons, as well as on Titan, may contain water mixed with methane and/or ammonia.

 Moon is half in darkness;  a plume of white stuff comes off the top, angles to the left
Photo: NASA
An ice volcano on Jupiter's moon Io spews a plume of gas 290 kilometers above its surface.

No way is Titan's volcanism similar to Earth's, where molten rock is expelled from the interior, since Titan's surface temperature is a below-frigid -181° Celsius (92 Kelvin). But even cryovolcanism would require some source of heat to thaw the solution and move it toward the surface. This heat may come from the tugging of Saturn's intense gravity.

Although Cassini's radar can measure surface temperature to within 1 or 2 degrees Celsius, warm spots have not been detected, so the surface changes may be caused by on-and-off volcanoes. "The flows may have been active weeks or months before our observation, and still be obliterated by a fast-forming crust," says Lopes.

However, it's also possible that the surface changes are due to something like mudflows, says Jeffrey Moore, a planetary geologist at the NASA Ames Research Center. "The flow-like features we see on the surface may just be icy debris that has been lubricated by methane rain and transported downslope into sinuous piles like mudflows."

The "Why bother?" paragraph

So who cares about this moon around Saturn? The growing evidence for ice volcanoes, "Makes Titan a more interesting body than otherwise," says Lopes. "There is a lot of organic material on Titan. If there is activity, and water, and heat, you start to get the possibility of something prebiotic," some molecules that could form the basis for primitive life. Flowing slush requires some source of heat underground, and although "Titan's surface is so cold that it's not very exciting from a life point of view, if there is volcanic activity, and there are warm areas, that makes things more exciting for sure," she says.

What if you can't see the stars?

Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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