Saturn Moon Coated in Fresh Powder

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Winter Wonderland
Image courtesy Paul Schenk, LPI

Skiers, get your poles ready: Saturn's moon Enceladus appears to be cloaked in drifts of powdery snow around 330 feet (100 meters) thick, scientists announced this week.
The researchers think superfine snowflakes are blasted out of geyser-like jets, which emanate from long fissures called tiger stripes on the moon's southern hemisphere. Some of the snow from these plumes falls back to the moon's surface, coating older fractures and craters in a slow process of accumulation.
"The particles are only a fraction of a millimeter in size ... even finer than talcum powder," study leader Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, said in a statement. "This would make for the finest powder a skier could hope for."


"Rocket" Moon
Image courtesy SSI/NASA

Enceladus's icy geysers were first seen spewing from the moon's south polar region in 2005 in pictures from the Cassini spacecraft. Cassini's backlighted photographs—including the one above shot in October 2009—make the 300-mile-wide (500-kilometer-wide) moon appear to be rocket-propelled.
Further Cassini data have since shown that the active tiger-stripe fissures are warmer than the surrounding icy terrain, hinting that the jets are being driven by a subsurface liquid ocean. Cassini's close flybys of the moon also revealed that Enceladus's geysers may contain the chemical ingredients for life.


Snowy Landscape?
Image courtesy Paul Schenk, LPI

According to the new study, as Enceladus's powdery snow falls back to the surface, it softens the contours of the underlying landscape.
This high-resolution Cassini picture, for instance, shows a region just north of the moon's active geysers. The rims of older craters and fissures appear to have been smoothed by the blanket of snow, while the edges of newer fractures are more distinct.
Based on such images, Schenk estimates that snow has accumulated to depths of 250 to 400 feet (75 to 125 meters) in places.



Steaming Geyser
Photograph by Mark Thiessen, National Geographic

Earthly geysers, such as Castle Geyser in Yellowstone National Park (pictured), emit steam and scalding water.
Enceladus is so far from the sun, however, that its surface temperature is about -330°F (-200°C), causing water vapor spewed from its geysers to condense into ice crystals.


Its Own Ring
Image courtesy SSI/NASA

Scientists estimate that Enceladus's low gravity—about one percent that of Earth—allows some of the ice emitted by the polar geysers to jet into space rather than falling back to the moon's surface.
Enough material escapes to form an entire ring of Saturn, called the E ring, shown in this Cassini picture taken in September 2006. The wispy E ring is so tenuous that astronomers didn't see it until 1967.



Fine Spray
Image courtesy SSI/NASA

A plume of fine water-ice particles from Enceladus stretches into space in a September 2008 Cassini picture.
Scientists previously estimated that if Enceladus's geysers were to shut off, Saturn's E ring would dissipate within a few hundred to a few thousand years. This would mean that the entire E ring—and the geysers that feed it—could be quite recent features that we just happen to be seeing at the right time.
But the large accumulations of snow on Enceladus's surface prove otherwise, Schenk argued today at a joint meeting of European and American planetary scientists in Nantes, France.
That's because the snowfall on Enceladus is incredibly light, with accumulations of less than a thousandth of a millimeter a year. For snow to build up to depths of hundreds of feet would require tens of millions of years, he said—indicating that the geysers have been active for a very long time.