[meteorite-list] NASA Cassini Spacecraft Captures Ocean-Like Spray At Enceladus

[Ron Baalke]

June 22, 2011

Dwayne C. Brown 
Headquarters, Washington                           
202-358-1726 
dwayne.c.brown at nasa.gov 

Jia-Rui Cook 
Jet Propulsion Laboratory, Pasadena, Calif. 
818-354-0850 
jccook at jpl.nasa.gov 

Markus Bauer 
European Space Agency 
011-31-71-565-6799 
markus.bauer at esa.int 

RELEASE: 11-196

NASA CASSINI SPACECRAFT CAPTURES OCEAN-LIKE SPRAY AT SATURN MOON

WASHINGTON -- NASA's Cassini spacecraft has discovered the best 
evidence yet for a large-scale saltwater reservoir beneath the icy 
crust of Saturn's moon Enceladus. The data came from the spacecraft's 
direct analysis of salt-rich ice grains close to the jets ejected 
from the moon. 

Data from Cassini's cosmic dust analyzer show the grains expelled from 
fissures, known as tiger stripes, are relatively small and usually 
low in salt far away from the moon. But closer to the moon's surface, 
Cassini found that relatively large grains rich with sodium and 
potassium dominate the plumes. The salt-rich particles have an 
"ocean-like" composition and indicate that most, if not all, of the 
expelled ice and water vapor comes from the evaporation of liquid 
salt-water. The findings appear in this week's issue of the journal 
Nature. 

"There currently is no plausible way to produce a steady outflow of 
salt-rich grains from solid ice across all the tiger stripes other 
than salt water under Enceladus's icy surface," said Frank Postberg, 
a Cassini team scientist at the University of Heidelberg, Germany, 
and the lead author on the paper. When water freezes, the salt is 
squeezed out, leaving pure water ice behind. If the plumes emanated 
from ice, they should have very little salt in them. 

The Cassini mission discovered Enceladus' water-vapor and ice jets in 
2005. In 2009, scientists working with the cosmic dust analyzer 
examined some sodium salts found in ice grains of Saturn's E ring, 
the outermost ring that gets its material primarily from Enceladean 
jets. But the link to subsurface salt water was not definitive. 

The new paper analyzes three Enceladus flybys in 2008 and 2009 with 
the same instrument, focusing on the composition of freshly ejected 
plume grains. The icy particles hit the detector target at speeds 
between 15,000 and 39,000 mph (23,000 and 63,000 kilometers per 
hour), vaporizing instantly. Electrical fields inside the cosmic dust 
analyzer separated the various constituents of the impact cloud. 

The data suggest a layer of water between the moon's rocky core and 
its icy mantle, possibly as deep as about 50 miles (80 kilometers) 
beneath the surface. As this water washes against the rocks, it 
dissolves salt compounds and rises through fractures in the overlying 
ice to form reserves nearer the surface. If the outermost layer 
cracks open, the decrease in pressure from these reserves to space 
causes a plume to shoot out. Roughly 400 pounds (200 kilograms) of 
water vapor is lost every second in the plumes, with smaller amounts 
being lost as ice grains. The team calculates the water reserves must 
have large evaporating surfaces, or they would freeze easily and stop 
the plumes. 

"This finding is a crucial new piece of evidence showing that 
environmental conditions favorable to the emergence of life can be 
sustained on icy bodies orbiting gas giant planets," said Nicolas 
Altobelli, the European Space Agency's project scientist for Cassini. 

Cassini's ultraviolet imaging spectrograph also recently obtained 
complementary results that support the presence of a subsurface 
ocean. A team of Cassini researchers led by Candice Hansen of the 
Planetary Science Institute in Tucson, Ariz., measured gas shooting 
out of distinct jets originating in the moon's south polar region at 
five to eight times the speed of sound, several times faster than 
previously measured. These observations of distinct jets, from a 2010 
flyby, are consistent with results showing a difference in 
composition of ice grains close to the moon's surface and those that 
made it out to the E ring. The paper was published in the June 9 
issue of Geophysical Research Letters. 

"Without an orbiter like Cassini to fly close to Saturn and its moons 
-- to taste salt and feel the bombardment of ice grains -- scientists 
would never have known how interesting these outer solar system 
worlds are," said Linda Spilker, NASA's Cassini project scientist at 
the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. 

The Cassini-Huygens mission is a cooperative project of NASA, the 
European Space Agency and the Italian Space Agency. The mission is 
managed by JPL for NASA's Science Mission Directorate in Washington. 

For more information about Cassini, visit: 

http://www.nasa.gov/cassini 
	
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