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article imageJupiter moon Europa may yield unexpected chemistry

By Lee Labuschagne     Oct 5, 2010 in Science
Greenbelt - The frigid ice of Jupiter's moon Europa may be hiding more than a presumed ocean: it is likely the scene of some unexpectedly fast chemistry between water and sulphur dioxide at extremely cold temperatures.
Although these molecules react easily as liquids (both are well-known ingredients of acid rain) Mark Loeffler and Reggie Hudson at NASA's Goddard Space Flight Center in Greenbelt, Md., now report that they react in their solid form as ices with surprising speed and high yield at temperatures hundreds of degrees below freezing.
Because the reaction occurs without the aid of radiation, it could take place throughout Europa's thick coating of ice. This would change current thinking about the chemistry and geology of this moon and perhaps others in the Solar System.
"When people talk about chemistry on Europa, they typically talk about reactions that are driven by radiation," says Goddard scientist Mark Loeffler, who is first author on the paper being published in Geophysical Research Letters. That's because the moon's temperature hovers around 86 to 130 Kelvin (minus 300 to minus 225 degrees Fahrenheit). In this extreme cold, most chemical reactions require an infusion of energy from radiation or light. On Europa, the energy comes from particles from Jupiter's radiation belts. Because most of those particles penetrate just fractions of an inch into the surface, models of Europa's chemistry typically stop there.
"Once you get below Europa's surface, it's cold and solid, and you normally don't expect things to happen very fast under those conditions," explains co-author Reggie Hudson, the associate lab chief of Goddard's Astrochemistry Laboratory.
"But with the chemistry we describe," adds Loeffler, "you could have ice 10 or 100 meters (about 33 or 330 feet) thick, and if it has sulfur dioxide mixed in, you're going to have a reaction."
"This is an extremely important result for understanding the chemistry and geology of Europa's icy crust," says Robert E. Johnson, an expert on radiation-induced chemistry on planets and a professor of engineering physics at the University of Virginia in Charlottesville.
From remote observations, astronomers know that sulphur is present in Europa's ice. It originates in the volcanoes of Io, another one of Jupiter's moons. It then becomes ionized and transported to Europa, where it gets embedded in the ice.
Additional sulphur might come from the ocean that is thought to lie beneath Europa's surface. "However," says Johnson, "the fate of the implanted or any subsurface sulphur is not understood and depends on the geology and chemistry in the ice crusts."
In experiments that simulated the conditions on Europa, Loeffler and Hudson sprayed water vapor and sulphur dioxide gas onto quarter-sized mirrors in a high-vacuum chamber. Because the mirrors were kept at about 50 to 100 Kelvin (about minus 370 to minus 280 degrees Fahrenheit), the gases immediately condensed as ice.
Despite the extreme cold, the molecules reacted quickly in their icy forms. "At 130 Kelvin, which represents the warm end of the expected temperatures on Europa, this reaction is essentially instantaneous," says Loeffler. "At 100 Kelvin, you can saturate the reaction after half a day to a day. If that doesn't sound fast, remember that on geologic timescales (billions of years) a day is faster than the blink of an eye."
To test the reaction, the researchers added frozen carbon dioxide, also known as dry ice, which is commonly found on icy bodies, including Europa.
"If frozen carbon dioxide had blocked the reaction, we wouldn't be nearly as interested," explains Hudson, "because then the reaction probably wouldn't be relevant to Europa's chemistry. It would be a laboratory curiosity."
But the reaction continued, which means it could be significant on Europa as well as Ganymede and Callisto, two more of Jupiter's moons, and other places in the Solar System where both water and sulphur dioxide are present.
The ultimate test of the laboratory experiments will be whether evidence of any reaction products can be found in data collected during remote observations or future visits to Europa.
More about Jupiter, Europa, Jupiters moons, Solar system, Chemistry
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