Europa, a moon of Jupiter, appears as a thick crescent in this enhanced-color image from NASA's Galileo spacecraft, which has been orbiting Jupiter since 1995. The view combines images taken in violet, green and near-infrared filters in 1998 and 1995. The colors have been stretched to show the subtle differences in materials that cover the icy surface of Europa.
Credit: NASA/JPL/University of Arizona
Chemistry experiments in the lab have mimicked the conditions on Jupiter's icy moon Europa, providing new details about how molecules could react on the alien world, scientists announced.
The research suggests that water and sulfur dioxide ? the key ingredients in acid rain ? can react quickly and easily on top of Europa's thick shield of ice and even inside it.
Underneath that ice, a liquid ocean is thought to exist. Scientists already knew that the compounds can react in liquid form, but they needed the new experiment to learn that the reaction also could take place on the surface of the Jovian satellite, where the temperatures are ?hundreds of degrees below freezing.
Furthermore, since the reaction occurs without radiation, it could take place throughout Europa's coating of ice, the researchers said. If this is the case, the discovery could alter scientists' understanding of the nature of Europa and perhaps other moons. [Photos of Europa and other Jupiter moons]
"This is an extremely important result for understanding the chemistry and geology of Europa's icy crust," said Robert E. Johnson, an expert on radiation-induced chemistry on planets. Johnson, who is a professor of engineering physics at the University of Virginia in Charlottesville, was not involved in the new research.
Chemistry on Jupiter's moon
Mark Loeffler, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., was the lead author of the study. "When people talk about chemistry on Europa, they typically talk about reactions that are driven by radiation," he said.
Since Europa's surface temperature hovers between minus 225 to minus 300 degrees Fahrenheit (minus 143 to minus 184 degrees Celsius), most chemical reactions need to be facilitated by an infusion of energy from radiation or light.
On Europa, that energy typically comes from charged particles in belts of radiation on Jupiter, formed by the planet's magnetic field. These particles can travel the roughly 417,000 miles (671,100 km) to the moon, but they penetrate just a fraction of an inch into its surface. Consequently, models of Europa's chemistry usually 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," said Reggie Hudson, co-author of the study and the associate lab chief of Goddard's Astrochemistry Laboratory.
"But with the chemistry we describe," Loeffler added, "you could have ice roughly 33 or 330 feet (10 or 100 meters) thick, and if it has sulfur dioxide mixed in, you're going to have a reaction."
Testing the waters
Remote observations have shown that sulfur is present in Europa's ice. The chemical element originates in the volcanoes of Io, another of Jupiter's moons, and the molecules become ionized and transported to Europa, where they can become embedded in the ice.
Additional sulfur might come from the liquid ocean thought to lie beneath Europa's ice.
Still, "the fate of the implanted or any subsurface sulfur is not understood and depends on the geology and chemistry in the ice crusts," Johnson said.
In their study, the researchers simulated the conditions on Europa and experimented with spraying water vapor and sulfur dioxide gas onto quarter-sized mirrors in a high-vacuum chamber.
The mirrors were chilled to temperatures of about minus 280 to minus 370 degrees Fahrenheit (minus 173 to minus 223 degrees Celsius), and when sprayed, the gases immediately condensed as ice.
Even in the form of ice, though, the concentrations of water and sulfur dioxide decreased as they reacted to form different chemical products as well as charged ions.
Loeffler and Hudson found that in spite of the extremely cold temperatures, the molecules of water and sulfur dioxide reacted quickly in their icy forms.
At about minus 225 degrees Fahrenheit, which represents the warm end of the expected temperatures on Europa, the reaction was effectively instantaneous, Loeffler said. At minus 280 degrees Fahrenheit, the reactions are completed after half a day to a day, he said.
"If that doesn't sound fast, remember that on geologic time scales ? billions of years ? a day is faster than the blink of an eye," Loeffler said.
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, because then the reaction probably wouldn't be relevant to Europa's chemistry," Hudson said. "It would be a laboratory curiosity."
But the reaction continued, making the findings significant not only for Europa but other places where both water and sulfur dioxide are present ? including two more of Jupiter's moons, Ganymede and Callisto.
But wait, there's more
The researchers also found that the chemical reaction produced an unexpectedly high yield, and that the charged ions that were produced will react with other molecules. The discovery could blaze new trails for future chemical studies.
"The Loeffler and Hudson results show that really interesting acid-base reactions are going on," said Robert Carlson, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who collaborates with the two researchers but was not a co-author of the study. "I am anxious to see what might happen when other species are added and how the minor concentrations of sulfur dioxide on the satellite surfaces affect the overall chemistry."
The researchers hope future observations of Europa made by passing spacecraft can corroborate their laboratory findings.
The results of the study were published in the Oct. 2 issue of the journal Geophysical Research Letters.
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