ASU Research E-Magazine: Livin' La Vida Europa

Jack Farmer only has to listen to the sounds coming from across the hall to assess the progress of NASA’s Galileo mission to the distant, icy world of Europa, one of Jupiter’s many moons.

“On a daily basis we’re getting oohs and aahs from over there,” Farmer said in January 1999.

Farmer is a geology professor and director of ASU’s Astrobiology Program. The laboratory across the hall belongs to Ronald Greeley, Regents Professor of Geology and a member of the Galileo imaging team. The activities in Greeley’s lab hold special interest for Farmer. Greeley, like Farmer and a dozen other ASU scientists who belong to the NASA-sponsored Astrobiology Program, seeks to determine if life exists beyond Earth.

Galileo’s extended mission to Europa has opened the way to some intriguing possibilities. With lead responsibility for geologic interpretation of Europa and another of Jupiter’s moons, Callisto, Greeley has played a critical role in documenting the evidence.

Galileo’s mission to Jupiter originally was scheduled to end in December 1997. But NASA extended the mission for two years once it became apparent that Europa probably contains the three basic ingredients for life: a steady energy source (from the tidal push-pull of Jupiter), water, and organic compounds.

Europa resembles a bloodshot, mottled, and disembodied alien eyeball. The images that provoked the appreciative noises from Greeley’s team show a highly disrupted surface of long linear and curved bands. In these areas, chunks of soft ice or pockets of water appear either to have erupted directly onto the ice-encrusted surface or to have broken through it.

“Those places are very high priority for further exploration,” Greeley says.

NASA plans to launch an orbiter to Europa in 2003. The mission’s prime objective: search for an ocean beneath the icy surface. When more detailed mapping of Europa’s surface begins in 2008, the orbiter will pay special attention to these apparently recently active areas.

“That’s the big problem,” Greeley explains. “We don’t know if these features we see are forming today or if they’re relicts from the past.”

If the orbiter does find a subsurface ocean on Europa, NASA plans to follow-up with a landing mission. The lander might even be designed to melt through the ice, depending on its thickness, into the ocean below.

About the size of Earth’s moon, Europa consists mostly of rock surrounded by an icy shell up to 60 miles thick. Scientists calculate that the tidal forces of Jupiter may be strong enough to drive volcanism, which could have melted a layer of water between Europa’s icy surface and its rocky interior.

image of the surface of Europa
The icy crust of Europa is scarred by a dense network of rifts. One such rift is a double ridge about 2.6 kilometers (1.6 miles) wide and 300 meters (330 yards) high. Energy from Europa’s interior drives intense faulting and disruption and may provide enough heat to keep water from freezing beneath the surface. The area shown is about 14 by 17 kilometers.

Local hotspots and black smokers might also exist on Europa, just as they do on Earth’s ocean floor. Scientists have taken water samples near these superheated plumes issuing from terrestrial black smokers and found them teaming with microorganisms. According to one theory, life on Earth actually originated near such environments. If it happened on Earth, why not Europa?

And now scientists are beginning to wonder about Callisto. Based on data collected by the Voyager spacecraft in 1979, scientists expected Galileo to find little of interest on Callisto. They have been surprised.

Magnetic field measurements taken by Galileo suggest that there could be a salty ocean beneath Callisto’s cratered crust. The same measurements could be explained, however, if other conductive materials such as graphite were distributed throughout the ice.

Images of Callisto, unlike Europa, show no signs of water flowing onto the surface. “If there is an ocean, it’s pretty deep,” Greeley says.

Yet why does Callisto have so few small impact craters? An old, presumably geologically inactive surface like Callisto’s should be riddled with many small craters, much like Earth’s moon.

“Some process is erasing those small craters, or they didn’t form to begin with, which is a little hard to buy,” Greeley says. “We think they did form, but then some process removed them.”

The landslides observed around some of Callisto’s craters exhibit further evidence that different processes have shaped the moon’s surface. Ganymede, the moon that orbits Jupiter between Callisto and Europa, should have similar surface properties, Greeley adds. And yet Ganymede exhibits no such landslides.

Still, most scientists would want to see far more evidence before building a case for life on Callisto, Greeley says. They have been calling for more evidence from Mars, as well, especially in the wake of the controversy surrounding the Martian meteorite known as ALH84001.

The controversy began in August 1996 when a research team led by scientists at NASA’s Johnson Space Center in Houston announced that it had found nanofossils, bacterially produced minerals, and other evidence for life in ALH84001. Among the researchers was NASA senior scientist Everett Gibson Jr., who received his doctoral degree in geochemistry from ASU in 1969.

Even some undisputed aspects of the meteorite’s history once would have left scientists shaking their heads. Blasted from Mars millions of years ago by an impacting comet or asteroid, it eventually fell to Earth in Antarctica.

There it shared the fate of thousands of other meteorites now sitting in scientific collections.

The flow of the Antarctic ice sheet works as a conveyor belt, concentrating meteorites in certain areas where, gradually exposed by wind, the dark stones make easy pickings against the white ground. ALH84001 was the first meteorite from the Allan Hills ice field that U.S. scientists cataloged following the 1984 field season, hence the code.

During a March 1997 lecture at ASU, Gibson likened his team to detectives preparing evidence for a grand jury. Although the controversy still simmers three years later, the scientific jury has overwhelmingly rejected the team’s evidence.

John Bradley, who received his ASU doctoral degree in chemistry in 1981, soon emerged as one of the leading skeptics. Bradley, executive director of MVA Inc., a microanalytic company in Norcross, Georgia, co-authored three studies from late 1996 to mid-1998 that challenged the NASA researchers’ findings.

“These three papers in combination basically invalidate much of their evidence,” Bradley said in a news release from the Georgia Institute of Technology, where he serves as an adjunct faculty member.

Bradley and his co-authors have published evidence that the features in the Martian meteorite had formed geologically, not biologically; that the crystals in the meteorite were formed at temperatures too high for organisms to exist; and that the meteorite’s purported nanofossils were merely fractured mineral surfaces.

ASU Astrobiology Program members Peter Buseck and Laurie Leshin also have gathered data contradicting the NASA team’s claims.

Gibson and his co-authors had cited the presence of the mineral pyrrhotite as possible evidence of past Martian bacterial life. But Buseck, Regents Professor of Chemistry and Geology, ASU research associate Mihaly Posfai, and their co-authors noted in the May 8, 1998, issue of the journal Science that the bacteria actually prevent the mineral’s formation.

Leshin, an ASU assistant professor of geology, has already conducted laboratory tests on all but a few of the 14 known Martian meteorites during her young career. In 1997, she even spent two months living out of a tent in Antarctica hunting for meteorites—Martian or otherwise.

“They call it the harshest deep field camp on the continent,” Leshin says. “We specifically go to places where it’s really, really windy because that allow us to find the meteorites.”

Leshin uses isotopes, forms of a common element that differ only in their atomic weights, as an environmental tracer to reconstruct the conditions under which the carbonate and olivine minerals formed in ALH84001.

Both the carbonates that contained the nanofossils, and the olivine, which is found in close proximity to the carbonates, formed at temperatures catastrophically high for biological processes.

Nevertheless, the chemical and isotopic compositions of the carbonates are unlike anything ever seen on Earth.

“There’s a lot of ongoing work to sort out this one little rock that contains 1 percent carbonate. It’s not the ideal sample if one wanted to go look for life on Mars,” she says.

Still, Leshin, among others, regard the overall impact of the ALH84001 controversy as positive because of the scientific discussions it has stimulated about the minimum requirements for life to exist.

In that respect, the once lowly ALH84001 has honed scientific minds for the next such discovery. As University of Washington oceanographer John Delaney said during a lecture at ASU in January 1997, “There will be no more fundamental discovery than the discovery of life on another planet.”—Steve Koppes