[2] SPACE: ICE 19 KILOMETRES THICK ON EUROPA (pp419-421) A trio of unrelated new studies show that while Jupiter's moon Europa should have the ingredients necessary for life, finding any bugs on the icy moon would be difficult because its watery ocean lies beneath an impenetrable, frozen shell that is several miles deep.
Research led by Christopher Chyba of the SETI Institute shows comets could have long ago supplied raw material for jumpstarting life, including carbon, nitrogen, sulfur and phosphorus.
These ingredients were stirred into life's recipe on Earth, yet
no one knows how they actually assembled and evolved into the first microorganisms. "We now know that enough of the right materials should have been present to support a Europan biosphere," Chyba said.
His study, done with Elisabetta Pierazzo of the Planetary Science Institute in Arizona, appears in the May issue of the journal Icarus.
Chyba and other astrobiologists would love to poke a hole in the ice of Jupiter's moon Europa to search for life in the liquid ocean that some have said might exist just a mile or so below. However, two other new studies poke serious holes in that idea.
Thick shell
Research led by Paul Schenk of the Lunar and Planetary Institute in Houston suggests the outer ice layer of Europa is too thick for any practical drilling project. Based on fresh analysis of images of Europa's craters taken by the Voyager and Galileo spacecraft, Schenk and his colleagues say the crust is at least 11 miles (19 kilometers) thick.
"It is possible the crust could be locally thinner," Schenk told SPACE.com. But, he said, "it is unlikely that the crust could thin to a few kilometers and be stable."
Schenk compared craters on Europa to those on two other Jovian moons, Ganymede and Callisto, which are thought to have oceans roughly 60-120 miles (100-200 kilometers) under their frozen exteriors. The researchers also looked at craters on Earth's Moon.
All the craters were caused by ancient impacts of asteroids or comets.
Small craters on all the moons had much in common. But the larger ones on Europa, instead of having rims and central uplifts common to other craters, are surrounded by sets of concentric troughs and ridges.
The features may be due to the wholesale collapse of the crater floor, Schenk says. Material underlying the icy crust rushed in to fill in the void, dragging the overlying crust, fracturing it and forming the concentric rings.
The study will be detailed in the May 23 issue of the journal Nature.
[Late last year, another group of researchers studied other features of Europa's craters and
concluded the ice was at least 2 miles (3 kilometers) thick.]Hope for life
Most experts, Schenk included, are confident Europa harbors a liquid ocean. It is kept liquid via warmth generated by tidal forces that tug at the insides of the planet as it orbits giant Jupiter.
The only real questions are depth and content.
Schenk said his study does not rule out life in Europa's oceans. It just means it will be very difficult to detect anything that might be there. The work also implies life would have to use something other than sunlight as an energy source.
"It is unlikely that photosynthesis is important or feasible on Europa" now, Schenk said. He added, however, that the crust might have been thinner several hundred million years ago. It is even possible, he said, that the liquid ocean was once exposed. "If that were true, then a variety of organisms could evolve, depending on chemistry and time."
Europa could support life without sunlight.
On Earth, life exists on ocean floors, where no sunlight reaches. Chemicals from hot vents energize hardy critters, which in turn support other organisms. Europa may have similar vents.
But living Europan bugs could only be found by drilling through the ice, a tremendous robotic undertaken regardless of the depth. Engineers at NASA's Jet Propulsion Laboratory are at work on a robot that would melt its way through the ice. It is limited to 1.8 miles (3 kilometers).
Going to Europa
Either way, plans to send a landing craft to Europa have not gotten off the ground. Nothing is likely before the year 2030. Even a planned orbiter, now estimated to cost a billion dollars, is
on hold.Chyba, of the SETI Institute, has long advocated such a mission.
"My current feelings about Europa exploration can primarily be categorized as depression," Chyba said earlier this month at an astrobiology meeting at the Space Telescope Science Institute (STScI).
Chyba said if a lander is sent to Europa, it would not necessarily need to drill clear through the ice to find signs of life.
Fossils of past life, or even possibly dormant life, might exist near the surface.
Chyba recommended landing in a smooth area, where some researchers say water might once have reached the surface, then drilling about 3 feet (1 meter) down, below the area where intense radiation would kill life, and then melting ice and sampling its contents.
"I think that Europa is of exobiological interest comparable to Mars," Chyba said, "and I think we need to figure out how to get there."
Chyba pointed out that while Mars missions are cheaper, over the course of several years and many missions the price tag for Mars Exploration easily exceeds a billion dollars. He called on NASA to give equal consideration to Europa, even if that means investing a lump sum in a single mission.
Next Page: More holes, and details of raw material delivery
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