Using Robotic Probes to Identify Fossils on Jupiter’s Europa

6a00d8341bf7f753ef0134864b395f970c.jpg If extraterrestrial life exists on Jupiter's moon Europa, instead of deploying probes to drill past its ice shell to look for alien life in the ocean below, robotic probe might just find fossils on the icy surface. Europa might not only sustain, but foster life, according to the research of  University of Arizona's Richard Greenberg, a professor of planetary sciences and member of the Imaging Team for NASA's Galileo Jupiter-orbiter spacecraft.

"A prospector sent there could possibly find extraterrestrial life within our lifetimes," suggested Greenberg. However, rather than deploying complex equipment to try and penetrate an uncertain distance into the ice, the remains of marine life on Europa could be available right on the outer shell for landers to find. Scientists theorize that the constant upheaval this Jovian moon undergoes could drag organisms upward," Greenberg explained.

"The reason I like Europa so much is that it’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features – and giving it the potential to harbor life. It’s one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life. Europa is also easier to get to than Enceladus or Titan. As such, I think it ought to be one of the highest-priority exploration targets for robotic space probes. (Human exploration would be nice, too, but if you think radiation exposure on the way to Mars is hard, you don’t even want to consider putting people in the Jovian system!)"

Europa, similar in size to Earth's moon, has been imaged by the Galileo Jupiter-orbiter spacecraft. Its surface, a frozen crust of water, was previously thought to be tens of kilometers thick, denying the oceans below any exposure. The combination of tidal processes, warm waters and periodic surface exposure may be enough not only to warrant life, but also to encourage evolution.

With Jupiter being the largest planet in the solar system, its tidal stresses on Europa create enough heat to keep the water on Europa in a liquid state. More than just water is needed to support life. Tides also play a role in providing for life. Ocean tides on Europa are much greater in size than Earth's with heights reaching 500 meters (more than 1,600 feet). Even the shape of the moon is stretched along the equator due to Jupiter's pull on the waters below the icy surface.

The mixing of substances needed to support life is also driven by tides. Stable environments are also necessary for life to flourish. Europa, whose orbit around Jupiter is in-sync with its rotation, is able to keep the same face towards the gas giant for thousands of years. The ocean is interacting with the surface, according to Greenberg, and "there is a possible that extends from way below the surface to just above the crust."

"The real key to life on Europa," Greenburg adds, "is the permeability of the ice crust. There is strong evidence that the ocean below the ice is connected to the surface through cracks and melting, at various times and places. As a result, the , if there is one, includes not just the liquid water ocean, but it extends through the ice up to the surface where there is access to oxidants, organic compounds, and light for photosynthesis. The physical setting provides a variety of potentially habitable and evolving niches. If there is life there, it would not necessarily be restricted to microorganisms."

Tides have created the two types of surface features seen on Europa: cracks/ridges and chaotic areas, Greenberg said.The ridges are thought to be built over thousands of years by water seeping up the edges of cracks and refreezing to form higher and higher edges until the cracks close to form a new ridge.

The chaotic areas are thought to be evidence of the melt-through necessary for exposure to the oceans.

The tidal heat, created by internal friction, could be enough to melt the ice, along with undersea volcanoes – a combination of factors would give organisms a stable but changing environment — exactly the type that would encourage evolution.

Astronomers believe, based on images from the Galileo mission and magnetometer readings, that Europa has an icy shell over a liquid ocean, with a solid rocky core at the centre. There’s some disagreement between scientists on how thick the ice is — estimates range from 10 to 100,000 metres — but observations have yielded reports of a number of “double-ridges” on the surface that are believed to be cracks in the crust.

The cracks are the most likely place for life to take root. They get sunlight (unlike the rest of the ocean below the ice) and are also subjected to strong currents.

If a 100 kilometer-deep ocean existed below Europa’s ice shell, it would be 10 times deeper than any ocean on Earth and would contain twice as much water as Earth's oceans and rivers combined. 

The scarcity of craters seen on Europa suggests the ice shell is no older than 50 million years old, hinting that it underwent complete turnover in that time. The force behind this extraordinary activity is the gravitational pull Europa experiences from Jupiter. This leads to tidal forces roughly 1,000 times stronger than what Earth feels from our moon, flexing and heating the Jovian moon and constantly stirring its crust.

Ice — probably newly frozen ocean water — apparently regularly gets pushed up from below, leading double ridges typically 330 feet high (100 meters) to form and cover at least half of Europa's surface. Parts of the surface also could partially melt from below, creating rafts of ice that break loose and tumble around.

Scientists are now developing drills to penetrate Europa’s ice shell. Evidence for life might be found right on the surface of the ice, however, making drilling unnecessary. 

This process creates the "chaotic terrain" that comprises roughly 40 percent of the ice shell, and also sends matter both upward and downward.

"If there are organisms in Europa's ocean, one could well imagine that all over the surface there might be frozen chunks of that stuff," Greenberg said. "People are talking about various kinds of drills and melting down through the ice, and I think we can jump past that and sample the ocean from the surface."

One of the best places to look for any fossils on Europa would be newly formed double ridges, Greenberg said. "The ridges that crisscross others are going to be the most recent ones," he explained. "One could then imagine landers scooping up the ice and analyzing it."

Chaotic terrain would also be another good area to explore, as would an active crack in Europa's crust. "If we can land right next to an active crack, there's a good chance we could sample some of the most recent ice," Greenberg said. "If we could put a penetrator into it, we could even sample water as it comes up."

If any microbes did manage to make their way to Europa's surface, the constant stream of radiation from Jupiter would likely break their proteins down over time, assuming such life would have proteins at all, said planetary geologist Brad Dalton at NASA's Jet Propulsion Laboratory.

Still, experiments of Dalton's have suggested orbiters could investigate the infrared signature of Europa's icy crust to look for tattered remnants of life. Landers could conduct even more detailed analyses — for instance, using "lab-on-a-chip" devices on melted ice samples to look for biomolecules, he added.

Also "there's always the possibility that we could find structures — something analogous to skeletal remains," Greenberg noted.

Of course, if there is life in Europa's ocean, it remains uncertain whether it would indeed get lofted up via geological processes to its surface. Conversely, if no life is seen on Europa's surface, that does not mean there is no life in Europa's ocean.

"My point is only why wait to look for life at the hardest place on Europa to get to?" Greenberg said. "Why not go to the easy place first?"

Casey Kazan

Sources:

The Ice Fracture Explorer

http://www.wired.com/wiredscience/2010/08/mission-to-europa/

http://www.astrobio.net/exclusive/3528/hunting-for-fossils-on-europa

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