Mars or Europa? Which could show life first?

By Jeremy Hsu

Special to SPACE.com

updated 6:39 p.m. ET Oct. 12, 2009

A dusty red planet and an icy moon of Jupiter may hold the best hopes for scientists trying to track down extraterrestrial life, at least in this solar system.

Mars and Europa each hold the promise of liquid water and possibly life. Mars has a history that suggests water once flowed in rivers and lakes, and it may still harbor liquid water deep underground. The more distant Europa could hide a churning ocean filled with life forms beneath its icy surface, as the moon gets gravitationally squeezed by Jupiter.

Future space missions have targeted both destinations to send new robotic explorers. But the red planet represents a much closer and better known target for space explorers.

"We're much farther down the road with Mars than Europa," said Jack Farmer, an astrobiologist at the University of Arizona.

Mars invites a deeper look
Liquid water probably once filled the valleys and basins on Mars, but now the planet's surface resembles a barren, dusty badland. Any living organisms that may have existed must have gone extinct or underground.

"My view is that habitable environments on Mars are likely to only be found in the deeper subsurface where we might have a hydrosphere," Farmer told SPACE.com. "Liquid water is unstable at the surface of Mars today."

Some ice water or snowfall could temporarily become liquid at the surface, such as when NASA's Phoenix Mars Lander possibly found some liquefied globules clinging to its struts. Still, that would hardly last long enough under freezing or vaporizing conditions to sustain life.

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Microbial life that could eke out an existence also seems unlikely to survive the cosmic radiation that scours the surface of Mars. But astrobiologists remain excited about possibly finding signs of past life on the surface, where minerals that only form in water may have preserved certain remains.

"A lot of these kinds of mineralogical targets are water indicators, and we know where a lot of these deposits are now," Farmer said. He noted that sulfate minerals do a decent job of preserving organic compounds produced by organisms on Earth, and sometimes even microfossils. Silica and other clay minerals have also turned up during searches by the Mars rovers and orbiters.

Upcoming missions to Mars could perhaps even tap into any liquid reservoirs hidden deeper below and search for existing life, if they have the right equipment.

Europa's ocean: fact or fiction?
A more challenging target for astrobiologists sits farther out in the solar system, where the icy moon Europa beckons with hints of a salty ocean beneath its crusty exterior.

"Europa's a very appealing target for astrobiology, and particularly from the standpoint of what life forms might be working in a sub-surface ocean," Farmer noted. "The challenge with Europa is that we don't know for sure if there's a sub-surface ocean."

Some studies have suggested that Europa holds an ocean up to three times deeper than Earth's oceans. But other models have suggested that no such ocean exists, and that perhaps the moon only harbors pockets of ice-brine slush. The debate largely depends on how much heat Europa can generate from tidal flexing, when Jupiter squeezes the moon with its gravitational pull.

Still, Farmer suggested that life could perhaps exist even within a "snowball slush" mixture between the solid ice chunks. Such slush appears to have erupted onto the moon's surface at times due to icy volcanic eruptions, and any material that came up might have carried signs of life with it — although living organisms would perish quickly due to the harsh radiation bombardment at the moon's surface.

"If you drop down a ways below the depth where radiation is affecting surface materials, you might be able to access biosignatures frozen out in the ice there," Farmer said.

A recent study suggested that Europa may hold hundreds of times more oxygen than scientists had previously imagined. That has lent to the sense of optimism about prospects for life on the slush ball.

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