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.
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.
Send in the robots
Still other worlds may beckon just as strongly as Mars or Europa, such as Saturn's icy moon Enceladus. Yet the race to find extraterrestrial life may ultimately come down to where humans decide to send the robots.
Planned missions or mission proposals exist for the robotic exploration of both Mars and Europa. NASA's Mars Science Laboratory (MSL) is slated to put an SUV-sized rover down on the red planet after launching in late 2011, and has an onboard organic chemistry lab that can do a general assessment of surface conditions. That would represent a "baby step for an actual life detection mission," Farmer said.
The Europeans also continue to refine their ExoMars rover concept that could go a step farther and actually drill down into Martian regolith. A successful biosignature reading would likely lead to a mission to return Martian samples to Earth, where scientist could run definitive tests. Farmer sees this as a real possibility within the next decade.
Europa may ultimately lag behind Mars in the ongoing search for life, if only because it represents a greater unknown and poses steeper mission challenges.
"If you were going to look for life on convecting snowball, you're going to use a different kind of approach than if you were going to penetrate an ocean three times deeper than any on Earth," Farmer pointed out. The extreme different views of Europa each require very different mission approaches.
A joint mission between NASA and the European Space Agency would likely first send an orbiter to better examine Europa, and perhaps figure out whether a sub-surface ocean truly exists to warrant deep drilling. But that won't likely launch until 2020 at the earliest. And besides, scientists may not yet have the technology to create a lander capable of surviving Europa's harsh surface environment.
"You would have to land on surface with no real atmosphere, freezing temperatures and high radiation, and survive there while you drill," Farmer said. "We can't do that yet."