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."
Editor's Note:This feature article
is part of SPACE.com's weekly Mystery Monday series.
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