When imagining locations where extraterrestrial life could potentially dwell, few places inspire the imagination like one of Earth's closest neighbors. For centuries, man has looked to Mars and imagined it as a home for other beings. Over the last fifty years, various missions to the red planet have sought to determine the probability of such an evolution. But how likely is life on Mars?
A habitable environment
When searching for life, most astrobiologists agree that water is key. All forms of terrestrial life require water, and while it is possible that life could evolve without the precious liquid, it is easier to search for conditions that are known to be optimal, rather than conditions we suppose could be." [5 Bold Claims of Alien Life ]
This raises a problem on Mars. The planet today is dry and barren, with most of its water locked up in the polar ice caps. The planet's thin atmosphere allows radiation from the sun to irradiate the surface of the planet, adding to the environment's challenges. If life exists on Mars today, it is likely hiding beneath the surface, perhaps hiding in a shielded water source.
But Mars wasn't always a desolate wasteland. Scientists think that, in the past, water may have flowed across the surface in rivers and streams, and that vast oceans covered the planet. Over time, the water was lost into space, but early conditions on the wetter planet could have been right for life to evolve.
Over the last four billion years, Earth has received a number of visitors from Mars. Our planet has been bombarded by rocks blown from the surface of the red planet, one of the few bodies in the solar system scientists have samples from. Of the 34 Martian meteorites, scientists have determined that three have the potential to carry evidence of past life on Mars.
A meteorite found in Antarctica made headlines in 1996 when scientists claimed that it could contain evidence of traces of life on Mars. Known as ALH 84001, the Martian rock contained structures resembled the fossilized remains of bacteria-like lifeforms. Follow-up tests revealed organic material, though the debate over whether or not the material was caused by biological processes wasn't settled until 2012, when it was determined that these vital ingredients had been formed without the involvement of life.
Scientists also found structures resembling fossilized nanobacteria on the Nakhla meteorite, a chunk of Mars that landed in Egypt. They determined that as much as three-fourths of the organic material found on the meteorite may not stem from contamination by Earth.
A third meteorite, the Shergotty, contains features suggestive of biofilm remnants and microbial communities.
All of these samples provide tantalizing hints of the possibility of life in the early history of the red planet. But a fresh examination of the surface has the potential to reveal even more insights into the evolution of life on Mars.
Searching for life
The first closeups of the red planet came from flyby photos taken by Mariner 4. Followed up with images from the Viking orbiters, the crafts provided detailed images of the surface that revealed that the surface of Mars was dry, but contained landforms that indicated it may not have always been so. Gullies, ocean beds, signs of erosion, and the vast polar caps inspired hope that Mars could have at one point hosted life.
When NASA set the first lander down on the Martian surface, one of the experiments performed sought traces for life. Though Viking's results were deemed inconclusive, they paved the way for other probes into the environment of the red planet. [Mars Explored: Landers and Rovers Since 1971 (Infographic)]
Exploration of the red planet was put on hold for more than two decades. When examination of the planet resumed, scientists focused more on the search for habitable environments than for life, and specifically on the search for water. The slew of rovers, orbiters, and landers revealed evidence of water beneath the crust, hot springs – considered an excellent potential environment for life to evolve – and occasional rare precipitation. Although the most recent lander, Curiosity, isn't a life-finding mission, there are hopes that it could pinpoint locations that later visitors might explore and analyze.
Future mission to Mars could include sample returns, bringing pieces of the Martian crust back to Earth to study. More experiments could be run by hand on Earth than can be performed by a remote robot explorer, and would be more controlled than meteorites that have lain on Earth.
Are we the Martians?
The transfer of material from Mars to Earth and presumably back again has sparked some debate about the possibility of contamination early in the history of life. Some scientists argue that a meteorite from Earth could have traveled to Mars — or vice versa. Debates rage over whether or not tiny organisms would be hardy enough to survive the voyage through a freezing, airless, radiation-filled vacuum and kick off life at its new home.
The idea of such seeding is not limited to interactions with Mars. Some have proposed that debris from outside the solar system could even be responsible for spawning life on Earth. But in terms of the red planet, it is possible that scientists might one day find life on Mars — and it could be a close relation.
— Nola Taylor Redd, SPACE.com Contributor