It’s no secret that the possibility of life on Mars has tantalized us for a long time now. Even today, despite the knowledge that there are no intelligent Martians, the idea works its way into science fiction, as movies like John Carter, The Last Days on Mars, and the animated film Mars Needs Moms demonstrate. Even the character of Spock on Star Trek was originally conceived as a Martian!

But the concept isn’t limited to science fiction; scientific interest in life on Mars continues. Scientists, of course, aren’t expecting to find any little green men, but it’s still an open question whether some form of microbial life could exist there, especially given Mars’ relatively Earth-like past and the presence of chemicals necessary for life as we know it. But is there a real possibility that Mars could hold some sort of life? Is the planet’s apparent similarity to Earth really enough to allow for life to have formed?

Perhaps most significantly, is the planet habitable to life today?

While there are no firm answers to these questions, Dr. Pamela G. Conrad, an astrobiologist and mineralogist with NASA, wrote a piece for the journal Science exploring what we know so far about the potential habitability of Mars—and how upcoming missions to the red planet could finally answer some of these questions.

Definitions matter

"Habitability" may seem like a straightforward term, but in cases like this, specificity is important. Mars is clearly not habitable to humans. And there’s always the possibility that life we know on Earth is not the only kind of possible life. In that case, there’s no way to know the conditions in which it could exist. In the article, Conrad defines habitability to mean that the planet meets the needs of the simplest Earth microbes, intentionally not considering the possibility of other forms of life.

The bare minimum requirement is the presence of water, according to Conrad. And we now know there’s water on Mars. But water is just a minimum; chemistry, Conrad points out, is not enough by itself to make a planet habitable. All the right chemicals might be present, but if the physical conditions aren’t appropriate, those chemicals won’t undergo the reactions necessary to form life.

Before the late heavy bombardment, a period when impacts were common throughout the Solar System, Mars had a magnetic field which protected the planet from ionizing radiation. This means that, from about 4.1 billion to 3.8 billion years ago, it was more habitable than it is now. Additionally, the planet had a thicker atmosphere at the time, which shielded the planet from UV light from the Sun, which also poses a challenge to life.

Given all that, Conrad says, the planet had the right conditions and could have sustained Earth-like microbial life at the time. Whether it did, says Conrad, is a matter of timing. “The emergence and sustenance of both life and its habitat require a convergence of the right chemicals and physical conditions in the right place at the same time,” she explains in the article.

A brief history of missions to Mars

Whether those conditions were right, and at the right time, is a very complicated question to answer. Our attempts to come up with an answer have largely relied on our visits to Mars—which have provided essential data in the quest to determine habitability. Conrad lays out the history of some of the missions, their discoveries, and how those have shaped the current understandings of Mars’ potential habitability.

Besides producing some inconclusive findings regarding the presence of life, the Viking landers also evaluated the planet’s climate using its meteorology packages. This set the precedent for missions to follow, Conrad argues, by teaching us that taking a wide range of physical samples is key to determining whether the planet is habitable.

Since then, scientists have explored the Martian landscape with rovers, which were able to drive around, guided by orbiters, to find evidence of habitability—either in the present or past of Mars. Together with the Viking landers and other missions, the rovers have returned data that is now being used to plan further landings by missions from NASA and the European Space Agency.

In order to really understand whether Mars is or was ever habitable, a better understanding of the historical conditions in the atmosphere is important. That’s what the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission, which landed in September, aims to accomplish. It studies the chemistry of the upper atmosphere, which will help piece together the planet’s atmospheric history. The present chemistry may provide clues as to how the current condition was reached.

Meanwhile, we have a full year of data from the MSL (Mars Science Laboratory), which landed in 2013. The MSL has regularly measured the surface radiation, ground and air temperature, wind speed and direction, and atmospheric pressure.

Possibilities

All of this data on the present conditions of Mars is essential, Conrad explains. One problem in learning about the historical conditions on Mars has been that some of the parameters necessary for extrapolations into the past aren’t so easily preserved in the rock record. But by combining our geological data on the past with measurements of the planet’s current environment, researchers will be able to find out the rate at which the environment is changing.

By mapping out the milestones on the way from the ancient, wet Mars to the current, dry, inhospitable Mars, researchers hope to learn how the current environment was shaped and whether life could fit into the picture. This is an ongoing process. Future missions will address these questions with more sophisticated scientific equipment. NASA’s Mars 2020 mission will save samples for return to Earth and a more detailed analyses.

As to whether there actually is life on the red planet, ESA’s ExoMars mission, planned for 2016, is intended to finally provide some definitive answers by drilling two meters beneath the Martian surface to look for chemical evidence of life.

Science, 2014. DOI: 10.1126/science.1259943 (About DOIs)