Imagine a field geologist hiking a dusty landscape. She spies a ridge of rock, climbs to it, whacks off a protruding bit with a hammer. She stoops to pick up the broken piece, turning its freshly fractured face upward. From its color and crystals, she deduces its composition. She drops the rock and makes a note in her field notebook. Then she walks on.

On Earth, this whole process takes only minutes. On Mars, where robots substitute for human geologists, the same operation takes a day, sometimes several. Mars scientists eagerly anticipate a time when we’ll see human geologists walking on Mars, using their brains and hands to rapidly increase the rate at which we learn about Mars’s past. But by their very presence, human astronauts could endanger our search for life on Mars, contaminating the planet with the throngs of Earth life we bring with us.

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Why even look for life on Mars? Right now, the only place in the entire universe that we know for sure possesses life is Earth. Yet we also know that it’s easy to launch rocks from Earth and Mars into space as ejecta from large asteroid impacts, taking spores of bacteria and fungi with them. Earth life has almost certainly been transported to Mars, and if Mars ever had life, it’s almost certainly been transported to Earth. Could life from one planet have taken root on the other? Did it originate on both worlds, or only one? If we find life on Mars, is it possible that life started there and came to Earth? And what would that mean for us?

These questions underlie our scientific exploration of our planetary neighbor. They’re why the Spirit and Opportunity rovers were sent to “follow the water,” and why Curiosity was sent in search of “habitable environments.” Yet given the fact that there could have been transport of life between Earth and Mars, it’s possible that any life found on Mars could resemble Earth life, making it difficult to tell the difference between the two. That’s one reason that the concept of planetary protection is crucially important. Planetary protection isn’t just about preserving our own planet from the Andromeda Strain; it’s also about preventing the forward contamination of Mars with stowaway Earth life, ruining an alien ecosystem before we had a chance to study it.

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Since 1967, Article IX of the Outer Space Treaty has guided how spacefaring nations sterilize space robots to prevent such contamination. It’s impossible to perfectly sterilize a spacecraft, but humans have done their best. (All three NASA rovers were required to be cleaned so that they would carry no more than 300,000 bacterial spores to the surface of Mars, for instance.)

Unfortunately, there’s no way to make a human that clean. On our skin alone, we carry a thousand different species of bacteria, possibly a trillion individuals. Multiply that by every biological system within us, and the bio-burden is difficult to fathom. Technology can only do so much; once you land humans on Mars, contamination is inevitable.

So how can we examine its potentially life-supporting environments — such as pockets of ice or briny water buried a meter or few beneath the surface which the Outer Space Treaty calls “special regions”— without contaminating them? We simply can’t. For now, that reality means that missions to Mars are actually disallowed from visiting any such environments.

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In other words: We study Mars to look for life, but we do not allow ourselves to study the parts of Mars that may actually contain life. It’s a rather absurd situation. And before too long, the question of planetary protection may be moot; private companies and nations that did not sign the Outer Space Treaty have both expressed interest in sending humans to Mars. Some private individuals are imagining one-way trips for humans to Mars on missions that accept much more risk than any round-trip NASA or ESA (European Space Agency) one would. The most likely outcome for such missions is a crashed spaceship whose contents — including its human passengers — lie exposed on the Martian surface, contaminating everything.

We may one day discover life on Mars, but the longer it takes us to do so, the more difficult it will be to prove that humans didn’t carry it there. We have perhaps two more decades before the inevitable contamination of Mars. Should we relax the restrictions on robotic investigation of “special regions,” accepting that soon all such protections will be moot? Should we work through international law and treaty to prevent humans from traveling to Mars, to preserve an “ecosystem” that may, after all, contain no life, and limit our horizons in the process? Or should we shift our focus of the search for life elsewhere, to worlds like Europa and Enceladus where humans are unlikely to travel for the foreseeable future?

My answers to these questions are pragmatic. I think contamination is inevitable; therefore, we should do the most we can to explore special regions with relatively sterile robots in the time that remains before humans arrive. Ideally, the first “human” exploration of Mars could be through direct control of landed robots from humans staying in Mars orbit. This way, we’d enjoy the advantages of the human brain and also the toughness and relative sterility of landed robots, advancing science as much as we can before we forever complicate the search for life on Mars.