Finding definitive evidence of extraterrestrial life will be one of the biggest scientific discoveries in history because it will settle one of our most profound questions: are we alone in the universe? Given our limited ability for space travel, the most likely place we'll discover alien life is on Mars. The search for signs of Martian life is one of the primary motivations behind recent spacecraft missions to the Red Planet. But even if life on Mars does (or did) exist, we still face a major challenge in our efforts to discover it. If our only experience with life is on Earth, how will we know alien life when we see it?
Life on Mars clearly won't look anything like the space-faring invaders of War of the Worlds. Many of the geological and atmospheric features that make life possible on Earth don't exist on Mars. Mars is a small planet with a very thin atmosphere, and it doesn't appear to have the kind of moving, geologically active continental plates we have on Earth. For decades, Mars was thought to be primarily a frozen, dormant, lifeless place. But it turns out that Mars is relatively warm; while its surface is often colder than any place on Earth, during Martian summers the temperature can rise above 90 degrees Fahrenheit. And recent spacecraft surveys of Mars have found evidence for liquid water. The presence of moderate temperatures and liquid water means that Mars, while not supporting a civilization of sentient beings, could harbor microbial life. Our search for alien life is primarily a search for Martian microbes.
We should be cautiously optimistic because our experience on Earth clearly shows that, in the face of extremely long odds, life finds a way.
Researchers have focused their exploration on regions of Mars that are most likely to support microorganisms, or that could have done so during a wetter Martian past. The Mars rover Curiosity, loaded with instruments to look for habitable environments, was deliberately targeted to an area of Mars that appears to be an ancient lake bed. In a set of reports released in December, the Curiosity science team provided one of the most compelling and in-depth analyses of a potentially habitable Martian environment.
The Curiosity rover explored a portion of a long-dry, ancient lake bed called Yellowknife Bay. Yellowknife Bay has been dry for nearly four billion years, but at one point it was extensively covered by water. The Curiosity science team found evidence that the water held levels of salts and minerals that would have made the lake "habitable to a broad range of prokaryotic microorganisms," of the types that we're familiar with on Earth. These habitable conditions may have persisted for tens of millions of years. Is that long enough for life to arise? We have no idea, but scientists aren't ruling out the possibility.
Finding an environment that was habitable billions of years ago is obviously not the same thing as finding direct evidence for life—such as fossils, metabolic by-products, or living organisms themselves. Even if such evidence exists, recognizing it won't be a trivial task. Definitively identifying fossilized microbes on Earth is often challenging, and our ability to scour the Martian landscape for fossils is extremely limited. The organic compounds that are metabolic by-products of living things are also often produced by inanimate processes. Curiosity collected samples that are rich in organic carbon, but researchers can't tell if those compounds are signs of life or simply the remains of carbon-rich meteorites. And if living microbes exist today on Mars, they will probably be impossible to cultivate in a spacecraft lab—much like most of Earth’s microorganisms, which can't be grown in the lab and are only studied through DNA-based technologies. Martian microbes would be unlikely to have DNA.
So even if there is abundant life on Mars, it's going to be a major challenge to find it. However, we should be cautiously optimistic because our experience on Earth clearly shows that, in the face of extremely long odds, life finds a way. Microorganisms exist in some of the most extreme environments on Earth, including ice-covered Antarctic lakes, deep-ocean hydrothermal vents, gold mines two miles underground where sulfur is the primary food, and even up in the atmosphere, nine miles above the Earth's surface. Earth life is so resilient, so capable of adapting to new environments, that NASA takes major precautions to avoid contaminating Mars with terrestrial bacteria.
If we ever discover life on Mars, it will almost certainly happen many years in the future, and the discovery will require a much more extensive exploration of the planet than we've carried out so far. Mars continues to be a major scientific priority because our study of it is a study of our place in the universe. How likely is life in the universe? Is life on Earth a fluke, an unlikely, roll of the cosmic dice? Or is life truly at home in the universe, a common and natural consequence of geology and chemistry on billions of planets? Serious thinkers have made arguments for both views, but the question won’t be truly settled until we find extraterrestrial life.