Preface

Mars is the only planet in the Solar System where life beyond Earth may have arisen, may still exist, and is within our reach. In principle, Mars is habitable: it has an atmosphere, water and a solid surface, receives warmth from the Sun, and may occasionally have liquid water for short periods. In the past, when its atmosphere was denser, it certainly had larger amounts of surface water. Mars lies at the outer edge of the Sun’s habitable zone. Where, then, are its inhabitants? Images returned by rovers show an apparently empty world: a planet-sized, dry, cold, dusty wilderness, sometimes windy or foggy. Rocky, sandy and icy deserts cover the planet. Its surface consists of lava or rock debris broken up by impacts. Yet life might still hide there. Tiny microbes or spores might lie dormant in a crack or below ground, awaiting warmer, wetter conditions. Planetary scientists and astrogeologists seek places most likely to allow survival: potential habitats of today or the past, where water exists or once existed. All life on Earth is related through DNA. If life exists on Mars, it might be related to terrestrial life: it may have reached Earth in a Martian meteorite, or a meteorite from another solar system may have seeded both planets. Life might also have arisen independently on Mars, or never existed there at all. Within the foreseeable future, we cannot reach another nearby place with such a good chance of hosting independently evolved life. Exoplanets are too distant for probes. Finding living organisms or fossil traces could help us understand how living matter arises from nonliving matter, and what life is. We may eventually examine many possible habitats, including caves and underground cracks, and find nothing alive. What about life forms that evolved on Earth? They might be able to settle on harsh Mars. Some people propose terraforming it and building human settlements. Mars differs so much geologically from Earth that it could never become another Earth. It could, however, become a different Mars, where organisms from Earth gradually make the surface more hospitable and begin a second evolutionary path, eventually producing new forms. Perhaps millions of years from now, when the Sun is warmer and Earth becomes too hot for our distant descendants, more complex life could appear on Mars. Its polar caps and ground ice would melt, evaporate or sublime, and after millions of years of evolution intelligent Martians might truly arise. Yet perhaps this could happen without terrestrial organisms, in a completely different way. Today we already—or still—have technology to investigate potential Martian habitats. If we found nothing, we could send Earth organisms to Mars, and might even artificially enable some to survive its harsh conditions. In a few million years, we will discover how important humanity was in the history of Mars. The history of Mars, which—as we believe—only we know about, has just begun.