One of the most elusive questions about our very existence is the origin of life on Earth. There are many possibilities for how it arose and many more about whether life always arises the same way it arose on Earth. In an attempt to define, design, and detect life elsewhere in the universe, Seed’s Veronique Greenwood explores the role of biosignatures—organic traces of Earth-like organisms—in searching for life beyond our planet.
But the closest reasonable place to look for life outside Earth? That has to be Mars. Today, Mars is dry, desolate, and frozen. Its atmosphere is so thin that it would take 140 Martian atmospheres all stacked atop one another to give you the same pressure we find here on Earth. Why would we even consider a place like this to be hospitable to life? There are three pieces to the argument: imaging from space, exploration of the soil, and the theory of Mars’ history.
From space, we’ve determined that Mars has clouds, which means water in the atmosphere. We’ve seen dried up riverbeds with meanders, oxbows, and lake/lagoon areas, which means a past with liquid water. We’ve found icecaps that grow and retreat with the seasons at the poles. And we’ve found that it has a magnetic field like Earth does, only it’s both exceedingly weak and permanent, which means there’s no molten core continuously generating it like we have on our planet.
From landers and rovers—starting with the Viking landers launched in 1975—we’ve learned a ridiculous amount about the surface of Mars. First off, we’ve found all the minerals and elements necessary for the kind of life we have on Earth. We’ve found evidence for not only rivers and lakes on Mars, but also dried up oceans that have left frozen ground water behind. We’ve found mineral deposits at the bottoms of craters and riverbeds that only arise from biological processes on Earth. And we find it in many different locations, meaning that Mars’ present state is not the way Mars has always been.
So finally, we come to the theory side, where we try to put these observations together. Clearly, from the observations of riverbeds, discoveries of unusual minerals on the surface, and frozen water just beneath the soil’s surface, Mars wasn’t always like it is now. It used to have liquid water, which means it used to have a thicker atmosphere. Like all planets at the beginning of the solar system, Mars probably had a molten core that produced a strong magnetic field, shielding it from solar radiation and keeping the atmosphere intact. That means water, water everywhere! On Earth, everywhere there’s water, there’s life. Was the same true on Mars?
Contrast that with the big differences between a young Mars and a young Earth. There are only two: First off, Mars was farther away from the Sun, which means it was cooler. But second, Mars is much smaller than Earth and has a volcano bigger than any in the solar system, Olympus Mons. We think that’s how Mars’ core cooled so quickly. Once the core cools and solidifies, the magnetic field weakens terribly. When the field weakens, the Sun’s radiation strips the atmosphere off of the planet. It cools, freezes, dries up, and becomes uninhabitable.
Yet Mars is our best hope for finding life beyond Earth. We don’t yet know whether there ever was life on Mars, but it was a lot like Earth for the first billion years of the solar system, and we certainly had life by then. It may have been like life on Earth, it may have been completely different, or it may not have been there at all. But it’s the first question about extraterrestrial life we should be able to answer. And all the things we know about Mars have me feeling optimistic that there once was life there. All we have to do is find evidence of it.
Originally published November 9, 2009