The biggest clue for alien life? The mineral kingdom. | Carnegie astrobiologist Robert Hazen

One of the greatest unanswered questions in science is: Is this a lonely universe in which we are the only living world? Or is the cosmos teeming with life with lots of civilizations asking that exact same question?

I think if we're going to find an answer, it may very well lie in the realm of minerals 'cause minerals tell stories. When we look at Earth's early minerals, we see a dramatic change. Before life, there were maybe 1,000, maybe 2,000 mineral species. We have over 6,000 identified today because when life came along, it changed the early Earth's environment. It put oxygen into the atmosphere. It had other chemical processes of metabolism that produce new minerals.

And those minerals then become a very sure sign of life and the co-evolution of the geosphere and the biosphere. So how is it possible for minerals to provide clues about life, especially life on other worlds? In some cases, it would be obvious if we found a nice big shell or a bone or a tooth made out of minerals—well, that's a pretty clear sign.

And there's other kinds of fossils as well that might point to the living world. And minerals would have to be involved in those discoveries. But there are other more subtle signs. There's certain chemical signs, there's certain physical signs, and these are called biosignatures.

Much of the effort in looking at Mars and other worlds in our solar system is to seek out those biosignatures. For example, there's lots of different ways to form pyrite, but the biological pyrites are always very, very fine-grained. Nanoscale. The microbes that precipitate the pyrite make a different kind of pyrite.

And so we look for the mineral species that can form biologically in ways that are different from the ways they form if there's no life around. And sometimes it's just the suite of minerals, there's certain combinations of minerals that can only form if you had a living world. For example, certain bright blue and green copper minerals or purple cobalt minerals that are just beautiful—but they're clues for life as well.

And maybe it's even more subtle. Maybe it has to do with the whole diversity of the mineral kingdom that you see. The Moon only has about 300 minerals. Mars, maybe 400, maybe 450. We haven't quite figured that out yet. Earth, 6,000. That's a sign of life.

And so minerals in many different ways, some obvious, some subtle, point to a biosphere. And with the trillions of trillions of trillions of planets out there, it's inconceivable that we're alone. If nature does something once, it's likely to do it all the time.