If you're looking for signs of past life on Earth, some of the evidence is obvious. "Finding a trilobite fossil is a no-brainer, we all can understand that one, or a dinosaur bone." Jack Mustard, a Brown University geologist.

He says less obvious signs of ancient life can be found in glass. Specifically, impact glass--which forms when asteroids slam into the planet, rapidly heating and melting the rocks around them. "Impact glass on the Earth can preserve biological material in a kind of capsule, a time capsule if you will."

Here on Earth, for instance, scientists have found impact glass containing ancient plant matter, and other chemical signatures of life.

Jack and his colleague Kevin Cannon have now spotted the same sort of impact glass on Mars, using the spectrometer on the Mars Reconnaissance Orbiter. Which means Martian glass might also hold evidence of life. If the Red Planet ever harbored life, that is.

The glass is more than just a potential time capsule. Its mineral structure also makes it an attractive snack for certain microbes. "Microbes eat geologic materials. But it can be a really slow process. Glass on the other hand is something that is kinda like having a Dorito for a microbe, because you can just start chewing away at it, and it's quite delicious, and you just munch right through it." Meaning it also might be worth inspecting glass for anything living on it—not just trapped inside. The findings are in the journal Geology. [Kevin M. Cannon and John F. Mustard, Preserved glass-rich impactites on Mars]

So does this study suggest we should abandon the old "follow the water" mantra when looking for life? "I certainly think it would not lend itself to, let's change the course of this ship and follow the glass. But I think where we're at is developing significantly interesting hypotheses to explore." And the Mars 2020 mission, which will collect samples for eventual return to Earth, might just be the perfect opportunity to start.

—Christopher Intagliata

[The above text is a transcript of this podcast.]