David Kipping

And so that oxygen, if we detect it on another planet, whether it be Mars, Venus, or an exoplanet, whatever it is, that was long thought to be evidence for something doing photosynthesis.

Because if you took away all the plant life on the Earth, the oxygen wouldn't just hang around here.

It's a highly reactive molecule.

It would oxidize things.

And so within about a million years, you would probably lose all the oxygen on planet Earth.

So that was conventionally how we thought we could look for life.

And then we started to realize that it's not so simple because A, there might be other things that life does apart from photosynthesis.

Certainly the vast majority of the Earth's history had no oxygen, and yet there was living things on it.

So that doesn't seem like a complete test.

And secondly, could there be other things that produce oxygen besides from life?

A growing concern has been these false positives in biosignature work.

One example of that would be photolysis that happens in the atmosphere.

When ultraviolet light hits the upper atmosphere, it can break up water vapor.

The hydrogen splits off to the oxygen.

The hydrogen is a much lighter atomic species, and so it can actually escape certainly planets like the Earth's gravity.

That's why we don't have any hydrogen or very little helium.

And so that leaves you with the oxygen, which then oxidizes the surface.

And so there could be a residual oxygen signature just due to this photolysis process.

So we've been trying to generalize.

And certainly in recent years, there's been other suggestions of things we could look for in the solar system beyond nitrous oxide.