Nathalie Cabrol

It's kind of a two or none equation.

That's a very good question.

The thing is that if you draw the parallel with Earth, it took 82% of Earth's history, geological history, to go from very simple life, microbial life, to complexity.

And when I'm saying complexity, I'm not even talking about us.

I'm talking about animals.

So Mars is smaller, lost its magnetic field very fast and lost its atmosphere very fast.

Life also appeared on Earth very fast.

So the condition being quite similar at that time between the Earth and Mars, let's assume for a moment that life appeared on Mars, it would have been simple life.

when conditions started to degrade, which was less than a billion years after the planet had formed.

So everything at the surface would have disappeared, except maybe for morphological traces of the interaction between life and its environment.

So on Earth, the best example are what we call stromatolites.

These are rock formations that are built by microbes.

So we know that.

We know how to recognize them.

You could have chemical traces as well.

There is some interesting question marks right now about carbon isotopes at Gale Crater because we found an abundance of C12, which normally is used by life on Earth, but it can be produced by other things.

So it's not that it's a real biosignature in itself, but it's intriguing.

We have now the C12 and we have methane.

But going back, it's a time on Mars 3.5 billion years ago where you have lots of destructions, where you have lots of impact cratering, etc.

But we still have very old rocks that survived from that time.