Nathalie Cabrol

So right away, Viking put on the fast track and the idea was, so Mars looks so much like Earth could have been, although it's arid and there is little atmosphere, et cetera.

Could there be life?

And of course, behind this, at the time, there were people like Klein and Sagan, Carl Sagan.

just, you know, thinking about how can we test the idea of biology of life on Mars.

So this is what Viking did.

But of course, at the time when the two landers arrived on Mars, we didn't have the context of the geology of the environment.

We didn't have much data at all.

So the data that Viking collected

sent back was very confusing.

Some people still think today that we discovered life on Mars at the time because some of the experiment turned out to show a strange signal.

But most of the community think that it can be explained by chemical reaction that we see today.

So it was so confusing that NASA decided to say, okay, if we want to be serious about looking for life on Mars, we have to understand the environment because life and environment co-evolve.

So as cause or effect, a planet is going to give you the physical chemical environment for life to happen.

These are the boundaries.

But once life is here, it's going to change everything.

One of the biggest impact of life was to inject oxygen into the atmosphere of the Earth two billion years ago.

And that changed everything, including our signature in space.

So...

There is this co-evolution.

So if you want to understand one, you have to remove the other from the equation.