A lot of my role is advocacy, and as a scientist, you're an advocate, too, because you are coming up with a theory and having to convince your fellow scientists that you're right.

If you think of the Apollo capsule coming into Earth with a parachute, the Mars atmosphere is just so thin, you've got to find some way of slowing yourself down really rapidly.

We like to talk about pioneering Mars rather than just exploring Mars, because once we get to Mars, we will set up some sort of permanent presence.

I always like to say just think you were a doctor with only one patient. You might understand how that person gets sick, how they get better, but you understand nothing about the progression of disease or how humans in general get ill. Now take an Earth scientist: you only have one planet to study.

As chief scientist, it's sort of my job to look at bridges between what we do and to see the connections. But when we try to understand how are planets around other stars habitable... to looking back at the Earth - how are the changes that are taking place, how are they going to affect humanity?

Humans can actually read a landscape, go through a lot of rocks - crack them open, throw them, pick up the next one. Rovers are great - they do amazing science - but it is a lot more tedious process; they go much less far than a human can cover in a day.