One of these wavelengths corresponds to hydrogen, another wavelength would be iron. Another wavelength would be oxygen. And so you go to the part of the spectrum you are interested in and you see you've got a big peak there.

As Odyssey circled Mars, it started to pick up gamma rays. The strength of the signal indicated how much of any element it had found.

We were hopeful that we would see spots where water had been concentrated. We weren't sure we would see that. But it's these kind of things that we're expecting to see and actually very hopeful that we would see.

The data was radioed back from Odyssey to NASA to the University of Tucson, and finally to Boynton's desk.

Building up the picture takes a long time bacause any given spectrum that comes back is only 20 seconds worth of data. We get a new spectrum every 20 seconds, so we actually slowly build up an image in our database.

As the data came through, a picture started to build.

When I first saw the signal, I was looking through it and first trying to find the hydrogen signal. And then when I saw it, it was so big, I couldn't believe it. I actually had to do some checks to see could this be real, or somehow did we mess things up.

It could only mean one thing. There is water ice on Mars today. And there is masses of it.

Boynton and his team had discovered a vast area of ice in the southern hemisphere. A frozen ocean over 5,000 km wide.

There is plenty of water there. What we found is just in the surface. If we melted that, it would be enough to fill Lake Michigan two times over. There actually is probably a lot more 'cos we don't know how deep the ice goes. That's just in the upper metre there's that much. It could be 10 meters deep. It could be 100 meters deep. There's a lot of ice there.

The ice they have found is trapped in the ground, a permafrost just like in Antarctica. Crucially, it's located at latitudes on Mars where conditions are relatively mild, where there's a real chance of life surviving.

One of the most interesting things about this is it's not right up near the South Pole or right not near the North Pole. It actually goes down to temperate latitudes. It goes all the way down to about 45 degrees latitude. This is like the equivalent of Paris France on Earth. This is really a place where it's much more likely we can find life I feel.

spectrum: Physics. The distribution of a characteristic of a physical system or phenomenon especially: 1 The distribution of energy emitted by a radiant source, as by an incandescent body, arranged in order of wavelengths. 2 The distribution of atomic or subatomic particles in a system, as in a magnetically resolved molecular beam, arranged in order of masses.

peak: Physics The highest value attained by a varying quantity