The Mars Express SPICAM instrument, which is monitoring the martian atmosphere, is addressing the question of the habitability of Mars. If there is life on the surface of Mars now, could it survive?

So this experiment is measuring, for instance, the solar ultraviolet (UV) light which is penetrating to the surface. It can also measure ozone and water.

It can measure oxidants as well -- products that are scavengers for life. To do this, the SPICAM team monitors a star when it is rising or setting on the horizon, and looks at how light from the star is being absorbed by the atmosphere. That light absorption helps them derive properties of the atmosphere.

They have found a very interesting correlation between ozone and water. It seems that wherever there is an increase in atmospheric water, there is a corresponding disappearance of ozone. So, on Mars, when water goes up, ozone goes down.

We are very much dependent on a layer of ozone on Earth; it protects Earth against UV radiation. Recent measurements on Earth indicate that the water content of the Earth's atmosphere has increased from the water evaporation of all the land surfaces and the oceans, maybe as an effect of global warming.

So if it's true that an increase in atmospheric water vapor causes the amount of ozone to go down, then we have to be worried, because that could mean our ozone layer will be further reduced.

Earth obviously has much more water than Mars, and we still do have an ozone layer, so it's not a perfect correlation. But this planetary result from Mars could be giving us a warning. It's an interesting application where you explore another world, and suddenly you realize that there's something you may have overlooked while on Earth.

The detection of methane on Mars is also very intriguing, because it relates to the search for life. Methane is a gas that, on Earth, is produced mainly through biological activity.

The amount we found with Mars Express is very tiny, only 10 parts per billion. But in martian conditions, methane could not last more than 600 years. So methane is being produced somehow, and the source must be producing at least 150 tons of methane per year for us to be detecting it at 10 parts per billion in the atmosphere.

So what is the source? One explanation would be chemistry activities in the sub-surface. Methane could be produced where volcanic activity combines with water and basalt. But there is also the possibility that the methane is the result of methanogenic bacteria.

We have evidence for recent volcanic activity on Mars. There is some new evidence, in polar areas, of small volcanic cones a few hundred meters high. These are small volcanoes in the making, poking up through the ice.

There also seem to be recent volcanic activity in the calderas - large craters caused by the collapse of the magma chamber after a volcanic eruption. You can measure the age of the caldera in martian volcanoes by counting the impact craters inside.

From that, we've found that there are some calderas that are very much younger than we thought. Some of them are maybe between 100 and 150 million years old, but we even found some that are a few million years old. On a geological time scale, that's just like yesterday.

So we are discovering a new Mars, a Mars which is very active, with recent volcanism. Mars was probably quite dry and cold during most of the last 3 billion years, but it could have experienced temporary episodes where water was present in a few places.

So in terms of the search for life, the possibility of the habitability of Mars is much richer than we thought before. We see new possible niches where we should search for the signature of life.

There's still a lot of work to do to put all of our discoveries into context, instrument to instrument, but also with discoveries made by the Mars rovers.

The next step is to prepare new missions that will characterize some of the potential habitats for life on Mars.

We'll still follow the water, but we also want to understand the carbon cycle, determine what energy sources are available, search for hydrothermal hot spots, and look for any prebiotic chemistry, or even biotic chemistry. Then we could pinpoint areas that could be further explored with landers, to be followed by sample return missions.

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