Like Earth and Venus, the night side of Mars emits a subtle glow, scientists reported last week in the journal Science. The night-side air glow at the red planet, elusive until now, was detected by the European Space Agency's Mars Express orbiter.

In an interview with Astrobiology Magazine, Jean-Loup Bertaux, principal investigator for the Mars Express SPICAM instrument, explains what lights up the martian evening sky, and why our understanding of that process could aid future missions to Mars.

Astrobiology Magazine (AM): Let's talk about the paper you just published in the journal Science.

Jean-Loup Bertaux (JLB): Our paper is about light that was discovered on the night side of Mars. Up to now, there has been no detection of this natural light, or "air glow." It has been seen on the day side of Mars by Mariner 6, 7 and 9 in the ultraviolet, but there were no observations of the night side.

AM: How is this kind of light produced?

JLB: One way is through chemically produced emissions. That occurs in the atmosphere of the Earth, and has been seen in the atmosphere of Venus, and we have discovered for the first time it happening in the atmosphere of Mars.

AM: So the night glow is due to chemical reactions occurring in the atmosphere.

JLB: Yes. There are several mechanisms, but the one I am talking about is an emission that is produced when one atom of nitrogen is combining with one atom of oxygen to form a new molecule called nitric oxide, NO. When it recombines, it emits one photon. This photon may have several different wavelengths, but the wavelengths of these photons are quite typical of this nitric oxide molecule.

There had been similar observations of the planet Venus in the 1980s, first with the International Ultraviolet Explorer telescope, by Paul Feldman and colleagues at Johns Hopkins, and with the Pioneer Venus UV spectrometer, by Charlie Barth and his associates at the University of Colorado. The mechanism of producing that spectrum was already understood at that time, and now we see the same thing happening in the atmosphere of Mars.

So, what is happening? Why do you have this kind of emission? It's because in the upper atmosphere of Mars, on the day side, you have nitrogen molecules, N2. You have also oxygen molecules, O2. They are not the major constituents of the atmosphere, but still, they are there. They are then photo-dissociated by extreme ultraviolet light into nitrogen atoms and oxygen atoms.

They do not recombine until they sink down in the atmosphere, because when they go down the density is increasing. The increase in density results in an increase in the number of recombinations. And each time you have a recombination of nitrogen and oxygen atoms, one photon is emitted. On Mars, we find that the emission is at an altitude between 60 and 80 kilometers [37-50 miles]. This is proof of air moving down, but not just anywhere - it happens at high southern latitudes, at a place that was always dark at the time we made our observations.

When we found this emission, it was winter in the southern hemisphere, so the whole south pole region was totally in the dark. It is so cold at this time that even the atmosphere of Mars - the carbon dioxide - is freezing and forming a layer of frozen carbon dioxide around the South Pole. This frozen carbon dioxide layer has been measured to be about 1 meter thick. You have all this carbon dioxide air coming down there and condensing, and you must have some air coming in to replace that condensed air. So the light emission shows there is a flow of air coming down from the high altitudes, going down just above the South Pole.

This gives us an optical way to detect the airflow circulation of Mars. It is not so easy to understand atmospheric circulation, because you need some wind measurements, and we don't have many wind measurement instruments on Mars. We have a few on ground stations, but nothing at high latitudes.

So this technique is going to allow us to better study the circulation of the atmosphere of Mars. That's quite important from a technological point of view, because if you want to put something on the ground you have to do what is called an EDL - entry, descent, and landing. The probe has to go through the atmosphere with the use of parachutes, and then land. You can use the atmosphere as a kind of capturer of the spacecraft by using aerocapture or aerobraking. But you need to know how the atmosphere is behaving to really use those techniques.

We have general circulation models of the atmosphere, but I would say these models are not fully validated. Indeed, our study is one way to validate the models.

AM: You said the nitric oxide is coming from the photo-dissociation of O2 molecules high up in the atmosphere. Where is the oxygen coming from? From the photo-dissociation of water?

JBL: A little bit, but O2 is also produced by the dissociation of carbon dioxide in the upper atmosphere - that is the major source. So you have a continuous production of oxygen atoms and oxygen molecules.

AM: Is there anything that your discovery tells us about martian present day conditions or past conditions that we didn't know before?

JBL: We are really learning something about the present conditions. It proves that there are nitrogen atoms, but we already knew that. What is surprising is the way the atmosphere goes from the sunny side to the dark side. Apparently, it goes in a pattern that favors the descent of air near the South Pole. According to models, the flow will be much larger in the reverse situation, when it is winter in the North Pole. So we would expect to see a much larger emission at the North Pole when it is winter there.

There is a difference between the northern and southern winters because the orbit of Mars is eccentric. When it is winter at the South Pole, Mars is very far away from the sun, and when it is winter at the North Pole, Mars is near the sun.

AM: When you talk about the displacement of air coming down, I visualize wind rushing in to fill that void. It reminds me of weather systems on Earth, like when two fronts meet and produce storms. Is something like that happening on Mars?

JBL: I really would not describe it like that. Models are showing that during the southern winter, for instance, there is a wind pattern that circulates around the pole at a rather large velocity. It is a vortex situation, where air is circulating fast around the pole. I visualize that maybe the air is sent into some kind of whirling situation, and it goes down to the South Pole, almost to the ground, possibly.

AM: Is that at all associated with the martian dust devils?

JBL: No, I don't think so. Dust devils are quite local, I would say. And they don't occur in the winter night, they are produced when there is some sun and some dust particulate situation.

AM: So like Earth, Mars has a dynamic atmospheric system. Things are getting all mixed up, and it's not a dead world, in that sense, as it's often described.

JBL: No, the atmosphere of Mars is a living atmosphere. It's quite dynamic. But we still don't completely understand the atmosphere of Mars.

AM: Astrobiology Magazine sponsored a terraforming debate about Mars, and our participants pointed out that we have to understand how the Mars system works before we can start discussing things like altering the planet's environment. For one thing, the martian atmosphere is not like the Earth's atmosphere.

JBL: No, it is thinner, about a factor of about 150 times thinner. But still, about terraforming, I have some plans for that!

AM: Really? During our debate, Jim Kasting said he thought Mars could be terraformed for plants, after about 40,000 years of carbon burial, but he couldn't foresee Mars being suitable for humans until millions of years later.

JBL: I have a faster plan. The idea is you dig a big hole, maybe at a 45-degree inclination, in the Hellas crater, which is already at low altitude. Maybe you need a hole of 45 kilometers [28 miles], which is not so easy. But the whole atmosphere of Mars is going to go down into the hole. Two things would happen. First of all, the pressure at the bottom of the hole would be about 1 bar - much higher than at the surface. It would also be a much warmer temperature, because when you go down, the pressure increases and the temperature also goes up. You would have enough CO2 and enough water possibly to have a greenhouse effect and liquid water.

Salty water is liquid at several degrees lower than 0 degrees Celsius [32 Fahrenheit], depending on the amount of salt. In fact, salty water is much less demanding than pure water in terms of the amount of greenhouse warming needed for it to stay liquid. And life likes salty water. Possibly, such a situation might have happened in the past on Mars, without requiring a 45 kilometer-deep hole! But the combination of salt water and greenhouse warming may be needed in the future for us to sustain any life there.

AM: So you foresee a habitat for plants and people rather than an attempt to terraform the whole planet.

JBL: Yes, that is absolutely right. It would be local, but after that perhaps you can start the other project. You plant trees there, and they may produce carbon, and that would be a good starting point.

Astrobiology Magazine is a web-based publication sponsored by the NASA astrobiology program. It is presented here in cooperation with the magazine.