Why is Mars Dry?

by Christopher Yukna

" Channels resembling ancient rivers added to the scientific debate that ensued and continues today: what happened to the water that once flowed on Mars? "

Viking to Mars JPL

So what happened to the water on Mars? To understand this question a little better we should compare the Earth and Mars. Mars is dry, thousands of times drier than Earth. Why is that so? As a matter of fact all of the other terrestrial planets are significantly drier than our rather misnamed planet. Certainly, Venus and Mercury are closer to the Sun and thus perhaps this proximity has somehow removed or never allowed the formation of water on these planets. When the planets were coalescing from bands of gas to molten spheres of rock it is possible that being close to the Sun precluded large quantities of water from being included in the planetary crust and therefore would not in later millenniums be outgassed in volcanos etc. I had also heard of another theory on the origin of most of the water on Earth and that was that atleast some of our water came from years of bombardment by comets in early days of our planet. (Of course no one explained to me why those comets only managed to hit the Earth and not the other inner planets.)

Those possibilities are neither here nor there with respect to Mars. Mars is much farther from the Sun than the Earth and if it began with the same root stock as the Earth then it should have had as much water as the Earth. There is some evidence of this, massive flood plains, river basins, etc. seem to be indicated from features on the Martian surface. So where is the water?

I bought a book this summer " Red Mars " by Kim Stanley Robinson. A tremendous saga. It had a breathtaking scope and fanatical attention to detail.

The science and engineering seem to be where we will be going in the future.

In short it seemed a very plausible future scenario. There were only one or two minor items that bothered me. One of them was the discovery of large underground reserves of water. Something about this nagged at me. I mean it appears logical. There were once vast quantities of water on this planet hence this water must still be somewhere. Some people believe that along with frozen carbon dioxide there is significant amounts of water ice at the poles. At any rate, there is not enough water at the poles to equal the amount of water that once ran on the surface. So this water must be trapped under the surface of Mars.

Okay, Kim Stanley Robinson makes a great case for this in his book and I almost believed it. But in his book, to further the plot or to add a little zest to the story these underground aquifers are broken into and water once more flows on Mars.

It was the magnitude that shocked me. In this book seas the size of the Mediterrean break loose. I doubt that this facet of his portrait of Mars was something he simply made up. I think quite frankly that a lot of areologists would agree with this vision of Mars. But, I just couldn't believe that a planet so dry on the surface could be so wet in the interior.

So what happened to all the water? A darn good question and one that I mulled over for quite a while. I mean it couldn't have just been destroyed or could it? Recently, I read an article about some scientists who had developed a way to measure the OH values in the atmosphere.(Smog - Busters get radical over pollution, by Jeff Hecht in New Scientist, Sept 7th 1996, page 18.) OH is very reactive and tends to clean the air of pollution. The amounts of OH in our atmosphere are in the trillionths, so there is not a lot of OH in our atmosphere at any one time. Well, how does this all tie in? To begin with, the source of the OH in our atmosphere was listed in the article as being produced by ultraviolet radiation. Okay, here's what happen as I understand it; a photon of UV light strikes a molecule of H2O and separates a hydrogen atom from the water molecule. The free hydrogen atom connects often with another hydrogen atom thus becoming hydrogen gas. The OH wanders around in the atmosphere till it combines with another molecule (i.e. pollution). So OH values are important to know. This is pretty much cut and dried.

What does this have to do with Mars? Let's go back to the ancient Martian atmosphere. There were oceans on the surface of the planet and this means that there was plenty of water vapor in the atmosphere. Today as in the past Mars is constantly bombarded with intense UV light. Mars does not have much of an ozone layer and as everyone must know by now the ozone layer protects the Earth from most of the UV radiation reaches our planet. Now think about this; with all the UV light striking the water vapor in the ancient atmosphere of Mars there must have been lots of hydrogen gas and OH being produced. The hydrogen gas would have escaped the gravitational pull of Mars and gone into interplanetary space. Earth itself can not keep hydrogen from going off into space. Only the large gas giants like Jupiter and Saturn have gravitational fields sufficiently strong enough to permanently trap hydrogen. I can not tell you how many tons of hydrogen gas would be produced each year. This destruction of water may not have been instantaneous but since we are talking of atleast a billion years duration even a small loss over this much time could have eliminated all but the smallest traces of water.

We have looked at the removal of hydrogen from Mars but what happened to the other half of the water; the oxygen? This enormous quantity of oxygen would have had to go somewhere. I suspect that it went into the ground. I think that it would have oxidized the soil on Mars to a great extent. This is of course the state of the soil on Mars today. I remember when the Viking Landers touched down on Mars in 1976. One of the landers took a small sample of soil inside itself.

The probe added a minute amount of water to the soil sample and instantly there was oxygen. Everyone thought that at last we had found extraterrestrial life. But no, it was simply a curious chemical reaction. I believe that I understand better that curious chemical reaction. It was an echo from the seas of Mars.

Why is the Earth Wet?

Well, that wraps everything up or does it? Ancient Earth didn't have a significant ozone layer either. Even with an ozone layer OH is being produced. That means that hydrogen gas is also being produced. Earth can't trap permanently H2 anymore than Mars can. So why isn't our water slowly wasting away? I don't know why not but I have a real good guess. First in our upper atmosphere there is a lot energy and ozone so perhaps the escaping hydrogen combines with the ozone and produces water once again. Maybe, but just maybe it is something else. Now what I am going to suggest dated from when plants began producing oxygen. I have no idea what or if anything protected our oceans before that.

But today I suspect that there is a process which produces water in our atmosphere greater than any loss by UV light. Here goes. As we know the Earth is surround by a vast ozone shell high in the atmosphere. Also our little planet, unlike the other terrestrial planets has a large magnetic field. When cosmic rays (composed of more or less high energy protons) and the solar wind (also largely made up of protons) pass near the vicinity of the Earth they get trapped in the magnetic field and are directed to the poles. This is a simplification but fairly accurate. This causes the aurora borealis and aurora australis. Now what we have here is protons or atoms of hydrogen striking a layer of ozone. I don't really think you need to be a research chemist to realize that the end result is water. Is there any physical evidence that this what is occurring. I think so. A lot of the water vapor in our atmosphere is in the form clouds. These clouds generally are found within a few kilometers of the surface. There is an exception at the poles where there are ice clouds found at very high attitudes in the order of 100,000 feet. These clouds are not found any where else except the over the poles. Are these clouds the result of a chemical reaction between the cosmic rays and solar wind and our ozone layer. Probably.

Post Script:

If all of this is true it really underscores the importance of our fragile ozone layer. This report is not all good news for the ecologists. It raises the question of how much of the holes in the ozone layer are due to the ozone layer capturing hydrogen ions, a process that has been going on for billions of years. Another interesting question is: How much of our water is extraterrestrial in origin?

I hope in this last supposition that I haven't reinvented the wheel. I trust that this a whole new water cycle and not something that everyone else knew years ago. These ideas seem so simple, so obvious, it is as if I have overlooked something important, but I can't for the life of me figure out what it is.

Christopher YUKNA
Professor of English
Ecole des Mines
75 cours fauriel