Mystery of the missing atmosphere

November 16, 1999

As atmospheres go, it has mostly gone. Admittedly, if you plough into the Martian atmosphere at the speed of a meteorite, as the misguided Mars Climate Observer did in September, there is still enough there to tear you apart. But under most other circumstances, it is a poor excuse for an atmosphere. At the planet's surface, the pressure is a paltry 1 per cent of that on Earth.

Why should Mars have so little atmosphere when Venus and Earth have so much? Though it might simply have been born that way, there are plenty of hints that the atmosphere was once much thicker-the evidence of water, for example. Today the Martian surface is cold and exceedingly arid. But the surface bears unmistakable signs that liquid water once raged through flood channels and valleys, left shorelines in craters and may even have formed oceans in the Great Northern Basin. It's hard to be wet with an average temperature of about -53 ¡C, so liquid water implies warmth. And warmth implies a thick insulating atmosphere, replete with warming greenhouse gases such as carbon dioxide.

If the Martian atmosphere was once much thicker, where did all the gas go? Despite diligent searching, no one knows. But in the past year, NASA's Mars Global Surveyor-which itself used the atmosphere to brake and change orbit-has been collecting information that could answer that question. And its findings are not at all what its designers expected.

In the 1980s, researchers developed a theory for why Mars was once warm and wet. First they calculated how much CO2 it would take to melt the Martian ice and allow water to flow, and came up with a figure of between 5 and 10 bars (one bar is the pressure of about one Earth atmosphere). That's rather a lot for a planet with only a few millibars left today, so they had to explain where the CO2 might have disappeared to since. According to their picture, the atmosphere sowed the seeds of its own destruction.

When liquid water is around, a CO2 atmosphere becomes unstable-the gas dissolves, chemically weathers the silicate rocks on the planet's surface and is ultimately locked up in the form of carbonates. The proof is beneath your feet. There was a time when CO2 dominated the Earth's atmosphere, which was probably a good deal thicker than it is today. Now, despite humanity's eager attempts to redress the matter, CO2 has dwindled to a trace of its former glory, making up less than a thousandth of the air we breathe.

The reason is that over billions of years, chemical weathering has stored a great deal of CO2 as carbonates. According to Jim Kasting of Pennsylvania State University in University Park, who was one of the researchers who put together the warm, wet, early Mars theory-and one of the first to point out some of its flaws-if you released all the CO2 that is now locked up in the Earth's carbonate sediments you'd get about 60 atmospheres worth of the stuff.

If chemical weathering can destroy greenhouses so easily, why did the Earth not freeze as Mars did? The answer, the researchers decided, was recycling. On Earth, some of the CO2 from carbonates is recycled through plate tectonics. When carbonate-rich sediments start their journey down into the mantle at a subduction zone, where one plate slides under another, they are heated up and release CO2 back into the atmosphere, where it can warm the planet.

On cold little Mars, though, the recycling seems not to have been so good. Unlike Earth, Mars doesn't have enough internal heat to keep pushing lumps of its crust around, or to resurface itself with great big burps, as Venus may have done. There is little evidence that Mars's inner fires ever drove a system of plate tectonics, and while the planet may well have had some other ways of using its internal heat to recycle carbonates, they would have run out of oomph fairly early on as the planet's innards cooled down. CO2 recycling would have started to lag behind the production of new carbonates, and the atmosphere would have begun to shrink in earnest.

So far so good. Now all the researchers needed to do was find some carbonates on the planet's surface to confirm their story. The best technology for doing the job from space is infrared spectroscopy, which picks up features in the infrared spectrum unique to specific minerals. This year, Mars Global Surveyor's spectrometer, the Thermal Emission Spectrometer (TES), completed its first thorough study of the planet, covering almost three-quarters of the surface. According to the scientist in charge of the instrument, Phil Christensen of Arizona State University, Tempe, it has found that carbonates make up less than 15 per cent of the surface. Probably a lot less. "We're trying to be conservative with the 10 or 15 per cent-there's basically no discernible carbonate signature," says Christensen. "My guess is that the most profound discovery that TES will make and the most interesting paper we'll write is that there aren't carbonates on Mars, at the surface at least."

If Christensen's suspicions are correct, then Mars researchers face some intriguing choices. They must either find another way to get rid of the atmosphere or make do with less atmosphere in the first place-or possibly do a bit of both.

Take the other hiding places first. There is probably some CO2 frozen into the planet's soil, or hidden in dry-ice deposits underneath the water-ice exteriors of the polar caps (though other observations from Mars Global Surveyor are throwing some doubt on that second possibility). Reservoirs like these could account for ten times as much CO2 as is currently seen in the atmosphere. But since the current atmosphere is less than a hundredth of a bar, that isn't enough to explain the difference between past and present.

Then there could be carbonates hidden below the surface. The 13 Martian meteorites found on Earth all contain faint traces of carbonate, and the oldest of them, ALH 84001, has veins of carbonate running through it. It's conceivable that you could lose a fair amount of CO2 in the Martian underground. Again, though, it doesn't seem likely that you could get rid of a few bars of atmosphere without leaving any discernible carbonate sediments on the surface.

So perhaps the atmosphere quit the planet altogether. There are two ways this could have happened: very big impacts and very small impacts. Asteroids and comets hitting a planet's surface can throw swathes of the atmosphere off at such high speeds that they escape the planet's gravity for good. In the very early days of the Solar System, when the planets had only just been assembled, there was plenty of rubble left over. During this period, known as the late heavy bombardment, Mars was hit by dozens of large chunks and hundreds of smaller ones, all of which could mark the passing of parts of the atmosphere.

After asteroid impacts eroded the early Martian atmosphere from the bottom up, a subtler process could have nibbled at it from the top down. The upper atmosphere of the planet is constantly being buffeted by the solar wind. In itself this wind is fairly harmless, since it is thin and made of very light particles, but it also carries a magnetic field. This can pick up ions from the upper atmosphere, accelerate them and then slam them back into their fellows. "You can have ions slammed into the upper atmosphere at more than 400 kilometres per second," says Bruce Jakosky of the University of Colorado at Boulder. "It's like shooting pool. On the break shot you knock everything all to hell. You can knock stuff out of the atmosphere entirely." This process, called sputtering, is still thought to be eroding Mars's atmosphere today, though no one knows how quickly.

How do these different processes fit together? The biggest factor was probably impacts. According to Kevin Zahnle of NASA's Ames Research Center in California, the evidence suggests that they stripped off a huge amount of the original atmosphere-more than 99 per cent of it, in fact. That figure, he says, comes from looking at the ratios of different isotopes of xenon in the atmosphere.

The mixture of xenon isotopes in the Martian atmosphere today contains a far higher proportion of xenon-129 than is found in the Earth's atmosphere, or in the Sun. Xenon-129 is produced by the decay of iodine-129. For xenon-129 to be so predominant, the original atmosphere-in which the mixture of xenon isotopes was presumably similar to that in the rest of the Solar System-must have been more or less stripped off the planet before most of the radioactive iodine inside the planet had decayed. With hardly any other xenon around, the newly released gas would have quickly come to dominate the isotopic distribution, as it does today.

But though Zahnle's calculations suggest that impact erosion was a scourge of biblical proportions, it did not succeed in flaying away all the atmosphere. It's hard to say how thick that remnant atmosphere was, but it could have been a good bit thicker than it is today.

Zahnle thinks some of the atmosphere may have sat out the bombardment trapped in the crust, emerging only when it was safe to do so. In a paper presented at the Fifth International Mars Conference in Pasadena, California, this summer-the first really big meeting to be saturated with the heady new findings of the Mars Global Surveyor-Kattathu Mathew and Kurt Marti from the University of California, San Diego, described a new analysis of the gases trapped in the meteorite ALH 84001.

These ancient Martian gases apparently correspond to the time when the rock first formed. They bear a xenon ratio quite like that seen today, and so presumably postdate the first great flaying. But the meteorite's nitrogen isotopes set it apart from the modern Martian atmosphere. Today's atmosphere is highly enriched with the heavy isotope of nitrogen. But Mathew's samples of ALH 84001 show no such enrichment.

As it happens, sputtering is particularly good at removing light nitrogen. In the upper reaches of the atmosphere there is very little turbulence, and so a delicate isotopic layering takes place, with the lighter isotopes of each gas rising to the top. Since sputtering works from the top down, it is more likely to knock lighter isotopes out than the heavier ones. So the sample in ALH 84001 looks as though it comes from a time when sputtering had not yet begun-from a time when the upper atmosphere of Mars was protected against the depredations of the solar wind. And this is where another intriguing discovery from Mars Global Surveyor comes in.

While the spacecraft was using the upper atmosphere of Mars to change its orbit, it flew quite low over the planet's southern highlands-low enough for its magnetometer to pick up unexpected signals from the crust. Since then it has become clear that, although Mars has no global magnetic field today, in its youth it had a very strong one, traces of which were imprinted on its crust. Again, Mars was too small to keep up such exertions for long. The internal energy that drove its magnetic dynamo must have run out fairly quickly, since it is only in the oldest crust that the magnetic field's signature has been seen.

As long as the magnetic field was around, it would have shielded the planet from the depredations of the solar wind. So the post-bombardment atmosphere might have been able to stay reasonably thick-or at least thicker than it is today-for as long as the magnetic field held up.

But was there enough to explain the water? It's hard to say. Nobody knows how fast the sputtering is happening today, or how strong the solar wind was in the early Solar System. While most estimates have put sputtering loss at a tenth of a bar or so over the planet's lifetime, Jakosky-who made some of those predictions-thinks it could conceivably have been ten times more.

That still wouldn't add up to the pressure of between 5 and 10 bars that researchers originally thought they needed to explain a sustained, relatively wet period early on. But they may have overestimated the planet's requirements. The models that called for many bars of CO2 to explain the presence of liquid water did not take into account the formation of clouds. It turns out that, in principle, clouds of solid CO2 might have warmed Mars up quite nicely, even with an atmospheric pressure of only half a bar.

In November 1997, Francois Forget of Pierre and Marie Curie University in Paris and Raymond Pierrehumbert of the University of Chicago calculated that large dry-ice crystals in such an atmosphere could be very good at scattering thermal radiation back towards the ground while letting incoming visible and ultraviolet light through (Science, vol 273, p 1273). A thin but cloudy atmosphere could have warmed Mars during the earliest phases of its history and then been sputtered away when the cooling core shut down the magnetic field. As the atmosphere thinned, the soil would have been able to absorb most of the relatively small amount of CO2, and carbonate production could have been minimal.

The problem is that just because cooling clouds can be found in a model, doesn't mean they were ever there in real life. And Kasting points out that while some sorts of cloud may have warmed the surface, others might have cooled it-just as different clouds affect the temperature in different ways on Earth.

Then there's the possibility that it was never really all that warm in the first place. Water can contrive to be liquid in some pretty cold places, at least fleetingly, and some think that a great many of the watermarks on Mars's surface may have formed in a few short, wet catastrophes. As Zahnle puts it, "I have seen evidence of liquid silicate lavas on the surface of the Earth: do I need to conclude that the global temperature was 1500 K? All I can fairly conclude is that the liquid was there, and that the liquid was hot." The river valleys might have formed through the action of groundwater heated by local volcanism or impacts. Or they might have formed under transient ice sheets that later sublimed away.

Maybe warmth came in very brief spurts. That would explain why, despite the presence of valleys, there is little evidence of sustained erosion in many of the old craters, and some of them maintain an almost Moon-like sharpness.

Victor Baker of the University of Tucson in Arizona believes that Mars has sometimes been very wet indeed thanks to gases from inside the planet forcing warm water from the depths of the crust out onto the surface. But these floods would have lasted only ten thousands years or so. Even a dozen such wet spells would add up to only a tiny fraction of Martian history, and leave the southern highlands untouched by erosion.

It shouldn't really come as a surprise that you can't make sense of a whole planet with a few space missions. But the complexities and seeming contradictions of Mars's past are forcing the lesson home. The history of Mars may be more complex than the "warm-and-wet-then, cold-and-dry-now" model allowed. Mars's first billion years may have thrown up all sorts of perplexing puzzles, and to solve them researchers will propose theories that stretch, like Jakosky's ideas, from the planet's molten heart to the very edge of space. The thin Martian atmosphere may make a poor planetary blanket, but as a springboard for speculation it's second to none.n
Oliver Morton is a science writer based in London

New Scientist issue: 20th November 99

Source: Geo-Marine Letters (vol 18, p 285)


New Scientist

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