快猫短视频

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鈥檚 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鈥攖he 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鈥檚 hard to be wet with an average temperature of about
鈥�53 掳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鈥檚 Mars
Global Surveyor鈥攚hich itself used the atmosphere to brake and change
orbit鈥攈as 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鈥檚 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鈥攖he gas dissolves, chemically weathers the silicate rocks on the
planet鈥檚 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鈥檚 atmosphere, which was probably a good deal thicker than it is today.
Now, despite humanity鈥檚 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鈥攁nd one of the first to
point out some of its flaws鈥攊f you released all the CO₂ that is
now locked up in the Earth鈥檚 carbonate sediments you鈥檇 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鈥檛 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鈥檚 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鈥檚 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鈥檚 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鈥檚
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. 鈥淲e鈥檙e trying to be
conservative with the 10 or 15 per cent鈥攖here鈥檚 basically no discernible
carbonate signature,鈥� says Christensen. 鈥淢y guess is that the most profound
discovery that TES will make and the most interesting paper we鈥檒l write is that
there aren鈥檛 carbonates on Mars, at the surface at least.鈥�

If Christensen鈥檚 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鈥攐r possibly
do a bit of both.

Take the other hiding places first. There is probably some CO₂
frozen into the planet鈥檚 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鈥檛 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鈥檚 conceivable
that you could lose a fair amount of CO₂ in the Martian underground.
Again, though, it doesn鈥檛 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鈥檚 surface can throw swathes of the atmosphere off at
such high speeds that they escape the planet鈥檚 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.
鈥淵ou can have ions slammed into the upper atmosphere at more than 400 kilometres
per second,鈥� says Bruce Jakosky of the University of Colorado at Boulder. 鈥淚t鈥檚
like shooting pool. On the break shot you knock everything all to hell. You can
knock stuff out of the atmosphere entirely.鈥� This process, called sputtering, is
still thought to be eroding Mars鈥檚 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鈥檚 Ames Research Center in
California, the evidence suggests that they stripped off a huge amount of the
original atmosphere鈥攎ore 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鈥檚 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鈥攊n which the mixture of xenon
isotopes was presumably similar to that in the rest of the Solar
System鈥攎ust 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鈥檚 calculations suggest that impact erosion was a scourge of
biblical proportions, it did not succeed in flaying away all the atmosphere.
It鈥檚 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鈥攖he first really big meeting to be saturated with the heady new
findings of the Mars Global Surveyor鈥擪attathu 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鈥檚 nitrogen
isotopes set it apart from the modern Martian atmosphere. Today鈥檚 atmosphere is
highly enriched with the heavy isotope of nitrogen. But Mathew鈥檚 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鈥攆rom 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鈥檚 southern highlands鈥攍ow 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鈥檚 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鈥攐r at least thicker than it
is today鈥攆or as long as the magnetic field held up.

But was there enough to explain the water? It鈥檚 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鈥檚 lifetime, Jakosky鈥攚ho made some of those
predictions鈥攖hinks it could conceivably have been ten times more.

That still wouldn鈥檛 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鈥檚 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, Fran莽ois 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鈥檛 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鈥攋ust as different clouds affect the temperature in different ways on
Earth.

Then there鈥檚 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鈥檚
surface may have formed in a few short, wet catastrophes. As Zahnle puts it, 鈥淚
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.鈥� 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鈥檛 really come as a surprise that you can鈥檛 make sense of a whole
planet with a few space missions. But the complexities and seeming
contradictions of Mars鈥檚 past are forcing the lesson home. The history of Mars
may be more complex than the 鈥渨arm-and-wet-then, cold-and-dry-now鈥� model
allowed. Mars鈥檚 first billion years may have thrown up all sorts of perplexing
puzzles, and to solve them researchers will propose theories that stretch, like
Jakosky鈥檚 ideas, from the planet鈥檚 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鈥檚 second to none.