NASA scans the polar wastelands.(около 5000
знаков.)
Mars
holds a special place in the human imagination as the planet most like the
Earth. It has an atmosphere, seasons, and distinctive polar ice caps. The ice
caps, first observed by Giovanni Cassini in 1666, immediately raised
tantalizing questions. Are they made of water ice like the giant glaciers that
smother Antarctica? Are they the frozen remains of long-vanished oceans? If
they melted, could Mars become a habitable place? NASA's Mars Global Surveyor,
currently in orbit about the Red Planet, is finally providing some solid
answers.
The Surveyor has already revealed
unexpected details about the size and structure of Mars's northern cap. By the
end of February, the spacecraft will begin mapping, for the first time, the
topography and composition of the even more poorly understood southern polar
ice cap. The new information (along with upcoming data from the Mars Polar
Lander, which will arrive in December) will strip away many of the lingering
mysteries of the Martian poles.
On Mars, the presence of water--essential
for life, past or present--is always an issue of great interest. "Some
people have proposed that there were oceans early in Martian history; others
have said there were not. "But for all of those theories, one needs to
understand the water cycle: how much water there was, where it went to, and
where it's at now." If scientists find substantial reserves of frozen
water, it would bolster the view that Mars was once a balmy, moist world where
life could have started.
Until about two months ago, planetary
astronomers believed that the southern cap contained nothing but frozen carbon
dioxide, also known as dry ice. New research suggests otherwise: a thick sheet
of carbon dioxide ice would be too soft to stay stable. "The thought now
is that carbon dioxide ice is so weak that it would flow away, like a glacier,
even at very low temperatures," Zuber explains. "So to maintain the
topography of the south polar cap, there has to be water ice in there
stiffening it up."
Zuber and her colleagues also analyzed
Mars's much larger northern polar cap. The ice cap is cut by deep troughs and
chasms; some of these depressions extend down over a mile to the base of the
planet's crust. Many researchers off guard. "There are
no troughs of that kind in any of the ice caps on Earth," said Global.
"We don't know how this formed
Zuber's results confirmed that the northern
cap is composed entirely of water ice, in some areas interspersed with layers
of wind-blown dust and sediment. That piece of good news came as no surprise,
because summer temperatures at the cap (which has an elevation several miles
lower than the southern cap) are high enough to vaporize frozen carbon dioxide.
But the Global Surveyor also produced the first accurate measurement of the
size of the northern cap--and that was a surprise.
Seven hundred and fifty miles across, and
up to two miles thick, the northern cap has a volume just half that of the
Greenland ice sheet. It may sound large, but doesn't contain nearly enough
water to account for the flood channels and other erosion features that appear
all over the place on Mars. "It's not even close to what is generally
believed to have once been on the surface," says Zuber. Scientists like
Michael Carr at the U.S. Geological Survey who believe oceans once covered much
of Mars face a serious challenge from the Global Surveyor studies. The northern
cap contains no more than one tenth the amount of water needed to fill an ancient
ocean. On the other hand, the fissures and ring of residual ice around the
perimeter of the cap suggest it has lost a great deal of water over the
millennia.
The Global Surveyor has also provided some clues about the way water
circulated about on Mars in the distant past. The northern ice cap sits nestled
within a deep depression that covers essentially the entire northern hemisphere
of Mars and drops in elevation as it nears the pole. The cap "looks
something like a hockey puck in that depression," David Smith of NASA's
Goddard Space Flight Center reported at the AGU press conference. Researchers
are not sure how the giant lowland formed (perhaps through a large impact), but
they do know that it has been there since very early in Martian history, and so
has clearly played an important role in the planet's water cycle.
"Before we made these measurements of
the northern hemisphere, it used to be thought that the only way you could get
water to the north pole of Mars was through the atmosphere," Zuber says.
But because the northern cap lies at a lower elevation than the rest of the
planet, "water than you put down almost anywhere in the northern
hemisphere is going to flow toward the pole. It is quite probable, then, that
you once had standing bodies of water at high northern latitudes. They might
not have persisted for very long, because we don't know how warm it was and
things may have frozen over quickly. But you clearly could get the water up
close to the pole."
Clearly, Mars was not always the frozen
wasteland it is today. What happened? Some of the ancient water could have been
lost to the atmosphere and then, over countless millennia, ejected into space
through complicated interactions with the Martian magnetic field. Some might
still be locked in aquifers and other formations beneath the surface. And some
may exist in the southern polar cap--but not much. The southern cap is
significantly smaller than the northern one. Even if the Mars Global Surveyor
finds water ice in the south, it won't come close to eliminating the water
shortage, according to Zuber.
"We haven't either improved or
diminished the possibility of life on Mars," she says. "Essentially,
what we have done is exacerbate the problem of there being too little water on
Mars today compared to where there was earlier. Now those people who have
proposed oceans have a bigger task in explaining where the water went."
--Kathy Svitil
Posted 2/19/99
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