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The Snows of Mars

                              The Snows of Mars
            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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