Mars Is Melting

http://science.nasa.gov/headlines/y2..._southpole.htm

Mars is Melting
NASA Science News
August 7, 2003

The south polar ice cap of Mars is receding, revealing frosty mountains,
rifts and curious dark spots.

It's not every day you get to watch a planetary ice cap
melt, but this month you can. All you need are clear skies, a backyard
telescope, and a sky map leading to Mars.

Actually, you won't need the sky map because Mars is so bright and easy to
find.

Just look south between midnight and dawn on any clear night this month.
Mars is that eye-catching red star, outshining everything around it. It's
getting brighter every night as Earth and Mars converge for a close
encounter on August 27th.

Mars has gotten so big in recent weeks that even a backyard telescope will
show details on the planet's surface: dust clouds, volcanic terrains, impact
basins. Best of all is the polar ice cap. The southern hemisphere of Mars is
tipped toward Earth and its bright southern cap, which reflects more
sunlight than any other part of the planet, is remarkably easy to see.

Don't wait too long to look, though, because the ice is melting.

Like Earth, Mars has seasons that cause its polar caps to wax and wane.
"It's late spring at the south pole of Mars. The polar cap is receding
because the sun is shining on it," explains planetary scientist Dave Smith
of the Goddard Space Flight Center. "Southern summer on Mars begins
September 29th; by then much of the polar cap will be gone."

Watching the polar cap disappear is fun.

The shrinking cap develops rifts, dark spots, and a ragged border. Lately,
for instance, amateur astronomers using 8-inch and larger telescopes have
been watching a frosty mountain range emerge from the ice. Says Smith,
"these are the Mountains of Mitchel"--named after the Ohio astronomer who
first spotted them 150 years ago. A bold dark rift called Rimas Australis
cuts through the polar ice just south of those mountains. (These features
are visible in Thomas Williamson's photograph of Mars at the beginning of
this story.)

Something else to look for is the "Cryptic region"--a dark zone hundreds of
km wide. Even after the ice above it recedes, the Cryptic region remains
remarkably cold according to infra-red cameras onboard NASA's Mars Global
Surveyor spacecraft. No one is sure what the Cryptic region is, "but it's
probably big enough to see from Earth," notes Smith.

Here's an amazing fact: The seasonal polar caps are made of martian air
(carbon dioxide or CO2) that freezes during winter. Depending on the time of
year, more than a quarter of the martian atmosphere can be found lying on
the ground around the poles.

As seasons come and go, this CO2 shifts back and forth--lying on the ground
during cold months, floating through the air during warmer months. The
world-wide air pressure rises and falls by 25%.

For comparison, the air pressure inside a hurricane on Earth is often only a
few percent lower than ambient. You can experience a full 25% difference in
pressure by traveling from sea level to the top of a 9000 ft (3000 m)
mountain. Just try running a 100 yard dash up there.

The south polar cap is vaporizing now, which means CO2 is rushing back into
the atmosphere. "Remember, though," adds Smith, "there are two polar caps on
Mars--north and south. While the south polar cap is vaporizing the north
polar cap is growing. It's a balancing act. Overall air pressure will be
greatest when there's the least amount of CO2 on the ground." The next such
peak is due in early October--that is, early southern summer on Mars.

The boost in pressure has some interesting consequences. It won't make the
martian atmosphere thick by Earth-standards. At best the air pressure on
Mars is 100 times less than Earth. But it might become thick enough in some
places for liquid water to flow.

Liquid water is normally impossible on Mars because the air pressure is so
low. On a warm summer day, ice doesn't melt. It vaporizes. But a small boost
in pressure could be enough to allow water to flow under a warm summer sun.
Southern summer, therefore, might be a good time for future human explorers
to visit.

On the other hand, thicker air also encourages dust storms, which are a big
problem on Mars. Small dust clouds stirred by sun-warmed winds sometimes
grow to encircle the entire planet. In 2001 such a storm lasted for months
and frustrated astronomers who couldn't see through the haze.

Will that happen again this year? No one knows.

When the seasonal polar cap finally vanishes, Smith recommends looking for
the permanent polar cap. "The permanent cap is made of frozen water hiding
beneath the seasonal cap of CO2," he explains. While the seasonal cap is
wide-ranging (90o to 60o latitude) and shallow (only 1-meter deep), the
permanent cap is compact and about 3-km deep. "It harbors a mass of water
comparable to the mass of the martian moon Phobos." To amateur astronomers
peering through telescopes, the water-ice cap will look like a tight white
knot within 10o latitude of the pole.

Dark "cryptic" spots. Mountainous rifts. A treasure trove of water. There's
a lot to look for around the south pole of Mars. Grab a telescope and see
for yourself!

Editor's note: In this story we talk about the martian polar caps "melting"
or "vaporizing." That's not exactly right. A physicist would say more
accurately "the polar caps are subliming." In other words, the frozen
CO2--better known as "dry ice"--transforms directly from a solid to a gas
without going through an intermediate liquid phase.