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."

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.

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!

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