Mars is Melting
The south polar ice cap of Mars is receding, revealing
frosty mountains, rifts and curious dark spots.
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.
Above: Amateur astronomer Thomas Williamson of New Mexico took this picture of Mars on August 1st. He used an 8-inch telescope and a digital web camera. 
Don't wait too long to look, though, because the ice will soon be gone.
Like Earth, Mars has seasons that cause its polar caps to wax and wane. "It's late spring at the south pole of Mars," says planetary scientist Dave Smith of the Goddard Space Flight Center. "The polar cap is receding because the springtime sun is shining on it."
As the cap shrinks it 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. (are visible in Thomas Williamson's photograph of Mars at the beginning of this story.)
Left: A brightness map of the martian south pole one Mars-year ago. The Cryptic region is the blue-green area around the 4 o'clock position. Reds and yellows denote frozen CO2. This map was created by Dave Smith using data from the 1 micron detector of the laser altimeter onboard Mars Global Surveyor. 
Here's an amazing fact: The seasonal polar caps are made of martian air 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. (The atmosphere is 95% CO2; that's why the seasonal polar caps are made of dry ice.)
As seasons come and go, carbon dioxide 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.
NASA spacecraft have detected frozen water beneath the surface of Mars. Liquid water, on the other hand, is scarce. Why? On a warm summer day, ice doesn't melt, it vaporizes--skipping directly from solid to gas. This happens because the air pressure is so low. But a small boost in pressure could be enough to allow ice to melt and water to flow under a warm summer sun. Southern summer, therefore, might be a good time for future human explorers to visit. (For more information read Science@NASA's Making a Splash on Mars.)
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.
Below: In early August, look for bright Mars rising above the southeastern horizon after 10 p.m.. The planet is even easier to find between midnight and dawn, when it hangs high and bright in the southern sky. (These instructions apply to observers at mid-northern latitudes.)
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.
Learn more about NASA's Mars Exploration Program
Approaching Mars (Science@NASA) -- Earth and Mars are converging for a close encounter in August. The red planet is already an appealing target for sky watchers.
Mars Dust -- (Science@NASA) Using only backyard telescopes, amateur astronomers are enjoying great views of dust clouds on Mars
Landmarks mentioned in this story: The Mountains of Mitchel (CNN); The Mountains of Mitchel (Mars Global Surveyor) The Cryptic region (Arizona State University); Rimas Australis and more (University of Arizona)
Laser Altimeter Provides First Measurements of Seasonal Snow Depth on Mars -- (NASA/GSFC) A global view of how Mars changes with the seasons has been provided by extremely precise observations from nvestigations on NASA's Mars Global Surveyor (MGS) spacecraft.
More images of Mars from the Association of Lunar and Planetary Observers (ALPO)
Why Study Weather and Climate on Mars? (NASA/Ames)
Making a Splash on Mars -- (Science@NASA) On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help.
Once Upon a Water Planet -- (Science@NASA) Today the Red Planet is dry and barren, but what about tomorrow? New data suggest that the long story of water on Mars isn't over yet.