Before the era of space probes, many astronomers believed that the Martian atmosphere was a relatively dense blanket that might even be able to support life. But from the 1960s onward, orbiting spacecraft and landers beamed back the disappointing news: The air on Mars was thin, desperately cold and composed mainly of life-choking carbon dioxide. It may not always have been so. The same spacecraft have also found evidence that Martian air might once have been as thick as the Earth's.

Mars' atmosphere is laden with red dust, and its pink skies are some of the most scenic in the solar system. But any space tourist would certainly need a pressure suit to survive. At the surface. Mars has an atmospheric pressure of no more than 10 millibars. On Earth, you would have to travel to 120,000 feet, four times the height of Mount Everest, to reach such thin air.

What gas there is on Mars is mainly carbon dioxide, a greenhouse gas. But there is not nearly enough of it to warm up the chilly planet. Mars' atmosphere contributes only about 12°F to the average temperature of -65°F. On Earth, carbon dioxide makes up less than one percent of the air, but raises temperatures by around 63°F.

Scientists once thought that Mars' atmosphere was mainly made up of nitrogen, like the air on Earth. But the Viking landers uncovered the more inhospitable truth in 1976, when they found that only three out of every hundred air molecules were nitrogen. Mars was not so short of nitrogen in the past. Atoms come in various isotopes—each with a different weight. Martian air contains a higher proportion of heavy nitrogen atoms than Earth's air: Many of the light isotopes have escaped.

If the young Mars did have a thicker atmosphere, the puzzle of its water-based features would be solved. The very thin, very cold Martian air means that water cannot exist as a liquid now—it appears as ice or vapor. Only flowing water in a thicker, warmer atmosphere could have shaped the channels recorded by the Mars orbiters.

So where is all the missing air? Some of it may have simply drifted away. The Red Planet has low gravity— only one-third of that on Earth—that is too weak to hang on to many air particles. Other parts of the atmosphere were removed more violently. Mars was bombarded by meteors in its early history, and much of the atmosphere was literally blasted into space. But other planets also came into the line of fire—and still managed to cultivate healthy second atmospheres.

One theory suggests that some of the Martian air was carried away by the solar wind. On Earth, the air is shielded by our planet's magnetic field. On Mars, the magnetic field is too weak to offer much protection. Atoms in the upper atmosphere are ionized—given an electrical charge—by sunlight. The solar wind sweeps these charged particles out into space. Lighter atoms are the easiest to pluck away, and the heavyweight molecules that make up the majority of the air now are the only ones left.

The liquid water that excavated rivers and channels on the young planet may also have destroyed much of the atmosphere. Carbon dioxide probably dissolved in the water, to be deposited later as carbonate minerals. The discovery of carbon in Martian meteorites supports this idea. The same process takes place on the Earth, with one essential difference—Earth has volcanoes. Carbon dioxide is absorbed into the Earth's surface in just the same way as it is on Mars, and is eventually blasted back into the atmosphere by lava flows.

But the lockup of carbon dioxide in minerals is permanent on Mars: There are no plate tectonics to free the trapped carbon. There is evidence that Mars has had violent volcanic epochs in the past, but the planet's surface is still now. Mars' missing atmosphere is irretrievable—and the Red Planet will remain a cold, dry and inhospitable desert.