The Italian astronomer Giovanni Schiaparelli (1835-1910) started a myth when he said he'd seen canali (channels) on Mars. These were later found to be an optical illusion and a scientific impossibility—spectroscopic analysis of light from Mars showed a dry place with so little atmosphere that water would have boiled away instantly. Yet Mars wasn't always a desert. Space probes have discovered the remains of dried-up riverbeds, flood plains and shallow seas, so what happened in the distant past to freeze-dry the entire planet?

Planetary scientists were slow to discover Mars' secret history. The first probes to fly past the Red Planet returned only a handful of images, which showed Moon-like cratered plains. And when Mariner 9 orbited Mars in 1971, it took many months for the spacecraft to begin mapping the planet—almost all of Mars7 features were obscured by a global dust storm. But when the storm eventually cleared, Mariner 9's pictures proved to be worth the wait. They showed enormous volcanoes, far bigger than any found on Earth. They also showed a multitude of features that suggested liquid water had once scoured the planet's surface. These included vast canyons, eroded craters, chaotic terrain of broken rock caused by sudden flooding, and long, riverlike channels fed by tributaries that run downhill.

Some researchers sought to dismiss these features as the results of runny lava flows rather than of water. But when the Viking orbiters sent back more detailed images, the similarity to terrestrial water features was striking. Distinct lines of sediment could be seen along valleys and canyons, suggesting the remains of melted glaciers. And the low plains that dominate the northern hemisphere have even been interpreted as the remains of a Martian sea, dubbed "Oceanus Borealis/' and estimated to be four times the size of the Earth's Arctic Ocean.

Opinions differ on how much water once existed on Mars. But the fact that it did exist is now certain. What we don't know is where the water went. The atmosphere on Mars today is only one-hundredth as dense as that of the Earth. In the low atmospheric pressure, any liquid water would evaporate instantly. So Mars7 atmosphere must have been much denser in the past. The diameter of Mars is half that of the Earth, and its lower gravitational field would have made it easier for its atmosphere to escape into space, particularly if knocked off by space debris. In addition, its weak magnetic field may have been unable to prevent the solar wind from progressively stripping the atmosphere as it passed.

On Earth, the geological process of plate tectonics recycles carbonates from rocks into the air, as continental plate movements redistribute the molten mantle. Mars lacked the energy for this process. If carbon and oxygen from the air got chemically locked into the Martian rocks, they stayed there—shrinking the atmosphere further. In a reverse greenhouse effect, the thinner the atmosphere got, the colder it became. Perhaps 2 billion years ago, much of the remaining atmosphere became frozen carbon dioxide—or dry ice—and the last of the water retreated below the surface, finding refuge at the planet's poles.