Today the Red Planet is dry and barren, but what about tomorrow? New data suggests that the long story of water on Mars isn't over yet.

By Trudy E Bell and Dr Tony Phillips

When Orson Welles broadcast "The War of the Worlds" in 1938, many listeners were ready to believe in Martians. After all, astronomers had long debated markings on the Red Planet that might be aquaducts or fields of vegetation. Why not warlike aliens as well?

Among laypeople (and some scientists) the notion that Mars was "Earth-Like" - warm, wet and verdant - persisted for decades, until the first spacecraft visited the Red Planet. The Mariner missions of the late 1960's revealed the real Mars: heavily cratered, dotted with extinct volcanoes, colder than Antarctica and drier than the Sahara desert. There were no trees, no canals, no Martians - and very little atmosphere! "The War of the Worlds" was a fantasy after all.

Subsequent missions mostly confirmed a new paradigm: Mars was once wet, but now it is dry. Spacecraft photos of Mars reveal signs of ancient rivers, lakes and maybe even an ocean. They might have been filled with water billions of years ago, but something happened - no one knows what - and the planet became a global desert.

This image by artist Duane Hilton is a fake! It shows a standing pool of water on Mars - impossible today, but what of the future?

Wherever the moisture went, new data suggest it might not be gone for good. Indeed, water may have flowed on Mars literally as recently as "yesterday or last year," declares James Garvin, Chief Scientist for Mars exploration at NASA headquarters. Evidence is mounting that water lies beneath the Martian terrain, he says. Furthermore, every few centuries weather conditions might become clement enough for that water to "come and go" on the surface as well.

The first hints of water near Mars' surface came in 2000 when the Mars Orbiter Camera (MOC) on board NASA's Mars Global Surveyor spacecraft spotted hundreds of delicately filigreed gully systems. Individual gullies are just 10 metres wide (earlier missions couldn't photograph such small features) and a whole system might cover only a dozen city blocks. Their sculpted terrain, cut-bank patterns, and fan-shaped accumulations of debris look hauntingly similar to flash-flood gully washes in deserts on Earth.

Dozens of the gully systems appear on the shaded sides of hills facing the polar ice caps. Their geometry suggests that "swimming-pool volumes of water could be entombed underground until suddenly it's warm enough for an ice plug to burst, letting all the water rush down the slopes," Garvin says.

Many of the gully systems look extraordinarily recent - sharply carved and crossing older, wind-scoured features. Their appearance is so fresh, in fact, that it has excited planetary geologists such as MOC designer Mike Malin to think that Mars "may have experienced massive, short-term climate changes, where water could come and go in hundreds of years." Indeed, Garvin said, scientists wonder whether liquid water might exist on Mars now, buried in some areas perhaps 500 metres underground, and that "there might be a dynamic cycling of the atmosphere going on even today."

The accumulated debris (or "apron") from this gully on Mars covers sand dunes that may have formed less than a century ago. [more]

MOC's findings are corroborated by data from another instrument on the spacecraft, the Mars Orbiter Laser Altimeter (MOLA). For 27 months - longer than a Martian year (one Martian year is 687 Earth days) - MOLA gauged the daily height of the Red Planet's polar icecaps, meticulously recording how much frozen material accumulated in winter and eroded (sublimed or evaporated) in summer in each hemisphere. MOLA documented that each ice cap has a volume as great as the Greenland ice cap on Earth.

Although the upper crust of frost is clearly carbon dioxide, scientists are now convinced that much of both caps' supporting mass must be frozen water-structurally, "dry ice can't stand up two miles high," Garvin remarked.

MOLA and MOC measured how the polar caps shrink in each hemisphere's summer. They shrink so much, in fact, that if the observed trends were continued for just a few centuries, nearly a third of each polar cap could evaporate into Mars's atmosphere. That would pump the atmospheric pressure up from 6 millibars to 30 or 40 mb (the Earth's atmospheric pressure is about 1000 mb) - high enough pressure for liquid water to be stable on the planet's surface under certain temperature conditions. Thus, perhaps as recently as just a century or two ago, Mars might have been "clement enough for ponds of water" to have dotted its surface like desert oases, Garvin said - and current trends suggest it might become so again.

All these observations reopen a venerable question: was there - or is there - life on Mars?

"Following the water makes sense if you're prospecting for biology," Garvin declared. "If we could find evidence of preserved liquid water on Mars, that would be the Holy Grail."

Looking for water is in fact a prime mission of the Mars Odyssey spacecraft, whose high-gain antenna unfurled on February 6, 2002, and whose instruments began mapping Mars at the end of that month. Odyssey's multispectral camera is imaging Mars simultaneously at numerous infrared wavelengths (from 8 to 20 micrometres) with unprecedented football-field resolution, seeking thermal and mineral "fingerprints" hinting of seeps, volcanic vents, or underground reservoirs.

In this false-colour map of Mars, soil enriched in hydrogen is indicated by deep blue. Source: the neutron spectrometer onboard NASA's 2001 Mars Odyssey spacecraft. [more]

Initial science data released on March 1st 2002 is already tantalising scientists. Within its first week, Odyssey's gamma-ray spectrometer had detected significant amounts of hydrogen in Mars's south polar regions-possibly indicating the presence of frozen water in the upper few feet of the Martian soil.

"These preliminary Odyssey observations are the 'tip of the iceberg'," Garvin concluded. Perhaps he was speaking quite literally!