oday 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.

March 12, 2002: 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.

Wherever the moisture went, new data suggest it might not be gone for good.
Indeed, water may have flowed on Mars literally as recent 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

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 meters 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 said.

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 meters underground, and that “there might be a
dynamic cycling of the atmosphere going on even today.”

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

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 micrometers) with unprecedented football-field resolution, seeking
thermal and mineral “fingerprints” hinting of seeps, volcanic vents, or
underground reservoirs.

Initial science data released March 1 is already tantalizing scientists.
Within its first week, Odyssey’s gamma-ray spectrometer has 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

“These preliminary Odyssey observations are the ‘tip of the iceberg’,”
Garvin concluded. Perhaps he was speaking quite literally!