Once Upon a Water Planet
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.
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 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 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.
Left: The accumulated debris (or "apron") from this gully on Mars covers sand dunes that may have formed less than a century ago. [more]
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 remarked.
Right: Click to view a movie depicting seasonal changes in the martian ice caps, as measured by the Mars Orbiter Laser Altimeter on board the Mars Global Surveyor spacecraft. The amplitude is exaggerated for illustrative purposes
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.
Above: In this false-color 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 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 soil.
"These preliminary Odyssey observations are the 'tip of the iceberg'," Garvin concluded. Perhaps he was speaking quite literally!
The Jet Propulsion Laboratory manages both the Mars Global Surveyor and Mars Odyssey missions for NASA
Mars Exploration -- (JPL) explore the Red Planet yourself at NASA's Mars Exploration Program home page.
The Case of the Missing Mars Water -- (Science@NASA) Plenty of clues suggest that liquid water once flowed on Mars, but the evidence remains inconclusive and sometimes contradictory.
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.
Layers of Mars -- (Science@NASA) Mars Global Surveyor has spotted terrains on Mars that resemble sedimentary rock deposits. Did they form under water?
Floods at Mars' equator are recent, scientists say -- (SpaceFlightNow.com) University of Arizona scientists conclude that water flowed near Mars' equator only 10 million years ago.
NASA'S Global Surveyor sees possible climate change on Mars -- a NASA press release
Evidence Suggests Mars Has Been Cold and Dry -- (USGS) On the other hand: The mineral olivine, an iron-magnesium silicate that weathers easily by water, has been found in abundance on Mars. The presence of olivine implies that chemical erosion by water is low on the planet and that Mars has been cold and dry throughout most of its geologic history.
Seasonal Variations of Snow Depth on Mars -- (GSFC) graphs, maps and animations.
Dusty Streaks = Canals? -- Crystal-clear images of Mars captured by the Hubble Space Telescope reveal dusty streaks reminiscent of the legendary Martian "canals."
The Planet Mars: A History of Observation and Discovery -- By William Sheehan, The University of Arizona Press, Tucson.
Desertification of arid lands -- Mars is a desert today, yet it was likely wet long ago. Find out here how moist places become deserts on Earth.
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