Mars: Signs of a Watery Past
Mars has provoked more speculation on the possibilities of life beyond Earth than any other planet in the Solar System. The presence of water is a prerequisite for the existence of life. Therefore, "follow the water" has been NASA's chief guideline for the exploration of the red planet. Although Mars experiences seasons like on Earth and has polar caps composed of carbon dioxide and water ice, today it is bone-dry and frigidly cold. But evidence is rapidly accumulating that Mars was once much wetter, with a more clement climate. This evidence comes from orbiting satellites and from data collected by roving landers.
Since the 1970s, space probes to Mars have revealed numerous features apparently carved by flowing water, such as winding, branched valleys resembling dried-out streambeds and giant outflow channels gouged by catastrophic floods. Recent high-resolution imagery from the Mars Global Surveyor Mars Orbiter Camera and the Mars Odyssey THEMIS reveals numerous examples of branched valleys that form tightly-packed, integrated drainage systems. These channels originate at topographic high points, the valleys widen "downstream", some even displaying inner valleys. The valley networks exhibit morphometric characteristics, including network densities, comparable to those of terrestrial drainage basins. These features were most likely produced by rainfall, during wetter, warmer periods in the past.
Minerals also furnish important clues about former climates on Mars. Most diagnostic are those that form under relatively narrow climatic ranges or that deposit from hydrous solutions at or near the surface. Examples of such minerals include evaporites (e.g., halite), clay minerals (e.g., kaolinite), carbonates (e.g., calcite), and hematite, a mineral generally deposited from water solutions.
The two Mars Exploration Rovers currently analyzing the chemical and mineral makeup of soils and rocks have found many telltale signs of water. In early January 2004, Spirit landed at Gusev crater, site of presumed delta or lake deposits near the mouth of a river-like channel. Shortly thereafter its twin, Opportunity, touched down at Meridiani Planum in an area where orbiting satellites had detected hematite (see Fig. 1 at right). At the latter site, the Alpha Particle X-ray Spectrometer and mini-TES infrared spectrometer, which fingerprint chemical elements and molecules, discovered sulfate-rich rocks in the walls of Eagle crater. Embedded within and weathering out of this rock, like blueberries in a muffin, are tiny round spherules of hematite (Fig. 2). These martian "blueberries" resemble small iron oxide concretions, like those commonly found weathering out of the Navajo Sandstone, famous for its spectacular scenic outcrops, in Zion and Canyonlands National Parks, Utah. The terrestrial hematite concretions deposited from iron-rich, oxidizing groundwater solutions that traveled through relatively porous sandstone, whereas their martian counterparts precipitated from sulfate-rich evaporite rocks.
The Mössbauer Spectrometer, which pinpoints iron-bearing minerals, found jarosite (Fig. 3). On Earth, jarosite forms by the oxidation of iron sulfides in the presence of acidic water, as at the metal sulfide deposit of Rio Tinto, Spain. The relatively acidic solutions from which it precipitates may account for the scarcity on Mars of carbonate minerals that would dissolve in acid. Martian rocks also show elevated levels of bromine relative to chlorine, evidence of evaporation from a concentrated brine, like Dead Sea salt. They further display patterns like ripples and cross-laminations, which on Earth result from stream or lake currents. Analogous features also occur at nearby Endurance crater, Meridiani Planum.
An extensive area (approximately 300,000 km2) of light-toned outcrops, similar to those examined by Opportunity, has been observed by the Mars Global Surveyor and Mars Odyssey spacecraft, presently orbiting Mars. This suggests that these features are not unique to one spot, but could form parts of what may have once been an extensive evaporite deposit, requiring a fairly large body of water that persisted over long enough periods of time to build up a 0.5 km thick pile of sediments.
Meanwhile, the other rover, Spirit, which had initially found nothing more exciting than lots of basaltic rocks and soils, more recently headed toward the nearby Columbia Hills, where it too detected hematite. High levels of sulfur, chlorine, and bromine were also found in these rocks, suggesting that they had either been chemical altered by migrating aqueous fluids or had been formed by evaporation.
The minerals that the Rovers have encountered so far — hematite, jarosite and other sulfates containing significant amounts of chlorine and bromine, as well as the geological context — layered rocks, cross-laminations, ripples — point to deposition from hydrous solutions, presumably as evaporites. Although many signs of water activity had previously been seen in satellite imagery, the new mineralogical data coupled with high resolution pictures of the surface strengthens the case for a formerly much wetter, warmer Mars, that may have once provided a hospitable environment for life's beginnings.
Where did all the martian water go? While some of its water may have been irretrievably lost to space, perhaps most of it is still locked in cold storage in various underground reservoirs. The existence of ice mixed into the regolith and/or at the poles has long been suspected. Recent measurements by the Mars Odyssey Gamma-Ray Spectrometer and Neutron Spectrometer indeed suggest substantial deposits of water ice, or other hydrous species, buried beneath the surface at high latitudes. Perennial water ice at the south polar cap has been discovered by spectrometers on both Mars Odyssey and the European Space Agency's Mars Express orbiter. These findings may be just the tip of the iceberg.
Please address all inquiries about this research to Dr. Vivien Gornitz.