Martian invasion: probing lively puzzles on the red planet - Mars

Science News, Nov 8, 2003 by Ron Cowen

Magnified images that reveal the specific shapes of mineral will provide additional clues to the origin of the iron oxides at either landing site, notes Bell. For instance, large, rounded grains of hematite suggest transport by a flow of water, while fine, flatter grains point toward the rust-formation mechanism.

Christensen says his favorite explanation for the hematite at Meridiani Planum is that a warm or hot spring percolated through the rocks there. The water, he suggests, could have come from a frozen lake covering layers of iron-bearing sediment. If some underground heat source, such as an erupting volcano, melted the ice, the water would have percolated through the sediment. If this scenario holds true, Opportunity "would see a hematite cement" filling in the rock pores, Christensen says.

Even if the hematite's origin remains ambiguous, trace amounts of other minerals could serve as additional markers of past water. Consider goethite, the water-bearing iron mineral named for the German poet Johann Wolfgang yon Goethe, who dabbled in mineralogy. Goethite formation requires water, and the mineral, when heated, can slowly convert to hematite. Small amounts of goethite would clinch a watery origin for the hematite at Meridiani Planum.

There is at least one way that liquid water could have been present on Mars without leaving behind carbonate fingerprints. Proposed by Christensen and other planetary scientists, this hypothesis could have important implications for future missions to Mars, especially in determining where to look for life.

If the Martian surface were somewhat warmer in the past--just above -20[degrees]C instead of today's average of -60[degrees]C--frozen water could liquefy as thin films at the boundary between layers of ice and dust, Jakosky notes. At that temperature, the chemical reactions that would lead to the formation of carbonates proceed so slowly that little carbonate would be made before the water refroze.

Supporting this idea, Christensen notes that many of the features on Mars that appear to have been sculpted by flowing water, including channels, don't require liquid water to last very long or to cover vast stretches of the planet. For example, the sudden melting of a reservoir of ice, creating a flash flood lasting for just a few weeks on a section of the Martian surface, would suffice to create a channel.

NEW SYNTHESIS, NEW MISSIONS Brief interludes when water was liquid also seem to have occurred recently. A camera aboard the Mars Global Surveyor spacecraft spied thousands of gullies at high latitudes where water would typically be frozen but under some circumstances could liquefy for brief periods. Free of craters and other blemishes typically acquired by older surfaces, the gullies look remarkably young, suggesting that water flowed there as recently as a few million years ago.

Such episodic flows would permit dormant organisms "to revive and repair themselves every few million years," Jakosky and his colleagues argue in the summer 2003 Astrobiology. Studies of life in extreme environments on Earth suggest that, "even at very low temperatures that would allow metabolism but not necessarily growth, organisms could effectively reset any damaged systems, including DNA, and thus allow very long-term survival," the team says. "Mars today appears to be right at the edge of being habitable by microorganisms."