Discoveries From Venus And Mars

PLANETARY EXPLORATIONS have been striking pay dirt lately, revealing new information about Venus' atmosphere and Mars's geologic history. The discoveries—and more are promised—are leading to new insights into our planetary neighbors.

Detection of the important and reactive hydroxyl radical in the atmosphere of Venus could lead to a better understanding of the planet's climate. And discovery of almost pure silica on Mars suggests the planet might have once been wet and habitable. Also, at C&EN press time, scientists were anticipating that the $460 million Phoenix Mars Lander would touch down on the red planet on May 25 and begin probing the martian surface.

Now that the hydroxyl radical, which helps clear pollutant molecules from Earth's atmosphere, has been found in Venus' atmosphere, scientists may begin to learn how this reactive species affects other molecules there, such as CO2, ozone, and oxygen.

Sensitive spectrometers aboard the European Space Agency's Venus Express spacecraft, which is orbiting that planet, detected infrared emissions from HO^., reports planetary scientist Giuseppe Piccioni of the Instituto di Astrofisica Spaziale e Fisica Cosmica, in Rome, and colleagues (Astron. Astrophys., DOI: 10.1051/0004-6361:200809761).

Scientists are particularly interested in Venus' atmosphere because the planet is approximately the same size and density as Earth. But whereas Earth's CO2 is largely sequestered in its rocks, Venus' CO2 covers the planet in a thick atmospheric blanket that also includes clouds of sulfuric acid, a layer that keeps the planet at a sweltering 450 ºC.

The new results add to understanding of the chemistry and dynamics of the venutian atmosphere, scientists say. On Earth, the reaction of atomic hydrogen with ozone generates atmospheric HO∂. The Venus Express group posits that the same may be true on Venus, although the radical is produced in much smaller quantities there.

Previously, "we didn't suspect that ozone could be the main mechanism of production of HO∂, due to ozone's relatively low abundance" on Venus, Piccioni says. But recent models suggest that large enough quantities of ozone may exist on the night side of Venus—where the unstable molecules can enjoy longer lifetimes—to account for the levels of HO∂ that the team measured.

The discovery also indicates that the hydroxyl radical may play a significant role in the oxidation of CO and SO2 at high altitudes and so might help answer the long-standing question of how Venus' CO2 atmosphere remains chemically stable, notes Yuk L. Yung, a geological and planetary sciences professor at California Institute of Technology.

And on Mars, the serendipitous discovery of large quantities of nearly pure silica—a material formed only in hot, watery environments—was announced last year, but now has been given the approving stamp of publication (Science 2008, 320, 1063).

NASA's Mars exploration rover Spirit, which has been forced to drag itself backward because of a broken right front wheel, churned up brightly colored soil that the rover's spectrometer determined was more than 90% SiO2.

Two possible processes could produce the mineral deposits: acidic steam in a hydrothermal vent removes other rock, leaving the silica behind; or hot springs dissolve the silica and carry it elsewhere. On Earth, such environments are hotbeds of microbes, suggesting that the martian silica might be a good place to look for evidence of life.

Should the University of Arizona's Phoenix Mars Lander arrive safely, the craft would dig holes into the martian soil and perform the first-ever wet chemistry experiments on another planet. Tension was high among Phoenix scientists because the landing involves a series of risky maneuvers, and a single large rock could unbalance the craft.