By recreating the Martian surface in the laboratory, NASA scientists may have begun to answer two questions: why the Martian surface is so red, and why organic life has not yet been found there.
By recreating the Martian surface in the laboratory, NASA scientists may have begun to answer two questions that have been plaguing scientists for years: why the Martian surface is so red, and why organic life has not yet been found there.
One answer, say the scientists, could be an important reactive oxygen molecule, called a superoxide anion,
or negatively charged oxygen (O2-
To test this hypothesis requires some way to test for hostile conditions in a laboratory. The researchers therefore first had to replicate a harsh Martian environment, but in a controlled recipe that they could sample reliably and easily. Their recipe: chilling a mineral sample under an ultraviolet lamp with just the right mix of gases included.
So after adding tiny (100-milligrams/0.0035-ounce) grains of a mineral called labradorite found in Martian soil, the research team filled test tubes with the gases found in a Mars-like atmosphere. The surface of Mars is cold, so the researchers chilled the tubes to minus 30 Celsius (minus 22 degrees Fahrenheit). They also bombarded the tubes with ultraviolet radiation to simulate the Sun's rays.
The result: superoxide ions formed on the surface of the mineral grains.
This finding is reported in the September 15 issue of Science. The principal investigator of the study is planetary geochemist Albert Yen of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Yen's co-authors are JPL chemist Sam Kim, JPL materials scientist Mike Hecht, chemist Martin Frant of Chemotics Consulting in Newton, MA, and planetary scientist Bruce Murray of the California Institute of Technology in Pasadena.
Experiments in the 1970s suggested that organic chemicals should form naturally on Mars, and that Mars should also get organics from meteorites. Yet when the two Viking landers arrived on Mars in 1976, they did not find any organic chemicals. However, the landers did find an oxidizing substance on Mars that went as deep as 10 centimeters (4 inches) beneath the surface. Viking tests showed that nutrients added to the Martian soil seemed to be consumed. Some scientists took this as evidence of life on Mars, but others said the same endpoint appears from non-biological chemical reactions. According to Yen, if "oxygen was released and the added nutrients were decomposed in the Viking experiments," several oxygen-reaction (redox) steps "may have occurred to produce the observed results."
Because Mars has a thinner ozone layer than Earth, its surface is constantly bombarded with ultraviolet rays from the Sun. This ultraviolet exposure not only damages organic molecules, it also converts oxygen into superoxides. The superoxides then combine with any organic carbon molecules present to produce carbon dioxide gas. In this way, superoxide might decompose organic molecules formed on Mars or deposited there by meteorites.
Usually, superoxides are consumed in reactions with water. But Mars has little if any surface water, so superoxides can remain unaltered in the Martian soil. To what depth and what rate these oxygen reactions occur are two key parts of the puzzle that remain to be discovered. "The thickness of the oxidizing layer," says Yen, "is unknown and the actual
depth could be much deeper than the shallow depths sampled by Viking."
Researchers have suggested that the oxidant in the Martian soil could be hydrogen peroxide (H2O2), potassium superoxide (KO2-), barium superoxide (BaO2-) or other chemicals. However, because formation of other oxidants is "unlikely under Martian conditions," Yen believes simple superoxide (O2-) is at work on Mars. In this scenario, a combination of ultraviolet radiation and superoxides essentially would sterilize the Martian surface, stripping it of organic precursors to life. "These oxygen radicals can explain the reactive nature of the soil and the apparent absence of organic material at the martian surface," conclude the Science article authors.
The surface of Mars is now cold and dry (although there is some water ice in the Northern polar ice caps). Because of its mostly waterless conditions, modern-day soil rusting is instead probably not caused by a combination of water and iron, but rather by chemical reactions between iron-rich soil and superoxides. "If ferric oxides formed on Mars early in its history," Yen points out, "they should remain until present day." While this cold gaseous process is slower than liquid water oxidization of iron, superoxides could still be an important current cause of the rusting apparent in iron-rich Martian soil.
This is not to say that life does not exist on Mars - only that if superoxides cover the Martian surface, their sterilizing reaction would inhibit life from thriving on the top layer of soil. To avoid the effects of the oxygen radicals, life would have to learn to survive at least four inches below the Martian surface. "We believe that the superoxide ions formed at the immediate surface (ultraviolet penetration depth) of Mars will be sufficiently mobile to explain the reactivity of subsurface soils analyzed by Viking," said Dr. Yen. "At present, we do not have sufficient data to determine how deep the reactive layer will extend (it could be a few meters or a few tens of meters)."
Getting Below the Surface
On Earth, life thrives in deep places: microorganisms have been discovered in boiling hot underground vents and in frigid Antarctic ice sheets. Some even believe that there may be more life existing below the surface of the Earth than above it.
On Mars, living below the surface makes a lot of sense. One essential requirement for all known life is liquid water, and the presence of underground water on Mars is still a possibility. The underground organisms would be untouched by superoxides, and they would be shielded from scorching ultraviolet radiation. This new finding implies that future Martian exploration missions may have to dig deeper to find evidence of life on Mars.
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