The Phoenix Lander was the first of NASA’s Scout class missions. It landed in the Martian arctic (the region around the Martian North Pole). The purpose of Phoenix was to study the history of water in the Martian arctic, and to examine the boundary between ice and regolith (the dirt on the Martian surface) to help to determine whether it might support life forms. The Phoenix mission did not make a direct search for life forms. Thus far, the only probes to search directly for the presence of life were NASA’s Viking 1 and Viking 2 landers (VL1 and VL2), sent to Mars in 1976. Costing a billion dollars each, VL1 and VL2 landed at Chryse Planitia and Utopia Planitia, respectively, two regions of Mars that were considered to be less risky for landing the probes due a lack of crevices, hills, or large boulders, which might have flipped the probes on landing.
From an astrobiology perspective, these sites also were considered to be less interesting than the icy poles, such as where Phoenix landed. Nevertheless, VL2 and VL2 transmitted interesting results. One instrument, called the Labeled Release (LR) Experiment, produced an analysis of regolith samples which seemed to indicate the presence of microorganisms. Because of the findings of other experiments, however, many scientists rethought the LR results and believed them to be the effect of some non-biological property of the regolith. New understanding of Mars gained over the last couple of decades, however, have cast some doubt on the rethinking, so that Martian microbes actually may prove to be a simpler explanation of the LR results, compared to proposed non-biological causes. Meanwhile, the LR Principal Investigator, Gilbert Levin, has maintained since 1976 that the LR did detect life on Mars. Today, many scientists have come to take all or some of Levin’s proposals seriously. This is due, in part, to an evolving understanding of the Martian environment with regard to water and organic compounds.
As far as organic material goes, back in the days of Viking, it was assumed that organics should exist on the Red Planet, since like Earth, Mars has been bombarded with comets and meteorites containing organic material from space. However, when an instrument carried by VL1 and VL2 known as the GCMS failed to identify organic material, people started to propose all sorts of mechanisms by which organic material could have been destroyed. Recently, however, the Phoenix lander identified perchlorate, whose presence would have turned any organic material sought by Viking into chlorinated organic compounds -which in fact the Viking GCMS devices did detect. Since these compounds were used as cleaning agents on the spacecraft prior to launch, the finding of chlorinated organic compounds by the Viking GCMS was thought to represent a false positive. Now, the entire story must be reassessed.
While the atmospheric pressure on Mars is much lower than it is on Earth, it is such that at the surface it is possible to have transient liquid water at certain times of the day, at certain times of the year. It also is possible for water to exist as liquid for long periods of time on Mars, provided that it is very salty water -a scenario that has been proposed to explain the observations that have been made in recent years, suggesting that water flowed on the surface of the Red Planet fairly recently, even though the atmospheric pressure at the surface is no more than 7 millibars. Analysis of meteorites that were catapulted from the Martian crust and made their way to Earth shows high levels of various salts in the rock, and so this hypothesis of briny seas and rivers on Mars of the past really makes a lot of sense. And if there was water, then probably life arose on Mars and still exists there today, since life is pretty tenacious and evolves to take advantage of changing environments.
At least one Mars meteorite, ALH84001, preserves fossils of microorganisms that lived on Mars billions of years ago. While several investigators have criticized biological interpretations of various features in the meterorite, in recent years, Dr. David McKay of NASA’s Johnson Space Center and his colleagues have countered with yet stronger evidence. Much of this evidence has to do with several features of crystals made of magnetite, which could have emerged only as a result of Darwinian selective processes that have been observed in nature only in connection with biology.
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