The prestigious journal Science recently exploded most of the media's "life-site" hype over the latest-and still laudable-Mars mission.1 Twelve peer-reviewed articles tell the true story: no lake and no life chemistry. However, Spirit Rover's remarkable instruments did reveal at least five relevant discoveries.
First, if watery flows ever marked the Martian surface, such instances were minor and intermittent at best. Gusev Crater is not an ancient lake bed, but even if it had been, liquid water is but one of many requirements for life.
Second, although sulfate salts found on Mars are indicative of past liquid water, those salts formed during the late heavy bombardment era when comets pelted Mars with frozen water (4.0 to 3.8 billion years ago). Sulfuric acid emissions from Mars' volcanoes would have quickly turned that water to sulfate salts. That same sulfuric acid would work against any chance for life to emerge.
A third finding of the Mars Exploration Mission reveals that the dominant geologic processes on Mars are physical (meteor impacts and dust storms) rather than chemical. Without a high level of chemical activity, no naturalistic origin of life would be possible.
Another life-critical discovery uncovered by the mission shows that blockage of Mars' surface sunlight (from dust in the atmosphere) is highly variable-ranging from 50 to 90 percent in just the five-month period of study. Such blockage, as well as such high variability in the blockage, renders sunlight an unlikely energy source for life.
Finally, dust storms (with particles from 0.001 to 3.0 mm) repeatedly bury and uncover Mars' surface. During the burial episodes the dust depth varies from 5 to 60 cm. At the very least, this frequent deposition and removal of dust by storm winds reaching 300 miles per hour yields inhospitable conditions for life.
All of what the Spirit did find is exciting and helpful to planetary scientists interested in the details of the solar system's history and design. But the bottom line for astrobiologists is deeply disappointing-Mars is far less hospitable for life than anyone had envisioned. The hospitality of Earth seems all the more remarkable, more exquisitely rare, by comparison.