NASA's Mars Science Laboratory, Curiosity, is the seventh visitor to the red planet, and by far the most ambitious mission of planetary exploration ever attempted. Curiosity's task is to determine if its landing spot, a large crater known as Gale, has ever experienced the environmental conditions that might be conducive to life.
In their paper, Cabrol, McKay, and their collaborators identified Gale as an ancient crater gouged out by the impact of a giant meteorite. They showed that over geological time the crater was probably flooded several times.
The land surface area of Mars is about the same as on Earth: it is vast compared to the area a rover can explore, which raises the question, "Why was Curiosity landed at Gale crater?" For an answer, we consulted Thomson Reuters Web of Science to uncover the story of when planetary scientists identified Gale as an interesting target. The analysis reveals that in 1999 Nathalie A. Cabrol and Christopher P. McKay (NASA Ames Research Center) and colleagues suggested in a research paper that Gale was a prime site for an astrobiology mission (N.A. Cabrol, et al., “Hydrogeologic evolution of Gale crater and its relevance to the exobiological exploration of Mars,” Icarus, 139 (2): 235-45, June 1999; cited 30 times in Thomson Reuters Web of Science).
Because all forms of life on Earth require water, searches on Mars concentrate on features that have clear evidence of surface water in the past. The geological record of past water is recovered by scrutinizing the dry river valleys, gulleys carved by floods, and ancient lake beds.
In their paper, Cabrol, McKay, and their collaborators identified Gale as an ancient crater gouged out by the impact of a giant meteorite. They showed that over geological time the crater was probably flooded several times. Over billions of years, wind-blown dust fell into the crater lake, settling as sediments on the lake floor, which Curiosity, at this writing, is now beginning to explore.
The paper showed that the mountain peak inside the crater, known as Mount Sharp, has the terraces and a layered structure typical of formation in a lake environment. This debris pile towers 18,000 feet above the crater floor. Its rock strata record 2 billion years of climatic and geologic change on the red planet. Gale is diverse in its geological setting, thanks to its long duration as a crater lake.
In their groundbreaking research paper a dozen years ago, the NASA scientists ventured that “impact crater lakes are probably the most interesting places to search for life on Mars.”
Ultimately, their view was borne out in the selection process for Curiosity’s landing site. Professor Charles Cockell of the UK Centre for Astrobiology, Edinburgh, is an expert on how life colonized meteorite-impact craters on Earth millions of years ago. He says that “the presence of liquid water in contact with an impact-altered environment clearly makes Gale crater a particularly favorable location.”
In response to ScienceWatch’s inquiry about the 1999 paper, co-author McKay, now a co-investigator with the Mars Science Laboratory (MSL) team, observes, “When the project selected Gale as the landing site I was very pleased. Everything we’ve seen so far on the Martian surface has reinforced the view that this is a lake bed, as Nathalie Cabrol had predicted.”
McKay notes that at the landing site, “the water-lain deposits will be accessible to the rover, and MSL has the instruments to search for organics in these deposits. Hooray for that—it’s been many years in the planning.”
Dr. Simon Mitton, a Fellow at St. Edmund’s College, University of Cambridge, and vice-president of the Royal Astronomical Society, is ScienceWatch’s Physics correspondent.
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