The quest for evidence of life on Mars could be more difficult than scientists previously thought. A scientific paper published today details the investigation of a chemical in the Martian soil that interferes with the techniques used by the Curiosity rover to test for traces of life. The chemical causes the evidence to burn away during the tests.
In search of clues to life's presence on Mars -- now or in the past -- Curiosity checks Martian soil and rocks for molecules known as organic carbon compounds that are the hallmark of living organisms on Earth.
While trekking around the Rocknest sand dune in November 2012, the rover found evidence of perchlorate -- a salt comprised of chlorine and oxygen. When Curiosity heats a scoop of Martian soil to test it for organic carbon, perchlorates can cause a chemical reaction that destroys organic carbon. Daniel Glavin, an astrobiologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and first author on the new paper, said he now believes the troublesome perchlorates are likely prevalent throughout the Martian surface.
"The presence of perchlorates isn't good news for some of the techniques we're currently using with Curiosity," said Glavin. "This may change the way we search for organics in the future on Mars."
Curiosity mission scientists previously announced finding perchlorates last December at the 2012 Fall Meeting of the American Geophysical Union (AGU) in San Francisco. Now, in the Journal of Geophysical Research: Planets, an AGU publication, they provide much more detail about the evidence and examine its potential impact on Mars exploration. Five other papers with other findings from the Curiosity mission are publishing today in the journal Science.
Accounting for Perchlorate
The Curiosity rover's Sample Analysis on Mars (SAM) system tests soil samples by heating them in an instrument, called a pyrolysis gas chromatograph mass spectrometer, which breaks the samples down into their chemical components and determines precisely how much of each of those components is present in the sample. Any perchlorate salts in the heated sample decompose as the temperature goes above 200 degrees Celsius (392 degrees Fahrenheit) and release pure oxygen. Organic molecules in the sample exposed to this oxygen will then combust into carbon dioxide, destroying the molecular evidence of their presence. Luckily, Glavin said, some organic carbon would likely survive, either incased in more heat-resistant materials or detected before the breakdown of perchlorates.
Glavin noted that scientists can account for the destroyed organic carbon by assuming a certain baseline of perchlorate in the Martian soil. In future tests, scientists can calculate how much organic carbon burnt away with the decomposing perchlorates in order to estimate the original amount of organic material in the soil. The findings at Rocknest serve particularly well for this purpose because the site was originally chosen because it was unlikely to have any organic material.
"It will be absolutely critical as we move on to other samples to compare them to the Rocknest dune to infer the presence or absence of Martian organic material," said Glavin.
While Curiosity findings published today in Science do not use Rocknest as a perchlorate baseline, Glavin said the next batch of papers from the Sheepbed mudstone will use them for calibration.
Glavin added that Curiosity has the potential to avoid the perchlorate problem in the future by using techniques that do not involve heating the soil to the point where perchlorates break down. The rover already carries an apparatus capable of that, which it hasn't yet used. The system, which employs liquids in its chemical assays, is more complicated than those currently in operation, Glavin said. And, it requires additional testing before it can be used, which he expects could happen in the near future.
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From now through 30 September, a PDF copy of this published article is available for download at http://publications.agu.org/jgre20144/. Starting 1 October, the article -- in PDF and HTML versions -- will become available instead at JGR:Planets, http://dx.doi.org/10.1002/jgre.20144. Only journalists and public information officers of educational and scientific institutions who have registered with AGU can download the paper from the JGR:Planets location. Or, you may order a copy of the final paper by emailing your request to Thomas Sumner. Please provide your name, the name of your publication, and your phone number.
"Evidence for Perchlorates and the Origin of Chlorinated Hydrocarbons Detected by SAM at the Rocknest Aeolian Deposit in Gale Crater"
Daniel Glavin, Caroline Freissinet, Jennifer Eigenbrode, Anna Brunner, William Brinckerhoff, Pamela Conrad, Jason Dworkin, Heather Franz, Mildred Martin, Alexander Pavlov and Paul Mahaffy: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; Kristen Miller and Roger Summons: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; Arnaud Buch: Laboratoire Genie des Procedes et Materiaux (LGPM), Ecole Centrale Paris, Chatenay-Malabry, France; Brad Sutter: Jacobs Technology-ESCG, Houston, Texas, USA; P. Douglas Archer, Jr., and Douglas Ming: Astromaterials Research and Exploration Science Directorate, NASA Johnson Space Center, Houston, Texas, USA; Sushil Atreya: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA; Michel Cabane, David Coscia, Cyril Szopa and Samuel Teinturier: Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS), Univ. Pierre et Marie Curie, Univ. Versailles Saint-Quentin & Center National de la Recherche Scientifique (CNRS), Paris, France; Patrice Coll: Laboratoire Interuniversitaire des Systemes Atmospheriques (LISA), Univ. Paris-Est Creteil, Univ. Denis Diderot & CNRS, Creteil, France; John Grotzinger: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA; Laurie Leshin: Department of Earth & Environmental Science and School of Science, Rensselaer Polytechnic Institute, Troy, New York, USA; Christopher McKay: Space Science Division, NASA Ames Research Center, Moffett Field, California, USA; Rafael Navarro-Gonzalez: Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico D.F., Mexico; Andrew Steele: Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA.
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