Wednesday 15 January 2014

Examining an Ordovician Stromatolite with a tool to look for life on Mars.

The potential of there being life on Mars has been a stalwart of popular fiction for over a century, though to date no signs of actual life have been discovered. Recent discoveries of geological structures on Mars that indicate the presence of large bodies of open water in the early history of the planet. This new understanding of the planet makes the search for evidence of ancient life on Mars a more realistic prospect, though actually finding traces of such life remains a daunting prospect.

In a paper published in the journal Astrobiology on 9 September 2013, Alison Olcott Marshall and Craig Marshall of the Department of Geology at the University of Kansas describe the results of an examination of an Ordovician Stromatolite from Gunter Sandstone Member of the Gasconade Dolomite in Reynolds County, Missouri, with a Field Raman Spectrometer, a device similar to those planned for use on the ExoMars planetary mission proposed for launch in 2018.

Stromatolites are thought to be the most ancient macro-fossils (big fossils, things that can be seen in the field rather than by looking at rock samples under a microscope) found on Earth. They are also quite simple in origin, produced by simple microorganisms present early in the history of life on Earth, and of the kind that scientists feel might have also existed on early Mars. Stromatolites are formed by layers of micro-organisms forming biofilms on the surface of sediments in shallow water environments. Typically such films are buried by sediments periodically, with a new biofilm forming on the surface. Over time this builds up to a distinctive structure with layers of organic and inorganic material, a Stromatolite. The earliest Stromatolites appear in the fossil record of Earth about 3.5 billion years ago, and they are still being formed today. Importantly, Stromatolites do not need to be formed by any particular organism, any micro-organism prone to forming biofilms on shallow water sediments can potentially produce a Stromatolite.

Stromatolite from the Gunter Sandstone Member of the Gasconade Dolomite in Reynolds County, Missouri. Olcott Marshall & Marshall (2013).

A Raman Spectroscope is a form of spectroscope that uses back scattering from a laser beam shone on a sample to determine the nature of chemical bonds in a sample, and thereby determine information about its chemistry. The model used by Olcott Marshall & Marshall was a hand held device operated by a bluetooth connection from a laptop computer.

Olcott Marshall & Marshall were able to obtain distinctive signatures for the organic and inorganic layers of the Stromatolite, suggesting that the method would be valid for detecting Stromatolites of similar chemical composition on Mars, without the need to collect samples and return them to Earth for analysis. However they note that the (commercially available) device used in Missouri operated with a laser at a wavelength of 785 nm, whereas the model intended for use on Mars operates at 532 nm, a wavelength chosen for its utility in identifying minerals rather than fossiliferous material, and which may prove to be less in this context.

Representative spectra acquired from a siliceous region of the stromatolite. The band at ca. 464 cm¯¹ in this spectrum is assigned to the vsSi-O-Si symmetric stretching mode (A₁ symmetry mode) of α quartz. Olcott Marshall & Marshall (2013).

Representative spectra from disseminated carbonaceous 
material shows a narrow low-intensity band at ca
464 cm¯¹ assigned to the vsSi-O-Si symmetric stretching 
mode (A₁ symmetry mode) of α quartz and two broad intensity 
bands at ca. 1280 and 1540 cm¯¹ assigned to the D 
and G bands of disordered/polycrystalline sp² carbonaceous 
material, respectively. 
Olcott Marshall & Marshall (2013).

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