Petrographic thin section made from core sample is shown. This 30 micron thin slice of rock allows a view of the types of features thought to be microbial. Here, the blue layers are an epoxy added in to see void-space in the rock and the grey is sediment. The morphology of the orangey-brown layers are suggestive of microbial activity, such as they way they roll over themselves in the bottom left and smoothly drape over the triangular feature. This type of deposition demonstrates that the sediment had to have a degree of cohesive stickiness, such as that provided by the presence of microbial mats. Credit: USGS Core Research Center
As NASA has confirmed flowing saltwater on the surface of Mars, the hunt for life on the Red Planet has new momentum. Now, Eocene rock samples from the Green River formation could guide the search for astrobiology. Marshall and Nicholas A. Cestari found these Green River rocks have features that visually indicate the presence of life, and they argue that probes to Mars should identify similar indicators on that planet and double-check them through chemical analysis.
The researchers examined cored samples of rock from 50 million years ago that included sections of “microbial mats.”
“Microbial mats are essentially the microbial world’s version of apartment buildings – they are layered communities of microbes, and each layer represents a different metabolic strategy,” Olcott-Marshall said. “Generally, the photosynthetic microbes are at the top, and then every successive layer makes use of the waste products of the level above. Thus, not only does a microbial mat contain a great deal of biology, but a great number of chemicals, pigments and metabolic products are made, which means lots of potential biosignatures.”
At times during the Eocene, the Green River Formation’s water chemistry purged fish and other organisms from the lake, providing room for these microbes to thrive. “During these times, microbialites’ formed – these are rocks thought to be made by microbial processes, essentially the preserved remnants of microbial mats. The Green River Formation has a wide variety of these structures, and these features are why we went looking in these rocks, as microbialites are one life-detection target on Mars.”
First, the researchers visually inspected the cored samples for signs of biology by identifying geological signs associated with microbialites – such as “stromatolites.” “These are things like finely laminated sediments, where each lamination follows the ones below, or signs of cohesive sediment, things like layers that roll over onto themselves or are at an angle steeper than what gravity would allow,” Olcott-Marshall said. “These are all thought to be signs that microbes are helping hold sediment together.”
METHOD: They powdered rock samples in a ball mill, then used hot organic solvents like methanol to remove organic carbons preserved in the rocks. That solvent was then concentrated and analyzed with gas chromatography/mass spectroscopy. “GC/MS allows an identification of compounds, including organic molecules, preserved in a rock,” Olcott-Marshall said. “Viking was the first time that a GC/MS was sent to Mars, and there is one up there right now on Curiosity collecting data.”
RESULTS: The rock structures appeared biological hosted living organisms millions of years ago: analysis showed the presence of lipid biomarkers. (preserved remnant of a lipid, or a fat, once synthesized by an organism). “These can be simple or very complex. Different organisms make different lipids, so identifying the biomarker can often allow a deeper understanding of the biota or the environment present when a rock was formed. These are a type of biosignature.”
“There is a GC/MS on Curiosity right now, but there are only 9 sample cups dedicated for looking for biomarkers like these,” Olcott-Marshall said. “It’s crucial those nine samples are ones most likely to guarantee success.
1 of the goals of the 2020 rover mission is to identify samples for caching for eventual return to Earth.” http://news.ku.edu/2015/09/21/scientists-refine-hunt-mars-life-analyzing-rock-samples-western-us
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