For some time, scientists have suspected that life may have existed on Mars in the deep past. Owing to the presence of a thicker atmosphere and liquid water on its surface, it is entirely possible that the simplest of organisms might have begun to evolve there. And for those looking to make Mars a home for humanity someday, it is hoped that these conditions (i.e favorable to life) could be recreated again someday.
But as it turns out, there are some terrestrial organisms that could survive on Mars as it is today. According to a recent study by a team of researchers from the Arkansas Center for Space and Planetary Sciences (ACSPS) at the University of Arkansas, four species of methanogenic microorganisms have shown that they could withstand one of the most severe conditions on Mars, which is its low-pressure atmosphere.
The study, titled "Low Pressure Tolerance by Methanogens in an Aqueous Environment: Implications for Subsurface Life on Mars," was recently published in the journal Origins of Life and Evolution of Biospheres. According to the study, the team tested the survivability of four different types of methanogens to see how they would survive in an environment analogous to the subsurface of Mars.
Methanogenic organisms that were found in samples of deep volcanic rocks along the Columbia River and in Idaho Falls. Credit: NASA
To put it simply, Methanogens are ancient group of organisms that are classified as archaea, a species of microorganism that do not require oxygen and can therefore survive in what we consider to be "extreme environments". On Earth, methanogens are common in wetlands, ocean environments, and even in the digestive tracts of animals, where they consume hydrogen and carbon dioxide to produce methane as a metabolic byproduct.
And as several NASA missions have shown, methane has also been found in the atmosphere of Mars. While the source of this methane has not yet been determined, it has been argued that it could be produced by methanogens living beneath the surface. As Rebecca Mickol, an astrobiologist at the ACSPS and the lead author of the study, explained:
"One of the exciting moments for me was the detection of methane in the Martian atmosphere. On Earth, most methane is produced biologically by past or present organisms. The same could possibly be true for Mars. Of course, there are a lot of possible alternatives to the methane on Mars and it is still considered controversial. But that just adds to the excitement."
As part of the ongoing effort to understand the Martian environment, scientists have spent the past 20 years studying if four specific strains of methanogen - Methanothermobacter wolfeii, Methanosarcina barkeri, Methanobacterium formicicum, Methanococcus maripaludis - can survive on Mars. While it is clear that they could endure the low-oxygen and radiation (if underground), there is still the matter of the extremely low air-pressure.
Graduate students Rebecca Mickol and Navita Sinha prepare to load methanogens into the Pegasus Chamber housed in W.M. Keck Laboratory. Credit: University of Arkansas
With help from the NASA Exobiology & Evolutionary Biology Program (part of NASA's Astrobiology Program), which issued them a three-year grant back in 2012, Mickol and her team took a new approach to testing these methanogens. This included placing them in a series of test tubes and adding dirt and fluids to simulate underground aquifers. They then fed the samples hydrogen as a fuel source and deprived them of oxygen.
The next step was subjecting the microorganisms to pressure conditions analogues to Mars to see how they might hold up. For this, they relied on the Pegasus Chamber, an instrument operated by the ACSPS in their W.M. Keck Laboratory for Planetary Simulations. What they found was that the methanogens all survived exposure to pressures of 6 to 143 millibars for periods of between 3 and 21 days.
This study shows that certain species of microorganisms are not dependent on a the presence of a dense atmosphere for their survival. It also shows that these particular species of methanogens could withstand periodic contact with the Martian atmosphere. This all bodes well for the theories that Martian methane is being produced organically - possibly in subsurface, wet environments.
This is especially good news in light of evidence provided by NASA's HiRISE instrument concerning Mars' recurring slope lineae, which pointed towards a possible connection between liquid water columns on the surface and deeper levels in the subsurface. If this should prove to be the case, then organisms being transported in the water column would be able to withstand the changing pressures during transport.
The possible ways methane might get into Mars' atmosphere, ranging from subsurface microbes and weathering of rock and stored methane ice called a clathrate. Ultraviolet light can work on surface materials to produce methane as well as break it apart into other molecules (. Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan
The next step, according to Mickol is to see how these organisms can stand up to temperature. "Mars is very, very cold," she said, "often getting down to -100ºC (-212ºF) at night, and sometimes, on the warmest day of the year, at noon, the temperature can rise above freezing. We'd run our experiments just above freezing, but the cold temperature would limit evaporation of the liquid media and it would create a more Mars-like environment."
Scientists have suspected for some time that life may still be found on Mars, hiding in recesses and holes that we have yet to peak into. Research that confirms that it can indeed exist under Mars' present (and severe) conditions is most helpful, in that it allows us to narrow down that search considerably.
In the coming years, and with the deployment of additional Mars missions - like NASA's Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander, which is scheduled for launch in May of next year - we will be able to probe deeper into the Red Planet. And with sample return missions on the horizon - like the Mars 2020 rover - we may at last find some direct evidence of life on Mars!
Further Reading: Astrobiology Magazine, Origins of Life and Evolution of Biospheres
Posts
No comments:
Post a Comment