One of the most vexing (and exciting) puzzles on Mars is that of the detection of methane in the red planet's atmosphere. As methane breaks down quickly when exposed to ultraviolet light from the sun, there must be a production mechanism below the Martian surface replenishing the organic gas as detected by Mars orbiters and astronomical observations from Earth.
But is that mechanism geological or biological in origin? In ongoing research at the University of Arkansas, there's a focus on the latter.
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During experiments on specific types of hardy microorganisms that produce methane, known as methanogens, researchers have been able to identify two species of the single-celled bacteria that could set up home under the frigid Mars regolith.
Although these microbes live on Earth, their life-cycle is very alien to our everyday experience. Methanogens do not require sunlight, oxygen or organic compounds to live, instead they metabolize hydrogen (for their energy source) and carbon dioxide (for their carbon source). As a waste product, these microbes generate methane. Methanogens are commonly found in the guts of cows and other animals and happily live in sub-surface environments.
Could extraterrestrial methanogen-like life be eking out an existence on Mars?
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"The surface temperature on Mars varies widely, often ranging between minus 90 degrees Celsius (-130 degrees Fahrenheit) and 27 degrees Celsius (80 degrees Fahrenheit) over one Martian day," said Rebecca Mickol, space and planetary sciences doctoral student at the University of Arkansas.
"If any life were to exist on Mars right now, it would at least have to survive that temperature range."
In the study, Mickol worked with Timothy Kral, who has been working on methanogens and their hypothetical Mars cousins since the 1990s, and identified two hardy methanogens - Methanothermobacter wolfeii and Methanobacterium formicicum - subjecting them to the extreme freeze-thaw conditions they'd experience on Mars. They survived.
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"The survival of these two methanogen species exposed to long-term freeze/thaw cycles suggests methanogens could potentially inhabit the subsurface of Mars," said Mickol. The two methanogens were selected as one is a hyperthermophile and the other is a thermophile, meaning they survive in extremely hot environments (such as geothermal vents) and warm environments, respectively.
During the experiments, the methanogens weren't exactly happy campers, but they survived.
"The low temperature on Mars inhibited their growth, but they survived," said Mickol. "Once they got back to a warm temperature, they were able to grow and metabolize again. I wanted to see if these cold temperatures would kill them, or if they were able to survive and adapt."
Although this research doesn't identify how life may have been sparked on Mars, nor does it suggest that there is life on Mars, it does provide a clue how life can find a way even in the most extreme environments and identifies a possible methane production mechanism on the red planet.
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We know that Mars was once warmer and wetter than it is now, so it's conceivable that basic lifeforms may have taken a foothold in Mars' ancient past only to have been forced underground as the planet's atmosphere thinned, cooled and dried. As it turns out, Earth hosts methanogens that can survive Mars' extremes, does Mars also have similar microbes that are currently hiding, slowly metabolizing and waiting for warmer days?
Fascinatingly, observations of the Martian atmosphere suggests that the methane concentrations are confined to certain regions and may be a seasonal phenomena - a pattern that may hint at subsurface populations of Mars-methanogens taking advantage of the slight temperature variations throughout the Martian year.
This puzzle has only been complicated by NASA's Mars rover Curiosity that has not detected atmospheric methane inside Gale Crater - obviously more work is needed.
Source: University of Arkansas via Physorg.com