The more scientists learn about Mars, the more intriguing the Red Planet becomes as a potential haven for primitive life in the ancient past ... and perhaps even the present.
A study released today (March 23) reports that ancient Mars harbored a form of nitrogen that could potentially have been used by microbes, if any existed, to build key molecules such as amino acids. An unrelated study suggests that atmospheric carbon monoxide has been a feasible energy source for microbes throughout the Red Planet's history. Both papers were published today in the journal Proceedings of the National Academy of Sciences (PNAS).
"It's more support for this environment that would have had the ingredients that life would have needed," said Jennifer Stern of NASA's Goddard Space Flight Center in Greenbelt, Maryland, lead author of the nitrogen study. [The Search for Life on Mars: A Photo Timeline]
All life on Earth requires nitrogen, which is a critical component of amino acids and other biomolecules. But microbes can't just pull their nitrogen straight out of the air; atmospheric, or molecular, nitrogen (N2) features two atoms of the stuff linked in a tight triple bond, making it relatively inert and inaccessible.
Before life-forms can incorporate nitrogen into their metabolic processes, that bond must be broken; nitrogen must be "fixed" into different, more chemically reactive compounds, such as nitrate (NO3).
That process did indeed occur on Mars, Stern and her team reported in their study, which looked at measurements made by the Sample Analysis at Mars (SAM) instrument aboard NASA's Mars rover Curiosity.
SAM found significant concentrations of nitrate in soil and rock samples that Curiosity collected at three different spots near its landing site — Rocknest, John Klein and Cumberland.
The John Klein and Cumberland samples, which were drilled from a sedimentary mudstone, had previously allowed rover team members to conclude that, billions of years ago, the area was part of a potentially life-supporting lake-and-stream system. The discovery of fixed nitrogen contributes to this habitability picture.
"Had life been there, it would have been able to use this nitrogen," Stern told Space.com. [Ancient Mars Could Have Supported Life (Photos)]
Not necessarily signs of Mars life
While much of the nitrogen fixation on Earth is biological, Curiosity's discovery isn't evidence of Martian life. Nitrogen-nitrogen bonds can also be broken by the thermal shocks caused by lightning and asteroid or comet impacts.
Indeed, the Red Planet's fixed nitrogen may have been generated primarily by the numerous powerful impacts that occurred (on Mars and other bodies in the inner solar system) about 4 billion years ago, during a period known as the Late Heavy Bombardment, Stern said.
But nitrogen may get fixed on modern Mars as well. In 2005, Europe's Mars Express orbiter detected nitrogen oxide (NO) high in the Red Planet's atmosphere. It likely formed after sunlight split apart oxygen, carbon dioxide and molecular nitrogen, Stern and her co-authors said.
"This suggests that N is currently being fixed in the Martian thermosphere, although it is unknown how much, if any, is transported to the lower atmosphere and surface," the researchers wrote in their PNAS paper.
Curiosity hasn't been able to get to the bottom of this question so far.
"Right now, our experiment is not targeted to get us a nitrate signal big enough to get, for example, any nitrogen isotope data," Stern said. (Isotopes are versions of an element that contain different numbers of neutrons in their nuclei.)
"If you had a nitrogen isotope composition of the modern [Martian] atmosphere, which would be very different than the primordial atmosphere, that would tell us about whether it was being formed today or not," she added. "So it would be great to be able to target an experiment where we could get enough of a signal in the instrument to get that data."
Energy source for Martian life?
Life as we know it needs certain basic building chemical blocks (such as carbon and fixed nitrogen), liquid water and an energy source. In the other new PNAS paper, Gary King of Louisiana State University suggested that carbon monoxide (CO) could serve as an energy source on Mars, from ancient epochs all the way up to the present day.
While CO is toxic to many organisms, including humans, here on Earth, some microbes use it to drive their metabolism, gaining energy by oxidizing the substance into carbon dioxide (CO2).
Such life-forms are taking advantage of a relatively scarce resource, as Earth's atmosphere is just 0.3 parts per million (ppm) or so CO by volume. The Martian atmosphere, in comparison, contains 800 ppm CO currently, and concentrations of the stuff may have been much higher in the past. Therefore, CO seems like a plausible candidate for an energy source for Mars life, but the possibility hasn't drawn much scholarly attention, King wrote in the PNAS paper.
King set out to determine if Earth microbes could indeed utilize CO under conditions approximating those found on the modern Martian surface — low pressure, high CO2 concentrations (CO2 makes up 95 percent of the Red Planet's atmosphere), low oxygen levels and low to moderate temperatures, among other characteristics.
King specifically targeted the conditions that might prevail at features known as recurring slope lineae (RSL), seasonal dark streaks that have been observed by NASA's Mars Reconnaissance Orbiter in a number of locales. Some scientists think these streaks are caused by salty water at or near the Red Planet's surface.
He found that soil samples collected in three different salty systems on Earth — the Big Island of Hawaii, Chile's Atacama Desert and the Bonneville Salt Flats in Utah — did indeed take up CO under putative RSL conditions.
Other experiments, using the salt-loving (halophilic) microbes Alkalilimnicola ehrlichii MLHE-1 and Halorubrum str. BV1, demonstrated this capacity at the organismal level. A. ehrlichii MLHE-1, in fact, tolerated concentrations of the chemical perchlorate similar to those found in Martian soil.
"These results collectively establish the potential for microbial CO oxidation under conditions that might obtain at local scales (e.g., RSL) on contemporary Mars and at larger spatial scales earlier in Mars' history," King wrote in the new study.
King believes his results are also relevant to discussions of the human exploration of Mars. CO-oxidizing organisms such as A. ehrlichii MLHE-1 could be part of an effort to transform the Red Planet into a place more hospitable to humans, he said.
"In order to develop any kind of a soil system that could support anything complex, you would have to have a complex microbial community," King told Space.com.
"You would need a variety of biosynthetic capabilities. You would need a variety of different elemental transformation capabilities — maybe nitrogen fixers," he added. "These halophiles would be part of that."
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Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.