Based on data gathered by NASA’s Curiosity rover, researchers now report that the robot has discovered a local source of nitrogen that is “biochemically accessible” and is one of the key ingredients necessary to support the existence of life forms on ancient Mars, if they ever existed.
Finding an indigenous source of nitrogen “has significant implications for the habitability of Mars in the past and present,” Jennifer Stern of NASA’s Goddard Space Flight Center in Greenbelt, Md., told AmericaSpace.
Curiosity has detected an indigenous source of nitrogen (N2) in the form of nitric oxide (NO) that was released from the breakdown of nitrate (NO3) during heating of soil and rock sediments, gathered from the surface of Gale crater and carefully analyzed using the Sample Analysis at Mars (SAM) instrument inside the rover’s belly.
Stern is the lead researcher of a new paper about Curiosity’s SAM results published in the Proceedings of the National Academy of Science on March 23.
Nitrogen is essential for all known life forms and is one of the main elemental constituents of amino acids, as well as DNA and RNA, the key building blocks of life on Earth that encode the genetic materials that provide the instructions and basis for life.
“Life as we know it, on Earth, needs a source of nitrogen for biomolecules such as amino acids,” Stern elaborated to AmericaSpace.
The nitrogen in nitrate molecules “can be used by living organisms and may indicate the first stage in development of a primitive nitrogen cycle on the surface of ancient Mars and would have provided a biochemically accessible source of nitrogen.”
The Martian sediment samples in which the nitrates were found in the study were scooped up from windblown sand at the “Rocknest” ripple and drilled from mudstone deposits at the “John Klein” and “Cumberland” work sites in Yellowknife Bay during the first year of Curiosity’s mission on the Red Planet.
The SAM results from the two drill sites indicate the presence of the equivalent of up to 1,100 parts per million (ppm) (0.11 wt%) nitrates in the Martian soil.
Overall, the new discovery adds significantly to the body of evidence that liquid water once flowed on ancient Mars and was habitable for life.
“By itself, detection of nitrate does not have biological significance. Nitrate however, has significant implications for the habitability of Mars in the past and present,” Stern stated.
Curiosity had already detected the presence of significant amounts of phyllosilicate clay minerals in the sediments gathered by the drill samples at “John Klein” and “Cumberland,” proving that Mars possessed the ingredients necessary for a habitable zone at Yellowknife Bay. The rover also detected some organic molecules.
Based on all the data, the science team says that Yellowknife Bay is a dried out lakebed and that the mudstone deposits formed from sediment deposited at the bottom of a lake inside Gale Crater in the ancient past.
Clay minerals form in more neutral water more conducive to the formation of life compared to the more harshly aqueous acidic environments found in certain other regions investigated on the Red Planet.
“Detection of nitrate at the same site where we also find clay minerals that indicate sustained interaction with liquid water, indicates that nitrogen was available in a form that could be assimilated by biology, should it have been present,” Stern explained.
Nitrate is a source of fixed nitrogen.
How did the nitrate form on Mars?
“We know that atmospheric photochemistry can make nitrate, and this happens on Earth (although on Earth, most nitrate is made via bacterial nitrification).”
“We believe that on Mars, the source of energy to break the triple bond in N2 and ultimately make nitrate is either thermal shock from impacts in the past, or ongoing photochemistry, or a combination of both,” Stern replied.
So the nitrates discovered so far have a non-biological origin from meteorite strikes or lightning eons ago on Mars.
There is no evidence to suggest that the fixed nitrogen molecules found by the team were created by life, notes NASA. And there is no evidence that any life forms—past or present—have been detected on Mars.
“Discovery of indigenous Martian nitrogen in Mars surface materials has important implications for habitability and, specifically, for the potential evolution of a nitrogen cycle at some point in Martian history,” according to the PNAS paper.
“Finding a biochemically accessible form of nitrogen is more support for the ancient Martian environment at Gale Crater being habitable,” says Stern.
So far the team has not found evidence for an complex Earth-like nitrogen cycle in the Yellowknife Bay samples analyzed thus far.
“Despite the fact that these ancient aqueous environments represented by Yellowknife Bay sediments on Mars once had a high potential for habitability, our analysis of these sediments does not support the presence of a complex N cycle analogous to that which drives life on Earth.”
Stern told me that the team is looking for evidence of a biologically related nitrogen cycle on Mars in the form of molecular species, such as ammonium ions (NN4+) and amines.
“To say anything about a more complex nitrogen cycle, and thus, possible cycling of nitrogen by biology, we would have to find other intermediate N species, such as NH4+, which, on Earth, is fixed directly from atmospheric N2 by bacteria, prior to nitrification.”
“We would also need to find more complex N molecules, such as amines, to indicate that nitrogen was being taken up by biological processes.”
To date, none have been found, but that’s part of the ongoing research!
“So far, we have not found these other compounds, but we will keep looking,” Stern said
“Our results suggest that the search for stratigraphic evidence of an ancient martian N [nitrogen] cycle should continue.”
As of today, the rover is still investigating a beautiful patch of mineral veins at the Garden City outcrop at the base of Mount Sharp, as outlined in my recent story here.
Mount Sharp is comprised of sedimentary rock layers that record the history of ancient Martian environments and is the primary destination of the mission.
The mountain towers 3.4 miles (5.5 kilometers) into the Martian sky and dominates the center of the Gale Crater landing site, where Curiosity safely touched down on Aug. 5, 2012.
As of today, Curiosity’s odometer totals over 6.4 miles (10.3 kilometers) since landing inside Gale Crater on Mars in August 2012. She has taken some 226,714 images during over 945 Sols of exploration.
Stay tuned here for continuing updates from Mars and throughout our Solar System!
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