Novel theory explains carbon levels in the modern Martian atmosphere
Scientistsbelieve that Mars once played host to a much warmer and wetterclimate, but for that to be the case it must have once had a thickeratmosphere. There's a big problem with that theory, though, withdetected levels of carbon not playing nice with atmospheric losstheories. Now, a joint team from NASA's Jet Propulsion Laboratory(JPL) and the California Institute of Technology (Caltech) believes it may havesolved the problem, with a new theory that explains the issue bymeans of two simultaneous mechanisms.
Broadlyspeaking, there are two ways in which Mars could have lost carbondioxide from its atmosphere. The first is by it having been slowlyincorporated into rocks known as carbonates. While this was areasonable theory for some time, data gathered from numerous orbiterssuggested that there are nowhere near enough carbonates in theupper half mile (0.8 km) of the crust to account for the level ofcarbon that would have been present in the ancient atmosphere.
The secondpossibility is that the carbon escaped out into space due to interactions between solar winds and the upper atmosphere – a process known as sputtering. Results from NASA's Mars Atmosphere and Volatile Evolution mission (MAVEN) recently found that the planet is currentlylosing around 100 g (3.5 oz) of particles each and every second,strongly indicating that the majority of the historic atmosphericloss occurred in this way.
While that mightseem like the end of the issue, things get much more complicated whenyou start to look in detail at carbon isotope levels. The processesthat are determined to have led to the current state of theatmosphere must line up with how the levels of carbon-12 andcarbon-13 (isotopes that differ in the number of neutrons present ineach nucleus) in order for a theory to be deemed correct. Otherwise,we're missing a piece of the puzzle.
The change inthe ratios between the isotopes was determined by analyzingmeteorites containing volcanically released gases from deep withinplanet – the original atmospheric condition – and modern readingstaken by the Sample Analysis at Mars (SAM) instrument on NASA's Curiosity rover.
The sputteringprocess, as observed by MAVEN, tends to leave behind slightly more ofthe isotope carbon-13 than carbon-12. However, readings of the modernMartian atmosphere show far higher levels of carbon-13 than should bepresent if sputtering was solely responsible for the atmosphericloss.
To account forthat discrepancy, the team added a second mechanism to its model,known as ultraviolet photodissociation. This process involves UVlight hitting carbon dioxide molecules in the atmosphere, splittingthem into oxygen and carbon monoxide. The light then strikes thecarbon monoxide component and separates it into carbon and oxygen.Not only would the process give some carbon atoms enough energy toescape the atmosphere, but the lighter carbon-12 isotopes are farmore likely to escape than the weightier carbon-13.
The amount ofcarbon that escapes due to this process is minimal, but when it'seffects are plotted on such a long timescale, it makes a big impact,accounting for the previously unexplained carbon isotopic ratio.Overall, the new theory solves one of the big mysteries about the RedPlanet.
"Our papershows that transitioning from a moderately dense atmosphere to thecurrent thin one is entirely possible," says lead author Renyu Hu."It is exciting that what we know about the Martian atmosphere cannow be pieced together into a consistent picture of its evolution –and this does not require a massive undetected carbon reservoir."
The findings ofthe research were published in the journal Nature Communications.