Scientists believe that Mars once played host to a much warmer and wetter climate, but for that to be the case it must have once had a thicker atmosphere. There's a big problem with that theory, though, with detected levels of carbon not playing nice with atmospheric loss theories. Now, a joint team from NASA's Jet Propulsion Laboratory (JPL) and the California Institute of Technology (Caltech) believes it may have solved the problem, with a new theory that explains the issue by means of two simultaneous mechanisms.
Broadly speaking, there are two ways in which Mars could have lost carbon dioxide from its atmosphere. The first is by it having been slowly incorporated into rocks known as carbonates. While this was a reasonable theory for some time, data gathered from numerous orbiters suggested that there are nowhere near enough carbonates in the upper half mile (0.8 km) of the crust to account for the level of carbon that would have been present in the ancient atmosphere.
The second possibility 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 currently losing around 100 g (3.5 oz) of particles each and every second, strongly indicating that the majority of the historic atmospheric loss occurred in this way.
While that might seem like the end of the issue, things get much more complicated when you start to look in detail at carbon isotope levels. The processes that are determined to have led to the current state of the atmosphere must line up with how the levels of carbon-12 and carbon-13 (isotopes that differ in the number of neutrons present in each nucleus) in order for a theory to be deemed correct. Otherwise, we're missing a piece of the puzzle.
The change in the ratios between the isotopes was determined by analyzing meteorites containing volcanically released gases from deep within planet – the original atmospheric condition – and modern readings taken by the Sample Analysis at Mars (SAM) instrument on NASA's Curiosity rover.
The sputtering process, as observed by MAVEN, tends to leave behind slightly more of the isotope carbon-13 than carbon-12. However, readings of the modern Martian atmosphere show far higher levels of carbon-13 than should be present if sputtering was solely responsible for the atmospheric loss.
To account for that discrepancy, the team added a second mechanism to its model, known as ultraviolet photodissociation. This process involves UV light hitting carbon dioxide molecules in the atmosphere, splitting them into oxygen and carbon monoxide. The light then strikes the carbon monoxide component and separates it into carbon and oxygen. Not only would the process give some carbon atoms enough energy to escape the atmosphere, but the lighter carbon-12 isotopes are far more likely to escape than the weightier carbon-13.
The amount of carbon that escapes due to this process is minimal, but when it's effects 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 Red Planet.
"Our paper shows that transitioning from a moderately dense atmosphere to the current thin one is entirely possible," says lead author Renyu Hu. "It is exciting that what we know about the Martian atmosphere can now be pieced together into a consistent picture of its evolution – and this does not require a massive undetected carbon reservoir."
The findings of the research were published in the journal Nature Communications.