Over the past couple of years, Paul Godin has been leading an effort in my group to understand the warming potential of the ancient martian atmosphere, above he shows experimentally-derived values for CO2-CH4 CIA as measured using the Canadian Light Source. He just submitted a paper on this topic which is now under review.
By Dr. Paul Godin
We’ve discussed in previous blogposts about our group’s effort to better constrain the early Mars atmosphere by taking measurements at the Canadian Light Source (http://york-pvl.blogspot.com/2018/11/searching-for-liquid-water-on-mars-at.html and http://york-pvl.blogspot.com/2019/04/the-continuing-adventures-at-canadian.html). As a quick summary, geological features on the surface of present-day Mars imply that there was once liquid water on the surface. To have liquid water on the surface, a sufficiently strongly absorbing atmosphere is required to produce enough of a greenhouse effect to warm the surface above freezing temperatures. Since most ancient Mars modeling suggest that Mars did not have a dense atmosphere, the remaining possibility is that the gas composition of an ancient Mars atmosphere could be strongly absorbing. One idea was collision induced absorption (CIA) between CO2 and H2 molecules, and CO2 and CH4 molecules, could provide enough absorption to warm ancient Mars. The goal of the CLS trips was to experimentally measure this CIA effect to determine if it was as strong as predicted.
Well, it turns out two trips to the CLS was sufficient, and we have enough data to calculate the absorption cross-sections which are shown in the figure at the top of this article for CO2-CH4 CIA and below for CO2-H2 CIA.
In the images the solid black lines are our measurements from the CLS, the dotted red line the original theoretical prediction, the solid red line is the prediction scaled by a factor of 0.5 to provide better agreement with the results from this work, and the blue dotted line is another experimental measurement made by Turbet et al. Uncertainty is represented by the shading around the experimentally derived measurements.
The primary result of these investigations is that the CIA effect was half the strength that was originally predicted, as shown by the agreement between the solid red and black lines in the above images. But what does that mean for ancient Mars? To answer that we partnered with notable Mars climate modeler, Ramses Ramirez from the Tokyo Institute of Technology, who produced climate simulations of possible early Mars atmospheres using our CIA cross-sections.
He found that to achieve liquid water, an atmosphere of at least 3-bar total pressure that contained 10% methane was needed. For hydrogen, liquid water is possible with 5% hydrogen in 2-bar of surface pressure atmosphere. Ultimately this would suggest that methane is unlikely to provide sufficient warming, since a 3-bar atmosphere is beyond what most ancient Mars atmosphere simulations predict. However, a 2-bar atmosphere is plausible, thus warming due to CO2-H2 remains a valid theory.
Our paper on these results is currently undergoing peer-review. However, a pre-print of our initial draft can be found online at: https://www.essoar.org/doi/pdf/10.1002/essoar.10501589.1
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