1. Stability of the Martian Atmosphere

Since the earliest measurements of the Martian atmosphere in the 1960s, we have known its composition to be 95% carbon dioxide (CO₂). However, we still do not understand the processes that keep it at 95%. Initially, the measurement of 95% CO₂ actually came as a surprise. In a process known as photodissociation, this CO₂ is broken up by high-energy ultraviolet (UV) radiation from the Sun into CO and O, with the O atoms then combining to form O₂. O₂ itself can be photodissociated back into O, maintaining an equilibrium amount of O₂ in the atmosphere. The only known reaction that could convert CO and O (or O₂) back into CO₂ was the spin-forbidden and slow CO + O + M → CO2 + M (M being a third species for momentum conservation), and that would have resulted in equilibrium abundances of 4% O₂ and 8% CO, much higher than the observed 0.2% and 0.08% respectively. Shortly after, two new catalytic cycles were discovered [1, 2], enabling a speedy conversion of CO and O₂ back into CO₂ through reactions with OH and HO₂, which are derived from photodissociation of atmospheric water vapor (H₂O). With these new reactions however, the conversion back into CO₂ is so efficient that models now have the opposite problem of underpredicting O₂ and CO by as much as 10x. Decades of research and new observations after, this problem of the "stability of the Martian atmosphere" still remains unresolved.

In addition to disagreements between our latest models and the observations about the long-term equilibrium abundance of O₂, recent observations by both the Quadrupole Mass Spectrometer (QMS) and the ChemCam instruments on the Mars Science Laboratory (MSL) Curiosity rover found that the O₂ abundance can change by as much as 50% within months, much faster than the decadal timescale that the reactions we know in the Martian atmosphere to operate over. We are currently making new measurements and analyzing data from across multiple missions (including near the surface with MSL, in the middle atmosphere with the Trace Gas Orbiter (TGO), and in the upper atmosphere with MAVEN and the Emirates Mars Mission (EMM)) to pinpoint more precisely where in the Martian atmosphere the models and observations disagree, and to figure out what additional processes could control the CO₂, O₂, and CO abundances.

2. Methane at Mars

Methane (CH₄) in the Martian atmosphere has been controversial since its first detection in 2004. Today, the most extensive set of methane detections comes from the Tunable Laser Spectrometer (TLS) on the Curiosity rover, indicating an increase up to 0.7 parts per billion in concentration in the middle of the Martian year. Most of these TLS measurements were made at night, and interestingly, two daytime measurements made at the supposed seasonal peak did not detect any methane. TGO measurements over the same period of time has consistently not detected any methane, although the measurements were higher in the atmosphere (>10 km) and made in the day. Currently, we are studying the implications behind these seemingly contradictory datasets, and what new measurements can be made to provide more clues to this mystery.

3. Evolution of the Martian Atmosphere

More than 3.6 billion years ago, Mars had liquid water on its surface. Just like water on Earth today, this water carved the ancient Martian landscape, forming rivers, depositing deltas and collecting in standing lakes. There were even occasional waterfalls. Water was so prevalent then that this period of Martian history is referred to as the "Noachian Period" after the biblical flood.

Modern Mars, however, is exceedingly dry. Gone are the rivers and the lakes. With an average surface temperature of -60 ^oC, H₂O on Mars now mostly exists as ice in the polar caps and the subsurface. Although the atmosphere is 95% CO₂, the surface pressure of 6 millibars today is too low to produce sufficient greenhouse heating. For Mars to have been warm enough in the past to support liquid water on the surface, hundreds of millibars of CO₂ would have been required, and it is believed that most of this CO₂ has been lost to space.

To understand the past, we must first understand the present.

My research focuses on the processes behind the loss of the Martian atmosphere. One main process is photochemical escape. Photodissociation of CO₂ produces highly energetic atomic C and O fragments, which can then escape into space. Through my research, I have found the previously-unknown CO₂ photodissociation into C and O₂ to be a major contributor to the escape of C atoms from the Mars atmosphere today, making up as much as 60% of the total photochemical escape. However, even after including this process, the predicted amount of C in the atmosphere is significantly less than what is measured by the Imaging UltraViolet Spectrograph (IUVS) instrument on the MAVEN spacecraft. This implies that present-day escape rates are higher than what we are calculating from models, and we are currently exploring what can result in this discrepancy. These results about atmospheric loss from modern Mars is the foundation for extrapolating the various loss processes back in time to ancient Mars, which had a thicker and warmer atmosphere, and orbited a younger Sun that emitted more intense UV radiation.

4. The Saguaro Photochemical Model

I am also the maintainer for the Saguaro photochemical model, which I use for my research into Mars atmospheric composition and photochemistry. Incorporating the latest reaction constants and cross sections, Saguaro calculates the composition of the entire atmosphere from the surface to the exobase, and the rates of all known reactions that maintain or drive changes in the composition. Saguaro treats the atmosphere as 1-dimensional, enabling quick turnarounds when testing possible new processes that I can easily introduce, at the expense of inaccuracies in describing horizontal transport and circulation. The model is available on my Github.