Daniel Y Lo

Atmospheric O2 and CO at Mars

The Martian atmosphere is 95% CO₂. This CO₂ is continuously converted into CO, O₂ and O through photodissociation by high-energy solar ultraviolet (UV) radiation, and the 3 products can react with each other directly or indirectly to be converted back into CO₂, completing a cycle that maintains an equilibrium amount of CO₂, O₂, CO and O in the atmosphere. However, this equilibrium, and thus the processes maintaining this equilibrium, remain poorly understood. 1-D photochemical models have been underpredicting O₂ and CO abundances by as much as an order of magnitude relative to the observations, while 3-D global circulation models (GCMs) generally avoid the problem by picking an initial atmosphere consistent with the observations and then evolving the atmosphere while subjecting O₂ and CO to only transport effects, or taking advantage of the long photochemical lifetimes of the two species and stopping the models before instabilities set in.

In addition to model–observation disagreements on the equilibrium O₂ abundance, recent observations by the Sample Analysis at Mars (SAM) and ChemCam on the Mars Science Laboratory (MSL) Curiosity rover also found significant seasonal variability in O₂ abundances of as much as 50% about its mean value, much faster than the decadal timescale predicted by models. Analysis of the available oxygen sources at Mars suggests surface–atmosphere exchange could play a significant role in the overall balance of CO₂, CO, O₂ and O in the Martian atmosphere.

To solve this problem, we are 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 and how the models and observations disagree, and to explore what additional processes could resolve those disagreements.

Photochemical Escape of the Martian Atmosphere

Over the last 3.6 billion years ago, Mars has lost a significant fraction of its atmosphere, resulting in its transition from a warmer and wetter climate to the cold and dry environment today. One main process behind the loss of the Martian atmosphere is photochemical escape, where atmospheric species are broken up via photodissociation by solar UV radiation to form high-energy fragments, which can then escape Mars' gravity into space. Characterizing present-day escape rates is essential for building a foundation to reliably extrapolate 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. 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 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 still remains significantly less than what is measured by the Imaging UltraViolet Spectrograph (IUVS) instrument on the MAVEN spacecraft. This implies that present-day escape rates (and in turn the historical escape rates) are higher than what we are calculating from models, and we are currently exploring what can result in this discrepancy.

NASA Postdoctoral Program Fellow

N/A - GSFC

2026 - Present

Project Title: A New 4-Dimensional Multi-Instrument Perspective to an Old Problem: Mars Atmospheric O₂ and Its Variability

Advisor: Alain S. J. Khayat

Research Scientist

Space Science Institute - N/A

2024 - Present

Research Fellow

University of Michigan - Ann Arbor, Michigan

2021 - 2025

The Mystery of CO in the Martian Atmosphere

Mars Data Analysis Program - NASA (NNH24ZDA001N) - Awarded: 2024-05-13

Dates:  - 

Photochemical Escape of Atomic Carbon from Mars

Solar System Workings - NASA (NNH22ZDA001N-SSW) - Awarded: 2023-09-28

Dates:  -