The quantum yield of O(

Photochemistry studies the interactions between light and molecules. Ultraviolet radiation interacts with the atmosphere, and due to its energy, it can dissociate, excite, or ionize its constituents, which initiate other processes. A good knowledge of the interaction between photons of different energies with molecules and atoms is crucial for accurately modeling the atmospheric physics and for climate predictions. Despite its importance, photo-fragment dynamics lacks data because the experimental setup is difficult. We used the upper Martian atmosphere as a natural laboratory to measure the quantum yield O(1S) from CO2 + hv as a function of wavelength. We analyzed 4 years of continuous remote-sensing observations from the NASA MAVEN/IUVS spectrograph within a Bayesian framework analysis tool. We retrieved the quantum yield for the first time through its entire production spectral range, ≈80–126 nm, and achieved uncertainty from 10% to 20% on average. While at Lyman-α (121.6 nm), we achieved a precision of 2% by taking advantage of the properties of the upper Martian atmosphere.