Where does oxygen come from on Mars? (3 photos)

When NASA's Curiosity rover, located on the Red Planet since August 6, 2012, detected the presence of molecular oxygen (O2) in the thin atmosphere, it came as a surprise.





Old Curiosity taking a "selfie." Image in approximately natural colors.

Today, it is known that oxygen accounts for only about 0.13–0.16% of the Martian atmosphere, which is 95% carbon dioxide (CO2). This is negligible by Earth standards, but its presence deserves special attention.

So, where did O2, whose reserves are being replenished, come from on a cold, dry planet with no plants, algae, or any other organisms* capable of photosynthesis?

*To date, there is no convincing evidence of life on Mars. Therefore, we proceed from this fact.

Atmospheric Photochemistry

Research shows that the main source of oxygen on Mars is photochemical processes occurring in the upper atmosphere. Under the influence of ultraviolet radiation from the Sun, CO2 molecules and a small amount of water vapor (H2O) disintegrate, releasing oxygen atoms. Some of these atoms briefly combine to form O2 molecules, which, after a series of photochemical reactions, are again bound together as CO2.

This model perfectly explains the presence of oxygen in the Martian atmosphere. This article could have ended here, but...

Mysterious Seasonal Variations

In 2019, Curiosity, continuing its work in Gale Crater, discovered that atmospheric oxygen fluctuations exhibit a stronger seasonal dependence than predicted by the photochemical model. For example, O2 levels increase by almost 30% during the spring and summer, returning to baseline levels in the fall and winter.

In an attempt to explain this anomaly, scientists have put forward two hypotheses:

The Role of Martian Soil

Martian regolith is saturated with perchlorates—salts containing bound oxygen. Laboratory experiments coupled with modeling show that, when exposed to radiation, such compounds can decompose, releasing reactive oxygen-containing products, including molecular oxygen.



Whether this mechanism can fully explain the observed seasonality remains unknown, but it certainly fits the role of a potential source of "additional" oxygen.

Subsurface Water and Radiolysis

The second hypothesis involves subsurface ice deposits and the possible presence of brine pockets—localized accumulations of water with very high concentrations of dissolved salts, allowing it to remain liquid even at very low temperatures.

Given that Mars lacks a robust magnetosphere and a dense atmosphere, large amounts of cosmic radiation reach the surface. This radiation, penetrating the soil, can break down water molecules—a process known as radiolysis. This results in the formation of oxygen-containing compounds that can contribute to the observed seasonal variability of oxygen in the Martian atmosphere.

However, there is no direct evidence that this process significantly influences the seasonal O2 content of the Martian atmosphere.

Could it be life?

Despite the fact that oxygen is traditionally considered one of the best biomarkers, its concentrations on Mars are too low to attribute it to a biological origin. What has been recorded fits easily within the "abiogenic framework"—photochemistry, surface chemistry, and radiation chemistry.



Curiosity, staring straight into the soul

Therefore, it's not the presence of O2 itself that interests scientists, but its unusual behavior. Understanding this mechanism will allow us to better understand the chemical processes occurring on the Red Planet today. Consequently, this will help refine our assessments of how habitable Mars might have been in the past.

Ultimately, the search for possible traces of Martian life must begin with understanding whether our neighboring planet ever had conditions suitable for its formation. And so far, there's no definitive answer.