Chemistry & Climate in Mars’ Ancient Atmosphere

Unlike the Earth, which is continuously eroded by plate tectonics, Mars’ surface has uniquely preserved clues about the planet’s history from 4 billion years ago. From carved river beds to hydrated clays, profound evidence suggests that an ancient Mars hosted surface liquid water, but this would have required a much thicker and warmer atmosphere than Mars has today. How has Mars evolved over time? What kept early Mars warm, and was the planet ever habitable?

Water could have escaped away to space over time, supported by heavy isotopes left behind in the atmosphere and by present-day escape rates. Escape cannot explain all of the water loss though; crustal hydration is also important! (See our Science paper & press release from Caltech, NASA, & others.)

The loss of water to the surface (crustal hydration) could helped keep early Mars warm. My 2025 paper explains that crustal hydration released large amounts of hydrogen to the atmosphere, capable of warming the climate in episodes. Each warm episode was likely at least 100,000 years, and this process cumulatively could have warmed Mars for up to 40 million years. Warm-cold periods likely experienced very different atmospheric chemistries, with warm periods more reduced (H2-rich CO2 atmospheres) and cold periods more oxidized (CO2-rich and H2-depleted). A cartoon of the chemistry driving these atmospheres is shown on the right. For details, see my Nature Geoscience paper & press release from Harvard, Phys.org, UniverseToday, and others.

This finding is important for several reasons! (1) The Faint Young Sun Paradox (Sagan, 1976) is not unique to Mars and remains an outstanding question for early Earth too. Understanding what kept early Mars warm could help us learn about early Earth’s environment. (2) Redox gradients on early Earth’s surface are thought to be favorable for the emergence of life. What these hypothesized changes mean for Mars’ habitability remains an open question.

So far that’s just a hypothesis. How can we test it?

Minerals and salts on Mars’ surface today have locked in clues about the ancient atmosphere they formed in. These include (1) nitrates, (2) sulfates, and (3) phosphates, hydrated minerals, and carbonates.

(1) My work explains how nitrates could deposit in H2-rich warm climates (see my paper in Astrobiology, recently summarized in Scientific American), and in a follow up paper, I posited that some NO-measurements may come from nitrite salts formed during cool climates (see my GRL paper.) With returned samples from Mars, we could test for these differences. We could also use nitrogen isotopes to estimate the age of formations (see Jaylen Shawcross’s paper, which I co-mentored). Fun tangent from this story: nitrates are also relevant to the emergence of life at early Earth (see Maddy Christensen’s paper, which I co-mentored).

(2) Changes in atmospheric redox are testable with sulfur isotopes: their relative abundance can change in a predictable mass-dependent way from geology, but mass independent fractionation (S-MIF, or when isotopes depart from that linear relationship) primarily results from sunlight splitting SO2. The S-MIF signal is sensitive to the atmosphere the sulfur reacts in -- for large S-MIF signals to be preserved, both oxidized (SO2 or H2SO4) and reduced (OCS or S8) species must deposit. We show this happens and leaves large S-MIF signals when SO2 reacts in atmospheres that have an Earth-like rate of H2 entering from the surface (1e10 H2/cm2/s). However, CO2 atmospheres alone deposit mostly sulfate with no S-MIF and more H2-rich atmospheres deposit smaller S-MIF signals in OCS and sulfate. Our model results may be used to interpret future returned samples from Mars to test whether H2 helped warm the climate. This paper has been submitted to JGR:P. 

(3) The timescale of surface liquid water reservoirs can be constrained by the exchange of oxygen isotopes in ancient ponds. Oxygen isotopes were measured 26 years ago. The mass independent fractionation (MIF, that non-linear relation described above) puzzled the community ever since, and we provide the first explanation that is consistent for all three isotopes. It's a fun story: (1) icy climates lasting 10 million years or longer deposited not only water ice but also trace gases from the atmosphere (H2O2 and HNOx). These gases had the MIF signal that we show comes from CO2 photolysis. (2) As the ice sublimates, the gases concentrate. (3) After the ice melts, we show the gases exchange their isotopes with the water before evading. (4) The water exchanged isotopes with dissolved CO2 and formed carbonates with the now diluted, remaining MIF signal. The story shows that the mass dependent isotopes show the deposited water and ice both had to transport before depositing, telling about circulation patterns and where the meteorites formed. This paper is in prep for JGR:P.