Chemistry & Climate in Mars’ Ancient Atmosphere

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 atmosphere than what is observed today. Where did it all go?

Mars’ D/H ratio is 6x Standard Mean Ocean Water; this isotope fractionation suggests significant escape occurred. MAVEN estimates that Mars’ present day atmospheric escape rate is ~160-1800g of H per second.

Escape cannot explain all of the water loss though; crustal hydration is also important! (See our Science paper & press release from Caltech, NASA, & others.)

This begs the question: what did Mars’ atmosphere look like in the past? I found that crustal hydration released large fluxes of hydrogen to the atmosphere, capable of warming the climate for at least 100,000 years and up to 40 million years. We find three atmospheric redox states and climates could have occurred at early Mars: (i) warm CO2-H2 (0.1-40 Myr); (ii) cool CO2 (<10 Myr); (iii) cool CO-runaway state (>10 Myr). See my Nature Geoscience paper in review.

Nitrates and other volatiles have been observed on the surface of Mars. What atmospheric composition could correspond to present-day deposits? Was Mars’ surface ever habitable? I explained the formation of the nitrates in 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 (submitted to GRL).

MAVEN Research

Mars Atmospheric and Volatile EvolutioN has been orbiting Mars since September 2014. Its main goal is to study present day atmospheric loss at Mars. While photochemical loss is responsible for most of Mars’ atmospheric loss today, the nightside ionosphere also acts as a reservoir for escape processes. I joined MAVEN in 2016 under Dave Mitchell and Shaosui Xu (SSL/UC Berkeley) and used magnetic topology to probe the sources of the nightside ionosphere. (See our GRL paper here.)

You may be wondering what I mean by “magnetic topology.” Mars lacks an intrinsic global dynamo; however, strong remnant magnetism of the crust was found by Mars Global Surveyor. These crustal magnetic fields can connect to the planet (“closed” fields), or out to space (“open fields”). A third topology, “draped” occurs when the interplanetary magnetic field drapes around Mars. These topologies have unique affects on plasma motion, therefore affecting the sourcing of the nightside ionosphere.

Cartoon representation of Mars’ magnetic topology. (a) Closed , (b) open-to-day, (c) open-to-night. The yellow ring represents the “exobase” where below this, ion motion is dominated by collisions and above, ions become magnetized (both described below).

In low altitudes, ion-neutral collisional frequency exceeds the gyro frequency about the magnetic field, and at dusk ions are dragged by neutral winds into the nightside.

In high altitudes, the neutral atmosphere becomes less dense and collisions occur less often. The ions become “magnetized” as magnetic topology controls their motion. Along open magnetic fields that connect to the dayside, dayside ionospheric plasma can travel to the high altitude nightside.

Finally, magnetic field lines open to night allow energetic particles to precipitate into and ionize the neutral nightside atmosphere, producing ions. We find that this produces ~50% of the nightside ionosphere below 160 km.