The Martian Atmosphere
Chemistry 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.
This begs the question: what did Mars’ atmosphere look like in the past? Nitrates and chlorates have been observed on the surface of Mars. What atmospheric composition could correspond to present-day deposits?
Under Yuk Yung, I run a coupled 1D photochemical-radiative transfer model to consider the production and rain out of nitric acid in potential early Mars atmospheres. By comparing the rate of nitric acid rain out to the amount of present day nitrates found at the surface, constraints about Mars’ early atmosphere may be made.
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.
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.