Age of Martian Air: Time scales for Martian atmospheric transport

Quantifying the rate of transport of air from the boundary layer throughout the atmosphere is important for understanding the atmospheric composition of Mars. Here we examine this by performing MarsWRF simulations of an idealized "mean age" tracer, which yields the mean time since air was in the low-latitude boundary layer. The spatial distribution and seasonality of the mean age in low and mid-latitudes broadly follow contours of the mean meridional circulation, with the mean age increasing from 0 at the surface to a maximum of 60-100 sols in the upper atmosphere. Substantially older mean ages (exceeding 300 sols) are found in polar regions, with oldest ages in the lower atmosphere (50-100 Pa), above a near-surface layer with very young ages (around 20 sols). The annual maximum ages occur around the equinoxes, and the age in the polar lower atmosphere decreases during the autumn to winter transition. This autumn-winter decrease in age occurs because of mixing of polar and mid-latitude air when the polar vortex exhibits an annulus of high potential vorticity (PV) with a local minimum near the pole. The altitudinal and seasonal variations in the mean age indicates similar variations in the transport of dust into polar regions and the mixing of polar air (with, e.g., low water vapor and high ozone concentrations during winter) into mid-latitudes.