Sodium and Potassium Linewidths as an Atmospheric Escape Diagnostic at Mercury

Abstract The spatial distribution and linewidth of Mercury's sodium and potassium exospheres were observed using a combination of long‐slit and high‐resolution point spectroscopy. Effective temperatures were estimated from emission line profiles by forward modeling their Doppler broadening. These serve as an energy metric for collisionless gas that is inherently nonthermal. The Na gas at low and mid‐latitudes ranges from 1,200 to 1,300 K along the noon meridian, in agreement with MESSENGER scale heights, increasing by ∼200 K at the poles and terminator. This increase is attributed to the loss of low energy atoms to the surface during photon‐driven transport antisunward. An escaping potassium tail was measured for the first time, observed out to 10.4 R M with Na/K ∼95 at 5.8 R M . Emission linewidths increase sharply between the dayside and escaping tail, with Na growing from about 1,200 to 7,500 K, and K from 750 to 8,500 K by the time the gas reaches 4.3 R M downtail. Na D line profiles down the exotail also evolve from Gaussian to boxcar in shape. Both characteristics are interpreted as filtering of the nascent velocity distribution function, where low energy atoms on gravitationally bound trajectories are removed from the gas population, while high energy escaping atoms are retained. Na linewidths become invariant past 3.5 R M , placing this altitude as the ballistic apex of bound trajectories. In this way, Mercury's emissions prototype a novel technique toward a broader understanding of atmospheric escape, using emission line morphology to probe the transition between bound and escaping gas.