First Evidence of Persistent Nighttime Temperature Structures in the Neutral Thermosphere of Mars

Abstract

Using two Mars years of data collected by the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatiles EvolutioN spacecraft, we reconstruct the local solar time structure of the Martian equatorial thermosphere for the dawn and dusk sectors. The results indicate the presence of several persistent features near the dusk and dawn terminators appearing in the neutral temperature and in the O, Ar, N2, and CO2 densities. The dusk temperature features include a minimum at the terminator surrounded by two local maxima with amplitudes between 20 and 40 K. A nighttime temperature enhancement occurs at a local solar time of 4–5 hr and has an amplitude between 50 and 100 K relative to the surrounding temperatures. The observed enhancements are interpreted to be a result of either nighttime dynamical heating caused by converging and downwelling winds or of a terminator wave originating in the lower atmosphere.Plain Language SummaryNASA’s Mars Atmosphere and Volatiles EvolutioN (MAVEN) spacecraft has been collecting information about the Mars’ upper atmosphere since November 2014. In our study we examined the temperature in the upper atmosphere measured by a gas analyzer on MAVEN that periodically sniffs the air around it. By combining several years of data, we reconstructed a map of atmospheric temperatures dividing it into equatorial daytime, morning, evening, and nighttime regions. The results indicate that local patches of warm atmosphere extend about 300–600 km along the Martian equator (3–6 hr driving distance at highway speeds) on the nightside near morning and evening. One possible reason for these nighttime warm spots is the air currents in the Martian upper atmosphere. As the sun heats the dayside atmosphere, it expands and causes wind currents to blow away from the dayside region. These wind currents can extend away from the equator then return on the nightside colliding in the morning and evening regions. The resulting meeting of wind currents can heat the upper atmosphere. Warmer air escapes from the upper atmosphere faster. Because of this, these nighttime patches of warm air could have contributed to a faster escape of Martian atmospheric gases than previously believed.Key PointsPersistent neutral density and temperature enhancements are observed within 30 min of the dusk terminatorA persistent neutral density enhancement is also observed prior to the dawn terminator at around 4–5 local timeThe neutral density features are observed in all species and appear to be associated with enhancements in neutral temperature