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The Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission measures the structure and variability of the Martian upper atmosphere. We use NGIMS density profiles to derive upper atmospheric temperature profiles and investigate the thermal structure of this region. The thermal structure of the upper atmosphere is a critical component of understanding atmospheric loss to space, the main science objective of MAVEN, and measured temperatures serve as inputs to and constraints on photochemical and global circulation models. We describe proper treatment of the NGIMS data and correct for the horizontal motion of the spacecraft. Temperature profiles from week‐long, low‐altitude excursions executed by MAVEN, called Deep Dips, are used to investigate the diurnal variation of the temperature and the thermospheric gradient, which varies between 1.33 ± 0.16 and 2.69 ± 0.33 K km−1 on the dayside. NGIMS measurements acquired on nominal MAVEN orbits over more than a Martian year further elucidate the diurnal and latitudinal variations of the temperature. Diurnal variations of about a factor of 2, from 127 ± 8 to 260 ± 7 K, are observed high in the exosphere, and latitudinal variations of 39 ± 17 K are observed in this region. Comparisons indicate broad agreement between temperatures derived from MAVEN NGIMS with previous in situ and remote sensing observations of upper atmospheric temperatures. NGIMS temperatures are also shown to be consistent with predictions of a 1‐D model which includes solar UV and near IR heating, thermal conduction, and radiative cooling in the CO2   ν2 15‐μm band. Key Points: Temperature profiles derived from MAVEN NGIMS data reveal the thermal structure of the Martian upper atmosphereProper treatment of instrumental effects is discussed, including impact on temperatures at the base of the thermosphereDiurnal variations of 130 K and thermospheric gradients between 1.3 and 2.7 K km−1 are observed [ABSTRACT FROM AUTHOR]

Mars has lost most of its once-abundant water to space, leaving the planet cold and dry. In standard models, molecular hydrogen produced from water in the lower atmosphere diffuses into the upper atmosphere where it is dissociated, producing atomic hydrogen, which is lost. Using observations from the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution spacecraft, we demonstrate that water is instead transported directly to the upper atmosphere, then dissociated by ions to produce atomic hydrogen. The water abundance in the upper atmosphere varied seasonally, peaking in southern summer, and surged during dust storms, including the 2018 global dust storm. We calculate that this transport of water dominates the present-day loss of atomic hydrogen to space and influenced the evolution of Mars’ climate. [ABSTRACT FROM AUTHOR]

We examine 252 Mars Atmosphere and Volatile EvolutioN (MAVEN) passes through the Martian atmosphere for which the Neutral Gas and Ion Mass Spectrometer (NGIMS) altitude-density profiles show perturbations with amplitudes larger than 40% of the background density that persist above the nominal exobase as defined here. The density profiles exhibiting such perturbations are plotted as a function of atmospheric column density rather than altitude. This roughly removes the dependence of the altitude-density profiles on composition, scale height, and local solar time. Such density structures are of interest as they can affect the local heating rate and, possibly, the escape rate. We find that the observed structures dissipate at roughly the same atomic column density and they affect the composition of the exosphere, raising the O/CO 2 ratio as compared with orbits that do not exhibit significant perturbations. [ABSTRACT FROM AUTHOR]