Structure and Composition of the Neutral Upper Atmosphere of Mars from the MAVEN NGIMS Investigation

The Neutral Gas and Ion Mass Spectrometer (NGIMS) on the MAVEN mission is designed to characterize the state of the upper atmosphere and its response to perturbations from the sun and from lower atmosphere. These measurements complement other instruments on the MAVEN spacecraft and support the MAVEN science goal [Jakosky et al., 2015] of understanding atmospheric escape and over the course of martian history. We report NGIMS measurements of the neutral composition of multiple atomic and molecular species over hundreds of orbits since the science phase of this mission began late in 2014. Ion measurements from NGIMS are reported separately in other contributions to this session. The wide dynamic range, the high temporal resolution, and the use of two different ion source configurations of the NGIMS instrument [Mahaffy et al., 2014] allows neutral density structure and its variability to be characterized in detail. Large amplitude wave structure is observed on the lowest altitude portion of many orbits that dissipates at higher altitudes. Although this structure puts scatter in the retrieval of scale height temperatures these are nevertheless robustly secured by averaging over several orbits. The variability of the upper atmosphere temperature with latitude and local solar time is reported. The deep dip campaign approach to the homopause allows the mixing ratio of the major gases in the lower atmosphere to be measured. We compare these mixing ratios with those reported by the Sample Analysis at Mars investigation and previous Viking measurements. Jakosky, B. M., et al. (2015), The Mars Atmosphere and Volatile Evolution (MAVEN) Mission, Space Sci Rev, 21. Mahaffy, P. R., et al. (2014), The Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution Mission, Space Sci Rev, 185, DOI: 10.1007/s11214-11014-10091-11211.