Martian Upper Atmospheric Thermal Structure, Composition, and Water and their Significance for Atmospheric Escape and Evolution

Abstract

Mars has lost the majority of its atmosphere to space over the last 4 billion years, leaving the planet cold, dry, and oxidized. The NASA Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft arrived at Mars in 2014 to investigate the Mars-near space environment and the processes leading to atmospheric escape today, with the goal of understanding the evolution of Mars' atmosphere and climate through time. The Neutral Gas and Ion Mass Spectrometer (NGIMS) is a quadrupole mass spectrometer onboard MAVEN which directly measures the neutral and ionic composition of the Martian upper atmosphere as MAVEN flies through it. Using data obtained from NGIMS between 2014 and 2020, we investigate the thermal structure and neutral composition of this region. In addition, we report on the discovery of water in the upper atmosphere. Each of these three research efforts provides insight into the escape of the Martian atmosphere to space. The thermal structure of the Martian upper atmosphere is important for the determination of atmospheric escape rates because temperature is a measure of the distribution of velocities of the atoms that can escape to space and it determines the rate of many chemical reactions which alter the composition of the upper atmosphere. Using NGIMS measurements of Ar, N$_2$, and CO$_2$ densities, we calculate vertical profiles of temperature by assuming hydrostatic equilibrium and using the ideal gas law. We find the thermal structure of the upper atmosphere is highly variable, with large diurnal variations, and is consistent with a 1D energy balance model which includes solar ultraviolet and near infrared heating, thermal conduction, and radiative cooling by the CO$_2$ ν$_2$ 15 µm band. Determination of the composition of the upper atmosphere of Mars is necessary to understand the chemistry and energy balance in this region and ultimately what species escape to space. NGIMS measures the abundances of CO$_2$, Ar, N$_2$, O, He, and H$_2$. These data are the first vertical profiles of H$_2$, an important source of escaping H, in the Martian upper atmosphere. We investigate variations in the composition of the upper atmosphere with latitude, local time, and season. Transport-driven polar and nightside enhancements are identified in the ratios to CO$_2$ of N$_2$, O, He, and H$_2$. We also observe seasonal variations in the Ar mixing ratio associated with the seasonal deposition and sublimation of CO$_2$ at the polar ice caps. The discovery of water in the upper atmosphere of Mars has a direct impact on the desiccation of the Martian atmosphere and surface, because H$_2$O transported high into the upper atmosphere is rapidly destroyed by ions to ultimately produce H and O atoms which escape to space. We use NGIMS measurements of water-related ions, namely H$_2$O$^+$ and H$_3$O$^+$, to determine the water abundance in the upper atmosphere. The H$_2$O abundance varies with season, peaking in southern summer when Mars is closest to the Sun. Regional and global dust storms lead to a sudden splash of additional water into the upper atmosphere. We demonstrate that the proportion of escaping H atoms produced from H$_2$O is comparable to that from H$_2$, the classical source of escaping H, during most of the Martian year. H$_2$O becomes the dominant source of escaping H during global dust storms.