Using Mars Atmosphere and Volatile Evolution observations, we characterize the variability of water vapor in the Martian thermosphere during Mars Years 32-35. Near a fixed atmospheric pressure level of ~5 × 10-7 Pa, the typical water density is 1.3 (±0.8) × 103 cm-3 and the typical water mixing ratio is 10 (±6) ppm. Thermospheric water levels are higher during the southern spring and summer seasons when Mars is near perihelion and there is significant dust loading in the lower atmosphere. However, the seasonal variation is not the same from year-to-year, likely due annual differences in dust loading. Water vapor is highly correlated with lower atmospheric dust, and increases during both regional and global dust storms. Our results support previous work that found increased dust levels allow more water to be supplied directly to the thermosphere.

Mars and Venus have atmospheres but lack large-scale intrinsic magnetic fields. Consequently, the solar wind interaction at each planet results in the formation of an induced magnetosphere. Our work aims to compare the low-altitude (< 250 km) component of the induced magnetic field at Venus and Mars using observations from Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile EvolutioN (MAVEN). The observations from Mars are restricted to regions of weak crustal magnetism. At Venus, it has long been known the vertical structure of the induced magnetic field profiles have recurring features that enable them to be classified as either magnetized or unmagnetized. We find the induced field profiles at Mars are more varied, lack recurring features, and are unable to be classified in the same way. The solar zenith angle dependence of the low-altitude field strength at both planets is controlled by the shape of the magnetic pileup boundary. Also, because the ionospheric thermal pressure at Venus is often comparable to the solar wind dynamic pressure, the induced fields are weaker than required to balance the solar wind by themselves. By contrast, induced fields at Mars are stronger than required to achieve pressure balance. Lastly, we find the induced fields in the magnetized ionosphere of Venus have a weaker dependence on solar wind dynamic pressure than the induced fields at Mars. Our results point to planetary properties, such as planet-Sun distance, having a major effect on the properties of induced fields at nonmagentized planets.

Proton aurora are the most commonly observed yet least studied type of aurora at Mars. In order to better understand the physics and driving processes of Martian proton aurora, we undertake a multi-model comparison campaign. We compare results from four different proton/hydrogen precipitation models with unique abilities to represent Martian proton aurora: Jolitz model (3-D Monte Carlo), Kallio model (3-D Monte Carlo), Bisikalo/Shematovich et al. model (1-D kinetic Monte Carlo), and Gronoff et al. model (1-D kinetic). This campaign is divided into two steps: an inter-model comparison and a data-model comparison. The inter-model comparison entails modeling five different representative cases using similar constraints in order to better understand the capabilities and limitations of each of the models. Through this step we find that the two primary variables affecting proton aurora are the incident solar wind particle flux and velocity. In the data-model comparison, we assess the robustness of each model based on its ability to reproduce a MAVEN/IUVS proton aurora observation. All models are able to effectively simulate the data. Variations in modeled intensity and peak altitude can be attributed to differences in model capabilities/solving techniques and input assumptions (e.g., cross sections, 3-D versus 1-D solvers, and implementation of the relevant physics and processes). The good match between the observations and multiple models gives a measure of confidence that the appropriate physical processes and their associated parameters have been correctly identified, and provides insight into the key physics that should be incorporated in future models.

Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN’s (MAVEN’s) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of nightside UV discrete aurora observations. Initial analysis of these observations has shown that, near the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the aurora detection frequency has not yet been determined. In this work, we use IUVS discrete aurora observations, and MAVEN solar wind observations, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the SCFR, the detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure and moderately increases with IMF strength. Interestingly, the auroral emission brightness outside the SCFR is insensitive to the dynamic pressure. Inside the SCFR, the detection frequency is moderately dependent on the dynamic pressure and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti-radial directions. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars.

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind in the ionopause at Mars, with only 13% unmagnetized ionopauses observed over a 11-year period. We also find that the ionopause altitude decreases as the normal component of the solar wind dynamic pressure increases. Moreover, our results show that the ionopause thickness at Mars is mainly determined by the ion gyromotion and equivalent to about 5.7 ion gyroradii.

At Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H+ and H–. The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H–originating in the solar wind. The highest fluxes of H– were observed near perihelion and the southern summer solstice. We calculated an average ratio of ~4% between H– density and H+ density, implying a slightly smaller relative abundance than reported previously (~10%). We found that the fraction of H ENAs converted to H– increases with the solar wind energy, in agreement with laboratory measurements of the H–CO2 electron capture cross section.

Outside the Martian bow shock, charge exchange between solar wind protons and exospheric hydrogen produces energetic neutral atoms (ENAs) that travel towards Mars at the solar wind velocity. The penetrating ENAs deposit most of their energy near 150 km, but a fraction of them undergo enough collisions to be scattered back to space, resulting in a hydrogen albedo. Some of the penetrating ENAs are converted into protons upon reaching the collisional upper atmosphere. These protons can be measured by the Mars Atmosphere and Volatile EvolutioN’s Solar Wind Ion Analzyer (SWIA) during periapsis passes, providing information about the penetrating and backscatter populations. In this work, we perform the first detailed analysis of the backscatter and albedo using SWIA observations. We find that our calculated backscatter energy spectra are consistent with model predictions and that, as expected, the penetrating and backscatter particle fluxes increase with solar wind speed and decrease with solar zenith angle (SZA). We also find that the albedo, which has an average value of 0.20±0.16, decreases with solar wind speed and increases at high SZAs near the terminator.

In October 2014, the close encounter between Mars and comet Siding Spring produced a transient ionized layer in the upper atmosphere composed primarily of Mg⁺ ions. The layer was detected by instruments on three spacecraft, including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express. Analyses of the MARSIS data indicated the transient layer persisted up to ~19 hours after the comet’s closest approach. We report MARSIS observations that suggest the transient layer lasted at least 7 days – and potentially as long as 32 days – after closest approach. During this period, the transient layer was mostly confined to a narrow latitude range between 20°N-60°N and a longitude range spanning 275°E to 95°E. Since this period coincided with a highly active Sun, we discuss how solar flares may have contributed to the layer’s prolonged lifetime.