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The Martian proton aurora is a distinct aurora phenomenon resulting from the direct deposition of solar wind energy into Mars' dayside atmosphere. What solar wind parameters influence the aurora activity in the short term is yet unknown, as are the associated repercussions in the Martian atmospheric ion loss. Here we present observational evidence of synchronized proton aurora brightening and atmospheric ion loss intensifying on Mars, controlled by solar wind dynamic pressure, using observations by the Mars Atmosphere and Volatile Evolution spacecraft. The solar wind dynamic pressure possibly has a saturation effect on brightening proton aurora. Significant erosion of the Martian ionosphere during periods of high dynamic pressure indicates at least five‐to‐tenfold increase in atmospheric ion loss. An empirical relationship between ion escape rate and auroral emission enhancement is established, providing a new proxy of Mars' atmospheric ion loss with optical imaging that may be used remotely and with greater flexibility. Plain Language Summary Ion loss during solar events early in Mars history may have been the major cause to the long‐term evolution of the Mars atmosphere. Restricted by observation data, the relationship between ion loss and solar wind is still in debate. We have shown observational evidence for synchronized dayside proton aurora brightening and atmospheric ion loss intensifying on Mars, controlled by solar wind dynamic pressure using comprehensive observations by the NASA's MAVEN spacecraft. A power law model between ion escape rate and proton auroral emission enhancement is established for the first time, providing a new proxy of Mars' atmospheric loss with optical imaging that may be used remotely and with greater flexibility. The results presented in this study shed light on the direct control of dynamic pressure on interactions of the solar wind with Mars and other objects in solar system (e.g., Venus and Titan), and presumably stellar wind with exoplanets. Key Points Synchronized proton aurora brightening and atmospheric ion loss intensifying on Mars, both controlled by solar wind dynamic pressure The solar wind dynamic pressure possibly has a saturation effect on brightening proton aurora An empirical power law model between ion escape rate and proton auroral emission enhancement is established

SignificanceApproximately 3.5-Ga sedimentary rocks surveyed by the Mars Science Laboratory rover in Gale Crater, Mars, contain secondary mineral phases indicating aqueous alteration and release of cations from mafic minerals during sediment deposition in lakes. However, carbonate phases are not detected, and our model calculations indicate atmospheric CO2 levels at the time of sediment deposition 10s to 100s of times lower than those required by climate models to warm early Mars enough to maintain surficial water. Our results offer a ground-based reference point for the evolution of martian atmospheric CO2 and imply that other mechanisms of warming Hesperian Mars, or processes that allowed for confined hydrological activity under cold conditions, must be sought. Carbon dioxide is an essential atmospheric component in martian climate models that attempt to reconcile a faint young sun with planetwide evidence of liquid water in the Noachian and Early Hesperian. In this study, we use mineral and contextual sedimentary environmental data measured by the Mars Science Laboratory (MSL) Rover Curiosity to estimate the atmospheric partial pressure of CO2 (PCO2) coinciding with a long-lived lake system in Gale Crater at ∼3.5 Ga. A reaction–transport model that simulates mineralogy observed within the Sheepbed member at Yellowknife Bay (YKB), by coupling mineral equilibria with carbonate precipitation kinetics and rates of sedimentation, indicates atmospheric PCO2 levels in the 10s mbar range. At such low PCO2 levels, existing climate models are unable to warm Hesperian Mars anywhere near the freezing point of water, and other gases are required to raise atmospheric pressure to prevent lake waters from being lost to the atmosphere. Thus, either lacustrine features of Gale formed in a cold environment by a mechanism yet to be determined, or the climate models still lack an essential component that would serve to elevate surface temperatures, at least locally, on Hesperian Mars. Our results also impose restrictions on the potential role of atmospheric CO2 in inferred warmer conditions and valley network formation of the late Noachian.

Our recently developed nonlinear spectral gravity wave (GW) parameterization has been implemented into a Martian general circulation model (GCM) that has been extended to ∼130 km height. The simulations reveal a very strong influence of subgrid‐scale GWs with non‐zero phase velocities in the upper mesosphere (100–130 km). The momentum deposition provided by breaking/saturating/dissipating GWs of lower atmospheric origin significantly decelerate the zonal wind, and even produce jet reversals similar to those observed in the terrestrial mesosphere and lower thermosphere. GWs also weaken the meridional wind, transform the two‐cell meridional equinoctial circulation to a one‐cell summer‐to‐winter hemisphere transport, and modify the zonal‐mean temperature by up to ±15 K. Especially large temperature changes occur over the winter pole, where GW‐altered meridional circulation enhances both “middle” and “upper” atmosphere maxima by up to 25 K. A series of sensitivity tests demonstrates that these results are not an artefact of a poorly constrained GW scheme, but must be considered as robust features of the Martian atmospheric dynamics. Key Points Spectral gravity wave parameterization was introduced into a Martian GCM GW turn out to be extremely important in the Martian atmosphere Their dynamical effects are similar to those in the terrestrial mesosphere

Simultaneous measurements of ozone concentrations and vibrationally excited hydroxyl (OH*) emissions provide a method for determining atomic oxygen (O) and atomic hydrogen (H) concentrations, which are otherwise difficult to measure directly. This approach requires that the photochemical equilibrium conditions for ozone are satisfied. In this study, we use model simulations to examine the spatio-temporal distribution of ozone photochemical equilibrium in the nighttime Martian atmosphere and evaluate its relationship to detectable OH* emission layers. Our analysis reveals that ozone mostly maintains chemical equilibrium over large latitude–altitude regions during the second half of the Martian year ( Lₛ  = 180°–360°). However, these equilibrium conditions are partial consistent with observable OH* emissions only in limited areas: at high latitudes (60–70 km) during Lₛ  = 90°–120°, in northern mid-latitudes (45–50 km) during Lₛ  = 80°–135°, at equatorial latitudes (50–60 km) during Lₛ  = 80°–135°, and in southern mid-latitudes (55–65 km) during Lₛ  = 45°–90°. Graphical Abstract

It has been known for decades that carbon dioxide (CO2) ice clouds exist in the Martian atmosphere. According to remote sensing observations and previous modeling studies, the Martian CO2 ice crystals may be sufficiently large to generate halos. However, observations of CO2 ice crystal halos have not been reported so far. This study simulates the scattering and polarized phase functions at a wavelength of 0.48 μm based on state‐of‐the‐art light‐scattering computational capabilities. The specific CO2 ice crystal habits considered in the simulations include cubes, octahedrons, cubo‐octahedrons, and truncated octahedrons of various sizes. The halos produced by CO2 ice crystals peak at approximately 29° and 42°. Moreover, large CO2 ice crystals may cause strong scattering peaks at 155° and 180°. An ensemble of water (H2O) ice crystals and CO2 ice crystals with appropriate mixing fractions might be responsible for a halo occurrence recently observed on Mars. Plain Language Summary On Earth, optical phenomena, particularly halos, glories, and rainbows, caused by ice crystals and water droplets, are often observed in the sky. In the Martian atmosphere consisting of approximately 95% carbon dioxide (CO2) by volume, CO2 ice crystals can exist and result in optical phenomena similar to those observed on Earth. In the present study, the optical properties of CO2 ice crystals are computed to explain halos and other optical features caused by these particles. Because the habits (shapes) of CO2 ice crystals are different from those of water droplets and ice crystals, the optical phenomena produced by the former have different positions in the sky compared to the counterparts caused by water droplets and ice crystals. Furthermore, a halo observed recently on Mars might be caused by a mixture of water ice crystals and CO2 ice crystals. Key Points CO2 ice crystals in the Martian atmosphere can produce halos at approximately 29° and 42° Optical phenomena associated with light scattering by CO2 ice crystals can be used to estimate the particle habits and sizes A halo observed on Mars might be caused by an ensemble of H2O and CO2 ice crystals

Observations by the Compact Reconnaissance Imaging Spectrometer (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) over the range 440–2920 nm of the very dusty Martian atmosphere of the 2007 planet‐encircling dust event are combined with those made by both Mars Exploration Rovers (MERs) to better characterize the single scattering albedo (ω0) of Martian dust aerosols. Using the diagnostic geometry of the CRISM emission phase function (EPF) sequences and the “ground truth” connection provided at both MER locations allows one to more effectively isolate the single scattering albedo (ω0). This approach eliminates a significant portion of the type of uncertainty involved in many of the earlier radiative transfer analyses. Furthermore, the use of a “first principles” or microphysical representation of the aerosol scattering properties offers a direct path to produce a set of complex refractive indices (m = n + ik) that are consistent with the retrieved ω0 values. We consider a family of effective particle radii: 1.2, 1.4, 1.6, and 1.8 μm. The resulting set of model data comparisons, ω0, and m are presented along with an assessment of potential sources of error and uncertainty. We discuss our results within the context of previous work, including the apparent dichotomy of the literature values: “dark” (solar band ω0 = 0.89–0.90) and “bright” (solar band ω0 = 0.92–0.94). Previous work suggests that a mean radius of 1.8 μm is representative for the conditions sampled by the CRISM observations. Using the m for this case and a smaller effective particle radius more appropriate for diffuse dust conditions (1.4 μm), we examine EPF‐derived optical depths relative to the MER 880 nm optical depths. Finally, we explore the potential impact of the resulting brighter solar band ω0 of 0.94 to atmospheric temperatures in the planetary boundary layer.

Dust storms have a significant impact on the Martian atmosphere and climate. Previous studies have found that regional and global dust storms mainly occur in the Mars perihelion season. However, an anomalous spring regional dust storm occurred in the aphelion season of Martian year 35 (MY 35). The occurrence and evolution of this new type of large dust storm and its impact on the Martian atmosphere are not yet fully understood. Using Mars Climate Sounder (MCS) dust observations, this study investigates the evolutionary characteristics of the MY 35 anomalous spring storm during its pre-storm, onset, expansion, and decay phases, by comparing it with other types of regional dust storms. The evolution of the MY 35 anomalous spring dust storm is more similar to that of the MY 35 C storm, showing north–south mirror symmetry relative to the equator, suggesting that the two storms may have similar evolutionary mechanisms. Additionally, we analyze the effects of the anomalous MY 35 storm on the atmospheric thermal and dynamical structures using a combination of MCS temperature observations and LMD-GCM wind simulation results. Eastward winds in the high latitudes of both hemispheres and westward winds in the low-to-mid latitudes are significantly enhanced during the storm, corresponding to the change in the atmospheric thermal structure and the global circulation. Finally, we performed a preliminary analysis of changes in the wind field during the spring dust storm in the Chryse and Utopia plains, which are two potential landing areas for China’s Tianwen-3 Mars sample-return mission. The vertical profiles of the simulated horizonal wind in the two plains show that, during the E storm peak time, the change in daily mean wind speed is significant above 20 km, but relatively small in the atmospheric boundary layer below ~5 km. Within the boundary layer, the horizontal wind speed shows remarkable diurnal variation, remaining relatively low during the midday hours (10:00 a.m. to 4:00 p.m.). These results can provide necessary environmental parameters related to spring dust storms for China’s Tianwen-3 mission.

We report a detection of methane in the martian atmosphere by the Planetary Fourier Spectrometer onboard the Mars Express spacecraft. The global average methane mixing ratio is found to be 10 ± 5 parts per billion by volume (ppbv). However, the mixing ratio varies between 0 and 30 ppbv over the planet. The source of methane could be either biogenic or nonbiogenic, including past or present subsurface microorganisms, hydrothermal activity, or cometary impacts.