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Atomic H is a direct tracer of water loss at Mars. The recent discovery of annual enhancements in its escape rate near perihelion, in excess of the previously established theory, indicates that Mars has lost substantial amounts of water to space. However, these loss rates are often estimated assuming thermal properties for the exospheric H atoms and are therefore a lower limit. Past analyses of spacecraft observations delivered indirect evidence for the existence of an energetic non‐thermal H population without a clear detection. Here, we present the unambiguous observational signature of non‐thermal H at Mars, consistent with solar wind charge exchange as the primary driver for its production. The calculated non‐thermal escape rates are as high as ∼26% of the thermal rate near aphelion and solar minima. An active Sun would increase the present‐day escape rate and a younger energetic Sun likely contributed toward the significant historic loss of water from Mars. Plain Language Summary: The total water lost by Mars is determined by calculating the number of H atoms permanently escaping its upper atmosphere into space at present and extrapolating it back in time. The present‐day escape rate of H does not account for energetic H atoms, which were postulated to exist but not previously observed to date. Such a population would enhance the H escape rate further, thereby increasing the total volume of the water lost by Mars. This work presents the first confirmed signature of energetic H atoms in the atmosphere of Mars and finds that interaction of the neutral H population in its uppermost atmospheric layer, that is, its exosphere, with the solar wind results in its creation. The amount of energetic H atoms produced depends on the Mars' atmospheric conditions such as temperature and density of neutral H atoms in the exosphere, solar activity, and Mars' position around the Sun. Since the younger Sun was much more active than today, it is likely that the interaction of an early Martian exosphere with the stronger solar wind resulted in the loss of a larger fraction of H atoms into space than has been estimated to date. Key Points: Observations of H brightness at high altitude in Mars' atmosphere during aphelion solar minimum show an abrupt slope change above 5,000 kmHot H atoms produced via solar wind charge exchange can reproduce the observed profiles, providing evidence for nonthermal hydrogenSince this escape scales with solar wind flux, elevated loss rates early in Mars' history could result in much larger overall water loss [ABSTRACT FROM AUTHOR]

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high‐spectral resolution designed to resolve D and H Lyman‐α emissions. The high‐spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman‐α emissions. This work presents improved characterizations of the high‐spectral resolution mode using in‐orbit observations that allow for more robust detections of the faint D Lyman‐α emission line. Additionally, the instrument was re‐calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high‐spectral resolution instrument. Comparisons to Lyman‐α observations made with the low‐resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint‐emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data‐reduction pipeline with favorable implications for the science utility of D and H Lyman‐α emissions. Plain Language Summary: The MAVEN high resolution echelle (ECH) instrument has been used to observe ultraviolet emissions from Mars. Measurements made with MAVEN/ECH were made available to the public by using a data reduction pipeline that had been developed with in‐lab and theoretical analysis to convert observations into a form that was useable by the broader scientific community. Since MAVEN's orbit insertion in 2014, the ECH detector has been better characterized with lessons learned from in‐orbit performance. These improvements produced a new data reduction pipeline that has enhanced the detection of faint D Lyman‐α emissions that are critical to interpreting water loss from the upper atmosphere of Mars. Key Points: MAVEN/ECH uses HST/STIS, the only other UV echelle instrument deployed to space, for calibration and independent verificationThe MAVEN/ECH data reduction algorithm has been upgraded to improve detection of faint D Lyman‐α emissionsThe scientific implications of this work include doubling the previous data set of D measurements available for reliable interpretation [ABSTRACT FROM AUTHOR]

Ozone (O3) in the atmosphere of Mars is produced following the photolysis of CO2 and is readily destroyed by the hydrogen radicals (HOx) released by the photolysis and oxidation of water vapor. As a result, an anti‐correlation between ozone and water vapor is expected. We describe here the O3‐H2O relationship derived from 4 Martian years of simultaneous observations by the SPICAM spectrometer onboard the Mars Express spacecraft. A distinct anti‐correlation is found at high latitudes, where the O3 column varies roughly with the −0.6 power of the H2O column. The O3 and H2O columns are uncorrelated at low latitudes. To evaluate our quantitative understanding of the Martian photochemistry, the observed O3‐H2O relationship is then compared to that predicted by a global climate model with photochemistry. For identical model and observed abundances of H2O, the model underpredicts observed ozone by about a factor of 2 relative to SPICAM when using the currently recommended gas‐phase chemistry. Sensitivity studies employing low‐temperature CO2 absorption cross sections, or adjusted kinetics rates, do not solve this bias. Taking into account potential heterogeneous processes of HOx loss on clouds leads to a significant improvement, but only at high northern latitudes. More broadly, the modeled ozone deficits suggest that the HOx‐catalyzed photochemistry is too efficient in our simulations. This problem is consistent with the long‐standing underestimation of CO in Mars photochemical models, and may be related to similar difficulties in modeling O3 and HOx in the Earth's upper stratosphere and mesosphere. Plain Language Summary: The thin ozone layer on Mars is produced when the solar ultraviolet light breaks the CO2 molecules that compose 95% of its atmosphere. Conversely, ozone on Mars is readily destroyed by the hydrogen species released by water vapor. An inverse relationship is therefore expected between the quantities of ozone and water vapor. Quantifying this relationship provides important insight into the hydrogen chemistry that stabilizes the composition of the Mars atmosphere. We describe here the ozone and water vapor measurements performed during 4 Martian years (7.5 Earth years) by the SPICAM instrument onboard the Mars Express spacecraft. We then attempt to reproduce these measurements with a Mars climate model with photochemistry. Although the model reproduces the inverse relationship observed between ozone and water vapor, the ozone amount is underestimated by about a factor of 2 in the simulations. The ozone deficit suggests that the destruction by hydrogen species is too strong when one uses the currently recommended reaction rates. This problem is consistent with the long‐standing underestimation in Mars models of carbon monoxide, also destroyed by hydrogen species, and can be related to similar difficulties in modeling ozone in the Earth's upper atmosphere. Key Points: The relationship between the O3 and H2O columns on Mars is quantified from 4 Martian years of simultaneous measurementsThe O3 and H2O columns are distinctly anti‐correlated at high latitudes but are uncorrelated at low latitudesModel simulations using the observed amount of H2O and the currently recommended kinetics underpredict O3 by about a factor of 2 [ABSTRACT FROM AUTHOR]

The nighttime ozone and sulfur dioxide distributions were analyzed using the entire Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV/Venus Express stellar occultation data set. After the discovery of an ozone layer at 100 km in the mesosphere reported by Montmessin, Bertaux, et al. (2011, https://doi.org/10.1016/j.icarus.2011.08.010), 132 other detections were made during the entire 8 years long observing period of the SPICAV UV instrument. In the rare detections, the peak abundances of O3 accumulating in the mesosphere are observed with densities from 107 to 108 molecules⋅cm−3 at 85–110 km. The ozone layer is estimated to vary from 1 to 30 ppbv at 85–95 km while at 95–105 km the volume mixing ratio is expected within an interval from 6 to 120 ppbv. Below 93 km, a puzzling decrease of mixing ratio is observed towards midnight at 30°N. Our work also provides an improved sequel to the analysis of the sulfur dioxide survey previously made in the upper mesosphere by Belyaev, Evdokimova, et al. (2017, https://doi.org/10.1016/j.icarus.2017.05.002). On average, the SO2 content is found to remain constant throughout the vertical profile at a value of around 135 ± 21 ppbv between 85 and 100 km. Rapid and large variations prevent to conclude firmly on any time or space pattern of SO2. Plain Language Summary: Observations of light attenuation while bright ultraviolet stars were rising or setting behind Venus permitted us to characterize the quantity of carbon dioxide (the main atmospheric component), sulfur oxide, and ozone between 85 and 110 km of altitude at night. They were performed by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV instrument on board the ESA Venus Express mission in 2006–2014. For the first time, this data set has been entirely analyzed with the benefit of recent calibration's improvements enabled more faithful restitution of gases' abundances. For ozone, discovered on Venus in 2011, thanks to SPICAV, we have been able to detect maximum concentrations and to estimate its nightside characterization. At midlatitudes, a ratio between ozone and carbon dioxide densities, or O3 mixing ratio, decreases below 93 km at midnight. This puzzling result is tentatively explained by chemical compounds transported by winds from the dayside hemisphere to the nightside and which are known to quickly react with ozone and destroy it. Sulfur dioxide has been found to vary considerably on a short time scale (days) but, on average, the SO2 mixing ratio is constant within 85–100 km. Key Points: Ozone density is within 107 to 108 cm−3 in the 85–110 km altitude range from 2006 to 2015The altitude profile of the sulfur dioxide volume mixing ratio is vertically uniform around 135 ppbv between 85 and 100 kmTemporal variations of ozone and sulfur dioxide are analyzed [ABSTRACT FROM AUTHOR]

Winds derived by a digital tracking technique from ultraviolet (365 nm) images captured by the Venus Monitoring Camera (VMC) onboard the Venus Express spacecraft from 2006 to 2013 were used to study the atmospheric circulation at cloud top level (70 ± 2 km). This data set allows variations of the wind speed with both latitude and longitude to be studied and establishes their correlation with surface topography as well as local time dependence. Both zonal and meridional wind components show some correlation with topography. The minimum zonal wind speed was found at noon above Ovda Regio (10°S, 93°E), the highest region of Aphrodite Terra, one of the largest highlands in the equatorial region. The area of slow zonal wind extends to at least 30°S and shifts in the direction of superrotation in the afternoon and with increasing latitude (poleward). The observed deceleration of cloud top wind was recently attributed to the interaction of the gravity (mountain) waves generated by Aphrodite Terra with the atmospheric circulation. The present study was performed for different local time over the mountainous longitudes. The deceleration pattern in the zonal wind field is mainly conserved within a few hours around noon. Systematic longitude shift is observed in the afternoon in the direction of the evening terminator. Another area of perturbation of both zonal and meridional wind components is observed in the equatorial region around LT = 13–14 hr and may be explained by the solar tide. Plain Language Summary: Venus is completely covered with a thick cloud layer with its top at about 70 km. Surprisingly, recent observations show that the cloud level circulation is affected by the surface topography. In this paper we analyzed wind velocities derived from tracking of cloud features in the UV images acquired by the Venus Monitoring Camera onboard the European Space Agency's Venus Express orbiter during its operations from 2006 to 2013. The zonal wind at the cloud top decelerates by about 20% above the highest part of Aphrodite Terra and reaches its minimum at local noon. The zonal wind deceleration is explained by interaction of gravity waves generated by the surface relief with the atmospheric circulation. An additional deceleration occurs in the afternoon in the equatorial region and is probably caused by solar heating of the clouds. The combination of both effects results in a vast area of slow wind during the daytime. Key Points: A maximum deceleration of the mean zonal flow is observed at noon above the highest region of Aphrodite Terra, VenusThe mean zonal and meridional flows at cloud top level in the equatorial region are perturbed by a solar tide at 13–14 hrA dependence of the mean zonal and meridional flows on topography is observed from the equator to at least 30°S [ABSTRACT FROM AUTHOR]

The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the Solar Wind Anisotropies/Solar and Heliospheric Observatory (SWAN/SOHO) instrument in January 1996, 1997, and 1998 (low solar activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth radii (RE) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of Lyman Alpha Imaging Camera (LAICA) imager (∼50 RE), and encompassing the orbit of the Moon (∼60 RE). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere, which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the dayside, producing a bulge of H density between 3 and 20 RE, which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to Orbiting Geophysical Observatory number 5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 RE are larger than LAICA/Orbiting Geophysical Observatory number 5 by factors 1.1–2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to nonthermal atoms (not in the model). Key Points: We find that the geocorona extends to almost twice the distance of the Moonthe H exosphere is compressed by solar radiation pressure, forming a bulge on the daysidethis bulge is enhanced at low solar activity, possibly in relation with a population of Hatoms in satellite orbits [ABSTRACT FROM AUTHOR]

Hubble Space Telescope (HST) observations of the Martian hydrogen exosphere in Lyman α are presented in this paper for a period when Mars passed perihelion and southern summer solstice in its orbit. The peak intensity in the exospheric Lyman α brightness was recorded after Mars went past its perihelion, slightly after southern summer solstice. The increase in brightness as Mars approached perihelion was found to not be symmetric around the peak, making it impossible to fit the H escape flux trend with a single sinusoidal curve with the peak at perihelion. While the short-term (~30 Earth days) changes were not directly correlated with changes in the solar Lyman α flux, the long-term (~10 Earth years) trend in the data does show some correlation with solar activity. This suggests that the short-term changes brought about in the exosphere could be due to intrinsic changes occurring within the lower atmosphere. For example, thermospheric heating by dust can alter the cold-trapping mechanism for water vapor resulting in it being present in large quantities at higher altitudes (60-80 km), possibly enhancing the escape flux of H. Therefore, it is important to understand the drivers of atmospheric dynamics in the Martian atmosphere, which produce the yearly enhanced seasonal changes observed at Mars around periapsis and southern summer solstice in order to accurately determine the total amount of water lost over its history. [ABSTRACT FROM AUTHOR]