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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

A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H 2 O production rate varied from 0.3 kg s –1 in early June 2014 to 1.2 kg s –1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K –1 m –2 s –0.5 ), consistent with a thermally insulating powdered surface, is inferred.

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

Carbon dioxide (CO2) ice clouds have been routinely observed in the middle atmosphere of Mars. However, there are still uncertainties concerning physical mechanisms that control their altitude, geographical, and seasonal distributions. Using the Max Planck Institute Martian General Circulation Model (MPI-MGCM), incorporating a state-of-the-art whole atmosphere subgrid-scale gravity wave parameterization (Yiğit et al., 2008), we demonstrate that internal gravity waves generated by lower atmospheric weather processes have a wide-reaching impact on the Martian climate. Globally, GWs cool the upper atmosphere of Mars by ∼10 % and facilitate high-altitude CO2 ice cloud formation. CO2 ice cloud seasonal variations in the mesosphere and the mesopause region appreciably coincide with the spatio-temporal variations of GW effects, providing insight into the observed distribution of clouds. Our results suggest that GW propagation and dissipation constitute a necessary physical mechanism for CO2 ice cloud formation in the Martian upper atmosphere during all seasons.

Juno's microwave radiometer experiment (MWR) provided the first spatially resolved observations beneath the surface of Ganymede's ice shell. The results indicate that scattering is a significant component of the observed brightness temperature, which is a combination of the upwelling ice emission and reflected emission from the sky and from Jupiter's synchrotron emission (Brown et al., 2023). Retrieval of the sub‐surface ice temperature profile requires that these confounding signals are estimated and removed to isolate the thermal signature of the ice. We present data analysis and model results to estimate the reflected synchrotron emission component. Our results indicate reflected emission over a broad range of observed angles, due to surface roughness and internal scattering. Based on viewing geometry, direct specular reflection from a smooth surface at a narrow angle is not observed. A microwave‐reflective medium is indicated, that is, a very rough surface and/or non‐homogeneous subsurface. Plain Language Summary On 7 June 2021, Juno had a close flyby of Jupiter's moon Ganymede, flying approximately 1,000 km above the surface. During the flyby, Juno's six channel Microwave Radiometer (MWR) mapped a portion of Ganymede, providing the first resolved observations of Ganymede's sub‐surface ice shell. The observed brightness temperature is composed of upwelling thermal emission from the ice shell and reflected radiation from the sky and from Jupiter's synchrotron emission. To study the sub‐surface ice shell temperature profile, we present data analysis and model results to estimate the reflected radiation component. The radiation is reflected diffusively by a very rough surface and/or non‐homogeneous subsurface. Key Points Reflected radiation from the sky and from Jupiter's synchrotron is an important component for Juno microwave radiometer experiment (MWR) observations at 0.6 and 1.2 GHz Absence of specular reflection indicating that Ganymede has a rough surface Reflections originate mostly from internal scattering

Results of simulations with a new high-resolution Martian general circulation model (MGCM) (T106 spectral resolution, or ~67-km horizontal grid size) have been analyzed to reveal global distributions of gravity waves (GWs) during the solstice and equinox periods. They show that shorter-scale harmonics progressively dominate with height, and the body force per unit mass (drag) they impose on the larger-scale flow increases. Mean magnitudes of the drag in the middle atmosphere are tens of meters per second per sol, while instantaneously they can reach thousands of meters per second per sol. Inclusion of small-scale GW harmonics results in an attenuation of the wind jets in the middle atmosphere and in the tendency of their reversal. GW energy in the troposphere due to the shortest-scale harmonics is concentrated in the low latitudes for both seasons and is in a good agreement with observations. The vertical fluxes of wave horizontal momentum are directed mainly against the larger-scale wind. Orographically generated GWs contribute significantly to the total energy of small-scale disturbances and to the drag created by the latter. These waves strongly decay with height, and thus the nonorographic GWs of tropospheric origin dominate near the mesopause. The results of this study can be used to better constrain and validate GW parameterizations in MGCMs.

Monitoring excited hydroxyl (OH*) airglow is broadly used for characterizing the state and dynamics of the terrestrial atmosphere. Recently, the existence of excited hydroxyl was confirmed using satellite observations in the Martian atmosphere. The location and timing of its detection on Mars were restricted to a winter season at the north pole. We present three-dimensional global simulations of excited hydroxyl over a Martian year. The predicted spatio-temporal distribution of the OH* can provide guidance for future observations, namely by indicating where and when the airglow is likely to be detected.

A nonlinear spectral gravity wave (GW) drag parameterization systematically accounting for breaking and dissipation in the thermosphere developed by Yiğit et al. (2008) has been implemented into the University College London Coupled Middle Atmosphere‐Thermosphere‐2 (CMAT2) general circulation model (GCM). The dynamical role of GWs propagating upward from the lower atmosphere has been studied in a series of GCM tests for June solstice conditions. The results suggest that GW drag is not only nonnegligible above the turbopause, but that GWs propagate strongly into the upper thermosphere, and, upon their dissipation, deposit momentum comparable to that of ion drag, at least up to 180–200 km. The effects of thermospheric GW drag are particularly noticeable in the winter (southern) hemisphere, where weaker westerlies and stronger high‐latitude easterlies are simulated well, in agreement with the empirical Horizontal Wind Model (HWM93). The dynamic response in the F region is sensitive to the variations of the source spectrum. However, the spectra commonly employed in middle atmosphere GCMs reproduce the circulation both in the lower and upper thermosphere reasonably well.

Observations of excited hydroxyl (OH*) emissions are broadly used for inferring information about atmospheric dynamics and composition. We present several analytical approximations for characterizing the excited hydroxyl layer in the Martian atmosphere. They include the OH* number density at the maximum and the height of the peak, along with the relations for assessing different impacts on the OH* layer under night-time conditions. These characteristics are determined by the ambient temperature, atomic oxygen concentration, and their vertical gradients. The derived relations can be used for the analysis of airglow measurements and the interpretation of their variations.