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Recent findings of NO near Gale Crater on Mars have been explained by two pathways: formation of nitric acid (HNO3) in a warm climate or formation of peroxynitric acid (HO2NO2) in a cool climate. Here, we put forth two hitherto unexplored pathways: (a) deposition of nitric/peroxynitric acid onto ice particles in a cold atmosphere, which settle quickly onto Mars' surface and (b) solar energetic particle‐induced production of nitric/peroxynitric acid. The deposition rates are enhanced and NO production is more efficient under the higher atmospheric pressures typical of Mars' ancient atmosphere. Depending on the unknown rate at which nitric/peroxynitric acid is lost from the surface, the new pathways could result in larger NO‐levels than those detected by the Mars Science Laboratory. We predict a 2:1 ratio of nitrite:nitrate would have deposited in cool surface climates with an icy atmosphere, whereas orders of magnitude more nitrate than nitrite is expected from warm surface climates. Plain Language Summary The nitrogen oxides discovered in present‐day soil on Mars likely formed in the atmosphere before being deposited on the ground. Two possible mechanisms are deposition of nitric acid (HNO3) when Mars had a warm climate and deposition of peroxynitric acid (HO2NO2) during cold climate. The latter scenario involves processes that have not been considered previously and leads to a much faster deposition rate for nitrogen oxides than was reported in previous studies: solar energetic particles splitting N2 in the middle atmosphere, reactions of nitrogen oxides on the surfaces of ice particles in the atmosphere, and deposition of peroxynitric acid onto the Martian surface when surface pressure was higher. Depending on the unknown rate at which they are lost from the surface due to UV photolysis, the maximum accumulation rate for nitrogen oxides could be much larger than is required to explain the present day measurements. We predict that more nitrite would form than nitrate in a cool climate with an icy atmosphere, whereas in a warm climate much more nitrate than nitrite is expected. So, an investigation of the relative amounts of NO2:NO3 in the soil in the present‐day measurements could reveal the climate state under which the salts formed. Key Points In a cold climate, heterogeneous reactions with atmospheric ice particles would cause faster deposition of HNOx than dry deposition Formation of HNOx species is faster for earlier Martian climates of larger surface pressure Modeled NO accumulates to amounts greater than present‐day measurements, so we propose there may be a loss mechanism that is unidentified

Present-day water loss from Mars provides insight into Mars’s past habitability 1 – 3 . Its main mechanism is thought to be Jeans escape of a steady hydrogen reservoir sourced from odd-oxygen reactions with near-surface water vapour 2 , 4 , 5 . The observed escape rate, however, is strongly variable and correlates poorly with solar extreme-ultraviolet radiation flux 6 – 8 , which was predicted to modulate escape 9 . This variability has recently been attributed to hydrogen sourced from photolysed middle atmospheric water vapour 10 , whose vertical and seasonal distribution is only partly characterized and understood 11 – 13 . Here, we report multi-annual observational estimates of water content and dust and water transport to the middle atmosphere from Mars Climate Sounder data. We provide strong evidence that the transport of water vapour and ice to the middle atmosphere by deep convection in Martian dust storms can enhance hydrogen escape. Planet-encircling dust storms can raise the effective hygropause (where water content rapidly decreases to effectively zero) from 50 to 80 km above the areoid (the reference equipotential surface). Smaller dust storms contribute to an annual mode in water content at 40 − 50 km that may explain seasonal variability in escape. Our results imply that Martian atmospheric chemistry and evolution can be strongly affected by the meteorology of the lower and middle atmosphere of Mars. Mars Climate Sounder’s multi-annual observations of the vertical distribution of water and dust in the Martian atmosphere show that deep convection from dust storms transports water from the lower to the middle atmosphere, enhancing water loss to space.

The Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter is the latest of a series of investigations devoted to improving the understanding of current Martian climate. MCS is a nine‐channel passive midinfrared and far‐infrared filter radiometer designed to measure thermal emission in limb and on‐planet geometries from which vertical profiles of atmospheric temperature, water vapor, dust, and condensates can be retrieved. Here we describe the algorithm that is used to retrieve atmospheric profiles from MCS limb measurements for delivery to the Planetary Data System. The algorithm is based on a modified Chahine method and uses a fast radiative transfer scheme based on the Curtis‐Godson approximation. It retrieves pressure and vertical profiles of atmospheric temperature, dust opacity, and water ice opacity. Water vapor retrievals involve a different approach and will be reported separately. Pressure can be retrieved to a precision of 1–2% and is used to establish the vertical coordinate. Temperature profiles are retrieved over a range from 5–10 to 80–90 km altitude with a typical altitude resolution of 4–6 km and a precision between 0.5 and 2 K over most of this altitude range. Dust and water ice opacity profiles also achieve vertical resolutions of about 5 km and typically have precisions of 10−4–10−5 km−1 at 463 cm−1 and 843 cm−1, respectively. Examples of temperature profiles as well as dust and water ice opacity profiles from the first year of the MCS mission are presented, and atmospheric features observed during periods employing different MCS operational modes are described. An intercomparison with historical temperature measurements from the Mars Global Surveyor mission shows good agreement.

We present south polar winter infrared observations from the Mars Climate Sounder (MCS) and test three hypotheses concerning the origins of “cold spots”: regions of anomalously low infrared brightness temperatures, which could be due to enrichment in non‐condensable gases, low‐emissivity surface frost, or optically thick CO2 clouds. Clouds and surface frosts have been historically difficult to distinguish, but the unique limb sounding capability of MCS reveals extensive tropospheric CO2clouds over the cold spots. We find that both clouds and surface deposits play a significant role in lowering the infrared emissivity of the seasonal ice cap, and the granular surface deposits are likely emplaced by snowfall. Surface temperatures indicate the polar winter atmosphere is enriched by a factor ∼5–7 in non‐condensable gases relative to the annual average, consistent with earlier gamma ray spectrometer observations, but not enough to account for the low brightness temperatures. A large ∼500‐km diameter cloud with visible optical depth ∼0.1–1.0 persists throughout winter over the south polar residual cap (SPRC). At latitudes 70–80°S, clouds and low emission regions are smaller and shorter‐lived, probably corresponding to large‐grained “channel 1” clouds observed by the Mars Orbiter Laser Altimeter. Snowfall over the SPRC imparts the lowest emissivity in the south polar region, which paradoxically tends to reduce net accumulation of seasonal CO2 by backscattering infrared radiation. This could be compensated by the observed anomalously high summertime albedo of the SPRC, which may be related to small grains preserved in a rapidly formed snow deposit. Key Points The snowiest place in the south polar region is the south polar residual cap A separate class of small, short‐lived CO2 clouds predominate 70‐80 S

We have used observations from the Mars Climate Sounder (MCS) to investigate the north polar hood (NPH) water ice clouds, including the first systematic examination of the vertical and nighttime structure. We show that the NPH clouds are present between LS = 150° (early autumn) and 30° (late spring) and that the clouds always extend to the pole. The daytime (1500 LMST) and nighttime (0300 LMST) clouds both have one layer that extends in altitude from 10 to 40 km above the surface, and the layer falls from its peak with a constant mixing ratio. We find that the cloud optical depth is controlled by the atmospheric thermal structure. The nighttime optical depth values are often higher than the daytime, sometimes due to tidally driven diurnal temperature differences and other times (i.e., LS = 240°–330°) a result of low temperatures associated with the polar vortex at night. We conclude that polar hood clouds are primarily controlled by the temperature structure and form at the water condensation level.

The Ensemble Mars Atmosphere Reanalysis System (EMARS) dataset version 1.0 contains hourly gridded atmospheric variables for the planet Mars, spanning Mars Year (MY) 24 through 33 (1999 through 2017). A reanalysis represents the best estimate of the state of the atmosphere by combining observations that are sparse in space and time with a dynamical model and weighting them by their uncertainties. EMARS uses the Local Ensemble Transform Kalman Filter (LETKF) for data assimilation with the GFDL/NASA Mars Global Climate Model (MGCM). Observations that are assimilated include the Thermal Emission Spectrometer (TES) and Mars Climate Sounder (MCS) temperature retrievals. The dataset includes gridded fields of temperature, wind, surface pressure, as well as dust, water ice, CO2 surface ice and other atmospheric quantities. Reanalyses are useful for both science and engineering studies, including investigations of transient eddies, the polar vortex, thermal tides and dust storms, and during spacecraft operations.

We have used observations from the Mars Climate Sounder to investigate south polar hood water ice clouds (at 12 μm), including the first systematic examination of the vertical (5 km resolution) and nighttime structure. We find that the structure and evolution of the polar hood is controlled more strongly by atmospheric temperature variations than by intrinsic fluctuations in water vapor abundance. The clouds form as a belt during LS = 10°–70° (phase 1) and LS = 100°–200° (phase 2). During phase 1, the cloud belt extends over a wide latitude range, between 30°S and 75°S with a visible column optical depth between 0.075 and 0.15. The cloud belt then evaporates as temperatures warm. During phase 2, the cloud belt reappears due to an increase in water vapor as a partial band of low‐opacity clouds south of the Tharsis region and eventually becomes continuous in longitude, with a visible column opacity between 0.125 and 0.25. As the southern spring equinox approaches, the cloud belt shifts southward, following the seasonal cap edge. From LS = 140° to LS = 200°, the daytime belt lies about 15° farther south than the nighttime belt, due to tidally driven diurnal temperature differences. The vertical structure of the cloud belt is consistent within and between the two seasonal phases and is characterized by a thick lower cloud deck and an upper layer whose altitude shifts between the nighttime and daytime because of thermal tidal control of the condensation altitudes. Overall, the southern polar hood is observed to rapidly form and dissipate as the temperature crosses the saturation point of water vapor.

Nitrous oxide (N2O) is the fourth most important greenhouse gas in the atmosphere and is considered the most important current source gas emission for global stratospheric ozone depletion (O3). It has natural and anthropogenic sources, mainly as an unintended by-product of food production activities. This work examines the identification and quantification of trends in the N2O concentration from the middle troposphere to the middle stratosphere (MTMS) by in situ and remote sensing observations. The temporal variability of N2O is addressed using a comprehensive dataset of in situ and remote sensing N2O concentrations based on aircraft and balloon measurements in the MTMS from 1987 to 2018. We determine N2O trends in the MTMS, based on observations. This consistent dataset was also used to study the N2O seasonal cycle to investigate the relationship between abundances and its emission sources through zonal means. The results show a long-term increase in global N2O concentration in the MTMS with an average of 0.89 ± 0.07 ppb/yr in the troposphere and 0.96 ± 0.15 ppb/yr in the stratosphere, consistent with 0.80 ppb/yr derived from ground-based measurements and 0.799 ± 0.024 ppb/yr ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) satellite measurements.

The radiant energy budget of a planet is essential to understanding its surface and atmospheric processes. Here, we report systematic measurements of Mars’ emitted power, which are used to estimate the radiant energy budget of the red planet. Based on the observations from Mars Global Surveyor, Curiosity, and InSight, our measurements suggest that Mars’ global-average emitted power is 111.7 ± 2.4 W m−2. More importantly, our measurements reveal strong seasonal and diurnal variations of Mars’ emitted power. The strong seasonal variations further suggest an energy imbalance at the time scale of Mars’ seasons (e.g., ∼15.3% of the emitted power in the Northern autumn for the Southern Hemisphere), which could play an important role in generating dust storms on Mars. We also find the 2001 global dust storm decreased the global-average emitted power by ∼22% during daytime but increased the global-average emitted power by ∼29% at nighttime. This suggests that global dust storms play a significant role in Mars’ radiant energy budget.