Over the last 16 years, the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter (MRO) has acquired a large body of surface observations at visible and thermal infrared wavelengths. Primary differentiators between the MCS record and other surface datasets include mission duration, regular global coverage, and high emission angles associated with most observations. These data have been analyzed to generate a global median apparent thermal inertia map that smooths out artificial spatial variability (i.e., streaking along orbital ground tracks) common in other maps. At mid and low latitudes, high emission angle observations yield similar nighttime thermal inertia values compared to nadir observations, even if grazing angles should favor vertically rough materials oriented towards MCS (i.e., material poking out of the ground, presumably associated with high thermal inertia rocks and scarps). This result independently confirms that fines control the Martian thermal inertia, not rocks, and also suggests that most rock and bedrock exposures should be characterized by low aspect ratios (i.e., appear platy or flat, regardless of subsurface shape). Selected global temperature maps are also presented. They show the influence of polar processes, the latitudinal distribution of exposed ices, and physical properties of the Martian regolith on surface temperatures. Temperature controls at regional and global scales include the variation of insolation with season and latitude, atmospheric composition, global circulation, the presence of snow precipitations, and various regolith properties (i.e., albedo and thermal inertia).

The kinetic temperature of the Martian seasonal caps is controlled by the partial pressure of atmospheric CO2 at the surface. When carbon dioxide condenses, typically near the poles, light non‐condensable species (Ar, N2, CO, etc.) accumulate in the atmosphere, resulting in a decrease of the CO2 partial pressure and depressing the local frost point temperature. The buoyant air should mix laterally and vertically within the polar vortices. Observations show that the Martian seasonal caps' kinetic temperatures are ∼0–4 K below the expected CO2 frost point, depending on latitude and season, indicating atmospheric CO2 gas depletion at the surface/atmosphere interface. In the North and South, we find relatively similar non‐condensable peak enhancement factors (e.g., EFNC ∼ 6–8, up to ∼8.7 in the North) at most latitudes, confirming the efficient meridional mixing within the polar vortices, despite steep surface condensation gradients. In the South, this surface enhancement is similar to column‐integrated values derived from Gamma Ray Spectrometer data, indicating efficient vertical mixing. But in the North, the surface depletion is much larger than in the entire column, suggesting poor vertical mixing. Reduced infrared emission of the seasonal caps stemming from CO2 depletion is not a major energy balance factor. This work illustrates how the atmosphere's composition at the surface can be significantly different from column‐integrated values.

Gravity waves are one way Mars’s lower atmospheric weather can affect the circulation and even composition of Mars’s middle and upper atmosphere. A recent study showed how on-planet observations near the center of the 15 μm CO2 band by the A3 channel (635–665 cm−1) of the Mars Climate Sounder on board the Mars Reconnaissance Orbiter could sense horizontally short, vertically broad gravity waves at ≈25 km above the surface by looking at small-scale radiance variability in temperature-sensitive channels. This approach is extended here to two additional channels closer to the wings of the 15 μm CO2 band, A1 (595–615 cm−1) and A2 (615–645 cm−1), to sense gravity waves throughout the lower atmosphere. Using information from all three channels demonstrates that gravity-wave activity in Mars’s lowermost atmosphere is dominated by orographic sources, particularly over the extremely rough terrain of Valles Marineris. Much of this orographic population is either trapped or filtered in the lowest two scale heights, such that variations in filtering and nonorographic sources shape the gravity-wave population observed at 25 km above the surface. During global dust storms, however, gravity-wave activity in the first scale height decreases by approximately a factor of 2, yet trapping/filtering of what activity remains in the tropics substantially weakens. Exceptionally high radiance variability at night in the tropics during the less dusty part of the year is the result of observing mesospheric clouds rather than gravity waves.

Gravity waves in Mars’s atmosphere strongly affect the general circulation as well as middle atmospheric cloud formation, but the climatology and sources of gravity waves in the lower atmosphere remain poorly understood. At Earth, the statistical variance in satellite observations of thermal emission above the instrumental noise floor has been used to enable measurement of gravity wave activity at a global scale. Here is presented an analysis of variance in calibrated radiance at 15.4μm (635–665 cm−1) from off-nadir and nadir observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO); a major expansion in the observational data available for validating models of Martian gravity wave activity. These observations are sensitive to gravity waves at 20–30 km altitude with wavelength properties (λh=10–100 km, λz> 5 km) that make them likely to affect the dynamics of the middle and upper atmosphere. We find that: (1) strong, moderately intermittent gravity wave activity is scattered over the tropical volcanoes and throughout the middle to high latitudes of both hemispheres during fall and winter, (2) gravity wave activity noticeably departs from climatology during regional and global dust storms; and (3) strong, intermittent variance is observed at night in parts of the southern tropics during its fall/winter, but frequent CO2 ice clouds prevents unambiguous attribution to GW activity. The spatial distribution of wave activity is consistent with topographic sources being dominant, but contributions from boundary layer convection and other convective processes are possible.

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.

The remote sounding of infrared emission from planetary atmospheres using limb-viewing geometry is a powerful technique for deriving vertical profiles of structure and composition on a global scale. Compared with nadir viewing, limb geometry provides enhanced vertical resolution and greater sensitivity to atmospheric constituents. However, standard limb profile retrieval techniques assume spherical symmetry and are vulnerable to biases produced by horizontal gradients in atmospheric parameters. We present a scheme for the correction of horizontal gradients in profile retrievals from limb observations of the martian atmosphere. It characterizes horizontal gradients in temperature, pressure, and aerosol extinction along the line-of-sight of a limb view through neighboring measurements, and represents these gradients by means of two-dimensional radiative transfer in the forward model of the retrieval. The scheme is applied to limb emission measurements from the Mars Climate Sounder instrument on Mars Reconnaissance Orbiter. Retrieval simulations using data from numerical models indicate that biases of up to 10K in the winter polar region, obtained with standard retrievals using spherical symmetry, are reduced to about 2K in most locations by the retrieval with two-dimensional radiative transfer. Retrievals from Mars atmospheric measurements suggest that the two-dimensional radiative transfer greatly reduces biases in temperature and aerosol opacity caused by observational geometry, predominantly in the polar winter regions.

While the detection of CO2 ice has only been reported outside the Martian polar regions at very high elevation (i.e., Elysium, Olympus Mons, and the Tharsis Montes), nighttime surface observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter document the widespread occurrence of atmospherically corrected ground temperatures consistent with the presence of extensive carbon dioxide frost deposits in the dusty low thermal inertia units at middle/low latitudes. Thermal infrared emissivities, interpreted in conjunction with mass balance modeling, suggest micrometer size CO2 ice crystals forming optically thin layers never exceeding a few hundreds of microns in thickness (i.e., 10(-2)kgm(-2)) locally, which is insufficient to generate a measurable diurnal pressure cycle (<<0.1% of the Martian atmosphere). Atmospheric temperatures at middle/low latitudes are not consistent with precipitation of CO2 ice, suggesting that condensation occurs on the surface. The recurring growth and sublimation of CO2 ice on Martian dusty terrains may be an important process preventing soil induration and promoting dynamic phenomena (soil avalanching and fluidization and regolith gardening), maintaining a reservoir of micrometer size dust particles that are mobile and available for lifting. The discovery of this diurnal CO2 cycle represents an important step forward in our understanding of the way the Martian atmosphere interacts with the surface.

It has been established that dust in the atmosphere of Mars is not distributed homogeneously in the vertical but exhibits layering in the lower atmosphere. Recently published results also suggest a dust maximum in the middle atmosphere that predominantly occurs at 50-60 km altitude on the daytime hemisphere. We use measurements from the Mars Climate Sounder to investigate the distribution of dust above similar to 40 km altitude. Our results do not support the existence of widespread dust in the middle atmosphere of Mars inferred from earlier observations. The average dust extinction does not exceed 10(-6) km(-1) at 463 cm(-1) above 50 km altitude in atmospheric conditions without large dust storms. (C) 2015 Elsevier Inc. All rights reserved.

We present eight Mars Years of nearly continuous tracking of the CO2 seasonal cap edges from Mars Year (MY) 24 to 31 using Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and Mars Reconnaissance Orbiter (MRO) Mars Climate Sounder (MCS) thermal infrared data. Spatial and temporal resolutions are 1 pixel per degree and 10 degrees L-s (aerocentric longitude of the Sun). The seasonal caps are defined as the regions where the diurnal radiometric temperature variations at similar to 32 mu m wavelength do not exceed 5 K. With this definition, terrains with small areal fraction of defrosted regolith able to experience measurable diurnal temperature cycles are not mapped as part of the cap. This technique is adequate to distinguish CO2 from H2O ices, and effective during the polar night or under low illumination conditions. The present analysis answers outstanding questions stemming from fragmented observations at visible wavelengths: (1) the previously sparsely documented growth of the North seasonal caps (160 degrees < L-s < 270 degrees) is shown to be repeatable within 1-2 degrees equivalent latitude, and monotonic over the MY 24-31 time period; high repeatability is observed during the retreat of the caps in non-dusty years (similar to 1 degrees or less equivalent latitude); (2) the MY 25 storm does not seem to have impacted the growth rate, maximal extents, or recession rate of the North seasonal caps, whereas the MY 28 dust storm clearly sped up the recession of the cap (similar to 2 degrees smaller on average after the storm, during the recession, compared to other years); (3) during non-dusty years, the growth of the South seasonal cap (350 degrees < L-s < 100 degrees) presents noticeable variability (up to 4 equivalent latitude near L-s = 20 degrees) with a maximum extent reached near L-s = 90 degrees; (4) the retreat of the Southern seasonal cap (100 degrees < L-s < 310 degrees) exhibits large inter-annual variability, especially near 190 degrees < L-s < 220 degrees; (5) the recession of the MY 25 South seasonal cap is significantly accelerated during the equinox global dust storm, with surface temperatures suggesting increased patchiness or enhanced dust mantling on the CO2 ice. These results suggest that atmospheric temperatures and dust loading are the primary source of variability in an otherwise remarkably repeatable cycle of seasonal cap growth and recession. (C) 2014 Elsevier Inc. All rights reserved.

The Mars Reconnaissance Orbiter (MRO) performs radio occultation (RO) measurements on selected orbits, generally once per day. We have retrieved atmospheric profiles from two subsets of data, yielding a variety of new results that illustrate the scientific value of the observations. One set of measurements sounded the tropics in northern summer at a local time similar to 1 h before sunrise. Some of these profiles contain an unexpected layer of neutral stability with a depth of similar to 4 km and a pressure at its upper boundary of similar to 160 Pa. The mixed layer is bounded above by a temperature inversion and below by another strong inversion adjacent to the surface. This type of structure is observed near Gale Crater, in the Tharsis region, and at a few other locations, whereas profiles in Amazonis Planitia and Elysium Planitia show no sign of a detached mixed layer with an overlying inversion. We supplemented the measurements with numerical simulations by the NASA Ames Mars General Circulation Model, which demonstrate that water ice clouds can generate this distinctive type of temperature structure through their influence on radiative transfer at infrared wavelengths. In particular, the simulations predict the presence of a nocturnal cloud layer in the Tharsis region at a pressure of similar to 150 Pa (similar to 10 km above the surface), and the nighttime radiative cooling at cloud level is sufficient to produce a temperature inversion above the cloud as well as convective instability below the cloud, consistent with the observations. The second set of measurements sounded mid-to-high northern latitudes in spring, when carefully coordinated observations by the MRO Mars Climate Sounder (MCS) are also available. The differences between the RO and MCS temperature profiles are generally consistent with the expected performance of the two instruments. Within this set of 21 comparisons the average temperature difference is less than 1 K where the aerosol opacities are smaller than 10(-3) km(-1), at pressures of 10-50 Pa, whereas it increases to similar to 2 K where the aerosol opacities exceed this threshold, at pressures of 50-300 Pa. The standard deviation of the temperature difference is similar to 2 K, independent of pressure. The second set of RO measurements also provides unique information about the stability of the annual CO2 cycle and the dynamics near the edge of the seasonal CO2 ice cap. (C) 2014 Elsevier Inc. All rights reserved.

We exploit the relative stability and repeatability of the Mars atmosphere at aphelion for an inter-comparison of Mars Global Surveyor/Thermal Emission Spectrometer (MGS/TES) and Mars Reconnaissance Orbiter/Mars Climate Sounder (MRO/MCS) nighttime temperature profiles and aerosol opacity profiles in Mars years 25, 26, 29, 30, and 31. Cross-calibration of these datasets is important, as they together provide an extended climatology for this planetary atmosphere. As a standard of comparison we employ temperature profiles obtained by radio occultation methods during the MGS mission in Mars years 24, 25, and 26. We first compare both zonal mean TES limb sounding profiles and zonal mean MCS limb sounding profiles with zonal means of radio occultation temperature profiles for the same season (L-s = 70-80 degrees) and latitudes (55-70 degrees N). We employ a statistical z test for quantifying the degree of agreement of temperature profiles by pressure level. For pressures less than 610 Pa (altitudes > 3 km), the ensemble mean temperature difference between the radio occultation and TES limb sounding profiles found in these comparisons was 1.7 +/- 0.7 K. The ensemble mean temperature difference between radio occultation and MCS profiles was 1.4 +/- 1.0 K. These differences fall within the formal error estimates for both TES and MCS, validating the accuracy of the instruments and their respective retrieval algorithms. In the second phase of our investigation, we compare aphelion season zonal mean TES limb sounding temperature, water ice opacity, and dust opacity profiles with those obtained at the same latitudes in different years by MCS. The ensemble mean temperature difference found for three comparisons between TES and MCS zonal mean temperature profiles was 2.8 +/- 2.1 K. MCS and TES temperatures between 610 Pa and 5 Pa from 55 to 70 degrees N are largely in agreement (with differences < 2 K) when water ice aerosol opacities are comparable. Temperature differences increase when the opacities are dissimilar; TES profiles exhibit colder temperatures when TES water ice opacities are greater than those observed by MCS. Our comparisons reveal a possible systematic offset of TES and MCS temperatures at the highest altitudes resolved in the TES retrievals; TES temperatures are consistently colder than the corresponding MCS temperatures at pressures <= 1 Pa (altitudes >= 58 km). We otherwise find no evidence of systematic bias between TES limb sounding and MCS retrieved atmospheric quantities between 610 Pa and 1 Pa. Inter-annual variability is noted in comparisons of latitudinal temperature gradients from 55 to 70 degrees N, in the amplitude of inversions linked with thermal tides in the middle atmosphere, and in the abundance and vertical distribution of water ice aerosols from 55 to 70 degrees N during the aphelion season. (C) 2014 Elsevier Inc. All rights reserved.

Mars Climate Sounder (MCS) nadir oriented thermal infrared and solar channel measurements are compared with Thermal Emission Spectrometer (TES) measurements across multiple Mars years. Thermal infrared measurements were compared by convolving the TES data using the MCS spectral band passes. The MCS solar channel measurements were calibrated using Compact Reconnaissance Imaging Spectrometer for Mars observations to provide the proper gain factor (3.09 x 10(-3) W sr(-1) m(-2) mu m(-1)). The comparisons of the datasets show that day and night surface and atmospheric temperatures are within 3 K over the course of 5 martian years, after accounting for the local time differences. Any potential interannual variations in global average temperature are masked by calibration and modeling uncertainties. Previous work attributed apparent interannual global surface and atmospheric temperature variations to major dust storm activity; however, this variation has since been attributed to a calibration error in the TES dataset that has been corrected. MCS derived Lambert albedos are slightly higher than TES measurements acquired over the same season and locations. Most of this difference can be attributed to the spectral response functions of MCS and TES. Consistent with previous work, global albedo is highly variable (similar to 6%) and this variability must be taken into account when determining long term global trends. Vertical aerosol distributions were also derived from the calibrated MCS visible channel limb measurements, demonstrating the utility of the MCS visible channel data for monitoring of aerosols. (C) 2013 Elsevier Inc. All rights reserved.

Atmospheric thermal tides are global oscillations in atmospheric fields that are subharmonics of a solar day. While atmospheric tides on Earth are mainly relevant in the upper atmosphere, on Mars, they dominate temperature variations and winds throughout the atmosphere. Observations and model simulations to date have suggested that the migrating diurnal tide is the predominant mode in the Martian atmosphere, and that the semidiurnal tide is only relevant in the tropical middle atmosphere during conditions of high dust loading. New comprehensive observations by the Mars Climate Sounder in a geometry that allows coverage of multiple local times show that the semidiurnal tide is a dominant response of the Martian atmosphere throughout the Martian year. The maximum semidiurnal amplitude of similar to 16 K is found at southern winter high latitudes, which makes it the largest tidal amplitude observed in the Martian middle atmosphere outside of dust storm conditions. The semidiurnal tide can be successfully modeled due to recent advances of Mars General Circulation Models (MGCMs) that include the radiatively active treatment of water ice clouds. Tidal forcing occurs through absorption of radiation by aerosols and points to the vertical structure of dust and clouds and their radiative effects as being essential for our understanding of the thermal structure and the general circulation of the Martian atmosphere. As with terrestrial GCMs trying to quantify mechanisms affecting climate, future Mars modeling efforts will require microphysical schemes to control aerosol distributions, and vertically and temporally resolved measurements of temperature and aerosols will be essential for their validation.

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.

We present a single-scattering approximation for infrared radiative transfer in limb geometry in the Martian atmosphere. It is based on the assumption that the upwelling internal radiation field is dominated by a surface with a uniform brightness temperature. It allows the calculation of the scattering source function for individual aerosol types, mixtures of aerosol types, and mixtures of gas and aerosol. The approximation can be applied in a Curtis–Godson radiative transfer code and is used for operational retrievals from Mars Climate Sounder measurements. Radiance comparisons with a multiple scattering model show good agreement in the mid- and far-infrared although the approximate model tends to underestimate the radiances in realistic conditions of the Martian atmosphere. Relative radiance differences are found to be about 2% in the lowermost atmosphere, increasing to ∼10% in the middle atmosphere of Mars. The increasing differences with altitude are mostly due to the increasing contribution to limb radiance of scattering relative to emission at the colder, higher atmospheric levels. This effect becomes smaller toward longer wavelengths at typical Martian temperatures. The relative radiance differences are expected to produce systematic errors of similar magnitude in retrieved opacity profiles.

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.