Polar Hood Research Articles

Far infrared radiative properties of Mars atmospheric aerosols and their application to Mars Climate Sounder retrievals of aerosol profiles, aerosol columns and surface temperatures

Limb sounding of thermal emission in the infrared wavelength range is a powerful technique for measuring temperature and aerosols in the martian atmosphere. However, the long optical path may provide challenges to limb retrievals in high aerosol conditions. These can be mitigated by considering limb measurements in the far infrared, where opacities of most aerosols are lower than in the mid-infrared. We present analyses of radiative properties of Mars dust and water ice aerosols at far infrared wavelengths based on measurements by the Mars Climate Sounder (MCS) in limb geometry at mid- and far infrared wavelengths. For dust aerosols, derived far infrared radiative properties show a homogeneous behavior that is consistent with particle sizes in the order of 1 μm effective radius. Far infrared radiative properties for water ice aerosols exhibit a larger variability in local time and region, leading to significant differences between the aphelion cloud belt and the north polar hood, with the resulting parameters suggesting particle sizes around 3 μm or larger. Using the derived parameters, we develop a method for retrieving aerosol profiles from MCS limb measurements that combines information from mid- and far infrared spectroscopic channels. The use of far infrared channels enables aerosol profile retrievals from limb measurements that typically reach about a scale height deeper into the atmosphere than would be possible using mid-infrared channels only. The extended vertical range of the aerosol profiles allows the derivation of aerosol column optical depths through vertical integration, with dust column derivations in global or large-scale regional dust storms also being available by extrapolating dust profiles below the lowest retrievable altitude of a limb measurement. The quantification of aerosol columns allows us to retrieve surface brightness temperatures from MCS on-planet viewing measurements that are corrected for atmospheric contributions. We show that differences between surface brightness and top-of-the-atmosphere temperatures are typically within 20 K, with surface brightness temperatures generally being warmer (colder) than top-of-the-atmosphere temperatures at daytime (nighttime), except at high latitudes.

The Temporal Brightening of Uranus' Northern Polar Hood From HST/WFC3 and HST/STIS Observations

AbstractHubble Space Telescope Wide‐Field Camera 3 (HST/WFC3) observations spanning 2015 to 2021 confirm a brightening of Uranus' north polar hood feature with time. The vertical aerosol model of Irwin et al. (2023, https://doi.org/10.1038/s41550-023-02047-0) (IRW23), consisting of a deep haze layer based at ∼5 bar, a 1–2 bar haze layer, and an extended haze rising up from the 1–2 bar layer, was applied to retrievals on HST Space Telescope Imaging Spectrograph (STIS) (HST/STIS) observations (Sromovsky et al., 2014, 2019, https://doi.org/10.1016/j.icarus.2014.05.016, https://doi.org/10.1016/j.icarus.2018.06.026) revealing a reduction in cloud‐top CH4 volume mixing ratio (VMR) (i.e., above the deep ∼5 bar haze) by an average of 0.0019 ± 0.0003 between 40–80◦N (∼10% average reduction) from 2012 to 2015. A combination of latitudinal retrievals on the HST/WFC3 and HST/STIS data sets, again employing the IRW23 model, reveal a temporal thickening of the 1–2 bar haze layer to be the main cause of the polar hood brightening, finding an average increase in integrated opacity of 1.09 ± 0.08 (∼33% increase) at 0.8 µm north of ∼45°N, concurrent with a decrease in the imaginary refractive index spectrum of the 1–2 bar haze layer north of ∼40°N and longwards of ∼0.7 µm. Small contributions to the brightening were found from a thickening of the deep aerosol layer, with an average increase in integrated opacity of 0.6 ± 0.1 (58% increase) north of 45°N between 2012 and 2015, and from the aforementioned decrease in CH4 VMR. Our results are consistent with the slowing of a stratospheric meridional circulation, exhibiting subsidence at the poles.

Mars Global Surveyor's Mars Orbiter Camera (MOC) Wide‐Angle Images (1999–2006): 2. Data Investigation Into North Polar Hood Formation, Broad Brightness Changes in Acidalia, and Seasonal Frost in Hellas

AbstractMars Orbiter Camera Wide Angle images were taken from late 1999 through most of 2006, in both red and blue, providing nearly daily, nearly global coverage of Mars for approximately four Mars Years. Significant work has been published from those data, but there did not exist in a public archive a uniform, full time series of the processed data for easier analysis. The companion work to this (Robbins, 2023a, https://doi.org/10.1029/2022EA002673) presents that processing and quality control of the data, producing the full time series in both colors, in equirectangular and polar stereographic projections, in mosaics that have been binned into four different time intervals (daily, ΔLs = 2°, 5°, and 10° [≈4 through 19 days]) and as approximately true‐color composites. The multidimensional data set can be mined for surface and atmospheric variations on Mars. To demonstrate agreement with past work, several well‐known trends are demonstrated in a review‐style narrative format in Supporting Information S1, including changes after a global dust storm, seasonal frost variations at the poles, and surface variations. The data are analyzed further in the main text to demonstrate new qualitative and quantitative analyses of atmospheric and surface features: the North Polar Hood can form earlier than previously demonstrated, Acidalia has significant seasonal brightness variations that can be quantified, and the seasonal frost deposition and sublimation of Hellas can be quantified and shown to vary from year‐to‐year. These different investigations represent a small slice of what can be done now that these data are made available and easily accessible.

The Mars Atmosphere Water Ice Aerosol Climatology by MRO/CRISM: 5 Mars Years of Observations

AbstractWe use near‐infrared spectra returned from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) to provide retrievals of the column‐integrated optical depth of water ice aerosols for more than 5 Martian years between Mars Year (MY) 28 at Ls = 112° (27 September 2006) and MY 33 at Ls = 213° (30 August 2016). We have developed a radiative transfer model for this purpose that retrieves aerosol opacity from the water ice aerosol feature near 3.3 μm using CRISM hyperspectral observations for the first time. The resulting retrievals well depict the main features in the water ice aerosol climatology with repeatable patterns every Martian year. The aphelion cloud belt (ACB) is observed between 10°S and 30°N, with peak optical depth around Ls = 90°. Apart from the global dust storm during MY 28, modest interannual variability in the water ice aerosol optical depth is observed in the ACB. The north polar hood is observed at latitudes poleward of 45°N throughout northern spring and summer seasons and reappears in northern winter. The south polar hood is observed in early to mid‐southern autumn, reaching latitudes around 60°S, and re‐emerging during southern winter between 45°S and 60°S. A lack of water ice aerosols is repeatedly seen in the southern hemisphere throughout southern spring and summer. High optical depth in the ACB is observed over the volcanoes of Olympus Mons, Elysium, and the Tharsis bulge, as well as over Hellas Basin.

Seasonal variations of orographic clouds on Mars with MRO/MARCI observations and the Mars Planetary Climate Model

Although an appreciable amount of research has been conducted studying the Aphelion Cloud Belt (ACB) and Polar Hoods (PHs), much less attention has been given to orographic clouds on Mars. Orographic clouds (afternoon) are generally observed in northern Spring and Summer since they are associated with the significant Martian volcanoes. The water ice optical depths near 15h00 LTST (Local True Solar Times) provided by the Mars Color Imager (MARCI) of Mars Reconnaissance Orbiter (MRO) are used to investigate seasonal and spatial variations of orographic clouds in four volcanic regions: Alba Patera, Olympus Mons, Tharsis region and Elysium Mons. The seasonal variations of water ice clouds over volcanic areas need to be better understood. Thus, context will be provided using the meteorological fields from the Mars PCM (Mars Planetary Climate Model led by the Laboratoire de Météorologie Dynamique Paris, France). This study provides a general picture of how Martian afternoon water ice clouds correlate with Mars PCM’s meteorological variables: water ice column, atmospheric temperature, meteorological winds, and water vapor mixing ratio. Comparison between the model and the MARCI observations show that spatial and seasonal variations of atmospheric temperature, water vapor, and meteorological winds strongly control the water ice cloud distribution during northern hemispheric Spring and Summer.

Revisiting Atmospheric Features of Mars Orbiter Laser Altimeter Data Using Machine Learning Algorithms

AbstractThe Mars Orbiter Laser Altimeter (MOLA) instrument has been drawing a map of Mars' topography between September 1997 and June 2001. It has also been able to observe clouds during the mission duration, providing data for the low Martian atmosphere for nearly 1.5 Mars years. The Mars Global Surveyor, which carried MOLA, also carried two other instruments that also observed clouds during the same time period (the Mars Orbiter Camera and the Thermal Emission Spectrometer). Combining observations from these three data sets could provide a complete recap of most atmospheric structures during MY24 and MY25. However, previous studies of MOLA data set often had to use stringent detection criteria. Using machine learning clustering methods, we end up finding way more atmospheric returns. Our results are presented in the form of an atmospheric features catalog that we then use to compare MOLA observations with Mars Orbiter Camera and Thermal Emission Spectrometer results, but also with more recent missions. We study the development of recurrent phenomenon in the Martian atmosphere, like the aphelion cloud belt or the south polar hood, but also spontaneous events such as regional dust storms. Methods could be tuned even more finely by using more complex clustering methods or deep learning algorithms to clearly distinguish atmospheric structures.

Evaluation of the Capability of ExoMars-TGO NOMAD Infrared Nadir Channel for Water Ice Clouds Detection on Mars

As part of the payload of the 2016 ExoMars Trace Gas Orbiter (TGO) mission, the Nadir and Occultation for MArs Discovery (NOMAD) suite instrument has been observing the Martian atmosphere since March 2018. NOMAD is mainly dedicated to the study of trace atmospheric species taking advantage of a high-spectral resolution. We demonstrate that when NOMAD is observing in nadir mode, i.e., when the line-of-sight points to the centre of Mars, it can be also exploited to detect ice. In this study we present a method based on the investigation of nadir observations of the NOMAD infrared channel, acquired during Mars Years 34 and 35 (March 2018 to February 2021). We take advantage of the strong water ice absorption band at 2.7 µm by selecting the diffraction orders 167, 168, and 169. We derive the Frost and Clouds Index (FCI), which is a good proxy for ice mapping, and obtain latitudinal-seasonal maps for water ice clouds. FCI is sensitive to the Polar Hood clouds. Nevertheless, detections in the Aphelion Cloud Belt (ACB) are limited. This is consistent with previous observations showing different physical properties between the two main Martian atmospheric structures and making the ACB less detectable in the infrared. We hence derive the infrared nadir channel sensitivity limit for the detection of these clouds.

Flow patterns of Jupiter's south polar region

Previous studies of Jupiter's wind patterns revealed the southernmost two prograde jets at 58°S and 64°S (planetocentric), which we designate as the S5 and S6 jets, respectively, but the jets and the wind patterns further poleward were not well defined. The Juno mission has provided the first opportunity to study the South Polar Region (SPR). Here we use images from Juno's camera, JunoCam, to characterize these jets and the wind patterns further south. We measure the main wind systems using JunoCam images taken up to two hours apart. The S5 jet coincides with a slightly sinuous boundary in methane-band images. The S6 jet is faster, broader and highly undulating in latitude, generally coinciding with the sinuous edge of the methane-bright South Polar Hood, whose wave pattern is often regular with a mean wavelength of 25.5° (±2.6°) longitude. Peak wind speeds along the S6 jet range from 42 (±12) to 49 (±11) m/s, faster than previously recognized. Poleward of the S6 jet, at ~65–70°S, there is an irregular belt of chaotic cyclonic regions termed folded filamentary regions (FFRs), with several small anticyclonic white ovals (AWOs) on or near its southern edge. Some of these FFRs appear to be extending northeast into the S6 jet. More FFRs are scattered from ~70 to 80°S. Wind speeds in the FFRs are generally ~20–60 m/s, comparable to lower-latitude cyclonic circulations. We also generate semi-quantitative maps of local vorticity, and thence, mean zonal vorticity profiles as a function of latitude. These confirm the S6 jet as sinuous and the southernmost belt of FFRs as a stable belt. Further south, there is usually a weak cyclonic vorticity maximum near 78°S, which probably represents irregular structures such as FFRs. Supplementary ground-based images, spaced by up to 4 days, show westward drifts for the ~65–70°S belt, with a mean of ~ + 0.9°/day in System III longitude. AWOs can also be tracked, for months or even years. They drift westward with a uniform speed of +0.8°/day between 69.5 and 72.4°S, but towards higher latitudes, up to 76°S, they show a steep latitudinal gradient to faster (eastward) speeds, comparable to those measured around the south polar pentagon of circumpolar cyclones at ~80°S. Overall, the ~65–70°S belt and associated AWOs show dynamical behaviour similar to lower latitudes. Further south, there are no rapid continuous jets, but the loose enhancement of cyclonic structures near 78°S suggests a trace of zonal structure extending almost as far as the polar pentagon.

Dust and water ice variability and their interaction pattern during Martian low-dust and high-dust periods

Analysis of dust and water ice variation is carried out using observations from the Mars Climate Sounder (MCS), which includes the data for six years (during Martian years 29–34). This study only used the nighttime observations, as these profiles extend closer to the surface compared to the daytime. The correlation between dust opacity and water ice opacity is found to switch sign between the low-dust northern spring and summer (LS ​= ​0°–180°) and high-dust southern spring and summer seasons (LS ​= ​180°–360°). Density-scaled opacity profiles show the correlation between dust and water ice variability over the southern hemisphere and the tropics, which alters between 20 and 40 ​km altitudes. The positive correlation during the low-dust period over the latitudes 40–80°S is mainly controlled by the water ice cycle in the south polar hood clouds. Whereas the water ice cycle within the tropical cloud belt (TCB) serves as a primary controlling factor, and the presence of atmospheric dust prevails only in its formation stage within the latitudes -20 – 40°N. During the high-dust period, at southern latitudes, significant dust lifting and the associated temperature change are found to be the reason for the strong negative correlation with ice clouds. And in tropical latitudes, the significant positive relationship at relatively high altitudes (∼40 ​km) is possibly due to the presence of thin or haze clouds. The global dust storm occurrence only modulates the correlation behavior during the high-dust period, and the influence is seen above ∼40 ​km altitude that indicates an enhanced vertical advection sustained till late northern winter. Based on analysis of observational data and simulations with the MarsWRF model, it is found that the difference in correlation behavior between low and high-dust seasons could be explained from the variations in the planetary boundary layer height. Moreover, the dust and water ice interaction pattern has a prominent seasonal variation that is influenced by the water ice cycle, dust cycle, or the dust-ice microphysical relationship. It also has an altitudinal dependency, which changes between low-dust, high-dust, or global dust storm scenarios.

Martian cloud climatology and life cycle extracted from Mars Express OMEGA spectral images

A Martian water-ice cloud climatology has been extracted from OMEGA data covering 7 Martian years (MY 26–32) on the dayside. We derived two products, the Reversed Ice Cloud Index (ICIR) and the Percentage of Cloudy Pixels (PCP), indicating the mean cloud thickness and nebulosity over a regular grid (1° longitude × 1° latitude × 1° Ls × 1 h Local Time). The ICIR has been shown to be a proxy of the water-ice column derived from the Mars Climate Database.The PCP confirms the existence and location of the main cloud structures mapped with the ICIR, but also gives a more accurate image of the cloud cover. We observed a more dense cloud coverage over Hellas Planitia, the Lunae Planum region and over large volcanoes (Tharsis volcanoes, Olympus and Elysium Montes) in the aphelion belt.For the first time, thanks to the fact that Mars Express is not in Sun-synchronous orbit, we can explore the clouds diurnal cycle at a given season by combining the seven years of observations. However, because of the eccentric orbit, the temporal coverage remains limited. Identified limitations of the dataset are its small size, the difficult distinction between ice clouds and frosts, and the impact of surface albedo on data uncertainty. We could nevertheless study the diurnal cloud life cycle by averaging the data over larger regions: from specific topographic features (covering a few degrees in longitude and latitude) up to large climatic bands (covering all longitudes). We found that in the tropics (25°S – 25°N) around northern summer solstice, the diurnal thermal tide modulates the abundance of clouds, which is reduced around noon (Local Time). At northern midlatitudes (35°N – 55°N), clouds corresponding to the edge of the north polar hood are observed mainly in the morning and around noon during northern winter (Ls = 260° – 30°). Over Chryse Planitia, low lying morning fogs dissipate earlier and earlier in the afternoon during northern winter. Over Argyre, clouds are present over all daytime during two periods, around Ls = 30° and 160°.

High-resolution UV/Optical/IR Imaging of Jupiter in 2016–2019

Abstract Imaging observations of Jupiter with high spatial resolution were acquired beginning in 2016, with a cadence of 53 days to coincide with atmospheric observations of the Juno spacecraft during each perijove pass. The Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST) collected Jupiter images from 236 to 925 nm in 14 filters. The Near-Infrared Imager (NIRI) at Gemini North imaged Jovian thermal emission using a lucky-imaging approach (co-adding the sharpest frames taken from a sequence of short exposures), using the M′ filter at 4.7 μm. We discuss the data acquisition and processing and an archive collection that contains the processed WFC3 and NIRI data (doi:10.17909/T94T1H). Zonal winds remain steady over time at most latitudes, but significant evolution of the wind profile near 24°N in 2016 and near 15°S in 2017 was linked with convective superstorm eruptions. Persistent mesoscale waves were seen throughout the 2016–2019 period. We link groups of lightning flashes observed by the Juno team with water clouds in a large convective plume near 15°S and in cyclones near 35°N–55°N. Thermal infrared maps at the 10.8 micron wavelength obtained at the Very Large Telescope show consistent high brightness temperature anomalies, despite a diversity of aerosol properties seen in the HST data. Both WFC3 and NIRI imaging reveal depleted aerosols consistent with downwelling around the periphery of the 15°S storm, which was also observed by the Atacama Large Millimeter/submillimeter Array. NIRI imaging of the Great Red Spot shows that locally reduced cloud opacity is responsible for dark features within the vortex. The HST data maps multiple concentric polar hoods of high-latitude hazes.

MARCI-observed clouds in the Hellas Basin during northern hemisphere summer on Mars: Interpretation with the NASA/Ames Legacy Mars Global Climate Model

We present a study that is motivated by a population of water ice clouds in the Hellas Basin that has been observed by the MARs Color Imager (MARCI) instrument on the Mars Reconnaissance Orbiter (MRO) to persist throughout the majority of Northern Hemisphere (NH) summer. Although water ice clouds are present in Hellas at very low opacities throughout NH spring, they noticeably thicken after Ls 60° and continue to do so until their peak optical thickness is attained at Ls ~120°. They dissipate rapidly from Ls 120° to 150°, and by Ls 150°, the only clouds in Hellas are on the southern side of the basin and are indistinguishable from the polar hood clouds. We use the NASA/Ames Legacy Mars Global Climate Model (GCM), which is supported by the Agency's Mars Climate Modeling Center, to investigate the dynamical and microphysical mechanisms that control the formation and evolution of Hellas water ice clouds. We show that water is transported from the North Polar Residual Cap (NPRC) southward across the equator and into the Hellas region. Water vapor is confined down low by cloud formation in the aphelion cloud belt, thus limiting the effectiveness of transport by the zonal mean overturning circulation (i.e., the Hadley cell). Thus, contrary to the commonly held conceptual understanding that the Hadley cell controls cross-equatorial water transport during this season, we show that the southward transport of water is done primarily in the vapor phase by stationary eddies at nearly all latitudes, including across the equator. Clouds form near the surface in the basin as moist air mixes with cold polar air on the western side of Hellas and are subsequently transported clockwise around the north side of the basin by the low-level cyclonic circulation. The simulated Hellas clouds have small particle sizes (~3–5 μm), are very low, and exist as long as the NPRC is exposed.

The current weather and climate of Mars: 12 years of atmospheric monitoring by the Planetary Fourier Spectrometer on Mars Express

We used thermal-infrared spectra returned by the Mars Express Planetary Fourier Spectrometer (PFS/MEx) to retrieve atmospheric temperature profiles, surface temperatures, and column-integrated optical depths of dust and water ice. More than 2,500,000 spectra were processed to build this new atmospheric dataset, covering the full range of season, latitude, longitude, and local time. The data presented here span more than six Martian Years (from MY 26, Ls = 331°, 10 January 2004 to MY 33, Ls = 78°, 6 December 2015). We present an overview of the seasonal and latitudinal dependence of the above atmospheric quantities for the relevant period, as well as an assessment of the interannual variability in the current Martian climate. The general effect of suspended dust on atmospheric temperatures observed during the global dust storm of MY 28 is also presented. Atmospheric temperatures and aerosol opacity were successfully retrieved over cold surface areas and in the polar regions, including the polar nights. Rather than the aphelion cloud belt, the most prominent feature one can observe in the climatology of the Martian water ice clouds is the seasonal extent, pattern and thickness of the North polar hoods (NPH), where most of the thickest clouds are observed. The NPH also shows peculiar features, observed in detail here for the first time, with characteristic spatial and seasonal patterns that repeat very similarly every Martian year. By exploiting PFS/MEx capability to perform observations at different local times (LT), this dataset allowed the characterization of how dust and ice cloud optical depths vary throughout the day on Mars. We present observations of the dust daily cycle during non-dusty seasons; the daily variation of water ice opacity in the aphelion cloud belt; and the diurnal variation of the North and South polar hoods. With unprecedented spatial and temporal coverage and details revealed, this dataset offers new challenges to the Martian global circulation models and, at the same time, a new reference for the MYs complementary to those observed by previous orbiters.

MARCI observations of a wavenumber-2 large-scale feature in the north polar hood of Mars: Interpretation with the NASA/Ames Legacy Global Climate Model

The Mars Color Imager on the Mars Reconnaissance Orbiter has observed a prominent Fourier wavenumber-2 planetary-scale pattern in the north polar hood clouds that appears in early fall, strengthens as the seasons progress, becomes most pronounced during midwinter, and then disappears by mid-spring as the clouds dissipate. We use the NASA/Ames Legacy Global Climate Model, which is supported by the Mars Climate Modeling Center, to interpret this feature and find that it is caused by a trapped, yet deep, mechanically forced quasi-stationary wavenumber-2. We show that the perturbation temperature and wind fields set up by this forced wave control the boundaries of the polar hood, locking them to fixed topographic ridges and troughs. These perturbation fields also determine the longitudes where the polar hood clouds are thickest, which is generally in the cold sectors set up by the stationary wave. Moisture advection during midwinter, however, changes this pattern, leading to warm regions having the thickest clouds. Observations do not show this pattern, though there is some ambiguity in the data. The source of moisture for the polar hood during midwinter is water released by the retreating south polar seasonal CO2 ice cap, which is carried into the Northern Hemisphere by different components of the global circulation, and in different forms. The model also predicts a seasonal variation in the annular structure of the polar hood for which existing observations are unable to confirm. We discuss the implications of these results and speculate on explanations for the apparent model/observation discrepancies.

Mapping water ice clouds on Mars with MRO/MARCI

Observations by the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) in the ultraviolet (UV, Band 7; 320 nm) are used to characterize the spatial and temporal behavior of atmospheric water ice over a period of 6 Mars Years. Exploiting the contrast of the bright ice clouds to the low albedo surface, a radiative transfer-based retrieval algorithm is developed to derive the column-integrated optical depth of the ice (τice). Several relatively unique input products are created as part of the retrieval development process, including a zonal dust climatology based on emission phase function (EPFs) sequences from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a spatially variable UV-reflectance model for Band 7 (as well as for Band 6, 260 nm), and a water ice scattering phase function based on a droxtal ice habit. Taking into account a radiometric precision of 7%, an error analysis estimates the uncertainty in τice to be ∼0.03 (excluding particle size effects, which are discussed separately). Zonal trends are analyzed over the full temporal extent of the observations, looking at both diurnal and interannual variability. The main (zonal) features are the aphelion cloud belt (ACB) and the polar hoods. For the ACB, there can be an appreciable diurnal change in τice between the periods of 14h30–15h00 and 15h00–15h30 Local True Solar Time (LTST). The amplitude of this effect shows relatively large interannual variability, associated mainly with changes in the earlier time block. When averaged over the interval 14h00–16h00 LTST, the interannual differences in the ACB structure are appreciably smaller. When the MARCI τice are compared to those from the Thermal Emission Spectrometer (TES), there is a good correlation of features, with the most significant difference being the seasonal (LS) evolution of the ACB. For TES, the ACB zonal profile is relative symmetric about LS = 90°. In the MARCI data, this profile is noticeably asymmetric, with the centroid shifted to later in the northern summer season (LS = 120°). The MARCI behavior is consistent with that observed by several other instruments. The correspondence of MARCI τice zonal and meridional behaviors with that predicted by two Global Circulation Models (GCM) is good. Each model captures the general behavior seen by MARCI in the ACB, the polar hoods, and the major orographic/topographic cloud features (including Valles Mariners). However, the mismatches between GCM results and MARCI reinforce the challenging nature of water ice clouds for dynamical models. The released τice are being archived at Malin Space Science Systems at https://www.msss.com/mro_marci_iceclouds/.

High-altitude water ice cloud formation on Mars controlled by interplanetary dust particles

Submicrometre-size meteoric smoke aggregates form when interplanetary dust particles ablate and re-coagulate in the Martian atmosphere. The MAVEN (Mars Atmosphere and Volatile Evolution) satellite has detected pervasive ionized metallic layers due to meteor ablation at an 80–90 km altitude, which suggests a continuous supply of meteoric smoke particles that settle to lower altitudes. Until now, meteoric smoke has been neglected in general circulation model studies of the formation of Martian water ice clouds. Here we show that when meteoric smoke is included in simulations of the atmospheric circulation on Mars, mesospheric water ice clouds form at low pressures and in discrete layers, polar hood clouds extend to higher altitudes and the seasonal Hadley cell is weakened. Furthermore, we find that the middle atmosphere water ice clouds respond to and influence the diurnal and semidiurnal migrating thermal tides. We conclude that Mars atmospheric simulations that neglect meteoric smoke do not reproduce the observed spatial distribution of water ice clouds and miss crucial radiative impacts on the overall atmospheric dynamics. Particles from interplanetary dust ablating in Mars’ atmosphere control high-altitude water ice cloud formation, according to numerical simulations of the Martian atmosphere.

Retrieval of the water ice column and physical properties of water-ice clouds in the martian atmosphere using the OMEGA imaging spectrometer

Using spectral images recorded by the OMEGA instrument on Mars Express (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité), we are able to derive physical properties of aerosols in water-ice clouds on Mars for a distribution of pixels over an observed cloud formation. These properties, mean effective radius, reff, and optical depth (at 0.67 μm), τi, were used to estimate the water ice-column (WIC), and we found an empirical relationship between the WIC and an ice cloud index (ICI). The overall mean of retrieved reff is ∼2.2 μm, with a standard deviation of 0.8 μm, and cloud formations with reff between 4.4 and 5.4 μm are observed. The optical depth varies between 0.2 and 2.0. The OMEGA spectra are primarily sensitive to water ice mass due to absorption, and we find that the ICI, very easy to compute, is a good proxy for the mass of the water-ice column (WIC) along the optical line of sight. Our retrieval of physical properties is limited in time (to before 2010) by the exhaustion of coolant for one of the OMEGA channels, and in space (to equatorial observations between 140∘W and 90∘E) by the availability of surface albedo measurements. However, we used the ICI to compute WIC values for the entire OMEGA data set, which has near-global coverage for Mars years 26–32, and we present a climatology of the WIC derived from the OMEGA data, which features enhancements on the order of 1.2–1.6 pr. μm over the aphelion cloud belt, and 1.5–2.5 pr. μm over the polar hoods. The data set analyzed is for observations between 140°W and 90°E, and between 35∘S and 35∘N. No restriction is placed on season, but the majority of cloudy observations were during the aphelion period from Ls 35∘ to 135∘. This work was motivated by the ability of the OMEGA instrument to observe the distribution of water-ice cloud physical properties, and by the availability of new a priori data sets, especially multi-spectral, aerosol-free surface albedo retrieved from a subset of the OMEGA data featuring a cloud-free sky. The main limitations of the retrieval algorithm are linked to the uncertainties on surface albedo, the dust opacity, and the quantity of water-ice suspended in the atmosphere, which can lead to spectral fits with lower accuracy or unrealistic results. We present distributions of each retrieved parameter, goodness of fit, ICI, and cloud mass, and our investigation of relationships between each parameter. Our approach was to maximize the amount of data analyzed, apply stringent data quality cuts and take a statistical approach to interpretation.

Seasonal Variation in Martian Water Ice Cloud Particle Size

AbstractWe employ the complete record of Mars limb‐viewing observations by the Compact Reconnaissance Imaging Spectrometer for Mars, totaling 922 successful retrievals, to chart the seasonal variation in water ice cloud particle size. Low‐latitude clouds exhibit particle size sorting with altitude at all seasons, with particles ranging from >3.0 μm in effective radius to 1.0–1.5 μm. However, at a given altitude, the ice particle size often varies through the year. Ice particle sizes in the tropics follow a different seasonal cycle than ice mixing ratio. North polar clouds have a complex seasonal cycle, with a sloped size profile with altitude as the polar hood cloud forms in early fall trending to a roughly uniform 1.5‐μm profile by the end of winter. These data are an important metric to validate and improve general circulation model microphysical routines and to better understand how Martian water ice clouds influence the atmospheric radiative budget.

Martian north polar cap summer water cycle

A key outstanding question in Martian science is “are the polar caps gaining or losing mass and what are the implications for past, current and future climate?” To address this question, we use observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of the north polar cap during late summer for multiple Martian years, to monitor the summertime water cycle in order to place quantitative limits on the amount of water ice deposited and sublimed in late summer.We establish here for the first time the summer cycle of water ice absorption band signatures on the north polar cap. We show that in a key region in the interior of the north polar cap, the absorption band depths grow until Ls = 120, when they begin to shrink, until they are obscured at the end of summer by the north polar hood. This behavior is transferable over the entire north polar cap, where in late summer regions 'flip' from being net sublimating into net condensation mode. This transition or 'mode flip' happens earlier for regions closer to the pole, and later for regions close to the periphery of the cap.The observations and calculations presented herein estimate that on average a water ice layer ∼70 microns thick is deposited during the Ls = 135–164 period. This is far larger than the results of deposition on the south pole during summer, where an average layer 0.6–6 microns deep has been estimated by Brown et al. (2014) Earth Planet. Sci. Lett., 406, 102–109.

Stratospheric benzene and hydrocarbon aerosols detected in Saturn’s auroral regions

Context. Saturn’s polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented. Aims. We aim to bring new constraints on the benzene distribution in Saturn’s stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols’ radiative impact on the thermal structure. Methods. Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680–900 and 1360–1440 cm −1 . Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements. Results. On Saturn’s auroral region (80 ◦ S), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses & Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80 ◦ S than at the equator. This advocates for the inclusion of ion-related reactions in Saturn’s chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan’s stratosphere. The aerosol mass loading at 80 ◦ S is estimated to be 1−4 × 10 −5 gc m −2 , an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn. We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn’s summer stratosphere.