This study provides comprehensive intercomparisons of dust optical depths derived from Mars Climate Sounder on Mars Reconnaissance Orbiter with measurements from the orbiter instruments THEMIS on Mars Odyssey, CRISM on Mars Reconnaissance Orbiter, EMIRS on the Emirates Mars Mission, and from the surface-based instruments Pancam on the Spirit and Opportunity rovers and SSI on the Phoenix lander. We demonstrate good overall agreement between these dust optical depth datasets, in particular as these measurements were taken at different wavelengths (near IR to thermal IR) and with different measurement geometries (limb, downlooking from orbit, uplooking from the surface). To draw our conclusions, we use direct comparisons of co-located measurements from these instruments, plots of the dust optical depth differences averaged by latitude and Solar longitude, as well as modified Bland-Altman analysis that takes the measurement uncertainties into account. From the Bland-Altman analysis, we find that error estimates provided by the instruments reasonably represent the variability introduced by the two datasets but do not capture offsets. Based on our investigations, we propose potential physical causes such as surface reflectance factor estimation and thermal contrast, for some of the observed differences. We find a linear relation of the differences in dust optical depth between MCS and the other instruments with derived MCS water ice optical depth. Taking this relation into account, together with a constant parameter that accounts for general offsets between the datasets, significantly reduces differences.

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 mu m 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 mu m 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 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.

Orbit spin coupling is proposed as an alternative to planetary tidal models for the excitation of solar variability as a function of time. Momentum sourced from the orbital angular momenta of solar system bodies is deposited within the circulating fluid envelopes of the Sun and planets in this hypothesis. A reversing torque acts about an axis lying within the Sun's equatorial plane. The torque gives rise to tangential differential accelerations of solar materials as a function of longitude, latitude, depth, and time. The accelerations pulse in amplitude, and change sign, on timescales corresponding to the periods, beats, and harmonics of inner and outer planet orbital motions. In contrast to planetary tidal models, no special amplification mechanism may be required, as estimated peak accelerations are about 2 orders of magnitude larger than the largest tidal accelerations. Organized mass motions driven by the torque may be incorporated in dynamo simulations through the flow velocity term of the MHD induction equation. The spatiotemporal variability of flow velocities may then influence the variability with time of solar magnetic activity. We provide torque values at 1 day timesteps for the years 1660 to 2220. We discuss the time variability of the torque in juxtaposition with SIDC monthly sunspot numbers from 1750 to present. We investigate Hale cycle synchronization, and the variability with time of the total solar irradiance, with reference to outer and inner planet contributions respectively. We propose a 3 component model for understanding and simulating the solar magnetic cycle, which includes processes internal to the Sun, external forcing, due to orbit spin coupling, and a time-delay, or system memory, component. This model supplies a physical explanation for the observed variability with time of Schwabe cycle periods and Hale cycle periods from 1712 to present.

The recent addition of orbit-spin coupling torques to atmospheric global circulation models has enabled successful years-in-advance forecasts of global and regional-scale dust storms on Mars. Here we explore the applicability of the orbit-spin coupling mechanism for understanding and forecasting anomalous weather and climate events on Earth. We calculate the time history of orbit-spin coupling torques on the Earth system for the interval from 1860-2040. The torque exhibits substantial variability on decadal to bidecadal timescales. Deep minima recur at intervals from 15-26 years; eight such episodes are documented within the study period prior to 2020. Each of the identified torque minima corresponds in time to an episode of widespread drought in the Western USA extending over several years. The multiyear droughts of the 1930s, the 1950s, the mid-1970s, the early 1990s, and of 2011-2015 were each coincident in time with orbit-spin coupling torque minima. The upcoming torque minimum of 2030 is the deepest such minimum of the 180-yr study interval. A multiyear episode of widespread drought in the Western USA is likely to be underway by 2028 plus or minus 4 years (2 standard deviations). The potential benefits to societies of improved drought predictions justify an immediate high-priority effort to include forcing by orbit-spin coupling within state-of-the-art Earth system GCMs. Future targeted numerical modeling investigations are likely to yield forecasts with considerably lower uncertainties and with much improved temporal resolution in comparison to that obtained here.

Wind is the process that connects Mars' climate system. Measurements of Mars atmospheric winds from orbit would dramatically advance our understanding of Mars and help prepare for human exploration of the Red Planet. Multiple instrument candidates are in development and will be ready for flight in the next decade. We urge the Decadal Survey to make these measurements a priority for 2023-2032.

A vigorous regional dust storm substantially altered both the global atmospheric thermal structure and the magnitude and spatial distribution of dust loading within the Mars atmosphere between 1 and 9 June 2018. We examine the development and decay of this storm in latitude, longitude, altitude, and time, employing observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter. Dust layer top altitudes rose from seasonal-normal values of similar to 40 to similar to 70 km. Dust lofting to high altitudes was localized between 0 degrees and similar to 60 degrees W longitude, and between 60 degrees N and 60 degrees S latitude. Intensification of paired meridional overturning circulation cells within the study area is confirmed by strong nighttime dynamical heating in higher latitudes of both hemispheres. By the end of this episode, significant dust loading was present at altitudes greater than 50 km above all longitudes on Mars, and other dust lifting centers had been activated.

The largest of dust storms on Mars are global dust events (GDEs) that affect essentially every aspect of the Martian atmosphere but do not occur in every Mars year. The Mars Climate Sounder globally observed the most recent GDE in 2018 throughout its lifecycle. The event started shortly after the southern spring equinox, with the first Mars Climate Sounder signature detected on 2 June 2018 (Ls = 186.2°). It grew into a mature global event by the end of June (Ls ~ 198°), with drastic changes to the atmospheric temperature, dust, and water ice profiles. It decayed over several months, reaching background conditions around southern summer solstice in October 2018. The 2018 GDE was very similar to the 2001 event, including the seasonal development and temperatures. GDEs appear to be distinct from even the largest regional dust storms. GDEs have 50‐Pa zonal tropical temperatures >220 K while no regional dust storm exceeds 205 K.

The Martian global dust storm (GDS) of 2018 began soon after the southern spring equinox, which is quite early in the dust storm season. The origins of early‐season GDS, including those of 1977, 2001, and now 2018, have been mysterious, as atmospheric dynamical investigations and numerical modeling experiments have been unable to explain or reproduce the timing of these events. We employ a newly expanded catalog of historic Martian GDS for our investigation, which includes 2018 and the telescopically observed equinoctial dust storms of 1877 and 1909. All of the GDS of this catalog took place either (1) when orbit‐spin coupling torques on the Martian atmosphere were near peak values or (2) near times when the orbit‐spin coupling torques were changing most rapidly. The second category, here termed “Mode 2,” includes all six of the equinoctial GDS of the historic record, including 2018. Recognition of the existence of two triggering modes for GDS occurrence leads to a significant improvement in temporal resolution for both hindcasting and forecasting. Orbit‐spin coupling now provides explanations for the late‐season inception dates of the 1924 and 1973 storms, as well as for the equinoctial events. We provide conditional forecasts, with sub‐seasonal time resolution, for GDS occurrence and non‐occurrence in Mars years 35 through 40. We introduce a detailed working hypothesis for the genesis of equinoctial GDS that may be validated through numerical modeling. The characteristic timescale for frictional damping of an intensified Martian Hadley circulation is estimated to be O(10) sols.

Orbit-spin coupling torques on the Earth in November 2020 are larger than at any other time between 2000 and 2050. This affords an opportunity to observe the terrestrial atmospheric response to the putative torque in near real time.

The orbital motions and spin-axis rotations of extended bodies are traditionally considered to be coupled only by tidal mechanisms. The orbit-spin coupling hypothesis supplies an additional mechanism. A reversing torque on rotating extended bodies is identified. The torque effects an exchange of angular momentum between the reservoirs of the orbital and rotational motions. The axis of the torque is constrained to lie within the equatorial plane of the subject body. Hypothesis testing to date has focused on the response to the putative torque of the Martian atmosphere. Atmospheric global circulation model simulations reveal that an episodic strengthening and weakening of meridional overturning circulations should be observable and is diagnostic in connection with the triggering of Martian planet-encircling dust storms. Spacecraft observations obtained during the earliest days of the 2018 Martian global dust storm document a strong intensification of atmospheric meridional motions as predicted under this hypothesis. We review implications for atmospheric physics, for investigations of planetary orbital evolution with rotational energy dissipation, and for theories of gravitation.

The stability of the residual carbon dioxide cap near the south pole of Mars is currently not well understood. The cap's survival depends on its radiation budget, controlled by the visible albedo and infrared emissivity. We investigated the role of CO2 snowfall in altering the albedo and emissivity, leading to the observed asymmetry in the net CO2 accumulation at the two poles. Uncontaminated snowfall increases albedo, and lowers emissivity, due to scattering by optically thick clouds and granular surface deposits. Data from the Mars Climate Sounder (MCS) show that fall and winter snowfall is correlated with higher springtime albedo at both poles. For the seasonal CO2 deposits in each polar region >60° latitude, we find mean albedo values of 0.39 in the north and 0.51 in the south, and winter 32‐μm emissivity values of 0.84 in the north and 0.87 in the south. Using a radiative transfer model and the MCS data, we find that the north polar deposits have ∼10× higher dust content than those in the south, explaining the ∼31% lower albedo of the north seasonal cap during spring. Our model shows that greater amounts of snowfall can explain the ∼4% lower emissivity of the north polar seasonal cap. These findings demonstrate that winter snowfall and dust transport affect the composition of Mars' seasonal ice caps and polar energy balance. Snowfall and dust loading are therefore important in modeling the CO2 cycle on Mars, as well as the planet's long‐term climate variations.

We report observations by the Mars Climate Sounder showing strong diurnal variations in temperature and the vertical dust distribution during the 2018 (Mars Year 34) global dust event. The temperature field shows weak diurnal tidal activity at equatorial latitudes but a strong diurnal tide in middle to high latitudes with a maximum amplitude of 29 K in the lower atmosphere of the south polar region. The diurnal variability of dust is small in the equatorial region and increases toward higher latitudes. At middle and low latitudes, comparable dust amounts are found about 5–10 km higher in the atmosphere on the dayside than on the nightside. The dust reaches the highest altitudes in the late afternoon and is found at the lowest altitudes in the late night. In the southern high latitudes a persistent cold air mass with low dust content is identified on the nightside of the planet centered at 3–6 a.m. local time. The observed variations are well represented by model simulations with the Laboratoire de Météorologie Dynamique General Circulation Model. Comparisons between data and model results suggest that the diurnal variations in the dust are largely driven by the meridional circulation exhibiting diurnal tidal variations. The model results show that the compact air mass in the south polar region has a high potential vorticity, supporting its interpretation as a remnant of the southern polar vortex, which is forced toward the nightside of the planet due to the enhanced diurnal tide during the global dust event.

One global-scale dust storm and two larger-than-average regional-scale dust storms have been recorded by spacecraft observations within the dust seasons of the past two Mars years. Each of these st...

Deep convection, as used in meteorology, refers to the rapid ascent of air parcels in the Earth's troposphere driven by the buoyancy generated by phase change in water. Deep convection undergirds some of the Earth's most important and violent weather phenomena and is responsible for many aspects of the observed distribution of energy, momentum, and constituents (particularly water) in the Earth's atmosphere. Deep convection driven by buoyancy generated by the radiative heating of atmospheric dust may be similarly important in the atmosphere of Mars but lacks a systematic description. Here we propose a comprehensive framework for this phenomenon of dusty deep convection (DDC) that is supported by energetic calculations and observations of the vertical dust distribution and exemplary dusty deep convective structures within local, regional, and global dust storm activity. In this framework, DDC is distinct from a spectrum of weaker dusty convective activity because DDC originates from pre-existing or concurrently forming mesoscale circulations that generate high surface dust fluxes, oppose large-scale horizontal advective-diffusive processes, and are thus able to maintain higher dust concentrations than typically simulated. DDC takes two distinctive forms. Mesoscale circulations that form near Mars's highest volcanoes in dust storms of all scales can transport dust to the base of the upper atmosphere in as little as two hours. In the second distinctive form, mesoscale circulations at low elevations within regional and global dust storm activity generate freely convecting streamers of dust that are sheared into the middle atmosphere over the diurnal cycle.

COMPARISONS TO PREVIOUS EVENTS. D. M. Kass1, J. T. Schofield2, A. Kleinböhl1, D. J. McCleese3, N. G. Heavens4, J. H. Shirley1, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 2Retired, Pasadena, California, USA, 3Synoptic Science, Pasadena, California, USA, 4Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia, USA. (David.M.Kass@jpl.nasa.gov)

Introduction: Global, or planet‐encircling, dust events are the probably the most distinctive feature of the martian atmosphere. They can lift dust well into the middle atmosphere, enshroud the planet with dust for months, and have a profound effect on atmospheric temperatures and the overall circulation. The recent global dust event (GDE) on Mars provides a unique opportunity to gain insight into the behavior of these large-scale dust events as it was observed from the surface by the Curiosity rover [1] as well as from Mars orbit by several orbiters. Initial signatures of the MY34 GDE were observed on June 2, 2018 (Ls=186°). It peaked around July 7, 2018 (Ls=207°) and then decayed over several months, with background dust conditions reached in the second half of October 2018 (Ls=270°-280°). Previous orbital observations of GDEs on Mars typically yield dust column optical depth measurements that are usually only available on the dayside of the planet [2,3]. The Mars Climate Sounder (MCS) [4] onboard Mars Reconnaissance Orbiter (MRO) was observing the martian atmosphere nearly continuously over the duration of the GDE. MCS dust profiles are available on both the dayside and nightside part of the orbit, allowing the study of diurnal variations of dust. In addition, the MCS observations included frequent sideways scans [5] to extend the range of local times observed during the event. Here we show dust profiles measured during the GDE and evaluate variations in the observed dust distribution with local time. We compare the measured dust distributions with results from the LMD General Circulation Model (GCM) in order to isolate mechanisms for the observed variability. MCS Instrument and Retrievals: The Mars Climate Sounder [4] is a midand far infrared thermal emission radiometer on board the Mars Reconnaissance Orbiter. It measures radiances in limb and onplanet viewing geometries. From these radiance measurements, profiles of atmospheric temperature, dust, and water ice are retrieved from the surface to ~80 km altitude with a vertical resolution of ~5 km [6-8]. Temperature measurements exploit the atmospheric emission of the 15 μm CO2 band. Dust and water ice extinction retrievals are centered on major absorption bands at 22 μm and 12 μm, respectively. Dust retrievals during the GDE use the MCS far IR channel B1 at 32 μm in addition to the 22 μm band. Because the extinction efficiency at 32 μm is only about half of the value at 22 μm, the use of B1 detectors extends the retrievable altitude range in dusty conditions typically by 1-1.5 scale heights [9]. Results: For this analysis we consider zonal averages of temperature and dust profiles that were binned separately for day and night in seasonal steps of 5° Ls. Figure 1 shows the results of this analysis for the Ls-