James H. Shirley

Mars Climate Sounder limb profile retrieval of atmospheric temperature, pressure, and dust and water ice opacity

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.

Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols

[1] The first Martian year and a half of observations by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter has revealed new details of the thermal structure and distributions of dust and water ice in the atmosphere. The Martian atmosphere is shown in the observations by the Mars Climate Sounder to vary seasonally between two modes: a symmetrical equinoctial structure with middle atmosphere polar warming and a solstitial structure with an intense middle atmosphere polar warming overlying a deep winter polar vortex. The dust distribution, in particular, is more complex than appreciated before the advent of these high (∼5 km) vertical resolution observations, which extend from near the surface to above 80 km and yield 13 dayside and 13 nightside pole-to-pole cross sections each day. Among the new features noted is a persistent maximum in dust mass mixing ratio at 15-25 km above the surface (at least on the nightside) during northern spring and summer. The water ice distribution is very sensitive to the diurnal and seasonal variation of temperature and is a good tracer of the vertically propagating tide.

Water ice clouds over the Martian tropics during northern summer

[1] Atmospheric models suggest that infrared heating due to water ice clouds over the tropics of Mars during early northern summer has a significant impact on the thermal structure of the tropics at cloud level and of the middle atmosphere near the south pole. Retrievals from limb observations by the Mars Climate Sounder on Mars Reconnaissance Orbiter during early northern summer show that water ice clouds over the northern tropics are thinner and higher than in published model results. Later in this season, the latitudinal extent, apparent mass mixing ratio (and infrared heating rate), and altitude of nighttime tropical clouds significantly increase, reaching a maximum just before northern fall equinox. Published model results do not show this transition. By underestimating the altitude at which water ice clouds form, models also may underestimate the intensity of the meridional circulation at higher altitudes in the tropics during northern summer.

Discussion of the 1.04‐μm water ice absorption band in the Europa NIMS spectra and a new NIMS calibration

Measurements made by the Galileo Near Infrared Mapping Spectrometer (NIMS) have been used to calculate reflectance spectra for regions of Europa. Many of these spectra show features indicative of water ice. Others show asymmetric H 2 O absorption features suggesting hydrated non-ice constituents such as hydrated sulfates and carbonates. Often, both types of features are present. However, some previously published NIMS spectra with strongly developed ice features did not show the expected 1.04-μm ice absorption band. This discrepancy has led to questions regarding earlier interpretations of Europas spectral features. Recent work reveals that unexpected changes have occurred in the spectral range covered by each NIMS detector and that an additional calibration is required. Preliminary calibration values are given. When this new calibration is applied, the 1.04-μm feature emerges clearly in those spectra where it was expected but absent previously, thereby resolving the 1.04-μm band discrepancy.

Seasonal and diurnal variability of detached dust layers in the tropical Martian atmosphere

Evidence for widespread nonuniform vertical mixing of dust in Marss tropical atmosphere (in the form of features called “detached dust layers” or DDLs) is a challenge for atmospheric modeling. We characterize the seasonal, diurnal, and geographic variability of DDL activity in retrievals from observations by the Mars Climate Sounder onboard Mars Reconnaissance Orbiter. We find that dust injection above the boundary layer, which forms DDLs, is a spatially ubiquitous phenomenon in the tropics during the daytime, implying that it has a significant nontopographic component. DDL formation is more intense in northern spring and summer than in southern spring and summer but is still common when the zonal average dust distribution appears uniformly mixed. DDLs do not appear to follow the upwelling associated with Marss Hadley circulation or the extant climatology of local dust storm activity in the tropics. Geographic variability in the nightside vertical dust distribution does not always correlate with the dayside vertical dust distribution, implying that there is spatial and seasonal variability in the efficiency of dust deposition/removal processes. Nighttime dust removal is especially efficient over the Tharsis Montes during northern spring and summer, which suggests some association between water ice clouds and removal. Intense injection combined with efficient removal results in a high amplitude of diurnal variability in the dust distribution at 15–30 km above the surface of the tropics during much of the Martian year.

Extreme detached dust layers near Martian volcanoes: Evidence for dust transport by mesoscale circulations forced by high topography

Modeling suggests that thermal circulations over Marss highest volcanoes transport water vapor and dust from the surface into the middle atmosphere, forming detached layers in these constituents. Intense vertical mixing also takes place in regional and global dust storms, which can generate detached layers that are extreme in both altitude and magnitude. Here we employ observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter, taking advantage of improved vertical coverage in MCSs aerosol retrievals, to discover a new class of extreme detached dust layers (EDDLs). Observed during minimal dust storm activity and furthermore distinguished by their potentially large and measurable horizontal extent (>1000 km), these EDDLs cluster near Olympus Mons and the Tharsis Montes, from which they likely originate. The existence of these EDDLs suggests that vertical mixing by topographic circulations can be much stronger than previously modeled and more frequent than previously observed.

Discovery of a widespread low‐latitude diurnal CO2 frost cycle on Mars

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 kg m−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.

Numerical modeling of orbit-spin coupling accelerations in a Mars general circulation model: Implications for global dust storm activity

Abstract We employ the MarsWRF general circulation model (GCM) to test the predictions of a new physical hypothesis: a weak coupling of the orbital and rotational angular momenta of extended bodies is predicted to give rise to cycles of intensification and relaxation of circulatory flows within atmospheres. The dynamical core of MarsWRF has been modified to include the orbit-spin coupling accelerations due to solar system dynamics for the years 1920–2030. The modified GCM is subjected to extensive testing and verification. We compare forced and unforced model outcomes for large-scale zonal and meridional flows, and for near-surface wind velocities and surface wind stresses. The predicted cycles of circulatory intensification and relaxation within the modified GCM are observed. Most remarkably, the modified GCM reproduces conditions favorable for the occurrence of perihelion-season global-scale dust storms (GDSs) on Mars in years in which such storms were observed. A strengthening of the meridional overturning circulation during the dust storm season occurs in the GCM in all recorded years with perihelion-season global-scale dust storms. The increased upwelling produced in the southern hemisphere in southern summer may facilitate the transport of dust to high altitudes in the Mars atmosphere during the dust storm season, where radiative heating may further strengthen the circulation. Significantly increased surface winds and surface wind stresses are also obtained. These may locally facilitate dust lifting from the surface. Based on comparison to the historical record, there is a strong likelihood of a perihelion-season GDS in Mars year 33 and/or Mars year 34.

Orbit-spin coupling and the circulation of the Martian atmosphere

Abstract The physical origins of the observed interannual variability of weather and climate on Mars are poorly understood. In this paper we introduce a deterministic physical mechanism that may account for much of the variability of the circulation of the Mars atmosphere on seasonal and longer timescales. We focus on a possible coupling between the planetary orbital angular momentum and the angular momentum of the planetary rotation. We suspect that the planetary atmosphere may participate in an exchange of momentum between these two reservoirs. Nontrivial changes in the circulation of the atmosphere are likely to occur, as the atmospheric system gains and loses angular momentum, during this exchange. We derive a coupling expression linking orbital and rotational motions that produces an acceleration field varying with position and with time on and within a subject body. The spatially and temporally varying accelerations may interfere constructively or destructively with large-scale flows of geophysical fluids that are established and maintained by other means. This physical hypothesis predicts cycles of intensification and relaxation of circulatory flows of atmospheres on seasonal and longer timescales that are largely independent of solar forcing. The predictions of this hypothesis may be tested through numerical modeling. Examples from investigations of the atmospheric circulation of Mars are provided to illustrate qualitative features and quantitative aspects of the coupling mechanism proposed.

Europa's surface composition from near-infrared observations: A comparison of results from linear mixture modeling and radiative transfer modeling

Quantitative estimates of the abundance of surface materials and of water ice particle grain sizes at five widely separated locations on the surface of Europa have been obtained by two independent methods in order to search for possible discrepancies that may be attributed to differences in the methods employed. Results of radiative transfer (RT) compositional modeling (also known as intimate mixture modeling) from two prior studies are here employed without modification. Areal (or “checkerboard”) mixture modeling, also known as linear mixture (LM) modeling, was performed to allow direct comparisons. The failure to model scattering processes (whose effects may be strongly nonlinear) in the LM approach is recognized as a potential source of errors. RT modeling accounts for nonlinear spectral responses due to scattering, but is subject to other uncertainties. By comparing abundance estimates for H2SO4•nH2O and water ice, obtained through both methods as applied to identical spectra, we may gain some insight into the importance of ‘volume scattering’ effects for investigations of Europas surface composition. We find that both methods return similar abundances for each location analyzed; linear correlation coefficients of ≥ 0.98 are found between the derived H2SO4•nH2O and water ice abundances returned by both methods. We thus find no evidence of a significant influence of volume scattering on the compositional solutions obtained by LM modeling for these locations. Some differences in the results obtained for water ice grain sizes are attributed to the limited selection of candidate materials allowed in the RT investigations. This article is protected by copyright. All rights reserved.