R. Todd Clancy

A new look at dust and clouds in the Mars atmosphere: analysis of emission-phase-function sequences from global viking IRTM observations

Abstract An analysis of emission-phase-function (EPF) observations from the Viking Orbiter infrared thermal mapper (IRTM) yields a wide variety of results regarding dust and cloud scattering in the Mars atmosphere and atmospheric-corrected albedos for the surface of Mars. Several hundred of these EPF sequences were returned during the Viking Orbiter missions, in which the visual brightness (solar band λ = 0.67 μm , bandwidth of 0.3 – 3.0 μm) of the surface/atmosphere over a given region was viewed over a substantial range of phase angles as the spacecraft passed overhead. These observations include polar latitudes, the Viking Lander sites, and other regions of specific interest such as Olympus Mons and the Hellas basin. They also span a considerable range of solar longitude (Ls), including periods before, during, and after the global dust storms of 1977, and periods of polar hood formation at midlatitudes and spring clouds at northern polar latitudes. A multiple scattering radiative transfer model incorporating a bidirectional phase function for the surface and atmospheric scattering by dust and clouds is used to derive surface albedos and dust and ice optical properties and optical depths for these various conditions on Mars. The specific photometric function of the surface of Mars is not well constrained by the EPF observations, although a resolved phase coefficient of ∼0.01 mag/deg is indicated by low-phase-angle, low-dust-loading sequences for both bright and dark regions on Mars. The dust and cloud optical depths are well constrained by the increased scattering present at high phase angles (>70°) within the EPF data. Furthermore, the single scattering albedos ( ω 0 and phase functions of dust and cloud particles are also well defined for EPF sequences in which the dust optical depths are large (τ >0.5) and/or the range of observed phase angles is large (>80°). It is possible to fit all of the analyzed EPF sequences corresponding to dust scattering with ω 0 = 0.92 , compared to the value of 0.86 derived by Pollack et al. (1979) from Viking Lander observations. The derived dust scattering phase function agrees very well with the results of Pollack et al., although the resulting single scattering asymmetry parameter is 0.55 for both measurements rather than the value of 0.79 reported by Pollack et al. Arguments regarding the observed dust phase function and the observed ratios of infrared and visible dust opacities are presented to suggest that dust particle sizes may be 5–10 times smaller than previous analyses have indicated. Observed dust optical depths range from 2 to 3 at the peak of the global dust storms to 0.2 after the second dust storm of 1977; a dust opacity of 0.8 is determined for the south polar region during the second global dust storm of 1977. Three dust opacity determinations coinciding with Viking Lander measurements provide consistency with the Viking Lander dust opacity measurements. The surface elevation dependence of dust loading over Olympus Mons is derived from an EPF sequence during the first global dust storm of 1977. EPF sequences corresponding to cloud scattering indicate g = 0.66 for midlatitude fall clouds and g = 0.55 for nortth polar spring clouds, with ω = 1.0 . Cloud opacities range from 0.2 to 0.7, with a typical value of 0.5 for both the fall midlatitude and the string polar clouds. The global range of surface albedos resulting from this EPF analysis are ∼ 10% darker for bright regions and − 25% darker for dark regions on Mars than are currently provided by studies that do not explicity account for the effects of atmospheric scattering.

First Atmospheric Science Results from the Mars Exploration Rovers Mini-TES

Thermal infrared spectra of the martian atmosphere taken by the Miniature Thermal Emission Spectrometer (Mini-TES) were used to determine the atmospheric temperatures in the planetary boundary layer and the column-integrated optical depth of aerosols. Mini-TES observations show the diurnal variation of the martian boundary layer thermal structure, including a near-surface superadiabatic layer during the afternoon and an inversion layer at night. Upward-looking Mini-TES observations show warm and cool parcels of air moving through the Mini-TES field of view on a time scale of 30 seconds. The retrieved dust optical depth shows a downward trend at both sites.

Temperature minima in the average thermal structure of the middle mesosphere (70–80 km) from analysis of 40- to 92-km SME global temperature profiles

Global temperatures have been derived for the upper stratosphere and mesosphere from analysis of Solar Mesosphere Explorer (SME) limb radiance profiles. The analysis of additional SME wavelength radiances at 304, 313, and 442 nm provides for extension of the original SME 60- to 90-km temperature climatology (Clancy and Rusch, 1989a) to a much expanded altitude coverage of 40–92 km. The SME temperatures represent fixed local time observations at 1400–1500 LT, with partial zonal coverage of 3–5 longitudes per day over the 1982–1986 period. These new SME temperatures are compared to the CIRA 86 climatology (Fleming et al., 1990) as well as stratospheric and mesospheric sounder (SAMS); (Barnett and Corney, 1984), National Meteorological Center (NMC); (Gelman et al., 1986), and individual lidar and rocket observations. Significant areas of disagreement between the SME and CIRA 86 mesospheric temperatures are 10 K warmer SME temperatures in the 55- to 65-km altitude region and 10–20 K warmer SME temperatures at altitudes above 80 km. The 1981–1982 SAMS temperatures are in much closer agreement with the SME temperatures between 40 and 75 km. Although much of the SME-CIRA 86 disagreement probably stems from the poor vertical resolution of the observations comprising the CIRA 86 model, some portion of the differences may reflect 5- to 10-year temporal variations in mesospheric temperatures. The CIRA 86 climatology is based on 1973–1978 measurements. Relatively large (1 K/yr) 5- to 10-year trends in temperatures as functions of longitude, latitude, and altitude have been observed for both the upper stratosphere (Clancy and Rusch, 1989a) and mesosphere (Clancy and Rusch, 1989b; Hauchecorne et al. 1991). The SME temperatures also exhibit enhanced amplitudes for the semiannual oscillation (SAO) of upper mesospheric temperatures at low latitudes, which are not evident in the CIRA 86 climatology. The so-called mesospheric “temperature inversions” at wintertime midlatitudes, which have been observed by ground-based lidar (Hauchecorne et al., 1987) and rocket in situ measurements (Schmidlin, 1976), are shown to be a climatological aspect of the mesosphere, based on the SME observations. The winter midlatitude “temperature inversion” is evident in both winter hemispheres and appears more distinctly as a low-altitude (75 km) temperature minimum at southern midlatitudes. Furthermore, the SME temperatures indicate the related presence of low-latitude temperature minima at 80-km altitude during the equinoxes. Both the low-latitude semiannual minima and the midlatitude annual minima in middle mesospheric temperatures appear to result from the strong SAO in mesospheric temperatures.

Hubble Space Telescope observations of the Martian aphelion cloud belt prior to the Pathfinder mission: Seasonal and interannual variations

The presence of a globally extant equatorial belt of water ice clouds on Mars is quantitatively investigated using data from three seasons of our Hubble Space Telescope synoptic monitoring program (1993–1997). A subset of the 1996–1997 images covers the landing site of the Mars Pathfinder including a set of images taken after touchdown. Using multicolor imaging from the Wide Field Planetary Camera and the Wide Field Planetary Camera 2, we characterize both water ice cloud and dust optical depths as a function of latitude at several local times for each observing epoch. The analysis technique models calibrated data using a multiple scattering radiative transfer code. Our results support the initial results of Clancy et al. [1996a] regarding changes between the aphelion and perihelion climate of Mars and provide a more detailed look at the development and decay of the cloud belt. Comparing our dust optical depths to those of the Viking landers for the same seasons, we note a trend toward lower dust loading in late northern winter and in spring. Our observations of the Pathfinder site in July 1997 reveal a dust opacity in good agreement with that reported by Pathfinder [Smith et al., 1997b]. In addition, the serendipitous occurrence of a dust storm in Valles Marineris in late June 1997 allows us to derive a set of dust single scattering albedos for use in more accurately modeling the dusts radiative properties and effects.

Mars surface mineralogy from Hubble Space Telescope imaging during 1994–1995: Observations, calibration, and initial results

Visible to near-infrared observations of Mars were made with the Hubble Space Telescope (HST) during 1994–1995 with the goals of monitoring seasonal variability of the surface and atmosphere and mapping specific spectral units to constrain the planets surface mineralogy. This paper presents the details of the collection and calibration of the data, concentrating specifically on the near-IR data that were obtained exclusively for the surface mineralogy aspect of our HST Mars observing program. We also present some initial results from the calibrated data set. Our calibration procedures included the standard “pipeline” processing steps, supplemented by special procedures required for use with the linear ramp filters on the Wide Field/Planetary Camera 2 instrument, and an additional point spread function deconvolution procedure applied in order to realize the full potential spatial resolution of the images (23 to 64 km/pixel between August 1994 and August 1995). The calibration results in a set of images projected onto a standard map grid and presented in radiance factor (I/F) units, having an estimated ≈5% photometric accuracy based on the performance of HST and comparisons with previous ground-based and spacecraft Mars spectra. Initial scientific analyses of these data reveal (1) distinct red/blue color units within the classical bright regions, similar to those seen in Viking Orbiter images and possibly related to variations in nanophase and/or crystalline ferric mineral abundance; (2) near-IR spectral slope variations correlated with albedo on a large scale (darker is “bluer” near-IR slope) but exhibiting wider variations among many of the small-scale features visible in the data; (3) an absorption at 860 nm that occurs in all regions but which is 3 to 5% stronger in many of the classical dark regions than in the bright regions, possibly because of a greater abundance of a well-crystalline ferric phase like hematite or a very low Ca pyroxene or opx/cpx mixture; and (4) an absorption from pyroxene at 953 nm with a band depth that is inversely correlated with albedo (bright regions 0 to 5% deep; dark regions 7 to 15% deep) and which shows the highest band depth values in individual craters, calderas, and other small geologic units that are resolved in the images.

Mapping Mars water vapor with the Very Large Array

Abstract We observed Mars with the Very Large Array (VLA) in the spectral line mode on December 3/4 and 6/7 of 1990. Operating at a wavelength of 1.35 cm, we were able to map the 22-GHz water emission line around the atmospheric limb of Mars. The variation of the emission around the limb and the measured lineshapes yield information on the diurnal, latitudinal, and vertical distributions of water vapor in the Mars atmosphere. The VLA limb spectra yield a global average water column of 3.0 ± 0.8 pr μ m, which is significantly lower than that returned by the 1988 IR reflectance observations of Rizk et al. (1991) for the same Mars season ( L S = 340−350°, early northern spring). The Rizk et al. measurement of 10 pr μm is roughly twice that returned by Viking MAWD observations for this driest of Mars seasons (Farmer et al. 1977), whereas our 1.35-cm emission determination is roughly one half that of the Viking MAWD determination. This implies interannual variations in global water vapor of the same order as the seasonal variations observed by Viking. The latitudinal variation measured by this VLA experiment indicates a maximum water vapor abundance of 4.5 ± 1.0 pr μ m (75 ppmv for an altitude-independent mixing ratio) at equatorial latitudes, decreasing to 2.9 ± 0.7 pr μ m (50 ppmv) at midlatitudes. The polar (60–90°NS) water mixing ratio fell below 20 ppmv at altitudes of 15–50 km. We do not find significant diurnal (5 AM vs 5 PM) variation of water vapor at low latitudes. However, it does appear that the water column at mid latitudes (40–60°NS) is ∼30% smaller for the morning versus the evening limb. An analysis of the pressure-broadened 22-GHz lineshape indicates that water vapor was well mixed to an altitude of ≥45 km, although a vertical mixing gradient as large as 50% over the 0 to 50-km altitude range would fall within the measurement uncertainties.

CO2 ice clouds in the upper atmosphere of Mars

Recent (Sept. 1996–Sept. 1997) observations of Mars submillimeter CO lines from the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawaii have revealed surprisingly cold temperatures as typical of the Mars atmosphere over the 50–80 km altitude region. Measurements of these J=2->3 and J=3->4 rotational transitions of Mars atmospheric 12CO provide unique seasonal coverage of global-averaged, dayside temperatures within this poorly observed mesophere of Mars. At solar longitudes (Ls) of 8°, 90°, 150°, and 187° in 1996–97, the mean atmospheric temperature for the 70–80 km (2.0–0.3 µbars) altitude region is observed to be at or below 120 K, which is only 10–15 K above the vapor saturation temperature for the CO2 Mars atmosphere. Consequently, local CO2 saturation conditions at these altitudes would be likely to exist and, in fact, were observed during the 3 AM descent entry of the Pathfinder spacecraft, on July 4, 1997. We argue that the blue wave clouds imaged from the Pathfinder lander 35–100 minutes prior to sunrise on Sol 39 are evidence of such CO2 ice formation within the 60–100 km altitude region; and that the 4.3 µm CO2 lines of solar scattered flux, in Mariner 6 and 7 infrared limb spectra of Mars are a direct spectral identification of CO2 ice cloud formation in the dayside Mars mesosphere. Simple considerations of these Pathfinder and Mariner 6 and 7 observations suggest 0.1–0.3 µm cloud particle radii, and particle number densities of the order 10² cm−3. On the basis of a variety of such day and night time measurements, we assert that the mesosphere of Mars does not exhibit extreme (25 K) diurnal temperature variations as maintained by the Pathfinder meteorology team; and that it is often substantially colder (>20 K) than determined from the Viking descent measurements in 1976 due to temporal variations in dust loading of this region.

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) on MRO (Mars Reconnaissance Orbiter)

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) is a hyperspectral imager that will be launched on the MRO (Mars Reconnaissance Orbiter) spacecraft in August 2005. MRO’s objectives are to recover climate science originally to have been conducted on the Mars Climate Orbiter (MCO), to identify and characterize sites of possible aqueous activity to which future landed missions may be sent, and to characterize the composition, geology, and stratigraphy of Martian surface deposits. MRO will operate from a sun-synchronous, near-circular (255x320 km altitude), near-polar orbit with a mean local solar time of 3 PM. CRISM’s spectral range spans the ultraviolet (UV) to the mid-wave infrared (MWIR), 383 nm to 3960 nm. The instrument utilizes a Ritchey-Chretien telescope with a 2.12° field-of-view (FOV) to focus light on the entrance slit of a dual spectrometer. Within the spectrometer, light is split by a dichroic into VNIR (visible-near-infrared, 383-1071 nm) and IR (infrared, 988-3960 nm) beams. Each beam is directed into a separate modified Offner spectrometer that focuses a spectrally dispersed image of the slit onto a two dimensional focal plane (FP). The IR FP is a 640 x 480 HgCdTe area array; the VNIR FP is a 640 x 480 silicon photodiode area array. The spectral image is contiguously sampled with a 6.6 nm spectral spacing and an instantaneous field of view of 61.5 μradians. The Optical Sensor Unit (OSU) can be gimbaled to take out along-track smear, allowing long integration times that afford high signal-to-noise ratio (SNR) at high spectral and spatial resolution. The scan motor and encoder are controlled by a separately housed Gimbal Motor Electronics (GME) unit. A Data Processing Unit (DPU) provides power, command and control, and data editing and compression. CRISM acquires three major types of observations of the Martian surface and atmosphere. In Multispectral Mapping Mode, with the gimbal pointed at planet nadir, data are collected at frame rates of 15 or 30 Hz. A commandable subset of wavelengths is saved by the DPU and binned 5:1 or 10:1 cross-track. The combination of frame rates and binning yields pixel footprints of 100 or 200 m. In this mode, nearly the entire planet can be mapped at wavelengths of key mineralogic absorption bands to select regions of interest. In Targeted Mode, the gimbal is scanned over ±60° from nadir to remove most along-track motion, and a region of interest is mapped at full spatial and spectral resolution. Ten additional abbreviated, pixel-binned observations are taken before and after the main hyperspectral image at longer atmospheric path lengths, providing an emission phase function (EPF) of the site for atmospheric study and correction of surface spectra for atmospheric effects. In Atmospheric Mode, the central observation is eliminated and only the EPF is acquired. Global grids of the resulting lower data volume observation are taken repeatedly throughout the Martian year to measure seasonal variations in atmospheric properties.

Global imaging of Mars by Hubble space telescope during the 1995 opposition

Hubble space telescope (HST) imaging of Mars near the 1995 opposition resulted in excellent synoptic-scale images of the planet during the spring season in the northern hemisphere. Because this season coincides with the aphelion position of Mars in its orbit, it is therefore the most difficult for ground based observation because of the relatively small angular size of Mars. This is the first sequence of images fully utilizing the capability of the new Planetary Camera to produce global synoptic images of the planet. The images reveal bright, discrete clouds associated with topographic features superimposed on a zonal band of condensate clouds between latitudes -10° and 30° ; the maximum violet optical depth of the cloud band is about 0.3. In a few instances, the appearance of clouds beyond the morning terminator can be used to infer cloud heights of roughly 8 km. A large, dark albedo feature in the Cerberus region, observed for many years by ground-based observers, has almost disappeared in the 1995 HST images. Other aspects of Mars, such as the north polar cap, appear much as they did during previous oppositions. Although cloudy regions were observed by spacecraft during this season, the HST images uniquely reveal the global extent of significant optical depth clouds.

Mars ozone measurements near the 1995 aphelion: Hubble space telescope ultraviolet spectroscopy with the faint object spectrograph

Ultraviolet (225–330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2–0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (Ls) of 63.5°, corresponding to the late northern spring season on Mars, and very near to Mars aphelion. The most important result of these observations is the measurement of low-latitude ozone abundances (3.1−0.5+2.1), which are significantly (≥100%) elevated relative to the northern fall (Ls = 208°, pre-perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion-to-aphelion increase in the global Mars ozone column (from 1.5−1.0+0.4 to 3.1−0.5+2.1 μm atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with Ls, which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3 ± 2.5 μm atm is retrieved for a high northern latitude region (71–75°N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18 ± 0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.