Steven W. Lee

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.

The thermal inertia of Mars from the Mars Global Surveyor Thermal Emission Spectrometer

We have used Mars Global Surveyor (MGS) Thermal Emission Spectrometer thermal emission measurements to derive the thermal inertia of the Martian surface at the ∼100-km spatial scale. We have validated the use of nighttime-only measurements to derive thermal inertia as well as the use of a single wavelength band versus bolometric thermal emission measurements. We have also reanalyzed the Viking Infrared Thermal Mapper data set in a similar manner in order to allow a direct comparison between the two. Within the uncertainties of the fit of the data to the model, and the uncertainties inherent in the model, the thermal inertia has not changed substantially in the 21 years between the Viking and the MGS measurements. Although some differences are seen, they are most likely due to changes in albedo during the intervening years or to residual effects of airborne dust that are not fully accounted for in the thermal models. The thermal inertia values that we derive, between about 24 and 800 J m−2 s−1/2 K−1, are thought to better represent the actual thermal inertia of the Martian surface than previous estimates.

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.

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.

1995 observations of Martian dust storms using the Hubble Space Telescope

A synoptic monitoring program of Mars using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope has resulted in the observation of discrete dust activity for two dates following the 1995 opposition. Data obtained on April 8 (Ls=82°) and August 21, 1995 (Ls=145°) reveal the presence of local dust storms in the northern polar region. In addition, the August images show activity over the Hellas basin. We present a multispectral analysis of the data from which we derive lower bounds to the atmospheric dust loading. Our technique directly models the absolute photometry, and consequently also provides estimates of the cloud opacity and the ozone columns for the regions analyzed.

Mars: Coming to a Museum Near You!

The recent successful landing of Mars Pathfinder and the arrival of Mars Global Surveyor at its destination have launched a decade of exploration that is changing our understanding of Martian natural history and climate. The public will share in the excitement and discoveries of the renewed Mars exploration through MarsQuest, an outreach effort that includes a traveling museum exhibit, planetarium show, and educational program. n nMarsQuests 5000 square foot traveling exhibit will transport visitors to the surface of the Red Planet, where they can experience the spirit and thrill of exploration while learning about the similarities and differences between Earth and Mars. In the planetarium show, audience members will be surrounded with vivid images of Mars. Complimentary education and outreach programs will enlighten the public about key planetary science concepts, such as physical scales, how physical systems interact, planetary formation, planetary climate, and the possibility of life on other planets, with special attention directed toward new results from the suite of NASA missions to Mars.