Bruce C. Murray

Behavior of carbon dioxide and other volatiles on Mars.

We have found that a rather simple thermal model of the Martian surface, in combination with current observations of the atmospheric composition, points strongly toward the conclusion that the polar caps of Mars consist almost entirely of frozen CO2. This study was based upon the following principal assumptions. 1) Carbon dioxide is a major constituent of the Martian atmosphere. 2) The blanketing effect of the atmosphere is small, and due principally to the absorption band of CO2 near 15 microns. 3) Lateral and convective heat transfer by the atmosphere is negligible. 4) The far-infrared emissivity of the Martian soil and of solid CO2 are near unity. 5) The reflectivities of the soil and of solid CO2 in the visible part of the spectrum are about 0.15 and 0.65, respectively. 6) Values for soil conductivity, density, and specific heat are those characteristic of powdered minerals at low gas pressure. 7) Water is a minor constituent of the Martian atmosphere, the maximum total amount in the atmosphere being 10 to 30 X 1O-4 g cm-2. In addition, several simplifications were made, which might have significant effects but should not alter our principal conclusions. Among these are the following. 1) Local blanketing or snowfall effects due to clouds or polar haze were ignored. 2) Dark and light areas were not differentiated in this study, although Sinton and Strong (6) have observed temperature differences between such areas. 3) The effects of local topography and microrelief were neglected. We believe that these must have quite significant effects at the higher latitudes, especially in connection with the evaporation of the remanent south polar cap. 4) Variation of reflectivity with angle of incidence of the sunlight was neglected. 5) Temperature dependence of soil conductivity and specific heat was ignored. 6) Effects of saturation of the soil by ice upon the thermal properties of the soil were neglected. Although in our main investigation we used certain specific values for the various relevant parameters, we also tested the effects of moderate changes in these quantities. Specifically, the soil conductivity was varied by a factor of 3, the albedo and emissivity of the surface were changed by 15 to 20 percent, and the effects of a gross amount of atmospheric blanketing were studied, as described. Only the last of these variations had any significant effect on the model, and other results of the atmospheric blanketing were in disagreement with other physical observations of the planet. Consequently, we find it difficult to avoid the conclusion that CO2 must condense in large amounts relative to H20. The main conclusions indicated by this study are the following. 1) The atmosphere and frost caps of Mars represent a single system with CO2 as the only active phase. 2) The appearance and disappearance of the polar caps are adequately explained on the presumption that they are composed almost entirely of solid CO2 with perhaps an occasional thin coating of water ice. 3) If the currently reported water-vapor observations are correct, water-ice permafrost probably exists under large regions of the planet at polar and temperate latitudes. 4) The geochemically anomalous enrichment of CO2 relative to N2 in the present Martian atmosphere may be a result of selective trapping of CO2 in the solid phase at and under the surface. 5) If the basic evaporation and condensation mechanisms for CO2 and H2O discussed in this article are correct, the possible migration of volatile organic compounds away from the warm temperate regions of the planet and their possible accumulation in the polar regions need to be carefully considered.

Practical limits on hyperspectral vegetation discrimination in arid and semiarid environments

Hyperspectral remote sensing is a promising tool for the analysis of vegetation and soils in remote sensing imagery. The purpose of this study is to ascertain how well hyperspectral remote sensing data can retrieve vegetation cover, vegetation type, and soil type in areas of low vegetation cover. We use multiple endmember spectral mixture analysis (MESMA), high-quality field spectra, and AVIRIS data to determine how well full-range spectral mixture analysis (SMA) techniques can retrieve vegetation and soil information. Using simulated AVIRIS-derived reflectance spectra, we find that, in areas of low vegetation cover, MESMA is not able to provide reliable retrievals of vegetation type when covers are less than at least 30%. Overestimations of vegetation are likely, but vegetation cover in many circumstances can be estimated reliably. Soil type retrievals are more than 90% reliable in discriminating dark-armored desert soils from blown sands. This simulation comprises a best-case scenario in which many typical problems with remote sensing in areas of low cover or desert areas are minimized. Our results have broad implications for the applicability of full-range SMA techniques in analysis of data from current and planned hyperspectral sensors. Several phenomena contribute to the unreliability of vegetation retrievals. Spectrally indeterminate vegetation types, characterized by low spectral contrast, are difficult to model correctly even at relatively high covers. Combinations of soil and vegetation spectra have the potential of generating mixtures that resemble an unmixed spectrum from different material, further confounding vegetation cover and soil type retrievals. Intraspecies spectral variability and nonlinear mixing produce uncertainties in spectral endmembers much larger than that only due to instrumental noise modeled here. Having established limits on linear spectral unmixing in areas of low cover through spectral simulations, we evaluate AVIRIS-derived reflectance data from the Mojave Desert, California. We show that MESMA is capable of mapping soil surface types even when vegetation type cannot be reasonable retrieved.

Diviner lunar radiometer observations of cold traps in the moon's south polar region

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

Nonlinear spectral mixing in desert vegetation

Linear mixing models are widely used in terrestrial remote sensing, with the errors in these models being often attributed to “nonlinear” mixing. Nonlinear mixing refers to the interaction of light with multiple target materials. Reflectance data from creosote bush in the Manix Basin of the Mojave Desert is used to show the existence and importance of nonlinear mixing in and region vegetation. It shows that the difference in the reflectance spectrum of plants against a soil background and the spectrum of the plant against a dark background is well represented by light that has interacted with both the soil and the plant.

Geological framework of the south polar region of Mars.

Abstract The first 4 months of Mariner 9 photography of the south polar region are discussed. Three major geological units have been recognized, separated by erosional unconformities. From oldest to youngest they are: cratered terrain, pitted plains, and laminated terrain. The latter unit is unique in occurrence to the polar region, volatiles are probably involved in its origin, and may still be present within the laminated terrain as layered ice. The residual south polar cap has been observed to survive the disappearance of the thin annual CO2 frost deposit and to last virtually unchanged in outline through the southern summer. That exposed deposit is inferred to be composed of water-ice. The residual cap appears to lie at the apex of an unusual quasi-circular structure composed of laminated terrain; a similar structure also appears to exist near the north pole.

Mercury's Surface: Preliminary Description and Interpretation from Mariner 10 Pictures

The surface morphology and optical properties of Mercury resemble those of the moon in remarkable detail and record a very similar sequence of events. Chemical and mineralogical similarity of the outer layers of Mercury and the moon is implied; Mercury is probably a differentiated planet with a large iron-rich core. Differentiation is inferred to have occurred very early. No evidence of atmospheric modification of landforms has been found. Large-scale scarps and ridges unlike lunar or martian features may reflect a unique period of planetary compression near the end of heavy bombardment by small planetesimals.

Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars

Following Svitek (Martian Water Frost: Control of Global Distribution by Small-Scale Processes, Ph.D Thesis, California Institute of Technology, 1992), analytic solutions are presented for the effective albedo, the effective emissivity, and the radiative equilibrium temperature in the shadowed portions of a spherical bowl-shaped crater. The model assumes that the surface is a Lambert scatterer with visual albedo and infrared emissivity each independent of wavelength across their respective spectral ranges. Absorption, emission, and multiple scattering from the walls of the crater are treated rigorously to all orders. For airless bodies whose surfaces are in radiative equilibrium, all shadowed portions of any individual crater have the same temperature, whose value depends on four quantities: the insolation (product of the solar constant and the sine of the solar elevation angle), the depth/diameter ratio of the crater, the visual albedo, and the infrared emissivity. As long as the crater is deep enough to have shadows, the lowest temperatures are for the shallowest crater—those with the smallest depth/diameter ratio. The model is applied first to the Moon and Mercury using a depth/diameter ratio of 0.2, which is typical of the lunar highlands according to Pike (Geophys. Res. Lett. 1, 291–294 (1974); in Impact and Explosion Cratering (Roddy et al., Eds.), pp. 489–509, Pergamon, New York, 1977). For Mercury and the Moon, temperatures in shadows in polar craters are below 102 K, so the sublimation rate of water ice calculated according to the model of Watson et al. (J. Geophys. Res. 66, 3033-3015 (1961)) is less than 1 cm per byr. The latitudinal extent of the cold zone on the Moon is greater than that on Mercury, although temperatures at the poles of the two planets are similar. The other application is to polar frosts on Mars. Illuminated water frosts in radiative equilibrium grow rougher, because the average temperature of a depression is greater than that of flat ground. Subliming CO_2 frosts, which are always at the same temperature, grow rougher at low solar elevation angles because the heat flux absorbed by a depression is greater than that for a flat surface. At high insolation rates (high Sun near perihelion) the average heat flux to a depression is less than for a flat surface. The latter evaporates faster, which makes the average surface smoother and leads to a high average albedo. This behavior helps explain the fact that the south CO_2 cap, which receives its greatest insolation near perihelion, has a higher effective albedo and therefore can survive the summer, whereas the north CO_2 cap has a lower effective albedo and disappears each year around summer solstice.

Periodic Insolation Variations on Mars

Previously unrecognized insolation variations on Mars are a consequence of periodic variations in eccentricity, first established by the theory of Brouwer and Van Woerkom (1950). Such annual insolation variations, characterized by both 95,000-year and 2,000,000-year periodicities, may actually be recorded in newly discovered layered deposits in the polar regions of Mars. An additional north-south variation in seasonal insolation, but not average annual insolation, exists with 51,000-year and 2,000,000-year periodicities.

Venus: Atmospheric Motion and Structure from Mariner 10 Pictures

The Mariner 10 television camieras imaged the planet Venus in the visible and near ultraviolet for a period of 8 days at resolutions ranging from 100 meters to 130 kilometers. Tle general pattern of the atmospheric circulation in the upper tropospheric/lower stratospheric region is displayed in the pictures. Atmospheric flow is symmetrical between north and south hemispheres. The equatorial motions are zonal (east-west) at approxiimnately 100 meters per second, consistent with the previously inferred 4-day retrograde rotation. Angular velocity increases with latitude. The subsolar region, and the region downwind from it, show evidence of large-scale convection that persists in spite of the main zonal motion. Dynamical interaction between the zonal motion and the relatively stationary region of convection is evidenced by bowlike waves.

Mariner 9 television reconnaissance of Mars and its satellites: Preliminary results

At orbit insertion on 14 November 1971 the Martian surface was largely obscured by a dust haze with an extinction optical depth that ranged from near unity in the south polar region to probably greater than 2 over most of the planet. The only features clearly visible were the south polar cap, one dark, spot in Nix Olympica, and three dark spots in the Tharsis region. During the third week the atmosphere began to clear and surface visibility improved, but contrasts remained a fraction of their normal value. Each of the dark spots that apparently protrude through most of the dust-filled atmosphere has a crater or crater complex in its center. The craters are rimless and have featureless floors that, in the crater complexes, are at different levels. The largest crater within the southernmost spot is approximately 100 kilometers wide. The craters apparently were formed by subsidence and resemble terrestrial calderas. The south polar cap has a regular margin, suggsting very flat topography. Two craters outside the cap have frost on their floors; an apparent crater rim within the cap is frost free, indicating preferentia loss of frost from elevated ground. If this is so then the curvilinear streaks, which were frost covered in 1969 and are now clear of frost, may be low-relief ridges. Closeup pictures of Phobos and Deimos show that Phobos is about 25 �5 by 21 �1 kilometers and Deimos is about 13.5 � 2 by 12.0 �0.5 kilometers. Both have irregular shapes and are highly cratered, with some craters showing raised rims. The satellites are dark objects with geometric albedos of 0.05.