J.B. Pollack

Saltation threshold on Mars: The effect of interparticle force, surface roughness, and low atmospheric density

Abstract The effect of interparticle forces, as well as changes in surface roughness, particle diameter and density, and atmospheric density and viscosity are considered in new estimates of saltation thershold on Mars. These estimates result in somewhat lower minimum values of predicted threshold speed than by use of the “universal” A - B curve, with the minimum threshold speed occuring at smaller values of particle diameter. It is shown that the new predictions are much closer to the limited low atmospheric density data available than if the older method of estimation is used.

Diurnal variations in optical depth at Mars

Abstract Viking lander camera images of the Sun were used to compute atmospheric optical depth at the two sites over a period of 1 1 3 Martian years. The complete set of 1044 optical depth determinations is presented, with error estimates. Optical depths in the morning ( AM ) are generally larger than in the afternoon ( PM ). The AM-PM differences are ascribed to condensation of water into atmospheric ice aerosols at night and their evaporation in midday. A smoothed time series of these differences shows several seasonal peaks. These are simulated using a one-dimensional radiative-convective model that predicts Martian atmospheric temperature profiles. A calculation combining these profiles with water vapor measurement from the Mars atmospheric water detector (MAWD, on the Viking orbiters)is used to predict when diurnal variations of water condensation should occur. The model reproduces a majority of the observed peaks and shows the factors influencing the process. Diurnal variation of condensation is shown to peak when the latitude and season combine to warm the atmosphere to the optimum temperature, cool enough to condense vapor at night, and warm enough to cause evaporation at midday. The diurnal variation is enhanced by increased water vapor and is sometimes enhanced, sometimes diminished, by enhanced dust loading, depending on the other conditions. Often the model predicts condensation only at altitudes of 25 km or more, while at other times the condensation reaches ground level. Agreement between model and observations is also evident on a time scale of hours, when the data are available at more than two times in a single day.

Vertical distribution of scattering hazes in Titan's upper atmosphere

Abstract Radial intensity scars of a Voyager 2 high phase angle image of Titan have been inverted to yield vertical extinction profiles at 1° intervals around the limb. A detached haze layer with peak particle number densities ∼0.2 cm −3 exists at all latitudes south of ∼45°N, and at an altitude of 300–350 km. The optical depth 0.01 level lies at a radius of 2932 ± 5 km at the equator and at a radius of 2915 ± 10 km over the poles (altitudes of 357 ± 5 and 340 ± 10 km, respectively). In addition to the haze layer at 300–350 km, there is a small enhancement in the extinction at ∼450 km which exists at all latitudes between 75°S and ∼60°N.

Derivation of midinfrared (5–25 μm) optical constants of some silicates and palagonite

Abstract The midinfrared 2000−400 cm −1 (5–25 μm) optical constants (real ( n ) and imaginary ( k ) indices of refraction) are presented for: (1) pyrophyllite; (2) kaolinite; (3) serpentine; (4) montmorillonite; (5) saponite; (6) palagonite; and (7) orthopyroxene. Comparison of the values derived here with those previously presented for serpentine, montmorillonite, and palagonite is generally quite good and discrepancies between values are probably due to either chemical differences between the actual samples or different techniques used to derive the values. For montmorillonite, saponite, and palagonite we were able to derive optical constants in the region of the H 2 O-bending fundamental near 6 μm. We find that if a pellet of pure material can be produced with a mirror-like surface then the optical constants of clays and other noncohesive materials can be readily derived.

Wind tunnel studies of Martian aeolian processes

In order to determine the nature of Martian aeolian processes, an investigation is in progress which involves wind tunnel simulations, geologic field studies, theoretical model studies, and analyses of Mariner 9 imagery; this report presents the preliminary results. Threshold speed experiments were conducted for particles ranging in relative density from 1.3 to 11.35 and diameter from 10.2 to 1290 μm to verify and better define Bagnold’s (1941) expression for grain movement, particularly for low particle Reynolds numbers and to study the effects of aerodynamic lift and surface roughness. Wind tunnel simulations were conducted to determine the flow field over raised rim craters and associated zones of deposition and erosion. A horseshoe vortex forms around the crater, resulting in two axial velocity maxima in the lee of the crater which cause a zone of preferential erosion in the wake of the crater. Reverse flow direction occurs on the floor of the crater. The result is a distinct pattern of erosion and deposition which is similar to some Martian craters and which indicates that some dark zones around Martian craters are erosional and some light zones are depositional. Analyses of the erosional and depositional zones associated with a 6 m raised rim crater on an open field and a 1.2 km natural impact crater tentatively confirm the wind tunnel results. Application of the wind tunnel results to Mars indicates that for flat surfaces, free stream winds in excess of 400 km/h are required for grain movement. However, lower velocities would be required in regions of high surface roughness, e. g. cratered terrain, and it is proposed that such regions could be zones of origin for some Martian dust storms. Analysis of the Coriolis effect on surface stress shows that surface streaks would be deflected about 15° from the geostrophic wind direction at mid-latitudes.

Windblown dust on earth, Mars and Venus

Abstract Attempts to predict threshold speeds for windblown dust and sand on Mars and Venus have raised new questions about the mechanism of soil and sand transport by wind. Estimates of threshold speeds on Mars and Venus are made including the effects of both Reynolds number and interparticle forces of cohesion. The current estimates show lower threshold speeds for small particles than previous estimates by Hess, who assumed that Reynolds number is the only important parameter. These estimates, on the other hand, show somewhat higher threshold speeds than those of Sagan and Bagnold, who assumed that the particles are cohesionless. An optimum particle size results, for which threshold speed is a minimum, contrary to Sagan and Bagnolds prediction.

Ice and minerals on Callisto: A reassessment of the reflectance spectra

Abstract Laboratory spectral reflectance measurements, including the 3 μm H2O fundamental region, of particulate mixtures of both hydrated silicates and palagonite with water ice are presented. In all cases, the reflectances of these mixtures, at and beyond 3 μm, are dominated by absorptions due to water ice until ≥50 weight percent of the non-ice component is present. The laboratory measurements are compared to two previously unpublished reflectance spectra of Callisto. This comparison provides direct support for previous suggestions that the measured reflectance of Callisto contains a significant spectral contribution from a non-ice component, especially at and beyond 3 μm. Theoretical modeling, using Hapkes (1981, J. Geophys. Res. 86, 3039–3054; 1986, Icarus, 67, 264–280) equations, is used to more thoroughly analyze the telescopic data. The modeling considers homogeneous (intimate) and heterogeneous (areal) mixtures of both three components (ice, serpentine, and magnetite) and two components (ice and serpentine or magnetite) as a function of the relative particle size and abundance of the individual components. Comparison of the resultant calculations to the measured reflectance of Callisto imply that (1) three component mixtures of ice-magnetite-serpentine provide a better comparison to the telescopic data than any two component mixtures of ice and serpentine or magnetite; (2) the ice-magnetite-serpentine intimate mixtures which provide the best comparisons to the telescopic spectra indicate that the water ice contributes more to the spectrum of the leading hemisphere than the trailing hemisphere of Callisto; (3) all ice-magnetite mixtures fail to provide the spectral shape exhibited by the telescopic spectra of Callisto at and beyond 3 μm, indicating that the non-ice component possesses its own characteristic spectral features in this region.

Nonthermal escape of hydrogen and deuterium from Venus and implications for loss of water

Abstract Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H + with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O + and H 2 . However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 10 10 cm −2 sec −1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10 −3 . This corresponds to an H 2 O mixing ratio of 1 × 10 −3 at the cold trap and ∼15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ∼1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H 2 O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ∼4.0 × 10 −6 .

Eolian erosion of the Martian surface, part 1: Erosion rate similitude

Abstract A similitude parameter is derived which is based on theoretical considerations of erosion due to sand in saltation. This parameter has been used to correlate wind tunnel experiments of particle flow over model craters. The characteristics of the flow field in the vicinity and downstream of a crater are discussed and it is shown that erosion is initiated in areas lying under a pair of trailing vortices. The erosion rate parameter is used to calculate erosion rates on Mars, reported in Part 2, to be published later.

Meridional Martian water abundance profiles during the 1988–1989 season

Abstract In this paper we report on measurements of the latitudinal distribution of atmospheric water vapor column abundance that were collected on 4 nights during spring and summer in the Martian southern hemisphere before and after the 1988 opposition. The profiles for early southern spring and southern autumnal equinox agree with those measured during the same season in 1977 by the Mars Atmospheric Water Detector (MAWD) on the Viking Orbiters, but the profiles during southern mid- and late summer show twice as much water in the southern hemisphere and planetwide as did the Viking MAWD. An equatorial water column abundance maximum was a relatively constant feature of the profiles acquired and 4 × 10 14 g was observed to appear and disappear at latitudes south of −30°. Based on the lack of global dust storms observed during the 1988–1989 spring and summer compared to the 1977 Viking measurements during the same period, the water abundance measurements reported in this paper represent observations in a relatively clear atmosphere.