Tomas Svitek

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.

Laboratory measurement of elastic anisotropy on spherical rock samples by longitudinal and transverse sounding under confining pressure

Knowledge of shear wave velocities in loaded rocks is important in describing elastic anisotropy. A new high-pressure measuring head was designed and constructed for longitudinal and traversal ultrasonic sounding of spherical rock samples in 132 independent directions under hydrostatic pressure up to 60 MPa. The velocity is measured using a pair of P-wave sensors and two pairs of S-wave sensors (TV/RV and TH/RH) with perpendicular polarization. An isotropic glass sphere was used to calibrate the experimental setup. A fine-grained anisotropic quartzite sample was examined using the P- and S-wave ultrasonic sounding. Waveforms are recorded by pairs of TP/RP, TV/RV and TH/RH transducers in a range of confining pressure between 0.1 and 60 MPa. The recorded data showed a shear wave splitting in three basic structural directions of the sample. The measurements proved to be useful in investigating oriented micro-cracks, lattice (LPO) and shape-preferred orientation (SPO) for the bulk elastic anisotropy of anisotropic rocks subjected to hydrostatic pressure.

Seismic anisotropy of serpentinite from Val Malenco, Italy

© 2015. American Geophysical Union. All Rights Reserved. Serpentinites, deformed in mantle subduction zones, are thought to contribute significantly to seismic anisotropy of the upper mantle and have therefore been of great interest with studies on deformation, preferred orientation, and elastic properties. Here we present a combined study of a classical sample from Val Malenco, Italy, investigating the microstructure and texture with state-of-the art synchrotron X-ray and neutron diffraction methods and measuring ultrasonic velocities both with a multi-anvil apparatus and a novel instrument to measure 3-D velocities on spheres. Both, results from diffraction methods and velocity measurements are compared, discussing advantages and disadvantages. From spherical velocities, elastic tensor properties can be derived by inversion. Also from quantitative texture measurements, elastic properties can be modeled by self-consistent averaging. Good agreement between the velocity and microstructural models is observed.

Microrovers for assisting humans on the moon and elsewhere: Microrover catalog, requirements, and general design conclusions

Microrovers, which we define as one to roughly ten kilograms, have been little studied for planetary exploration, and until this and related studies, have hardly been studied at all for how they could work with future human exploration of the Moon, Mars, and asteroids. For a variety of functions, microrovers have advantages over larger rovers because of their low cost, mass, and volume. We have produced a publicly accessible online catalog of terrestrial and planetary microrovers, defined a set of Level 1 requirements that could be applied to microrovers, and have derived several general design conclusions. Microrovers for use with and by astronauts have high merit and near-term feasibility, and could help enable safer, more efficient, more publicly engaging human exploration of the Moon, Mars, or asteroids.