Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Earth and Planetary Sciences

Language

English (en)

Date of Award

6-4-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Raymond E. Arvidson

Abstract

Two recent in situ Mars missions, the Phoenix Mars Lander and the Mars Exploration Rover Opportunity, have explored two quite different locations on the surface of Mars. The Phoenix lander investigated the polygonal terrain and associated soil and icy soil deposits of a high northern latitude site: 68.22° N, 234.25° E). The Opportunity rover, the only currently operational spacecraft on the surface of Mars, is located much closer to the equator: 1.95° S, 354.47° E), and has been exploring the plains and sedimentary rocks in Meridiani Planum. Concurrent with in situ Opportunity and Phoenix observations, the Compact Reconnaissance Imaging Spectrometer for Mars: CRISM) was in orbit around Mars collecting hyperspectral data. In this dissertation, surface and orbital data are used to explore and characterize surface material properties at the Phoenix and Opportunity sites.

The Phoenix soil physical properties experiments involved the analysis of forces determined from motor currents from the Robotic Arm: RA)’s trenching activities. Using this information and images of the landing site, soil cohesion and angle of internal friction were determined. Soil dump pile slopes were used to determine the angle of internal friction of loose soil: 38° ± 5°. Additionally, an excavation model that treated walls and edges of the RA’s scoop as retaining walls was used to calculate mean in situ soil cohesion values for several trenches in the Phoenix landing site workspace. These cohesions were found to be consistent with the stability of steep trench slopes. Cohesions varied from 0.20.4−0.2 kPa to 1.21.8−1.2 kPa, with the exception of a subsurface platy horizon unique to a shallow trough for which cohesion will have to be determined using other methods. Soil on a nearby polygon mound had the greatest cohesion: 1.21.8−1.2 kPa). This high cohesion value was most likely due to the presence of adsorbed water or pore ice above the shallow icy soil surface. Further evidence for enhanced soil cohesion above the ice table includes lateral increase in excavation force, by over 30 N, as the RA approached ice. The behavior of soil near the ice table interface is of particular interest considering that many of the high-latitude and mid–latitude regions of Mars are underlain by ice.

For the region traversed by Opportunity in the vicinity of Victoria crater, normalized spectral radiances from the Compact Reconnaissance Imaging Spectrometer for Mars: CRISM) were used to retrieve surface scattering properties. Estimates agree with those retrieved in previous photometric studies which used Opportunity–s Panoramic Camera: Pancam) data, and I was able to extend estimates of the Hapke single particle scattering albedo and asymmetry parameter: from the one–term Henyey Greenstein single particle phase function) to a greater spatial and spectral range. Results are useful for determining the boundaries between surface units that otherwise look relatively uniform spectrally. This work also provides photometric functions essential for converting spectra to a single viewing geometry which will yield more accurate spectral comparisons. Results were obtained through simultaneous modeling of surface and atmospheric contributions, iterating through surface scattering parameters until a Levenberg–Marquardt least squares best fit was achieved. Retrieved single scattering albedos range from 0.42 to 0.57: 0.5663 − 2.2715 micrometers), and retrieved asymmetry parameters range from −0.27 to −0.17: moderately backscattering). All surfaces become more backscattering with increasing wavelength. The majority of Victoria crater’s ejecta apron is more backscattering than surrounding regions, indicating a change in physical properties. Images taken when the rover traversed this unit show a cover of basaltic soil with superposed millimeter–scale hematitic spherules, providing agreement with previous analyses of lab experiments showing increased backscattering with the addition of hematitic spherules. Dark wind streaks on the apron appear smooth: low backscatter) because basaltic sands have partly buried spherules, lessening millimeter–scale roughness: in agreement with previous near–surface wind streak analyses). The CRISM–derived scattering parameters also show that bedrock–dominated surfaces are less backscattering than soil–covered surfaces, largely due to lower areal abundance of spherules. The ability to analyze surface unit spherule cover is important because it relates to a wetter period during which spherules formed in Meridiani.

DOI

https://doi.org/10.7936/K7M043GV

Comments

Permanent URL: http://dx.doi.org/10.7936/K7M043GV

Share

COinS