Date of Award
Doctor of Philosophy
Geological and Environmental Sciences
Dr. William A. Sauck
Dr. Essam Heggy
Dr. Wlodek Kofman
Dr. Johnson R. Haas
Airless, differentiated planetesimals in the inner solar system were presumed to have been depleted of most of their initial volatile content during formation. However, water-ice has been discovered at the lunar poles (Li et al., 2018), is inferred to exist in polar craters on Mercury (e.g., Butler, Slade & Muhleman, 1993, and suggested to survive beneath the dusty regolith of objects in the main asteroid belt if buried at sufficient depths—at least one meter for small-bodies in the outer main-belt (Schorghofer, 2008). Hence, the study of volatile occurrence, past or present, on airless, desiccated small-bodies provides insights into the timing, distribution and potential delivery mechanisms of water to the inner solar system.
One technique particularly well-suited to such studies is radar remote sensing, which can characterize the electrical and textural properties of desiccated planetary surfaces (e.g., Campbell, 2002). However, accurately interpreting radar observations requires disentangling the primary geophysical parameters that affect frequency, power and polarization, including surface topography, surface dielectric properties (i.e., its relative permittivity—dependent on mineralogy, density, temperature and ice content) and surface roughness at wavelength scales (e.g., Ostro et al., 2002). Unfortunately, asteroid surface mineralogies are not well-constrained due to a lack of clear spectral analogs among meteorites, and their surface texture at centimeter-to-decimeter (cm-dm) scales is poorly constrained due to lack of high-resolution surface images.
However, a unique opportunity arises to address the above uncertainties with the recent orbital mission to Asteroid Vesta by NASA’s Dawn spacecraft, which conducted the first orbital bistatic radar (BSR) observations of a small-body, using its high-gain communications antenna to transmit and Earth-based ground stations to receive (Palmer et al., 2017). To support accurate interpretation of Dawn’s BSR observations, the first dielectric model of Vesta is constructed (Palmer et al., 2015) by employing a mineralogical analogy with lunar basaltic soils to characterize the dielectric properties of the vestan regolith, adjusted for the temperatures and average surface density inferred from thermal observations by Dawn’s Visible and Infrared mapping spectrometer (Capria et al., 2014). Vesta’s surface dielectric constant is found to be uniform at ~2.4 at S- (2.3 GHz) and X-band (8.4 GHz) radar frequencies, suggesting that any variability in radar reflectivity can be attributed to variations in surface roughness (Palmer et al., 2015).
Subsequent power spectral analyses of Dawn BSR data reveal substantial radar reflectivity variability and therefore substantial variability in cm-dm surface roughness (Palmer et al., 2017). Unlike the Moon, surface roughness is not correlated with surface age, suggesting impact cratering alone cannot explain Vesta’s surface texture. Furthermore, heightened subsurface hydrogen concentration occurring within extensive smoother areas suggests that potential ground-ice presence (accessed by deep, impact-induced fracturing) may have contributed to shaping Vesta’s surface.
Finally, the feasibility of conducting a similar opportunistic BSR experiment at other small-bodies and moons is explored for several active and planned missions. Targets smaller than ~100 km in diameter require an onboard ultra-stable oscillator to achieve sufficient frequency stability for accurate power spectral analysis and interpretation of opportunistic BSR data in terms of surface roughness.
Palmer, Elizabeth M., "Exploring Potential Water-Ice Occurrence on Asteroid 4 Vesta Using Orbital Radar Observations by the Dawn Mission" (2018). Dissertations. 3359.