Low-Frequency Electrical Properties
for Subsurface Exploration
of the Earth and Planets, 15-R9709
Principal Investigators
Robert
E. Grimm
David E. Stillman
Inclusive Dates: 05/01/07 – Current
Background - Geoelectrical methods have been used for nearly a century for minerals and groundwater exploration and subsurface geology characterization. They generally employ arrays of electrodes at the ground surface to couple currents into the earth and to measure the frequency-dependent voltage and phase response. Geoelectrical methods can be used to map near-surface ice as a resource on the Moon and Mars. Biogeophysics is a developing field focusing on geoelectrical assessment of microbially enhanced soil remediation and also could be employed to search for life on Mars. The objective of this proposal is to perform laboratory measurements and develop pilot criteria for geoelectrical characterization of subsurface ice on the Moon and Mars and subsurface microbes on Earth and Mars.
Approach - Ice has a diagnostic low-frequency electrical signature, but careful laboratory experimentation is necessary to separate this signal from interfacial polarizations generated between soil and ice and to assess the effects of impurities in the ice. Low-frequency electrical responses intrinsic to microbes are caused by charge buildup under stimulus at cell membranes. Again, however, the electrical interactions between the target (microbes) and host (soil) materials have not been well characterized to date. Experimental mixtures of soil/ice and soil/microbes over a range of environmental conditions will determine if diagnostic criteria for characterization of these systems can be extrapolated to subsurface exploration of the Earth and other planetary bodies. Measurements are being performed in SwRI’s Planetary Electrical Properties and Geochemistry laboratory.
Accomplishments - This project determined that the electrical conductivity of salt-doped ices — for compositions and environmental conditions not previously reported in the literature — is largely consistent with prior theory. However, this theory predicts that the electrical behavior of ice at low temperatures is not similar to that observed for terrestrial glaciers and polar regions that are often used as planetary analogs. No interfacial polarization effects were observed that would obscure the ice response. The lab setup for culturing and measuring microbes has been completed, but this part of the research will be carried out at a later time.
