Ground-Penetrating Radar Investigations of Terrestrial Analogs
to the Martian Crust for 2011 Mars Scout Mission, 20-9470
Principal Investigators
Cynthia L. Dinwiddie
Robert E. Grimm
Inclusive Dates: 04/01/04 – 04/01/05
Background - The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), currently in orbit around Mars, will acquire the first global data set of Martian subsurface radar properties, including, perhaps, inferred depth to water. The SHAllow RADar (SHARAD) instrument is currently en route to Mars. Because only limited ground-penetrating radar investigations of the Earth have been made within the low-frequency range of MARSIS and SHARAD, the accurate interpretation of resulting data is uncertain. Terrestrial geophysical analogs to Mars provide the opportunity to collect low-frequency radar data from well-known, well-characterized sites so that uncertainties may be minimized when interpreting Mars radar data.
Approach - Staff from SwRI's Geosciences and Engineering Division and Space Science and Engineering Division and the Lunar and Planetary Institute conducted low-frequency ground-penetrating radar investigations of terrestrial analogs to Mars to develop benchmarks against which we may interpret MARSIS and SHARAD data. We used commercial ground-penetrating radar systems operating at frequencies similar to SHARAD to sound geophysical environments with complexity analogous to that expected on Mars. We used other ground-penetrating radar antennas to extend the field-tested frequency range up to 100 MHz, thus essentially covering the low-frequency range for planetary radars that may be used during the next decade. Transient electromagnetic and vertical electrical soundings were performed to measure the electrical conductivity of the subsurface to estimate absorptive losses in the radar signal.
Accomplishments - We characterized several field sites, including the Volcanic Tableland, California, and Craters of the Moon National Monument, Idaho, using geophysical methods and obtained an improved understanding of factors controlling radar signal attenuation and scattering at low frequencies. Principal findings include the following: 1) The combination of ground-penetrating radar with transient electromagnetic soundings or vertical electrical soundings enabled assessment of the extent to which conductive losses contributed to the penetration depth achieved by radar. 2) The first few meters of the subsurface strongly influence the ultimate penetration depth achieved by radar. Results indicated that the presence of conductive material in the first few meters, which may be below the radar resolution, can dramatically decrease the dynamic range of the radar-backscattered echoes and, thus, significantly decrease the penetration depth. Such an effect could also influence radar sounding performance on Mars, where a thin layer of conductive material, such as hematite, maghemite, clay, or silt, may significantly attenuate the radar signal. 3) Finally, operating at lower frequencies does not necessarily yield better penetration in an electrically conductive subsurface. Project results led to five conference publications, the convening of a workshop and the planning of a conference session, one journal article accepted for peer review, two journal articles in review, and two successful NASA proposals.
