Shallow Radar soundings of the Southern Highlands of Mars AGU Poster P13B-1280 1 1 2 3 4 5 5 1 6 Nathaniel E. Putzig , Roger J. Phillips , Jeffrey J. Plaut , Michael T. Mellon , James W. Head , Bruce A. Campbell , Lynn M. Carter , Anthony F. Egan , and Roberto Seu 2009 Dec 14 1 Southwest Research Institute, Boulder, CO, USA. 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 3 University of Colorado, Boulder, CO, USA. 4 Brown University, Providence, RI, USA. 5 Smithsonian Institution, Washington, DC, USA. 6 University of Rome “La Sapienza”, Rome, Italy. Contact: nathaniel @ putzig.com SYNOPSIS In the Southern Highlands between the edge of the South Polar Layered methods to suppress or enhance artifacts such as side lobes and surface clutter that Deposits and ~50°S, observations by the Shallow Radar (SHARAD) sounder from the might otherwise be mistaken as shallow reflections. We then use seismic interpretation II. CONFIDENCE ZONES Delineation is based on character of surface and near-surface radar returns. III. DISTRIBUTION Compared to maps of elevation, ice, geology Mars Reconnaissance Orbiter show a consistently strong surface reflection, often software to map zones of confidence (Panels II and III) and extract power information 0°E A High-confidence zone Low-confidence zone No-confidence zone MOLA elevation G followed by returns of lower power (-16 dB) at a delay of ~0.3–0.5 µs. Returns of for comparison to forward-modeling results (Panel IV). Smith et al. (2001) similar relative power and delay time occur at the Phoenix landing site and across the INTERPRETATION As with the Northern Lowlands [2], the detections in the south lower-power Northern Lowlands [1]. In both cases, we interpret the returns as reflections from a variable-power correspond roughly to the region where ground ice is inferred based on neutron- surface returns subsurface interface at a depth of ~25–45 m that is likely to be the base of ground ice surface returns spectrometer data [3] (Panel III, Ground-ice map). While the northern detections are consistent, high-power throughout these regions. surface returns also correlated geographically to the Vastitas Borealis interior unit [4], those in the 1 OBSERVATIONS The southern near-surface detections are most continuous laterally south cross numerous geologic boundaries [5] (Panel III, Geologic map). Given their in flat-floored craters (e.g., Mitchel, Secchi, and Steno) and in other relatively smooth distribution and their relatively uniform delay times, plausible subsurface sources of the 2 topographic depressions such as Sisyphi Planum and portions of Aonia Terra (Panel I). detections include shallow ice emplaced by vapor diffusion in the current era [6] and an smooth areas have 20°S Hellas In adjacent rougher terrain, we see a consistently strong surface reflection with ice-rich mantle emplaced during recent obliquity excursions [7]. However, we cannot returns at ~0.5-µs 30° Basin occasional returns from about the same delay time as in the smooth areas (Panel II). rule out other possibilities. For example, Mitchel Crater and environs are within the delay from surface F 40° Closer to the equator, the surface power is progressively more variable and the Circum-Hellas Volcanic Province [8] and volcanic flows could form near-surface layers. 50° Solis B subsurface returns less frequent. A careful analysis is required, as these shallow Returns in Sisyphi and Aonia could be associated with the Dorsa Argentea Formation 60° 70° returns push the vertical resolution limits of the SHARAD instrument. that appears to be ice-rich to greater depths [9] and may be much older than ice Planum discontinuous returns no clear returns 80° emplaced by equilibrium vapor diffusion or recent obliquity excursions. 270° 90° METHODS To discount surface and ionospheric sources for the returns, we use at ~0.5-µs delay at ~0.5-µs delay Aonia surface-clutter simulations and examine data acquired over a broad range of solar ACKNOWLEDGMENTS We thank the SHARAD Operations Center, Jack Holt and the 1 µs 1 µs 1 µs Terra zenith angles (primarily nighttime data, SZA > 90°). We apply a range of processing UT Sim Team, SeisWare, the MRO Mission, NASA, and ASI for their generous support. 3 I. DETECTION CHARACTERISTICS The most unambiguous detections are found in smooth terrains. 50 km 50 km 50 km 4 1 2

E 5 µs C

200 km

Region or crater SZA=125° with near-surface 676001 A returns identified 180° D

10 µs All regional radargrams Ground-ice table 0°E Mellon et al. (2004) A use the SI focused G Hellas Basin processing algorithm.

ʻblackbodyʼ color display

The University of Texas developed a radargram simulator (UT sim) using MOLA elevation (yellow line is nadir point) that represents energy from surface sources only. The Smithsonian Institution (SI), the Jet Propulsion Laboratory (JPL), and the SHARAD Operations Center (SHOC) developed focused-processing algorithms with different approaches to signal processing that may suppress or enhance side lobes and off-nadir surface scatter. SZA=130° 20°S 736501 B 30° 3 4 F 40° 50° B 5 µs 60° 70° 80° 270° 90°

769301 C E C 5 µs

180° D 819301 D Geologic units 0°E A SZA=125° SZA=134° SZA=130° Tanaka and Scott (1987) G 5 µs

IV. RADAR POWER AND POROSITY 757901 surface return We used SeisWare™ interpretation subsurface return software to delineate the surface and subsurface returns on the radargrams and extract the power for each horizon. E For the southern regions analyzed, we 308701 found a median power of -16 dB (see -34 -17 0 12 dB histogram at right) for the subsurface return power of subsurface return relative to that of surface Histogram of relative power for all F relative to the surface reflection. A similar 5 µs Solis Planum B regions analyzed ( in MOLA map). 60° analysis at the Phoenix site yielded a 70° median relative power of -11 dB. 80° Model 270° 90° To explore how power and delay time subsurface might be expected to vary for SHARAD 10 cm detections of ground ice, we used a dry layer numerical model to determine the signal 35 m 361301 F strength from the base of a zone containing ice-filled ice-filled pores. pores We ran cases varying the base porosity in 5 µs E all model layers (red) and in the ice layer C alone (blue; to simulate displacement of ice-free soil by ground ice). We also ran some layer D cases using a fixed porosity with a 30°S transitional boundary for the base of the ice 1398801 G 40°S (magenta), varying the vertical scale length Scott and Carr (1978) for ice saturation. Model results are 50°S consistent with a ground-ice interpretation 70 60 50 40 30 20 10°S 210° 240° 270° 300° 330° 0° 30° 60° 90° 120° 150°E for the SHARAD detections. REFERENCES [1] Putzig, N.E., et al., 2009, LPSC XL, Abstract 2477. [2] Putzig, N.E., et al., 2009, Geol. Soc. Amer., Abstract 20-13. [3] Mellon et al., 2004, Icarus 169, 324-340. [4] Tanaka et al., 2005, USGS Sci. Inv. Map 2888. [5] Tanaka and Scott, 1987, USGS Misc. Inv. Series Map I–1802–C. [6] Mellon, M.T., et al., 2008, J. Geophys. Res. 113, E00A25. [7] Head, J.W., et al., 2003, Nature 426, 797-802. [8] Williams, D.A., et al., 2009, Planet. Space Sci. 57, p. 895–916. [9] Plaut, J.J., et al., 2007, LPSC XXXVIIII, Abstract 2144.