Mini-RF Bistatic Observations of South Polar Craters on the Moon

Introduction: NASA’s Mini-RF instrument on the Lunar Reconnaissance Orbiter (LRO) is currently operating in concert with the Goldstone deep space communications complex 34 meter antenna DSS-13 to collect bistatic radar data of the Moon. These data provide a means to characterize the scattering properties of the upper meter of the lunar surface, as a function of bistatic angle, at X/C-band wavelengths (4.2 cm) and are being collected to address a variety of LRO science objectives. Here, we discuss efforts to address objectives related to the form and abundance of water ice and its vertical distribution.

Background: For each bistatic observation, the lunar surface is illuminated with a circularly polarized, chirped signal that tracks the Mini-RF antenna boresight intercept on the surface of the Moon. The receiver operates continuously and separately receives the horizontal and vertical polarization components of the signal backscattered from the lunar surface. The resolution of the data is ~100 m in range and ~2.5 m in azimuth but can vary from one observation to another, as a function of the viewing geometry. For analysis, the data are averaged in azimuth to provide a spatial resolution of 100 m. This yields an ~25-look average for each sampled location.

The data returned provide information on the structure (i.e., roughness) and dielectric properties of surface and buried materials within the penetration depth of the system (up to ~50 cm) [1-4]. The bistatic architecture allows examination of the scattering properties of a target surface for a variety of bistatic angles. Laboratory data and analog experiments, at optical wavelengths, have shown that the scattering properties of lunar materials can be sensitive to variations in bistatic angle [5-7].

Water ice can exhibit a strong response at radar wavelengths in the form of a Coherent Backscatter Opposition Effect (CBOE) and the circular polarization ratio (CPR) of the returned data can be a useful indicator of such a response—i.e., measured CPRs for icy materials typically exceed unity [8]. This effect has been observed in radar data for the floors of polar craters on Mercury [8,9]. However, ground-based radar observations of the lunar south polar region did not observe this effect [10]. This result was supported by later Mini-RF and Mini-SAR (Chandrayaan-1) monostatic observations of Cabeus (84.9°S, 35.5°W; 98 km dia.) [11] but was contradicted by Mini-RF bistatic observations that showed a clear opposition response, at S-band (l=12.6 cm) for Cabeus crater floor materials [12].

Observations/Results: Mini-RF S-band observations of the floor of Cabeus cover bistatic angles of 0.5° to 8.6° for incidence angles ranging from 82.4° to 86.6° (Figure 1). CPR measurements for the floor of the crater, as a function of bistatic angle,  show a clear opposition surge; something not observed for the floors of nearby, similar-sized craters that were sampled at S-band wavelengths (e.g., Casatus, Klaproth, Blancanus, and Newton A and G) [12]. The opposition peak of Cabeus floor materials has a width of ~2° and features a ~30% increase in CPR. The bistatic observations of the region surrounding Cabeus indicate that mean CPR values for the portion of its floor that was imaged by Mini-RF are less than that of the surrounding highlands for bistatic angles > ~1.8° but similar to that of nearby radar-facing slopes. Mean CPR values for the imaged floor of Cabeus are higher than that of surrounding highlands and nearby radar-facing slopes for bistatic angles of 0.5° to 1.8°. Mini-RF data for bistatic angles < 0.5° were not acquired during the bistatic campaign. However, Mini-RF monostatic data (i.e., bistatic angle of 0°) of the crater floor were acquired at an incidence angle of 48° [11] and ground-based CPR measurements at a bistatic angle of 0.37° and large (> 80°) incidence angles have been made [e.g., 14]. Elevated CPRs were not observed in either case.

Mini-RF X-band observations of the floor of Cabeus cover equivalent incidence angles but a smaller range of bistatic angles than have been sampled at S-band. Initial analysis of these data show no indication of a CBOE. Data for the floor of Amundsen crater (84.5°S, 82.8°E; 103 km dia.), however, do suggest an opposition response. Its character is distinctly different from the S-band response observed for Cabeus, though (Figure 2). This could indicate that, if water ice is present in Cabeus crater floor materials, it is buried beneath ~0.5 m of regolith that does not include radar-detectible deposits of water ice.

Figure 1. Plot of mean CPR versus bistatic angle for Cabeus, sampled in 5 bistatic S-band observations targeting Cabeus floor materials

Figure 2. Plot of mean CPR versus bistatic angle for Amundsen floor materials sampled in 3 bistatic X-band observations.

References: [1] Campbell et al. (2010), Icarus, 208, 565-573; [2] Raney et al. (2012), JGR, 117, E00H21; [3] Carter et al. (2012), JGR, 117, E00H09; [4] Campbell (2012), JGR, 117, E06008; [5] Hapke et al. (1998), Icarus, 133, 89-97; [6] Nelson et al. (2000), Icarus, 147, 545-558; [7] Piatek et al. (2004), Icarus, 171, 531-545; [8] Harmon J. K. et al., (1994) Nature, 369, 213–215. [9] Harmon J. K. and Slade M. A. (1992) Science, 258, 640–643. [10] Campbell D. B. et al. (2006) Nature, 443, 835–837. [11] Neish C. D. et al. (2011) JGR, 116, E01005. [12] Patterson G. W. et al. (2017) Icarus 283, 2-19; [13] Campbell et al., 2006, Nature 443, 835-837.