Modeling Radar Albedos of Laboratory‐Characterized Particles: Application to the Lunar Surface

• Published in: Journal of geophysical research. Planets 2019-11, Vol.124 (11), p.3025-3040
• Main Authors: Virkki, Anne K., Bhiravarasu, Sriram S.
• Format: Article
• Language: English
• Subjects: Albedo; Asteroids; Bulk density; Comets; Computer simulation; Crater evolution; Ice; Ice particles; Lunar craters; Lunar regolith; Lunar spacecraft; Lunar surface; Mathematical models; Mini‐RF; Moon; Numerical simulations; Optical observations; Particle size distribution; Radar; Radar astronomy; Radar data; Radar echoes; Radar observation; Radar scattering; Radar signatures; Radar wavelengths; Reconnaissance; Reflectance; Regolith; Rock properties; Solar system; Terrestrial planets; Water ice; Wavelengths
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2019JE006006

We analyze lunar images obtained by the Mini Radio Frequency instrument onboard the Lunar Reconnaissance Orbiter to interpret the anomalous radar scattering observed in some lunar craters. We compare the radar signatures of the crater floors to those of the regions external to the crater rim and show that there is a systematic difference between the radar albedo trends of the two regions. Using numerical simulations to compute radar scattering properties of rock and ice particles, we demonstrate that the difference is caused partly by the near‐surface bulk volume density or the underlying fine‐grained regolith and partly by the shape and size distributions of centimeter‐to‐decimeter‐scale rubble. We show that while the size distribution of wavelength‐scale particles plays the major role in the observed radar albedos, if the size distribution is fixed, the particle shapes can play a greater role than the composition; therefore, the icy material is indistinguishable from silicate‐rich material unless the abundance of ice in the particles plays a major role in the shape of the particles. Plain Language Summary We analyze images of lunar craters obtained by the Mini Radio Frequency instrument onboard the Lunar Reconnaissance Orbiter to understand why some lunar craters appear more reflective when observed using centimeter‐scale radar wavelengths compared to optical observations. It has been suggested that in some cases, the apparent reflectivity is caused by water ice in permanently shadowed craters. In addition to reflectivity, the radar echo polarization properties for the crater regions are remarkably different from the crater‐free “background” regolith. In order to fully interpret the received radar echoes, we study the role of different particle shapes, sizes, and materials using computer simulations. Our results suggest that younger craters are more reflective than older craters, that radar signatures between inner and outer regions of craters are different, and that ice is indistinguishable from the silicate‐rich lunar regolith in the current radar data. These results improve our current understanding of radar scattering from the lunar surface and are also relevant to radar observations of other solar system bodies such as asteroids, comets, and rocky planets. Key Points A systematic difference in the radar signatures of internal and external regions of lunar craters is shown The role of size, shape, and composition of wavelength‐scale particles on the