Passive Subsurface Radio Probes of Airless Bodies using High-Energy Cosmic Ray Showers via the Askaryan Effect

The surface of airless planetary bodies, such as the lunar regolith, are continually bombarded by cosmic rays, from GeV (10^{9} eV) up to ZeV (10^{21} eV) energies. Due to the lack of atmosphere, these cosmic rays impact the surface unimpeded with their full energy and produce extensive secondary particle cascades within the subsurface, extending for tens of meters at the highest energies. These particle cascades produce strong, coherent, wide-bandwidth, linearly-polarized radio pulses as they develop below the surface; this emission process is known as the Askaryan effect. This cosmic-ray-induced radio emission can be observed directly by a radio receiver but the emission will also reflect off any subsurface dielectric discontinuities, including subsurface ice layers and regolith-bedrock interfaces, and can then propagate back up to the surface. These direct and reflected radio pulses can be observed both from the surface or from an orbiting spacecraft and an analysis of these radio pulses, including the spectrum, amplitude, and polarization, allows for reconstructing the presence and properties of these subsurface layers, similar to an active radar sounding design. Laboratory and Earth-based measurements, in conjunction with detailed full-wave finite-difference time-domain (FDTD) electromagnetic simulations performed in this work, confirm that such pulses can be observed by an orbiting spacecraft using existing radio receiver technology. The advantage of this technique over orbital active radar sounders is that the cosmic ray source acts as an impulsive antenna embedded directly in the planetary subsurface, potentially very close to the targets of interest, avoiding both the decoherence and surface losses of traditional active radar sounding performed from orbit. This technique is used by the proposed Cosmic Ray Lunar Sounder (CoRaLS), a new orbital mission concept, that will have the ability to detect and characterize subsurface ice deposits in lunar permanently shadowed regions, but this passive cosmic-ray-induced radio sounding technique can also be used to perform passive subsurface radio probes on other airless planetary bodies.