A Novel Approach to Lunar Subsurface Exploration: the Cosmic Ray Lunar Sounder (CoRaLs)

The idea that Earth’s Moon may host substantial reserves of water ice buried beneath layers of regolith in the polar permanently shadowed regions (PSR) has excited much interest over the past decade.  Here, we present numerical modeling in support of a new instrument concept for exploration of the Moon’s PSR: the Cosmic Ray Lunar Sounder, or CoRaLS, under development via a NASA DALI (Development and Advancement of Lunar Instrumentation) grant.  CoRaLS is a passive radio-frequency (RF) receiver, tuned to receive direct and reflected radio signals from the subsurface of the Moon.  These signals are created via the Askaryan effect, in which high-energy cosmic rays incident on the lunar regolith collide with atomic nuclei and initiate relativistic cascades of charged particles.  These particle showers in turn create coherent, linearly-polarized, wideband radio pulses that propagate along the shower path, faster than the phase velocity of light in the regolith, in a Cherenkov cone. These signals can reflect from subsurface interfaces, or scatter from buried objects, thus providing an opportunity to probe the subsurface from orbit.  Because this radiation originates from within the regolith, using a passive sensor to detect reflected signals mitigates the effect of strong spurious off-nadir reflections that plague traditional active-source RF systems.  We posit that if deposits of relatively pure water ice reside within the upper ~10 m of the lunar surface in PSR, a CoRaLS-style instrument would be uniquely equipped to find it.

Radio pulses generated by the Askaryan effect have been observed in Earth’s atmosphere, in terrestrial glaciers, and in salt deposits in the laboratory. They have also been predicted, and should be expected, to develop in the lunar regolith.  Our work to date represents the first systematic investigation of the use of this well-established phenomenon in exploration of the lunar regolith, and in particular, prospecting for deeply buried ice in the lunar polar regions.

In this presentation, we show the results of a series of 3D finite-difference time domain (FDTD) numerical simulations of the electric field generated by Askaryan-induced radiation from cosmic ray showers in the lunar regolith.  Our simulation volumes consist of layered regolith with an embedded pure ice layer at 6 m depth.  We vary the thickness of the ice layer and explore the strength and nature of the electric field from direct and reflected Askaryan pulses as observed at a range of positions both on the surface and at a range of angles and elevations above the surface.  We find that in these simulations, we can detect reflections from pure ice layers as thin as 10 cm.  For thicker layers, we observe distinct reflections, with opposite polarity, from the upper and lower surfaces of the ice layer, that are stronger than and distinguishable from the direct signals.  These simulations inform our ongoing work to calculate likely event detection rates for a variety of mission architecture scenarios, as we continue to develop the CoRaLS concept.