PRIDE - Passive Radio Ice Depth Experiment - An Instrument to Measure Outer Planet Lunar Ice Depths from Orbit using Neutrinos

We describe a concept for an instrument to measure the thickness of the ice shell on a planetary body such as Jupiter's moon Europa. Unlike a high powered and massive device such as an ice-penetrating radar, the described instrument is a passive receiver of a naturally occurring signal generated by interactions of deep penetrating cosmic ray neutrinos. We discuss the basic concept and consider the instrument design requirements from the perspective of a NASA Outer Planet Orbiter Mission. We show results of simulations, compare signal-to-noise estimates, and examine possible components and configurations for the antenna, receiver, and electronics. We note some options that can be used to reduce mass and power. Finally, we present a list of issues that would need further study to produce a more concrete design. In the world of astrophysics, difficult problems can occasionally benefit from the use of results derived from seemingly unrelated areas. In the case at hand we explore how results from the world of high energy cosmic rays could potentially help solve a difficult measurement problem in planetary geology. Europa, one of the Galilean moons of Jupiter, is believed to be covered with an ice shell of unknown thickness, likely ranging from a few kilometers to tens of kilometers. Indirect measurements imply that under the ice is an ocean, which is warmed by tidal and volcanic heating, and is thought to be one of the best locations for life to have formed in the solar system outside of Earth. It is therefore of high scientific priority to gain a better understanding of the geology and structure of Europa by measuring the ice shell thickness. The question is then: "How can we best probe ice that is tens of km thick given the stringent mass and power requirements of a Europan explorer satellite?" The work described here was performed to determine whether the preceding measurement question could be answered with a reasonable instrument built to use the Extreme High Energy (EHE) cosmic ray neutrino signal to extract the ice depth on a planetary-sized body. All aspects of the instrument design are covered - the expected signal, the detector configuration, the sampling electronics, etc. Our expectation was that we would encounter a "show-stopper" that would make this instrument untenable, but to our surprise we did not find any obvious major shortcomings. We present here the overall concept and suggest ways PRIDE (Passive Radio [frequency] Ice Depth Experiment) could be realized. We begin with an examination of the expected neutrino signal, then look at antenna/detector characteristics, move on to detector configuration, and end with a discussion of the signal sampling electronics. Lastly, we present conclusions and identify issues for further study.