Using the Hottest Particles in the Universe to Probe Icy Solar System Worlds

We present results of our Phase 1 NIAC Study to determine the feasibility of developing a competitive, low cost, low power, low mass passive instrument to measure ice depth on outer planet ice moons, such as Europa, Ganymede, Callisto, and Enceladus. Indirect measurements indicate that liquid water oceans are likely present beneath the icy shells of such moons (see e.g., the JPL press release The Solar System and Beyond is Awash in Water), which has important astrobiological implications. Determining the thickness of these ice shells is challenging given spacecraft SWaP (Size, Weight and Power) resources. The current approach uses a suite of instruments, including a high power, massive ice penetrating radar. The instrument under study, called PRIDE (Passive Radio Ice Depth Experiment) exploits a remarkable confluence between methods from the high energy particle physics and the search for extraterrestrial life within the solar system. PRIDE is a passive receiver of a naturally occurring radio frequency (RF) signal generated by interactions of deep penetrating Extremely High Energy (> 10^18 eV) cosmic ray neutrinos. It could measure ice thickness directly, and at a significant savings to spacecraft resources. At RF frequencies the transparency of modeled Europan ice is up to many km, so an RF sensor in orbit can observe neutrino interactions to great depths, and thereby probe the thickness of the ice layer. Prior to our NIAC Phase 1 study, we analyzed the capability of PRIDE under the assumption of pure cold Europan ice, which we describe in [9]. Our goal in Phase 1 was to evaluate the promise of the concept in more realistic conditions. We made considerable progress in multiple areas: -We implemented several possible Europan ice models in detail, including impurity content and depth-dependent temperature and attenuation length. We identified a useful range of likely ice models for which event rates are in the hundreds or more per year and for which ice sheet depth measurement capabilities exist for thicknesses up to tens of km. -We adapted three existing, higher fidelity simulations used in the EHE astrophysics community by the ANITA, RICE, and LORD projects to the PRIDE application, and compared results to validate our expected event rates and other measurables. -We implemented models for several other ice moons: Ganymede, Callisto, and Enceladus, and implemented models for both Polar and Equatorial ice. We found that PRIDE is most promising for Europa and Enceladus, and that notably different depth measurement capabilities exist for polar vs equatorial ice due to colder polar temperatures. -We implemented a model of local water inclusions within ice sheets. We found that a variety of local water inclusions for both Enceladus and Europa should be detectable, but should not significantly compromise our ice depth measurements -We analyzed new observables to determine their ability to resolve ice depth independently of overall event rate, including event size, zenith angle to surface exit point and cascade location in the ice, and frequency content vs depth. We found promising results for some observables. -We performed an initial analysis of cosmic ray events, which are expected to be a major background for PRIDE. We found potential measurable quantities to distinguish cosmic ray events from neutrino events. -We leveraged existing efforts on developing low power digitizer technology to develop a chip applicable to PRIDE with 2 Gs/s rate, 1.3 GHz bandwidth, and expected favorable radiation hardness properties, which uses only 32 mW per channel, ideal for a deep space mission. -We established an MOU with Russian EHE neutrino researchers at the Lebedev Physical Institute. They are now part of the PRIDE collaboration via their own funding, and have contributed modifications to and analysis with the simulation they developed for the LORD EHE neutrino project.