  Ultra-high energy cosmic rays (UHECRs) are the most energetic particles
observed and serve as a probe of the extreme universe. A key question to
understanding the violent processes responsible for their acceleration is
identifying which classes of astrophysical objects (active galactic nuclei or
starburst galaxies, for example) correlate to their arrival directions. While
source clustering is limited by deflections in the Galactic magnetic field, at
the highest energies the scattering angles are sufficiently low to retain
correlation with source catalogues. While there have been several studies
attempting to identify source catalogue correlations with data from the Pierre
Auger Observatory and the Telescope Array, the significance above an isotropic
background has not yet reached the threshold for discovery. It has been known
for several decades that a full-sky UHECR observatory would provide a
substantial increase in sensitivity to the anisotropic component of UHECRs.
There have been several concepts developed in that time targeting the
identification of UHECR sources such as OWL, JEM-EUSO, and POEMMA, using
fluorescence detection in the Earth's atmosphere from orbit. In this white
paper, we present a concept called the Zettavolt Askaryan Polarimeter (ZAP),
designed to identify the source of UHECRs using radio detection of the Askaryan
radio emissions produced by UHECRs interacting in the Moon's regolith from low
lunar orbit.

High Energy Astrophysical Phenomena