  Potentially-habitable planets orbiting M-dwarfs are of intense
astrobiological interest because they are the only rocky worlds accessible to
biosignature search over the next 10+ years due to a confluence of
observational effects. Simultaneously, recent experimental and theoretical work
suggests that UV light may have played a key role in the origin of life on
Earth, and especially the origin of RNA. Characterizing the UV environment on
M-dwarfs planets is important to understanding whether life as we know it could
emerge on such worlds. In this work, we couple radiative transfer models to
observed M-dwarf spectra to determine the UV environment on prebiotic
Earth-analog planets orbiting M-dwarfs. We calculate dose rates to quantify the
impact of different host stars on prebiotically-important photoprocesses. We
find that M-dwarf planets have access to 100-1000 times less bioactive UV
fluence than the young Earth. It is unclear whether UV-sensitive prebiotic
chemistry that may have been important to abiogenesis, such as the only known
prebiotically plausible pathways for pyrimidine ribonucleotide synthesis, could
function on M-dwarf planets. This uncertainty affects objects like the
recently-discovered habitable-zone planets orbiting Proxima Centauri,
TRAPPIST-1, and LHS 1140. Laboratory studies of the sensitivity of putative
prebiotic pathways to irradiation level are required to resolve this
uncertainty. If steady-state M-dwarf UV output is insufficient to power these
pathways, transient elevated UV irradiation due to flares may suffice;
laboratory studies can constrain this possibility as well.
