  We show that the Deep Underground Neutrino Experiment (DUNE), with
significant but feasible new efforts, has the potential to deliver
world-leading results in solar neutrinos. With a 100 kton-year exposure, DUNE
could detect $\gtrsim 10^5$ signal events above 5 MeV electron energy. Separate
precision measurements of neutrino-mixing parameters and the $^8$B flux could
be made using two detection channels ($\nu_e + \, ^{40}$Ar and
$\nu_{e,\mu,\tau} + e^-$) and the day-night effect ($> 10 \sigma$). New
particle physics may be revealed through the comparison of solar neutrinos
(with matter effects) and reactor neutrinos (without), which is discrepant by
$\sim 2 \sigma$ (and could become $5.6 \sigma$). New astrophysics may be
revealed through the most precise measurement of the $^8$B flux (to 2.5\%) and
the first detection of the {\it hep} flux (to 11\%). {\it DUNE is required:} No
other experiment, even proposed, has been shown capable of fully realizing
these discovery opportunities.
