  Interpretations of the very rich second solar spectrum of the MgH molecule
face serious problems owing to the complete lack of any information about rates
of collisions between the MgH and hydrogen atoms. This work seeks to begin the
process of filling this lacuna by providing, for the first time, quantum
excitation, depolarization, and polarization transfer collisional rates of the
MgH ground state $X^2\Sigma$. To achieve the goals of this work, potential
energy surfaces are calculated and then are included in the Schr\"odinger
equation to obtain the probabilities of collisions and, thus, all collisional
rates. Our rates are obtained for temperatures ranging from $T \!\!=$2000 K to
$T \!\!=$15,000 K. Sophisticated genetic programming methods are adopted in
order to fit all depolarization rates with useful analytical functions of two
variables: the total molecular angular momentum and temperatures. We study the
solar implications of our results, and we find that the $X^2\Sigma$ state of
MgH is partially depolarized by isotropic collisions with neutral hydrogen in
its ground state $^2S$. Our findings show the limits of applicability of the
widely used approximation in which the lower-level polarization is neglected.
