| `Oumuamua (I1 2017) was the first macroscopic (l~100 m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that 'Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of `Oumaumua's observed properties can be explained if it contained a significant fraction of molecular hydrogen (H_2) ice. H_2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining `Oumuamua's shape. Back-tracing `Oumuamua's trajectory through the Solar System permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H_2-rich bodies plausibly form in the coldest dense cores of Giant Molecular Clouds, where number densities are of order n~10^5, and temperatures approach the T=3 K background. Post-formation exposure to galactic cosmic rays implies a tau ~ 100 Myr age, explaining the kinematics of `Oumuamua's inbound trajectory. |