  Small, close-in exoplanets are divided into two sub-populations: super-Earths
and sub-Neptunes. Most super-Earths are thought to have lost their primordially
accreted hydrogen-dominated atmospheres via thermally driven winds. We consider
the global chemical equilibrium of super-Earths and the lasting impacts of
their fleeting hydrogen atmospheres. We find that hydrogen is efficiently
sequestered into the interior, oxidising iron and endogenously producing
$\sim0.5-1.0\%$ water by mass. As the atmospheres of super-Earths are
continuously sculpted by mass loss and chemical equilibration, they remain
hydrogen-dominated by mole (number) fraction but become steam-dominated by
mass, which may be observable with JWST for planets transitioning across the
radius valley. One of the main effects of efficient sequestration of hydrogen
into the interior is to produce an under-dense bulk interior compared to that
of Earth. We predict bulk densities of super-Earths to be $\sim 5.0 \text{ g
cm}^{-3}$ for a $1M_\oplus$ planet, which is consistent with high-precision
mass measurements and also population-level inference analyses from atmospheric
escape models.
