  We provide estimates of atmospheric pressure and surface composition on
short-period rocky exoplanets with dayside magma pools and silicate vapor
atmospheres. Atmospheric pressure tends toward vapor-pressure equilibrium with
surface magma, and magma-surface composition is set by the competing effects of
fractional vaporization and surface-interior exchange. We use basic models to
show how surface-interior exchange is controlled by the planet's temperature,
mass, and initial composition. We assume that mantle rock undergoes bulk
melting to form the magma pool, and that winds flow radially away from the
substellar point. With these assumptions, we find that: (1) atmosphere-interior
exchange is fast when the planet's bulk-silicate FeO concentration is low, and
slow when FeO concentration is high; (2) magma pools are compositionally
well-mixed for substellar temperatures $\lesssim$ 2400 K, but compositionally
variegated and rapidly variable for substellar temperatures $\gtrsim$ 2400 K;
(3) currents within the magma pool tend to cool the top of the solid mantle
("tectonic refrigeration"); (4) contrary to earlier work, many magma planets
have time-variable surface compositions.
