  Terrestrial exoplanets likely form initial atmospheres through outgassing
during and after accretion, although there is currently no first-principles
understanding of how to connect a planet's bulk composition to its early
atmospheric properties. Important insights into this connection can be gained
by assaying meteorites, representative samples of planetary building blocks. We
perform laboratory outgassing experiments that use a mass spectrometer to
measure the abundances of volatiles released when meteorite samples are heated
to 1200 $^{\circ}$C. We find that outgassing from three carbonaceous chondrite
samples consistently produce H$_2$O-rich (averaged ~66 %) atmospheres but with
significant amounts of CO (~18 %) and CO$_2$ (~15 %) as well as smaller
quantities of H$_2$ and H$_2$S (up to 1 %). These results provide experimental
constraints on the initial chemical composition in theoretical models of
terrestrial planet atmospheres, supplying abundances for principal gas species
as a function of temperature.
