  N2 is abundant in Pluto's atmosphere and on its surface, but the supply is
depleted by prodigious atmospheric escape. We demonstrate that cometary impacts
could not have delivered enough N mass to resupply Pluto's N2 atmospheric
escape over time; thus Pluto's N2 is likely endogenous, and therefore was
either acquired early in its history or created by chemistry inside/on Pluto.
We find that cratering could excavate a considerable amount of N2 to resupply
the atmosphere against escape if the near-surface N2 reservoir is deep.
However, we find that this process likely falls short of that necessary to
resupply the atmosphere against escape by at least an order of magnitude. We
conclude that either the escape of N2 from Pluto's atmosphere was on average
much lower than the predictions for the current epoch, or that internal
activity could be necessary to bring N2 to the surface and resupply escape
losses. Observations made by the New Horizons spacecraft in mid-2015 will yield
further constraints on the provenance and evolution of Pluto's surface and
atmospheric N2, and could reveal evidence for past or present internal
activity.
