  We examine the spectrum of diffuse emission detected in the 17' by 17' field
around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits
He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with
originating in a two-temperature plasma, as well as a prominent low-ionization
Fe line. The cooler, kT=0.8 keV plasma differs in surface brightness across the
image by a factor of 9. This soft plasma is probably heated by supernovae. The
radiative cooling rate of the plasma within the inner 20 pc of the Galaxy could
be balanced by 1% of the kinetic energy of one supernova every 300,000 y. The
hotter, kT=8 keV component is more spatially uniform, ranging over a factor of
2 in surface brightness. The intensity of the hard plasma is correlated with
that of the soft, but they are probably only indirectly related, because
supernova remnants are not observed to produce thermal plasma hotter than kT=3
keV. Moreover, a kT=8 keV plasma is too hot to be bound to the Galactic center,
and therefore would form a slow wind or fountain of plasma. The energy required
to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy
is ~10^40 erg/s, which corresponds to the entire kinetic energy of one
supernova every 3000 y. This rate is unreasonably high. However, alternative
explanations for the kT=8 keV diffuse emission are equally unsatisfying. We are
left to conclude that either the diffuse emission is heated by an unanticipated
source of energy, or that a population of faint (< 10^31 erg/s), hard X-ray
sources that are a factor of 10 more numerous than CVs remains to be
discovered. (Abridged)
