  We report the detection of several emission bands in the CO Fourth Positive
Group from comet 103P/Hartley during ultraviolet spectroscopic observations
from the Hubble Space Telescope (HST) on 2010 November 4 near the time of
closest approach by NASA's EPOXI spacecraft. The derived CO/H2O ratio is
0.15-0.45%, which places 103P among the most CO-depleted comets. Apparently
this highly volatile species, whose abundance varies by a factor of ~50 among
the comets observed to date, does not play a major role in producing the strong
and temporally variable activity in 103P/Hartley. The CO emissions varied by
~30% between our two sets of observations, apparently in phase with the
temporal variability measured for several gases and dust by other observers.
The low absolute abundance of CO in 103P suggests several possibilities: the
nucleus formed in a region of the solar nebula that was depleted in CO or too
warm to retain much CO ice, repeated passages through the inner solar system
have substantially depleted the comet's primordial CO reservoir, or any CO
still in the nucleus is buried below the regions that contribute significantly
to the coma.
