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We report the detection of several emission bands in the CO Fourth PositiveGroup from comet 103P/Hartley during ultraviolet spectroscopic observationsfrom the Hubble Space Telescope (HST) on 2010 November 4 near the time ofclosest approach by NASA's EPOXI spacecraft. The derived CO/H2O ratio is0.15-0.45%, which places 103P among the most CO-depleted comets. Apparentlythis highly volatile species, whose abundance varies by a factor of ~50 amongthe comets observed to date, does not play a major role in producing the strongand temporally variable activity in 103P/Hartley. The CO emissions varied by~30% between our two sets of observations, apparently in phase with thetemporal variability measured for several gases and dust by other observers.The low absolute abundance of CO in 103P suggests several possibilities: thenucleus formed in a region of the solar nebula that was depleted in CO or toowarm to retain much CO ice, repeated passages through the inner solar systemhave substantially depleted the comet's primordial CO reservoir, or any COstill in the nucleus is buried below the regions that contribute significantlyto the coma.

The Carbon Monoxide Abundance in Comet 103P/Hartley during the EPOXI Flyby