 uncertainty 
in the outflow velocity is $\sim$25\%. 
Another source of error is the mismatch between our fluorescence model 
and the observed CO band intensities. 
For example, the model predicts that the (2,0) band is slightly brighter than the (1,0) band 
whereas the observations indicate the opposite.
However, this discrepancy can be attributed to a detector artifact, which
has $\sim$3$\times$ larger dark current than the surrounding region, near
the location of the (1,0) band. 
There are other discrepancies, of varying magnitudes (Figure~1, Table~2), between the measured and 
predicted CO band intensities. 
Taking all of these issues into account, we conclude that the true value of \ensuremath{\mathrm{Q_{\rm CO}}}\ could be up to 50\% 
different from our adopted value.

The relative abundance of CO in 103P is the ratio \ensuremath{\mathrm{Q_{\rm CO}}}/Q\ensuremath{_\mathrm{H_{\rm 2}O}}, 
where Q\ensuremath{_\mathrm{H_{\rm 2}O}}\ is the water production rate. 
Although Q\ensuremath{_\mathrm{H_{\rm 2}O}}\ was not measured during our \textit{HST\ } observations, 
an excellent record was obtained during ground-based infrared (IR) observations 
on November~4 \citep{Dello:2011}. 
Q\ensuremath{_\mathrm{H_{\rm 2}O}}\ \mbox{(molecules s$^{-1}$)} increased by a 
factor of $1.6$ from the first IR observation 
\mbox{($8.84 \pm 1.09 \times 10^{27}$,} 
\mbox{10:49--11:30 UT)} to the last one 
\mbox{($1.38 \pm 0.16 \times 10^{28}$,} \mbox{15:20--15:54 UT),} 
but Q\ensuremath{_\mathrm{H_{\rm 2}O}}\  appeared to be leveling off during the final 2.5~hr of the IR observations 
\mbox{($1.40 \pm 0.17 \times 10^{28}$,} \mbox{14:50--15:17 UT).} 
Our first CO observation was 5.5~hr before the first IR observation, 
and our second CO observation was 4.0~hr after the last IR observation. 
By comparing when the \textit{HST\ } and IR measurements were made relative to 
the periodic changes in the comet's overall level of activity (Figure~3), 
we find that \textit{HST\ } visit~6 was near the minimum activity level, and 
visit~9 was near the average activity level. 
The IR measurements were made during the rising portion of the activity phase curve, 
starting near the average level and ending at approximately the maximum. 
Assuming a peak-to-valley activity ratio of 2 for \ensuremath{\mathrm{H_{\rm 2}O}}\ \citep{Ahearn:2011}, 
we therefore conclude that Q\ensuremath{_\mathrm{H_{\rm 2}O}}\ was \mbox{$7.0 \times 10^{27}$} 
during visit~6 and \mbox{$1.0 \times 10^{28}$} 
during visit~9, yielding CO abundances of $\sim$0.31\% and $\sim$0.29\%, respectively. 
Given the large uncertainties discussed above, we estimate that the CO abundance 
was 0.15--0.45\%.

The CO abundance in 103P is apparently the lowest ever measured in a comet. 
Comets display a huge variation in the CO abundance, 
ranging from $\sim$0.4\% to $\sim$30\%  \citep{Bockelee:cometsII}, 
but the reason why is unknown. 
Although the most CO--rich comets are long-period Oort cloud objects, 
some long period comets have very low CO abundances 
($\sim$0.4\% for C/2001 WM1 [LINEAR]).
There is not much information available on the CO abundance in Jupiter family 
comets (JFCs), the dynamical class to which 103P belongs, owing to their low activity levels.  
Infrared CO emission was detected from JFC 21P/Giacobini-Zinner in October 1999, 
implying a CO abundance as high as $\sim$15\% \citep{Mumma:2000gz}. 
But IR CO emission was not detected from 21P several weeks later, when the comet was brighter, 
giving a 5$\sigma$ CO abundance upper limit of $\sim$3\% \citep{Weaver:1999gz}. 
IR observations of JFC 73P/Schwassmann-Wachmann~3-C gave a CO abundance of 
\mbox{$0.50 \pm 0.13$\%} \citep{Disanti:2007sw3}, 
comparable to the highest values allowed for 103P. 
A CO abundance of $\sim$4--6\% was measured from JFC 9P/Tempel, 
both before and after the outburst associated with the 
\emph{Deep Impact} event \citep{Mumma:2005t1,Feldman:2006t1}. 
A radio detection of CO emission from JFC 17P/Holmes yielded a 
CO abundance of $\sim$2--4\% \citep{Biver:2008holmes}, 
but the observations were made shortly after the spectacular outburst of the 
nucleus in October 2008 and at a relatively large heliocentric distance, 
both of which are rather unusual circumstances. 
Similarly, it is difficult to assess the compositional implications of the detection of 
radio CO emission from 29P/Schwassmann-Wachmann~1 \citep{Crovisier:2009jfc}, 
whose orbit never brings it within 5.7~AU of the Sun.

We assume here that all the observed CO is coming directly from the nucleus. 
In some comets, a significant fraction of the CO observed in the coma is
from ``extended sources'' (\textit{e.g.,\ } \ensuremath{\mathrm{H_{2}CO}}, \ensuremath{\mathrm{CO_{\rm 2}}}; 
\mbox{cf.,\  \citealt{Bockelee:cometsII}).} 
However, our high spatial resolution \textit{HST\ } observations 
strongly favor the ``native'' source sublimating directly from the nucleus. 
We calculate that the photodissociation of \ensuremath{\mathrm{CO_{\rm 2}}}\ and 
\ensuremath{\mathrm{H_{2}CO}}\ would contribute $<$10\% of the observed CO, 
assuming their relative abundances are 20\% and 1\%, respectively.
 
Similarly, the \ensuremath{\mathrm{H_{\rm 2}O}}\ production rates quoted here assume all the \ensuremath{\mathrm{H_{\rm 2}O}}\ is coming from 
sublimation at the nucleus. 
The results from the \emph{EPOXI}\  mission suggest that a substantial fraction of the \ensuremath{\mathrm{H_{\rm 2}O}}\ in
 the coma is produced by the sublimation of icy grains ejected in \ensuremath{\mathrm{CO_{\rm 2}}}\ jets \citep{Ahearn:2011}. 
 Ground-based IR observations show that the \ensuremath{\mathrm{H_{\rm 2}O}}\ spatial profile is more extended than 
 expected for a parent species \citep{Dello:2011}, suggesting at least one-third of the 
 observed \ensuremath{\mathrm{H_{\rm 2}O}}\ could be coming from an extended source (Dello Russo, priv.\ comm.). 
 If the icy grains presumed to be the source of the extended \ensuremath{\mathrm{H_{\rm 2}O}}\ emission have a different 
 CO/\ensuremath{\mathrm{H_{\rm 2}O}}\ ratio than is present in the bulk nucleus, the CO/\ensuremath{\mathrm{H_{\rm 2}O}}\ abundance reported 
 here might not reflect the true composition of the nucleus.

\section{IMPLICATIONS}

CO is predicted to be the dominant C-bearing gas in the outer parts of the solar nebula 
where comets formed \citep[cf.,][]{Fegley:1989}. 
However, CO is extremely volatile with a sublimation temperature of $\sim$20~K under 
solar nebula conditions \citep{Yamamoto:1983}. 
Thus, the CO content of ices incorporated into comets may have been very sensitive 
to the local conditions where the nucleus formed, or where the coldest part of the nucleus 
formed assuming there was significant radial mixing of material within the nebula, 
as suggested by the results from the \emph{Stardust}\  mission \citep{Brownlee:2006}. 
The capture of CO might have been enhanced by incorporation into a water ice clathrate 
\citep{Lunine:1989,Marboeuf:2010} or by trapping in amorphous water ice \citep{Barnun:2003}. 
But even if they were formed with significant amounts of CO, JFCs can subsequently 
lose