  Among the hot Jupiters that transit their parent stars known to date, the two
best candidates to be observed with transmission spectroscopy in the
mid-infrared (MIR) are HD189733b and HD209458b, due to their combined
characteristics of planetary density, orbital parameters and parent star
distance and brightness. Here we simulate transmission spectra of these two
planets during their primary eclipse in the MIR, and we present sensitivity
studies of the spectra to the changes of atmospheric thermal properties,
molecular abundances and C/O ratios. Our model predicts that the dominant
species absorbing in the MIR on hot Jupiters are water vapor and carbon
monoxide, and their relative abundances are determined by the C/O ratio. Since
the temperature profile plays a secondary role in the transmission spectra of
hot Jupiters compared to molecular abundances, future primary eclipse
observations in the MIR of those objects might give an insight on EGP
atmospheric chemistry. We find here that the absorption features caused by
water vapor and carbon monoxide in a cloud-free atmosphere, are deep enough to
be observable by the present and future generation of space-based
observatories, such as Spitzer Space Telescope and James Webb Space Telescope.
We discuss our results in light of the capabilities of these telescopes.
