  Absorption lines from water, methane and carbon monoxide are detected in the
atmosphere of exoplanet HR8799b. A medium-resolution spectrum presented here
shows well-resolved and easily identified spectral features from all three
molecules across the K band. The majority of the lines are produced by CO and
H2O, but several lines clearly belong to CH4. Comparisons between these data
and atmosphere models covering a range of temperatures and gravities yield log
mole fractions of H2O between -3.09 and -3.91, CO between -3.30 and -3.72 and
CH4 between -5.06 and -5.85. More precise mole fractions are obtained for each
temperature and gravity studied. A reanalysis of H-band data, previously
obtained at similar spectral resolution, results in a nearly identical water
abundance as determined from the K-band spectrum. The methane abundance is
shown to be sensitive to vertical mixing and indicates an eddy diffusion
coefficient in the range of 10^6 to 10^8 cm^2 s^-1, comparable to mixing in the
deep troposphere of Jupiter. The model comparisons also indicate a C/O between
~ 0.58 and 0.7, encompassing previous estimates for a second planet in the same
system, HR8799c. Super-stellar C/O could indicate planet formation by
core-accretion, however, the range of possible C/O for these planets (and the
star) is currently too large to comment strongly on planet formation. More
precise values of the bulk properties (e.g., effective temperature and surface
gravity) are needed for improved abundance estimates.
