Detection of abundant carbon monoxide in the brown dwarf Gliese 229B

We report the detection of carbon monoxide in the atmosphere of the brown dwarf Gliese 229B in spectra obtained at wavelengths near 4.7 mu m. The spectrum is consistent with mole fractions of CO of more than 50 parts per million (ppm), much higher than expected for gas in thermochemical equilibrium at the temperature of the atmosphere we observe. As in Jupiter, mixing in the upper atmosphere of Gl 229B may prevent equilibrium between CO and CH4 at low temperatures. The average flux from 4.55 to 5.10 mu m in Gl 229B is measured to be 3.6 +/- 0.2 mJy, lower than previous broadband measurements.