We have measured the spectrum of Than near 5 mu m and have found it to
be dominated by absorption from the carbon monoxide 1-0 vibration-rot
ation band. The position of the band edge allows us to constrain the a
bundance of CO in the atmosphere and/or the location of the reflecting
layer in the atmosphere. In the most likely case, 5 mu m radiation is
reflected from the surface and the mole fraction of CO in the atmosph
ere is qCO = 10(-5)(+10) ppm, significantly lower than previous estima
tes for tropospheric CO. The albedo of the reflecting layer is approxi
mately 0.07(-0.01)(+0.02) in the 5 mu m continuum outside the CO band,
The 5 mu m albedo is consistent with a surface of mixed ice and silic
ates similar to the icy Galilean satellites, Organic solids formed in
simulated Titan conditions can also produce Similar albedos at 5 mu m.
(C) 1996 Academic Press, Inc.