  Pluto's icy surface has changed colour and its atmosphere has swelled since
its last closest approach to the Sun in 1989. The thin atmosphere is produced
by evaporating ices, and so can also change rapidly, and in particular carbon
monoxide should be present as an active thermostat. Here we report the
discovery of gaseous CO via the 1.3mm wavelength J=2-1 rotational transition,
and find that the line-centre signal is more than twice as bright as a
tentative result obtained by Bockelee-Morvan et al. in 2000. Greater
surface-ice evaporation over the last decade could explain this, or increased
pressure could have caused the atmosphere to expand. The gas must be cold, with
a narrow line-width consistent with temperatures around 50 K, as predicted for
the very high atmosphere, and the line brightness implies that CO molecules
extend up to approximately 3 Pluto radii above the surface. The upper
atmosphere must have changed markedly over only a decade since the prior
search, and more alterations could occur by the arrival of the New Horizons
mission in 2015.
