Distribution and evolution of CH4, N2, and CO ices on Pluto's surface:: 1995 to 1998

We present new near-infrared spectra of the planet Pluto obtained at Lowell Observatory on 83 nights during 1995-1998. The dense temporal sampling of our observations enables us to measure with unprecedented detail cyclical changes in the depths of methane, carbon monoxide, and nitrogen ice absorption bands, modulated by Pluto's diurnal rotation. We show that CO, N(2), and weak CH(4) absorption band depths exhibit rotational patterns very different from those of Pluto's visible lightcurve, unlike the strong CH(4) absorption bands which are closely correlated with the visible lightcurve. Our observations are used to constrain the longitudinal distributions of the three ice species on Pluto's surface. The data also reveal a subtle, longer term strengthening of Pluto's strong near-infrared CH(4) bands, which is used to constrain the latitudinal distribution of CH(4) ice. We simulate the observed diurnal and seasonal spectral and photometric behavior of Pluto by means of model distributions of three terrain types. We see no evidence for changes in the distributions of Pluto's surface ices during the 1995-1998 interval. (C) 2001 Academic Press.