Diurnal and Seasonal Variations of Pluto’s surface composition through Spitzer Space telescope eyes

NASA’s New Horizons mission will encounter the Pluto system in July 2015. The payload of this mission has been designed to provide us with detailed knowledge on the physical and chemical characteristics of this icy world. From decades of ground-based visible and near-infrared (VNIR) observations we know that mixtures of CH₄, N₂ and CO, heterogeneously distributed, dominate Pluto’s surface. However, this composition is changing with a timescale of only months to years. Even though this mission will provide a very precise snapshot of the Pluto system, continuous monitoring before and after encounter are needed to gain a better understanding of the long term evolution of Pluto’s volatiles and atmosphere.Here we present broadband photometry of the Pluto-Charon system obtained in 2004 with IRAC/Spitzer, during the cryogenic phase of the mission. These data were acquired at eight equally spaced sub-observer longitudes. Two distinct spectral behaviors are readily apparent from the study of the albedos at 3.6, 4.5, 5.8 and 8.0 μm. Four longitudes show a lower albedo in channel-2 (4.5 μm) and a higher albedo in channel-1 (3.6 μm) suggesting a higher abundance of N₂ 4.29 μm) and/or CO 4.68 μm), and possibly CO₂ 4.27 μm) in these regions. This variability appears to be consistent with ground-based VNIR observations but the sparse 8-longitudes cadence precludes detailed comparison.In 2014 we conducted a second IRAC/Spitzer observational campaign to obtain data at 18 equally spaced longitudes in channel-1 and 2. These observations resulted in higher density light-curves. Along with spatial variability, the two datasets enable us to investigate temporal changes over a decade timescale.We show here a comparative study of the longitudinal variations of Pluto’s surface composition at two different dates separated by ten years. We investigate whether these variations indicate secular volatile transport or whether they are due to spatial variations of ices with latitude (sub-observer latitude changed from 33^o to 51^o from 2004 to 2014, both on the Northern hemisphere). Finally, we discuss what these variations tell us regarding the large picture of Pluto’s atmospheric and surface composition.