Subarcsecond Scale Imaging of the Pluto-Charon System at 1.1 and 1.4 mm

Abstract

Determining the surface temperatures of Pluto and Charon places strong constraints on the types of ices stable on their surfaces. This in turn yields information on the composition of these bodies, and constrains models of solar system formation. Prior to 2005, no direct measurements of the separated thermal flux densities (and therefore surface temperatures) of Pluto and Charon had been obtained. Their close proximity (maximum separation under 0.9") make spatially resolving the pair at thermal wavelengths a challenge. However, models of composition and thermal properties for the surfaces of each depend on accurate thermal measurements and call for high resolution imaging, now achievable at millimeter/submillimeter wavelengths. To better constrain the temperatures of Pluto and Charon, we used the Submillimeter Array (SMA) to image the system at 1.4 mm (21 May 2005) and are currently obtaining data at 1.1 mm (late July 2010). With baselines at the SMA up to 509 m in length, these observations achieve 0.5" and 0.4" resolution, respectively, and thus allow us to separate the pair, and independently measure the thermal brightness of each object. The 2005 observations were the first observations ever that separated the pair at a true thermal wavelength, finding the brightness temperature of Pluto to be 39±4 K while Charon was 48±12 K. At the meeting will present an improved analysis of this data. We will also present new results from our current campaign at 1.1 mm; we currently have data from July 9 and July 19, 2010, with more data expected. While the data obtained to date have not been fully analyzed, a qualitative analysis from imaging the July 19 data set shows the intensity ratio of Pluto to Charon to be about 0.75; e.g. Pluto has a significantly lower brightness temperature than Charon, as we previously saw at 1.4 mm.