Escape of Pluto's Atmosphere: In Situ Measurements from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on New Horizons and Remote Observations from the Chandra X-ray observatory

The escape rate of Pluto's atmosphere is of significant scientific interest. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) is a compact, energy by time-of-flight (TOF) instrument developed to help address this science goal. Pluto is known to have an atmosphere, and pre-encounter models have postulated a majority N₂ composition with free escape of up to ~10²⁸ molecules/sec. The expected major ionization product near Pluto is singly ionized N₂ molecules with pickup energies sufficient to be measured with PEPSSI. In the process of measuring the local energetic particle environment, such measurements can also provide constraints on the local density of Pluto's extended atmosphere, which, along with plasma measurements from the Solar Wind Around Pluto (SWAP) instrument, also on New Horizons, could allow the inference of the strengh and extent of mass-loading of the solar wind due to Pluto's atmosphere. Pluto's neutral atmosphere also provides a source population for charge exchange of highly ionized, minor ions in the solar wind, such as O, C, and N. This process allows these ions to capture one electron and be left in an excited state. That state, in turn decays with the emission of a low-energy (100 eV to 1 keV) X-ray. Observations of such solar wind charge exchange (SWCX) X-rays have been made in the past of the Earth's geocorona and Mars's extended atmosphere. The award of almost 40 hours of Director's Discretionary Time (DDT) for observing Pluto with the Chandra X-ray observatory near the period of closest approach of New Horizons to Pluto potentially enabled a remote determination of Pluto's global outgassing rate using the local solar wind flux as measured by the SWAP instrument. Preliminary anaysis of data returned from these observations reveal a definite interaction of Pluto with the solar wind, but at a lower strength than had been predicted. This work was supported by NASA's New Horizons project.