Pluto, Near and Far: PEPSSI Measurements of Energetic Particles During the New Horizons Flyby and Investigating a Pluto Torus of Circumsolar Neutral Gas
Abstract
The energetic particle environment at Pluto has been unknown, and little modeled, until this year’s historic encounter by the New Horizon (NH) spacecraft on 14 July 2015. The first energetic particle observations, made with the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument, were downlinked in August 2015. There are variations in the intensities of suprathermal (~3-30 keV/nucleon) ions that are associated with a combination of the position of the spacecraft relative to Pluto, the look direction of PEPSSI, and (potentially) temporal evolution in the system. We present the results of the near encounter with Pluto, to as close as ~11.6 Rp (1 Rp = 1187 km), which, early analysis shows, include large intensity variations associated with Pluto. We also present the concept of a neutral gas torus surrounding the Sun, aligned with Pluto’s orbit, and place observational constraints on it based primarily on comparison of NH measurements with a 3-D Monte Carlo model adapted from analogous satellite tori surrounding Saturn and Jupiter. Such a torus, or perhaps partial torus, could result from neutral N2 escaping from Pluto’s exosphere. Unlike other more massive planets, gaseous neutrals escape Pluto readily via Jeans escape (i.e., owing to the high thermal speed relative to the escape velocity). These neutrals are not directly observable by NH but, once ionized to N2+ or N+ via photolysis or charge exchange, are picked up by the solar wind, ultimately reaching ~50 keV or more, making these pickup ions detectable by PEPSSI. This work was supported by NASA's New Horizons project.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #47
- Pub Date:
- November 2015
- Bibcode:
- 2015DPS....4721026H