Observational Constraints on a Pluto Torus of Circumsolar Neutral Gas

Abstract:

We present the concept of a neutral gas torus surrounding the Sun, aligned with Pluto’s orbit, and place observational constraints based primarily on comparison of New Horizons (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, should result from neutral N₂ escaping from Pluto’s exosphere. Unlike other more massive planets closer to the Sun, neutrals escape Pluto readily owing, e.g., to the high thermal speed relative to the escape velocity. Importantly, escaped neutrals have a long lifetime due to the great distance from the Sun, ~100 years for photoionization of N₂ and ~180 years for photoionization of N, which results from disassociated N₂. Despite the lengthy 248-year orbit, these long e-folding lifetimes may allow an enhanced neutral population to form an extended gas cloud that modifies the N₂ spatial profile near Pluto. These neutrals are not directly observable by NH but once ionized N₂⁺ or N⁺ are picked up by the solar wind, reaching ~50 keV, making these pickup ions (PUIs) detectable by NH’s Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. PEPSSI observations analyzed to date may constrain the N₂ density; the remaining ~95% of the encounter data, scheduled for downlink in August along with similarly anticipated data from the Solar Wind Around Pluto (SWAP) experiment, should help determine the Pluto outgassing rates. Measurements from SWAP include the solar wind speed, a quantity that greatly enhances PUI studies by enabling us to directly account for the PUI distribution’s sensitive dependence on plasma speed. Note that anomalous cosmic ray Si observed at Voyager is overabundant by a factor of ~3000 relative to interstellar composition. This might be related to “outer source” PUIs, but the fact that N₂ and Si are indistinguishable in many instruments could mean that N₂ is actually driving this apparent Si discrepancy.