SM42B-03
Escape of Pluto's Atmosphere: In Situ Measurements from New Horizons and Remote Observations from Chandra
Thursday, 17 December 2015: 10:50
2009 (Moscone West)
Ralph L McNutt Jr1, Matthew E Hill2, Peter Kollmann3, Stamatios M Krimigis3, Lawrence E Brown4, Martha Bolz Kusterer3, Carey Michael Lisse3, Donald G Mitchell5, Jon Duane Vandegriff3, David J McComas6, Fran Bagenal7, Heather Alison Elliott6, Kimberly Ennico Smith8, Mihaly Horanyi9, Catherine Olkin10, Marcus Ryan Piquette7, Alan Stern11, Darrell F Strobel12, Jamey Szalay7, Philip W Valek13, Harold A Weaver Jr3, Scott Weidner6, Leslie Ann Young10, Eric Zirnstein6 and Scott J. Wolk14, (1)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (2)Johns Hopkins University, Laurel, MD, United States, (3)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (4)Applied Physics Laboratory Johns Hopkins, SES, Laurel, MD, United States, (5)JHU/APL, Laurel, MD, United States, (6)Southwest Research Institute, San Antonio, TX, United States, (7)University of Colorado at Boulder, Boulder, CO, United States, (8)NASA Ames Research Center, MS 245-3, Moffett Field, CA, United States, (9)University of Colorado at Boulder, Physics, Boulder, CO, United States, (10)Southwest Research Institute Boulder, Boulder, CO, United States, (11)Southwest Research Institute, Boulder, CO, United States, (12)Johns Hopkins Univ, Baltimore, MD, United States, (13)Southwest Research Inst, San Antonio, TX, United States, (14)Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States
Abstract:
The escape rate of Pluto's atmosphere is of significant scientific interest. It is a Group 1 science goal of the New Horizons mission. In addition, a Group 3 science goal of the mission has been to characterize the energetic particle environment of the Pluto system. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) is a compact, energy by time-of-flight (TOF) instrument developed to address both of these science goals. Pluto is known to have an atmosphere, and current models postulate a majority N2 composition with free escape of up to ~1028 molecules/sec. This is very similar to the physical situation of a variety comets observed in the inner heliosphere. However, the gravitational field of Pluto exerts a significant effect on the escaping neutrals, unlike at a comet. The ionization of neutrals emitted from comets results in heavy ions, which are accelerated by the convective solar-wind electric field. 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 will 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 should 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, which can be detected at Earth. Such observations have been made of comets since the X-ray emission discovery in 1996 and used to infer cometary outgassing rates. Similar observatins have been made 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 coinciding with the period of closest approach of New Horizons potentially enables a remote determination of Pluto's global outgassing rate using a known local solar wind flux as measured by SWAP.
