Once and future Pluto

1. Leslie Young Southwest Research Institute, Boulder CO Once and future Pluto

2. Small, cold, distant Pluto

3.

4.

5. Pluto and the Kuiper Belt The plot below shows the current locations and orbits of the Jovian planets (Jupiter through Neptune) and the current locations of various distant minor bodies. The orbits of the planets are shown in light blue and the current location of each object is marked by large dark-blue symbols. The current location of the minor bodies of the outer solar system are shown in different colors to denote different classes of object. Unusual high-e objects are shown as cyan triangles, Centaur objects as orange triangles, Plutinos (objects in 2:3 resonance with Neptune) as white circles (Pluto itself is the large white symbol), scattered-disk objects as magenta circles and "classical" or "main-belt" objects as red circles. Objects observed at only one opposition are denoted by open symbols, objects with multiple-opposition orbits are denoted by filled symbols. Numbered periodic comets are shown as filled light-blue squares. Other comets are shown as unfilled light-blue squares. Dual-status objects are shown as minor planets. The plot below shows the current locations and orbits of the Jovian planets (Jupiter through Neptune) and the current locations of various distant minor bodies. The orbits of the planets are shown in light blue and the current location of each object is marked by large dark-blue symbols. The current location of the minor bodies of the outer solar system are shown in different colors to denote different classes of object. Unusual high-e objects are shown as cyan triangles, Centaur objects as orange triangles, Plutinos (objects in 2:3 resonance with Neptune) as white circles (Pluto itself is the large white symbol), scattered-disk objects as magenta circles and "classical" or "main-belt" objects as red circles. Objects observed at only one opposition are denoted by open symbols, objects with multiple-opposition orbits are denoted by filled symbols. Numbered periodic comets are shown as filled light-blue squares. Other comets are shown as unfilled light-blue squares. Dual-status objects are shown as minor planets.

6. Heliocentric orbit of Pluto-Charon: the 3:2 resonance

8. It is thought that the early solar system contained many more objects in the 30-50 AU region than we see today. Objects in a protected resonance would be preserved, while others would have near encounters with Neptune and be ejected from the system. Were Pluto and the Plutinos formed with their current semi-major axis? This may imply a relatively homogenous population of Plutinos. On the other hand, Neptune (and its accompanying resonances) are thought to have migrated outward from its formation location. Were Pluto and the Plutinos formed at a range of closer distances and �swept up� in the expanding resonances? This may imply a diverse population of Plutinos.

9. 0 = sub-Charon point, Sub-Earth East longitude decreases with time.0 = sub-Charon point, Sub-Earth East longitude decreases with time.

10. Pluto rotates �sideways� (obliquity = 120�), and Charon orbits in Pluto�s equatorial plane. Pluto�s equinox (and the Pluto-Charon mutual event season) happens at perihelion. Pluto is tidally locked with Charon Charon is presumed to be tidally locked with Pluto Charon�s orbit has a small but surprising non-zero eccentricity

11. The currently most accepted model (mainly by process of elimination) for the formation of the Pluto-Charon binary is by giant impact, similar to the formation of the Earth-Moon system. However, detailed hydrocode models have trouble making a moon as massive as Charon. Are we missing something in moving the hydrocode to the outer solar system? Do we have Charon�s mass all wrong? Tidal forces should damp Charon�s eccentricity with a timescale of a few million years. Is the reported eccentricity correct? If so, how can we reconcile this with the observation that Pluto is tidally locked to Charon?

12. 9% have normalized reflectance < 0.2, 9% > 0.8 9% have normalized reflectance < 0.2, 9% > 0.8

13.

14. H2O, crystalline, at 60�20 K, seen at all longitudes (1.65 �m feature is diagnostic of crystalline vs. amorphous). Possible NH3 No N2, CO, or CH4 detected. Charon�s surface composition

15. The puzzle of the evolution of the surfaces of Pluto and Charon Old, irradiated frost deposits on Pluto should form large reddish hydrocarbons (tholins). Exposed N2-CH4-CO ice should preferentially lose N2, leaving a slag of CH4, CO, and possibly chemical products. Is the slag segregated vertically or horizontally? Amorphous ice rapidly crystallizes at temperatures above 120 K. Does the detection of crystalline ice on Charon mean it was recently much hotter than it is now?

16. Surface-atmosphere interaction

17. Pluto�s atmospheric structure Stellar occultations measure the effect of defocusing of an occulted star by Pluto�s atmosphere. The steep drops below half-light (in 1988 June 9 event) are due to haze or a thermal inversion. 2002 events show similar effects at ~0.2 light.

18. Pluto�s atmospheric composition

19. Pluto�s extended atmosphere

20. The puzzle of Pluto�s lower atmosphere An outstanding question for over a decade is the nature of the �kink� in the 1988 occultation lightcurve. Is this due to an absorbing haze or to colder atmospheric temperatures? The 2002 occultations also differ from isothermal lightcurves. Current radiative-equilibrium models cannot make thermal profiles that reproduce the observed occultation lightcurve. Is this related to other �energy crises� at ~10 �bar on the jovian planets (where CH4 is also the principle absorber and radiator)?

21. Seasonal change Surface temperatures react to changing heliocentric distance (30-49 AU) & subsolar latitude (�60�) Because the N2 is in vapor-pressure equilibrium, decreasing surface temperature lowers the surface pressure. Surface pressures for the coming decades are very model dependent (thermal inertia, albedo history of frost, emissivity). Occultations suggest pressures increase by factor of 3, 1988-2002. Surface temperature may stall 35 K (3.27 �bar), the N2 a/b phase transition.

22. The puzzle of Pluto�s seasonal changes Pluto�s south pole is just entering its arctic night after half a Pluto year of constant illumination. The expectation is that volatiles would flee the south pole for darker, colder areas. Why, then, is the south pole the brightest area on the mutual event maps? How does the surface pressure of Pluto now or in 2015 compare with the surface pressure in 1988? The answer depends on the thermal inertia of the surface and the distribution of frosts. If Pluto�s atmosphere is in hydrodynamic escape, models predict that ~1 km of the surface frost has been lost since Pluto was formed. The changing surface pressure affects the seasonally averaged loss rate.

23. Top 16 Observational Goals(in three groups) 1.1 Characterize geology and geomorphology 1.2 Surface composition mapping 1.3 Characterize the neutral atmosphere & its escape rate

24. Top 16 Observational Goals(in three groups) 1.1 Characterize geology and geomorphology 1.2 Surface composition mapping 1.3 Characterize the neutral atmosphere & its escape rate 2.1 Characterize variability 2.2 Stereo imaging. 2.3 Hi-res terminator maps 2.4 Selected hi-res composition maps 2.5 Ionosphere/solar wind 2.6 Search for H, H 2, nitriles, CxHy 2.7 Charon atmosphere search 2.8 Determine Bond albedos 2.9 Map temperatures

25. Top 16 Observational Goals(in three groups) 1.1 Characterize geology and geomorphology 1.2 Surface composition mapping 1.3 Characterize the neutral atmosphere & its escape rate 2.1 Characterize variability 2.2 Stereo imaging. 2.3 Hi-res terminator maps 2.4 Selected hi-res composition maps 2.5 Ionosphere/solar wind 2.6 Search for H, H 2, nitriles, CxHy 2.7 Charon atmosphere search 2.8 Determine Bond albedos 2.9 Map temperatures 3.1 Energetic particles 3.2 Refine bulk parameters 3.3 Search for magnetic field 3.4 Satellite & ring search

26. Examples of observatory-based observations of Pluto-Charon 1.2 Surface composition mapping High spectral resolution or wide wavelength range can be used to measure exotic spectral regions. 2.1 Characterize variability Continued observations of Pluto and Charon�s whole-disk spectra, colors, and albedos should be sensitive to changes in the surface from frost transport. Pluto�s cousin, Triton, has been seen to vary in the UV, visible, and near IR. Pluto is crossing the galactic plane, and the number of expected occultations is high beginning in 2005-2009. This will allow direct measurements of the changing atmosphere. 2.9 Map temperatures SIRTF could measure Pluto�s brightness temperature.

27.

30. PERSI Performance Requirements

32. REX Requirements and Specifications

34. SWAP and PEPPSI Requirements and Specifications

35. SWAP Solar Wind Plasma Sensor

36. PEPSSI Pluto Energetic Particle Spectrometer

37. LORRI Requirements and Specifications

39. This Europa image is at 300 m/pixel resolution, the same resolution as the New Horizons images taken with the PERSI/MVIC panchromatic imager at Pluto closest approach.

40. The Europa inset image is at 50 m/pixel resolution, the same resolution as the New Horizons high-resolution strips taken with the LORRI imager at Pluto closest approach.

41. Group 1 Traceability

42. Group 2 Traceability

43. Group 3 Traceability

44. Movie Time!