Published on November 15, 2007
Once and future Pluto: Leslie Young Southwest Research Institute, Boulder CO Once and future Pluto What we know now, what the puzzles are, and what we can learn from future observations and the Pluto-Kuiper Belt mission. Small, cold, distant Pluto: Small, cold, distant Pluto Slide3: Long thought to be an oddball in the outer solar system, more like the moon Triton than anything else… Slide4: Long thought to be an oddball in the outer solar system, more like the moon Triton than anything else… Pluto is now thought of by many as the first and brightest Kuiper-belt object Pluto and the Kuiper Belt: Pluto and the Kuiper Belt Pluto is one of an estimated 450,000 objects with diameters > 50 km between ~30 and 50 AU. Positions of known KBOs (as of last month) are plotted in red, white and magenta. Pluto (large white symbol) and the “plutinos” (small white dots) are in dynamically similar orbits, with semi-major axes of ~39 AU and periods 3/2 that of Neptune’s. This period places Pluto and the Plutinos in a protected 3:2 resonance with Neptune. Heliocentric orbit of Pluto-Charon: the 3:2 resonance: Heliocentric orbit of Pluto-Charon: the 3:2 resonance Even though Pluto’s orbit appears to cross Neptune’s, Pluto is protected from a Neptune collision because of a 3:2 resonance with Neptune and its inclined orbit. Malhotra et al 1997 Slide7: Pluto and Charon internal composition With accurate masses and radii, the interior composition and bulk rock fraction of Pluto, Charon, and KBOs can be modeled. McKinnon et al 1997 Slide8: Pluto, the Plutinos, and the puzzle of the drifting giants. 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. Slide9: Pluto & Charon rotational lightcurves Pluto has a very strong rotational lightcurve, from which we derive the period (6.4 days) and obliquity (120°). Charon has a very weak rotational lightcurve, which is consistent with a 6.4 day period. Buie et al 1997 Slide10: Orbit of Charon around Pluto 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 Tholen et al 1996 Slide11: The puzzle of Pluto’s big moon 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? Slide12: Pluto’s albedo and color HST observations (upper) and mutual event observations (B filter, lower left) both show large albedo variation. This is generally interpreted as bright, new frost and old, irradiated frost. Bright feature on the sub-Charon (at ~60° longitude) hemisphere, ~250 km across. The dark sub-Charon equatorial band is not all one color. Young et al 1999 Stern et al 1997 Young et al 2001 Slide13: Pluto’s surface composition N2 at 40±2 K (2.14 µm) CH4, both pure and mixed with N2 CO (2.35 µm) Concentrations of CO and CH4 is five times higher on the surface of Pluto than Triton. Doute et al 2000 Charon’s surface composition: H2O, crystalline, at 60±20 K, seen at all longitudes (1.65 µm feature is diagnostic of crystalline vs. amorphous). Possible NH3 No N2, CO, or CH4 detected. Buie and Grundy 2000 Charon’s surface composition The puzzle of the evolution of the surfaces of Pluto and Charon: 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? Surface-atmosphere interaction: Surface-atmosphere interaction Pluto’s surface is in vapor-pressure equilibrium with its atmosphere (c.f., Mars & Triton). Atmosphere is almost certainly dominated by N2, w/ surface pressure of 3-160 µbar. Gaseous CH4 has been detected, at ~1% mixing ratio. Gaseous CO has not yet been detected. Pluto’s atmospheric structure: 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. Elliot and Young 1992 Pluto’s atmospheric composition: Pluto’s atmospheric composition Titan, Pluto, and Triton form a triad for studying hydrocarbon and nitrile chemistry. Models of Pluto chemistry are underconstrained, since CO has not been detected, and only the CH4 column (not fraction) has been observed. after Summers et al 1997 Pluto’s extended atmosphere: Pluto’s extended atmosphere Pluto’s atmosphere is weakly bounded, with radius/(scale height) = 22 Pluto’s atmosphere is thought to be in hydrodynamic escape. The puzzle of Pluto’s lower atmosphere: 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 µbar on the jovian planets (where CH4 is also the principle absorber and radiator)? Seasonal change: 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 µbar), the N2 a/b phase transition. Hansen & Paige 1996 The puzzle of Pluto’s seasonal changes: 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. Top 16 Observational Goals(in three groups): 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 Top 16 Observational Goals(in three groups): 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 Top 16 Observational Goals(in three groups): 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 Examples of observatory-based observations of Pluto-Charon: 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. Slide27: New Horizons Pluto-Kuiper Belt Mission (PKB) Slide28: New Horizons PKB Mission Overview Launch in January 2006, last Jupiter-Pluto window until 2016. Jupiter/satellites encounter to be science intensive. Arrive Pluto in mid-2014 to mid-2016 (depends on ELV choice). Schematic Jupiter gravity assist trajectory. Slide29: New Horizons PKB Spacecraft, Payload, & Encounter Spacecraft: A 416 kg, highly-redundant spacecraft, based on Discovery/CONTOUR, but powered by a Cassini spare RTG. This is a far reduced spacecraft mass relative to Voyager (the most recent outer planets recon mission). Payload: Four advanced, highly miniaturized, instruments (24 kg total). The Pluto Kuiper Express mission cancelled in 2000 carried two instruments (15.5 kg total). Encounters: A six month Pluto-Charon encounter, which far exceeds the Voyager standard. HST and NGST resolution is exceeded for 150 days (±75 days of closest approach). Mapping resolution of 1 km average, 50 m best. Then on to explore several Kuiper Belt Objects (~1 every 18 months). PERSI Performance Requirements: PERSI Performance Requirements MVIC (Multispectral Visible Imaging Camera): 4-color and panchromatic scanning imager, 5000x128 pixels (panchromatic) and 5000x16 (color). LEISA (Linear Etalon Imaging Spectral Array): 1.25-2.5 µm spectral imager, 256x256 pixels. ALICE (Not an acronym, just a name): 500-1850 Å UVS. Slide31: PERSI Remote Sensing Package Objectives: MVIC: Global geology and geomorphology. Stereo and terminator images. Refine radii and orbits. Search for rings and satellites. Search for clouds and hazes. LEISA: Global composition maps, high resolution composition maps, temperatures from NIR bands. ALICE: UV airglow and solar occultation to characterize Pluto’s neutral atmosphere. Search for ionosphere, H, H2, and CxHy. Search for Charon’s atmosphere. REX Requirements and Specifications: REX Requirements and Specifications Slide33: REX Radio Experiment Objectives: Profiles of number density, temperature, and pressure in Pluto ’s atmosphere, including conditions at surface. •Search for Pluto’s ionosphere. •Search for atmosphere and ionosphere on Charon. •Measure masses and radii of Pluto and Charon, and masses of flyby KBOs. •Measure disk- averaged microwave brightness temperatures (4.2 cm) of Pluto and Charon. SWAP and PEPPSI Requirements and Specifications: SWAP and PEPPSI Requirements and Specifications SWAP Solar Wind Plasma Sensor: SWAP Solar Wind Plasma Sensor Objectives: Slowdown of the solar wind,as a diagnostic of Pluto’s atmospheric escape rate. Solar wind standoff Solar wind speed Solar wind density Nature of interaction of solar wind and Pluto’s atmosphere (distinguish magnetic, cometary, and ionospheric interactions) PEPSSI Pluto Energetic Particle Spectrometer : PEPSSI Pluto Energetic Particle Spectrometer Objectives: Measure energetic particles from Pluto’s upper atmosphere,as a diagnostic of Pluto’s atmospheric escape rate. LORRI Requirements and Specifications: LORRI Requirements and Specifications Slide38: LORRI Long Range Reconnasance Imager Objectives • Far-side maps • High-resolution closest approach images, including terminator and stereo imaging. Slide39: MVIC and LORRI Resolution 5 km 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. Slide40: MVIC and LORRI Resolution 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. Group 1 Traceability: Group 1 Traceability Group 2 Traceability: Group 2 Traceability Group 3 Traceability: Group 3 Traceability Movie Time!: Movie Time!