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Pluto’s thermal lightcurve: SPITZER/MIPS observations

This study focuses on the thermal lightcurve of Pluto using SPITZER/MIPS observations. The data suggests a measurable thermal inertia and relatively high bolometric emissivities. The observations also detect the presence of Charon and provide insights into its thermal properties.

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Pluto’s thermal lightcurve: SPITZER/MIPS observations

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  1. Pluto’s thermal lightcurve: SPITZER/MIPS observations E. Lellouch, J . Stansberry, D. Cruikshank, W. Grundy

  2. Introduction • Pluto has strong albedo contrasts and a well-marked visible lightcurve  a thermal lightcurve is expected • IRAS and ISO observations of Pluto-Charon have detected the lightcurve at 60 and 100 micron • ISO: the thermal lightcurve is roughly anticorrelated with the visible lightcurve, but shifted by ~ 25° • Modelling of ISO observations at 60,100,150 and 200 µm indicates (Lellouch et al. 2000) • A measurable thermal inertia  = (1.5-10)x104 cgs • Relatively high bolometric emissivities (e.g. 0.85 for CH4)

  3. SPITZER/MIPS Observations • Sept. 17-22, 2004 • Sub-earth latitude = 32° • 8 longitudes • 24, 70, 160 µm • Data reduction steps • MIPS Instrument Team reduction tools (see J. Stansberry’s talk) • 160um data was time-filtered to increase SNR                 • Increase in calibration uncertainty   •  Color corrected fluxes

  4. 24 micron 70 micron 160 micron

  5. First detection of Pluto-Charon at 24 micron • Lightcurve clearly detected at 24 micron • Amplitude (max/min) ~50 % • Lightcurve more noisy at 70 micron • Amplitude (max/min) ~30 % • Lightcurve not detected at 160 micron Min = 5.4 mJy

  6. Pluto-Charon brightness temperatures: • Decrease with increasing wavelengths • Lower than ISO at 70 and 160 micron • SPITZER 70 micron lightcurve has lower amplitude than ISO 60 micron lightcurve

  7. Thermophysical modelling • Thermophysical model (from Lellouch et al. 2000), including: • Sub-surface conduction (thermal inertia , thermal parameter )  = subsurface heat radiative timescale / diurnal timescale • Bolometric albedos (Ab) and emissivity (b), spectral emissivities () • Beaming (surface roughness – nominal =20°) • Proper geometry (e = s = 32°) • Surface distribution of terrains • Charon • 3 units on Pluto • N2 • CH4 • Tholins+H2O

  8. Charon’s emission • Charon has ~no visible lightcurve (Ag ~ 0.375)  constant thermal flux • Maximum Charon 24 µm flux = Minimum of 24 µm lightcurve = 5.4 mJy max. Charon brightness temperature : TB < 59 K • This maximum flux can be obtained from TPM with • b =  = 1 (water ice) • Ab = 0.22,  = 2, slope = 20° NOTE: Even if no beaming, and assuming instantaneous equilibrium with solar insolation ( = 0), flux < 5.4 mJy flux implies Ab > 0.33, i.e. a phase integral q > 0.88 : unlikely… - Charon has non-zero thermal inertia • Minimum Charon 24µm flux • Obtained by assuming Charon in equilibrium with diurnally-averaged insolation ( = ). Ab = Ag = 0.375. No beaming. Gives TB >49.5 KF(24 mic)=0.7 mJy • Note: Charon’s temperature measured from SMA = 56+/-14 K (Gurwell et al. 2005). Very nice but far too imprecise…

  9. C? Phase integral vs. albedo for planetary surfaces

  10. Charon’s emission • Charon has ~no visible lightcurve (Ag ~ 0.375)  constant thermal flux • Maximum Charon 24 µm flux = Minimum of 24 µm lightcurve = 5.4 mJy max. Charon brightness temperature : TB < 59 K • This maximum flux can be obtained from TPM with • b =  = 1 (water ice) • Ab = 0.22,  = 2, slope = 20° NOTE: Even if no beaming, and assuming instantaneous equilibrium with solar insolation ( = 0), flux < 5.4 mJy flux implies Ab > 0.33, i.e. a phase integral q > 0.88 : unlikely… - Charon has non-zero thermal inertia • Minimum Charon 24µm flux • Obtained by assuming Charon in equilibrium with diurnally-averaged insolation ( = ). Ab = Ag = 0.375. No beaming. Gives TB >49.5 KF(24 mic)=0.7 mJy • Note: Charon’s temperature measured from SMA = 56+/-14 K (Gurwell et al. 2005). Very nice but far too imprecise…

  11. Charon-corrected Pluto brightness temperatures • Decrease with increasing wavelengths • for nominal Charon model • <TB (24 mic)> ~ 50 K • <TB (70 mic)> ~ 42 K • <TB (160 mic)> ~ 35 K

  12. Pluto-only TB • Decreases with increasing wavelengths from 24 to 160 mic • Mixing of multiple temperatures? • Possible in theory, but does not work quantitatively (at least for simple 2-temperature model) • Emissivity effect? • Can be technically fit with single temperature and spectrally constant emissivity, but solution seems implausible: T ~ 55 K,  ~ 0.3 • More likely solution: a spectrally-variable surface emissivity (decreasing with wavelength)

  13. Pluto: thermal inertia from lightcurve phase • 24-mic lightcurve almost anticorrelated with visible lightcurve, but anticorrelation maximum if 24-mic lightcurve shifted by 14-17° • Elementary modelling of 24-mic data • Includes Charon + 2 types of Pluto terrains (« cold » and « hot » regions) • Fix temperatures of Charon and Pluto cold regions (TCH = 57 +/-2 K, Tcold = 40 +/- 5 K) • Take Cold / Hot relative proportions from visible lightcurve • Fit thermal lightcurve by solving for Thot and a global shift of thermal lightcurve

  14. Pluto/Charon lightcurve: elementary fit • Solution: Th = 51-55 K and shift = 15-18° • Suggests thermal parameter  ~2-3 • As expected, does not match 70 and 160-mic data

  15. Grundy and Fink 96 Lellouch et al 2000 Modified G & F HST Modified HST • Physical models • Includes Charon and three-unit models of Pluto – from Grundy et al. 2001 • Estimate geometric albedos of each unit from visible lightcurve fit and deduce bolometric albedos • Additional assumptions • -- T (N2) = 35 K • -- Emissivities • Tholin-H2O:  = b = 1 • CH4:b = 0.85, 24 mic = 0.35 , 0.7, 1 • Focus first on 24-mic lightcurve : solve for thermal parameter  of Pluto and for Charon emission « background » • Then model 70 and 160-mic data CH4 N2 Tholin-H2O

  16. CH4 N2 EMISSIVITY OF ICES (Stansberry et al. 1996)

  17. Grundy and Fink 96 Lellouch et al 2000 Modified G & F HST Modified HST • Physical models • Includes Charon and three-unit models of Pluto – from Grundy et al. 2001 • Estimate geometric albedos of each unit from visible lightcurve fit and deduce bolometric albedos • Additional assumptions • -- T (N2) = 35 K • -- Emissivities • Tholin-H2O:  = b = 1 • CH4:b = 0.85, 24 mic = 0.35 , 0.7, 1 • Focus first on 24-mic lightcurve : solve for thermal parameter  of Pluto and for Charon emission « background » • Then model 70 and 160-mic data CH4 N2 Tholin-H2O

  18. Fit of 24-mic lightcurve Need for better measurements here!

  19. Ag(N2) Ag(CH4) Ag(thol CH4 tholin PL CH (mJy) <T>CH (K) CH GF 0.76 0.53 0.10 1 1 7 3.9 57.2 3.5 .35 1 10 5.4 59 2 Lellouch 0.74 0.62 0.25 1 1 7 4.1 57.4 3 .35 1 10 5.1 58.6 2.3 Mod. GF 0.74 0.69 0.32 1 1 10 4.45 57.9 2.5 0.7 1 10 4.6 58.1 2.5 HST 0.78 0.83 (!?) 0.20 1 1 10 4.2 57.6 3 .35 1 7 3.9 57.2 3.5 Mod. HST 0.91 0.74 0.25 1 1 8 2.35 54.5 8 .35 1 7 2.15 54.1 10 24 micron fit: solution parameters Input parameters ! Fitted parameters • PL = 7 – 10 • CH = 2 – 10 (generally 2-3.5) • <TCHARON > = 54-59 K

  20. EMISSIVITY RESULTS • CH4: 24 mic = 0.7 - 1 give better fits than 24 mic = 0.35 • Models with spectrally-constant emissivities overestimate MIPS-measured TB at 70 and 160 mic (but would almost fit ISO 60 and 150 mic…) • Decrease of spectral emissivities of tholin-H2O regions at long wavelengths? Or  Calibration problem at 70 micron?

  21. Conclusions • Pluto’s thermal parameter  = 7-10, i.e. thermal inertia  = (3-5)x104 cgs: consistent and more accurate than ISO • Newest result: <T>CHARON = 54-59 K, i.e.  = 2-10 ( = 2-3.5 range favored, i.e.  = (1-2)x104 cgs) • Charon is not in instantaneous equilibrium with Sun, but probably has lower thermal inertia than Pluto. • Charon’s TI comparable to Saturn’s icy satellites, and Pluto’s to Galilean satellites. • Pluto’s TI enhanced by atmospheric conduction in porous regolith? • CH4 ice 24-mic emissivity not small (0.7-1) • Tholin-H2O emissivity decreases from 24 to 70 and 160 mic., but possible calibration error ?

  22. Charon’s emission • Charon has ~no visible lightcurve (Ag ~ 0.375)  constant thermal flux • Min. 24 µm flux = 5.4 mJy = max. Charon flux TB < 59 K • This maximum flux can be obtained from TPM with • b =  = 1 (water ice) • Ab = 0.22,  = 2, slope = 20° NOTE: Even if no beaming, and assuming instantaneous equilibrium with solar insolation ( = 0), flux < 5.4 mJy flux implies Ab > 0.33, i.e. a phase integral q > 0.88 : unlikely… - Charon has non-zero thermal inertia

  23. Range of Charon’s emission • Maximum model • <TB> = 59 K; obtained from thermophysical model (TPM) with Ab = 0.22,  = 2, slope = 20°, F(24 mic)=5.4 mJy • Minimum model: • Charon in equilibrium with diurnally-averaged insolation ( = ). Ab = Ag = 0.375. No beaming. Gives <TB> = 49.5 K, F(24 mic)=0.7 mJy • Nominal model: • <TB> = 57 K; obtained from thermophysical model (TPM) with Ab = 0.22,  = 3.5, slope = 20°, F(24 mic)=3.75 mJy • Note: Charon’s temperature measured from SMA = 56+/-14 K (Gurwell et al. 2005). Very nice but far too imprecise…

  24. Fitting Pluto 24:70 mic. color temperature • TB (70 mic) ~ 42 K • TB (24 mic) ~ 50 K • No solution for 2-temperature model • An (unlikely?) solution for Tsurf ~55 K and spectrally constant emissivity ~ 0.3 • More likely solution: spectrally variable surface emissivity ___ TB (70 mic) ….. TB (24 mic)   X =Charon min X =Charon nom X =Charon max X X X ___ TB (70 mic) ….. TB (24 mic)

  25. Fit of visible lightcurve

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