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

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)

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

24 micron 70 micron 160 micron

Lightcurve clearly detected at 24 micron Amplitude (max/min) ~50 % 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

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

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

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 24 µm flux Obtained by assuming Charon in equilibrium with diurnally-averaged insolation ( = ). Ab = Ag = 0.375. No beaming. Gives TB > 49.5 K F(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…

Phase integral vs. albedo for planetary surfaces

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 24 µm flux Obtained by assuming Charon in equilibrium with diurnally-averaged insolation ( = ). Ab = Ag = 0.375. No beaming. Gives TB > 49.5 K F(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…

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

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)

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

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

Physical models CH4 N2 Tholin-H2O 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 EMISSIVITY OF ICES (Stansberry et al. 1996)

Physical models CH4 N2 Tholin-H2O 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

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

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

 Calibration problem at 70 micron? 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?

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 ?

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

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…

Fitting Pluto 24:70 mic. color temperature ___ TB (70 mic) ….. TB (24 mic) X  X =Charon min X =Charon nom X =Charon max 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)

Fit of visible lightcurve