Plutos thermal lightcurve: SPITZERMIPS observations

1
Plutos 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.
  Stansberrys 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
• TholinsH2O

8
Charons 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 lt 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 lt 5.4 mJy flux implies Ab gt 0.33,
  i.e. a phase integral
• q gt 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 gt 49.5 K F(24
  mic)0.7 mJy
• Note Charons temperature measured from SMA
  56/-14 K (Gurwell et al. 2005). Very nice but
  far too imprecise

9
Phase integral vs. albedo for planetary surfaces
10
Charons 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 lt 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 lt 5.4 mJy flux implies Ab gt 0.33,
  i.e. a phase integral
• q gt 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 gt 49.5 K F(24
  mic)0.7 mJy
• Note Charons 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
• ltTB (24 mic)gt 50 K
• ltTB (70 mic)gt 42 K
• ltTB (160 mic)gt 35 K

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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

• 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

16
EMISSIVITY OF ICES (Stansberry et al. 1996)
17

• 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

18
Fit of 24-mic lightcurve
Need for better measurements here!
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24 micron fit solution parameters
 
 
Input parameters !
Fitted parameters

• ?PL 7 10
• ?CH 2 10 (generally 2-3.5)
• ltTCHARON gt 54-59 K

 
 
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• 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

• Plutos thermal parameter ? 7-10, i.e. thermal
  inertia ? (3-5)x104 cgs consistent and more
  accurate than ISO
• Newest result ltTgtCHARON 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.
• Charons TI comparable to Saturns icy
  satellites, and Plutos to Galilean satellites.
• Plutos 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
Charons emission

• Charon has no visible lightcurve (Ag 0.375) ?
  constant thermal flux
• Min. 24 µm flux 5.4 mJy max. Charon flux ?TB
  lt 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 lt 5.4 mJy flux implies Ab gt 0.33,
  i.e. a phase integral
• q gt 0.88 unlikely -? Charon has non-zero
  thermal inertia

23
Range of Charons emission

• Maximum model
• ltTBgt 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 ltTBgt 49.5 K,
  F(24 mic)0.7 mJy
• Nominal model
• ltTBgt 57 K obtained from thermophysical model
  (TPM) with Ab 0.22, ? 3.5, slope 20, F(24
  mic)3.75 mJy
• Note Charons temperature measured from SMA
  56/-14 K (Gurwell et al. 2005). Very nice but
  far too imprecise

24

• Fitting Pluto 2470 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)
25
Fit of visible lightcurve