Looking Inside Comet 67P/C-G

Looking inside Comet 67P/C-G

Anthony Lethuillier1, Paul A Von Allmen1, Mark D Hofstadter1 and the MIRO team

1Jet Propulsion Laboratory/Calif. Inst. Tech., Pasadena, CA, United States.

Image credit : ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

Image credit : ESA/Rosetta 2/32 I. The Rosetta mission & comet 67P/C-G

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and

interpretation Images credit : ESA/Rosetta Images credit : ESA/Rosetta VI. Conclusion The Orbiter: The Lander: • Remote and in-situ observations • In-situ measurements at the of the coma and nucleus surface of the nucleus • 11 instruments • Power for 3 days of operation • 2 with strong NASA participation • 10 instruments 3/32 I. The Rosetta mission & comet 67P/C-G

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion NASA, European Space Agency and Philippe Lamy Image credit : ESA/Rosetta 2003 2014

4/32 ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDATeamfor OSIRIS ESA/Rosetta/MPS

5/32 I. The Rosetta mission & comet 67P/C-G

Image credit : ESA/Rosetta

Image credit : ESA/Rosetta 6/32 ESA/Rosetta/Philae/CIVA

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam 7/32 I. The Rosetta mission & comet 67P/C-G

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA interpretation VI. Conclusion

Credit: xkcd 8/32 II. The internal structures of comets

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

See Weissman and Lowry, Structure and density of cometary nuclei, 2007

9/32 II. The internal structures of comets

• C-G’s nucleus seems to be primordial rubble pile I. The Rosetta mission & comet 67P/C-G II. The internal • Not formed from pieces of structures of comets larger parent bodies III. The Imhotep region IV. Thermal model & method • Cycles of sublimation and V. Results and recondensation forms an interpretation “eggshell” on surface VI. Conclusion

• Ejection and deposition of dust on the surface

10/32 II. The internal structures of comets My PhD: What does SESAME-PP tells us about the internal structure of comets (Lethuillier et al. 2016) ?

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

11/32 II. The internal structures of comets

How can the MIRO instrument help understand the internal structure and evolution of cometary nuclei ? • MIRO is a passive microwave radiometer I. The Rosetta mission & comet 67P/C-G II. The internal • Located on the Rosetta orbiter structures of comets III. The Imhotep region IV. Thermal model & • Combined with spectrometer to analyze method the coma V. Results and interpretation VI. Conclusion • Works at two frequencies (190 GHz, 1.6 mm and 562 GHz, 0.5 mm)

• Can help constrain the top layer of the cometary nucleus (down to 10 cm)

12/32 III. The Imhotep region

• Located on the main lobe I. The Rosetta mission of the nucleus & comet 67P/C-G II. The internal structures of comets • Was observed twice by III. The Imhotep region MIRO at very high spatial IV. Thermal model & method resolution (20/40 m). V. Results and interpretation • Overserved at 3 AU from VI. Conclusion the sun before (2014) and after (2016) perihelion ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

13/32 III. The Imhotep region

14/32 III. The Imhotep region

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from OSIRIS observations, Auger et al. 2015. 15/32 IV. Thermal model & method

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

16/32 IV. Thermal model & method

What assumptions are made ?

I. The Rosetta mission & comet 67P/C-G II. The internal • The ice is crystalline water ice structures of comets III. The Imhotep region IV. Thermal model & • The dust is chondritic in nature method V. Results and interpretation • No significant horizontal changes in the areas observed VI. Conclusion

17/32 IV. Thermal model & method

In order to calculate the illumination we use a 12 million facet shape model of the nucleus and SPICE kernels.

“Full” calculation I. The Rosetta mission • Compute insolation for each tile in the & comet 67P/C-G footprint II. The internal • Compute temperature profile for each tile in structures of comets the footprint III. The Imhotep region IV. Thermal model & • Compute antenna temperature from method weighted average V. Results and interpretation Approximate calculation VI. Conclusion • Compute insolation for each tile in the footprint • Compute beam averaged insolation • Compute average temperature profile from average insolation

18/32 IV. Thermal model & method

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

19/32 IV. Thermal model & method

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

20/32 IV. Thermal model & method

I. The Rosetta mission • The electrical properties of the water ice/dust/vacuum mixture is & comet 67P/C-G unknown. II. The internal structures of comets III. The Imhotep region • To calculate it we can use mixing laws but each mixing law only applies IV. Thermal model & to certain situation. method V. Results and interpretation • To overcome this problem we use the Hashin & Shtrikman bounds VI. Conclusion (1962).

21/32 IV. Thermal model & method

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

22/32 IV. Thermal model & method

Input parameters:

• Dust/Ice/Vacuum fraction of the top layer I. The Rosetta mission • Dust/Ice/Vacuum fraction of the bottom layer & comet 67P/C-G Explore II. The internal structures of comets Repeatparameter until a III. The Imhotep region globalspace minimum until a IV. Thermal model & for theglobal root mean square in method Run model for several comet days and nights minimum for V. Results and until it converges to a stable diurnal cycle found in the interpretation spacethe of root VI. Conclusion parametersmean square is found. Calculate the root mean square difference between the modeled and observed brightness temperatures 23/32 IV. Thermal model & method

Using a Python ensemble sampling toolkit for affine-invariant MCMC (emcee: The MCMC Hammer. Foreman-Mackey et al. 2013):

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

24/32 IV. Results and interpretation

• For the first time we obtained a good I. The Rosetta mission fit in both the SMM/MM channels for & comet 67P/C-G both observations of the Imhotep II. The internal region. structures of comets III. The Imhotep region IV. Thermal model & • Error bar in the model due to method uncertainties in the electrical and V. Results and interpretation thermal properties. VI. Conclusion • The 2016 fit can be improved by being more selective with the observed areas.

25/32 IV. Results and interpretation

I. The Rosetta mission & comet 67P/C-G • Big difference between both II. The internal layers structures of comets III. The Imhotep region IV. Thermal model & • We have a thermally insulating method layer on the top. V. Results and interpretation VI. Conclusion • The thermal inertia decreased between 2014 and 2016.

26/32 IV. Results and interpretation

I. The Rosetta mission Instrument Region observed Thermal inertia & comet 67P/C-G (J/m2/K/S1/2) II. The internal structures of comets MIRO All nucleus (2014) 10-50 III. The Imhotep region IV. Thermal model & MIRO Imhotep and Ash (September 2014) 10-30 method MUPUS Abydos (November 2014) 50 − 120 V. Results and interpretation MIRO Seth, Ash and Aten (September 2014) <80 VI. Conclusion VIRTIS Seth, Ash and Aten (September 2014) 40-160 MIRO Imhotep (October 2014 & July 2016) 60-64

27/32 IV. Results and interpretation

• At both dates we are in presence of top layer I. The Rosetta mission & comet 67P/C-G composed primarily of II. The internal porous dust (P > 70 %). structures of comets III. The Imhotep region IV. Thermal model & • Between both observations method there is small change in the V. Results and properties. interpretation VI. Conclusion • The change is not significant when compared to the error bar.

28/32 IV. Results and interpretation

• At both dates we are in presence of more compact I. The Rosetta mission bottom layer (P < 50 %). & comet 67P/C-G II. The internal • The water ice volume in the structures of comets III. The Imhotep region bottom layer is higher than IV. Thermal model & the dust volume (15-20 % method more). V. Results and interpretation VI. Conclusion • The models seem to imply that there is less water ice and more porosity in 2016 than in 2014.

29/32 IV. Results and interpretation

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

30/32 Conclusion

• We obtained for the first time a good fitting model to the high resolution measurements made by MIRO of the Imhotep region.

I. The Rosetta mission & comet 67P/C-G • We observe a decrease in water ice content and an increase in II. The internal porosity consistent with a sublimation of water ice in the subsurface structures of comets as the comet went by perihelion. III. The Imhotep region IV. Thermal model & method • To obtain a good fit, conservative assumptions were made, resulting V. Results and in error bars on the composition that are as big as the changes interpretation VI. Conclusion observed.

• Explore additional assumptions on the subsurface.

• Explore different geomorphological regions 31/32 What’s next ?

I. The Rosetta mission & comet 67P/C-G II. The internal structures of comets III. The Imhotep region IV. Thermal model & method V. Results and interpretation VI. Conclusion

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