1. 67P- The bigger Picture
H. Uwe Keller

2. Star and disk formation stages
Prestellar
Embedded stage
T. Greene Sci Am

3. O2 in comet 67P ROSINA team mtg. in June 2015 O2/H2O in percent range
Talk by Ewine F. van Dishoeck
Where does O2(ice) come from, how is it formed?
Not yet a conclusive concept
67P had to be formed at low T (20 to 30K)
=> 67P is a primordial body

4. Conclusion 67P abundance ratios similar to those found in
one warm pre-stellar core
But O2/H2O not well known
None of interstellar ice models can reproduce
O2/H2O as high as a few%
Does this rule out ‘inheritance’ model?
Lots of gaseous O2 in disks in comet-forming
zone
Need rapid cooling to get mixed O2-H2O ices, perhaps following accretion burst
Partial ‘reset’ scenario?

5. HDO/H2O as diagnostic origin water
Implications from O2?
Comets
67P
M. Persson
Was HDO/H2O set in cloud before solar system formed? (Visser et al. 2009)
‘Water is older than the Sun’ (Cleeves et al. 2014)

6. 67P a Pristine Body
Ample abundances of super volatile molecules such as CO2, CO, N2, H2S, Ar, and O2 clearly point to formation in the 10 to 30 K temperature range
Particularly the O2 finding may lead to a new understanding of the early solar system
High porosity and homogeneity of the body – not a rubble pile
Low tensile strength that allows the minute gravity to control the morphology
We need to publish our findings that clearly point to the formation as a primordial body. Björn’s paper needs to go out asap

7. Dichotomy of rough airfall covered north and consolidated south with large planes, no (few?) pits or sinkholes but strong activity!
Probable cause: limited insolation on the north. Only 10 to 20% of water production during nothern summer
If dichotomy is evolutionary process, the geometry of the rotation axis has been constant for many (50 ?) orbits
Question: how much does airfall contribute to formation of the rough “semi-cratered” surface?

8. Global airfall transport from south to north
ROLIS airfall features over large surface area aligned south – north
This supports global airfall transport from south to north during southern summer
Icy grains can survive until forthcoming perihelion approach – early activity of Hapi

9. Hapi is not an area of special physical consistence (crashed material) but rather a sink for airfall material at a gravitational low

10. Volume asymmetry by erosion
a ≈ 1530 m
b ≈ 1070 m
Erosion rate 1 m/orbit on the north side,
4 m/orbit on the south side needs about 150 orbits to cause observed asymmetry
Again: how stable is the rotation axis geometry?

11. The north – south erosion dichotomy
Indications are that the geometry of the rotation axis has prevailed for tens, possibly hundreds of orbits
ROSINA observes a higher CO2/H2O ratio coming from the south
Should the north not preserve the supervolatiles better than the scorched south?
Erosion (by water) in the south can be 10 m per orbit or more, hence probably larger than the orbital skin depth of heat penetration
As a consequence the south stays “pristine” and we expect more super volatiles such as CO2 and CO
The interpretation of relative abundances needs to take this into account if one discusses “heterogeneity”
Local activity strongly depends on the morphology and insolation history because the thermal scale lengths are small

12. Keller et al. Orlando DPS 2008
Thermal Skin Depths
Sample calculations for comet 9P/Tempel
Skin depth
τdiurnal = 105 s => xdiurnal= 1.5 cm
τorbital = s => xorb = 44 xdiurnal≈ 70 cm
Orbital skin depth is only 70 cm, smaller than the thickness of the surface layer that is removed by sublimation activity
Keller et al. Orlando DPS 2008

13. Schematics Surface recedes faster than the heat penetrates!
Orbital skin depth ~ 0.7 m
Diurnal skin depth ~ m
cometary surface
Erosion per orbit ~ 2 m
2 m
1 m
active area
See Gortsas et al. (2011) Icarus 212, 858
Surface recedes faster than the heat penetrates!
=> Material, a few centimetres below an active surface, was covered in previous orbits by metres of essentially unaltered nucleus material and has not been affected by earlier insolation
Keller et al. Orlando DPS 2008

14. Thermal Scale Lengths
Decimeter building blocks go with macro porosity. This could lead to deeper penetration of thermal energy.
Diurnal thermal skin depth is a few agglomerate diameters for porous material
It depends on the time τ of illumination (length of day) τ
If we have decimeter size building blocks they do not reach thermal equilibrium during a day
How is the seasonal (orbital) skin depth influenced by the macro porosity?
We need thermal models!
Interpretations of activity and abundance observations will need not only diurnal but also seasonal models

15. Heterogeneity
“Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse”
(Vincent et al. Nature 2015)
“Ultimately, regardless of the process creating the initial subsurface cavity, active pits indicate that large structural and/or compositional heterogeneities exist within the first few hundred metres below the current nucleus surface of comet 67P.”
Macro porosity should not be called heterogeneity
Do we really support such a general – in my mind misleading – statement?