67P- The bigger Picture H. Uwe Keller

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

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

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?

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)

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

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?

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

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

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?

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

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 = 2 108 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

Schematics Surface recedes faster than the heat penetrates! Orbital skin depth ~ 0.7 m Diurnal skin depth ~ 0.015 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

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

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?