C-G: Conundrums and Antagonisms a personal view by H. Uwe Keller

H. Uwe Keller - Conundrums Mixing on all scales See also A’Hearn et al. (2012) Altwegg et al. 2015 Science Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums How do comets survive? Bodies with extremely low density Extremely low tensile strength Super volatiles CO, CO2 Even Ar and N2 All observations point to 67P being primordial, formed in a very cold and begnin environment Estimates show that collisions with speeds of ½͏͏ km/s probable (Rickman, Origin WG) Peripheral fragments of big (50 km) bodies Radioactive heating - gravity More collisions Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums Bi - lobate One big body reduced and formed by sublimation? 100 orbits like the present should suffice to carve it However, how does one create the Neck? More volatile material? Binary => bi-lobate body (Massironi et al.) Head and Body very similar VIRTIS – no spectral difference Morphology very similar KBO: 25 % are binaries => Primordial binary system Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums How is dust lifted? The basic question: how does activity work? We have not yet made any progress! At 3.5 AU many more large (mm – cm) particles observed than expected Models assuming, that pressure of subliming ices lift the dust particles, cannot elevate dust smaller than cm-sized at >3.5 AU Super volatiles can just lift large particles (Early) activity driven by CO, CO2? Disintegration of large particles – how? Granada 18-Feb-15 H. Uwe Keller - Conundrums

Large particles more easy to lift, monomers impossible Bound orbits Large particles more easy to lift, monomers impossible (Gundlach, Blum, Keller, Skorov A&A submitted) Granada 18-Feb-15 H. Uwe Keller - Conundrums

Erosion of consolidated material How are the scares, fractures, and block-like structures at all scales produced? Fractal-like behavior Distribution of multi-meter sized blocks? Explosive events? Sublimation can only lift meter size Self propelled by sublimation activity? Bigger blocks rolled down or remnants? Global (south to north) distribution of “airfall” material (dust) probable, including icy particles Granada 18-Feb-15 H. Uwe Keller - Conundrums

Heterogeneity - Homogeneity Dust features observed by OSIRIS are tracers of activity – but at low levels hard to see Dust at first detected at Neck because of self-illumination (mainly Hathor), but also because Hapi is a long flat valley Now dust follows observations of ROSINA, that water density is correlated with insolated surfaces Strongest activity where most insolation Activity distributed like on a chess board On northern hemisphere only a few percent active => nucleus is globally homogeneous with respect to the potential to be active Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums Uniform Activity Granada 18-Feb-15 H. Uwe Keller - Conundrums Activity late Dec. 14/Jan. 15

Dust features radially oriented Near Neck jet features are bent Influence of converging gas streams from Hathor and Seth Hathor seems to be active Dust features above surface radially oriented How does this work if cliffs (walls) are active? Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums Granada 18-Feb-15 H. Uwe Keller - Conundrums

Unequal erosion north-south View from north: rough with flat depressions and steep cliffs View from sides (equator): nucleus “box-shaped”, smoother side walls North layer (Massironi) looks like a “roof” Partly overhanging, jutting out Orientation with rotation axis accidental? Caused by little insolation and hence limited activity? Takes at least 10 to 20 orbits to smooth north side if rotation axis is changed by 180° How stable can the present obliquity be? Is 67P a “young” JFC? Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums Are blocks in center of Hapi valley boulders dropped down from Hathor (or Seth)? Many of them probably are outcropping bedrock connecting to the mesa-like hill. Many “boulders” may be outcrops rather than loose rocks How does this affect the boulder statistics and interpretation? Granada 18-Feb-15 H. Uwe Keller - Conundrums

H. Uwe Keller - Conundrums Restricted activity For most comets only (small) parts of the surface need to be active to support the observed production rates This does not mean that the inactive areas have lost their capability to become active 67P can loose 5 to 10 m at active spots This creates a depression of a few 100 m depth over 20 orbits and sunlight is slowly cut off If surrounding walls do not erode by activity the comet is dead => Nucleus must be homogeneous (for activity) to sustain activity over long life times => spotty activity a sign of homogeneity Granada 18-Feb-15 H. Uwe Keller - Conundrums