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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

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

12. 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

13. 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

14. 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