1. Seasonal Mass Transport on 67P H. Uwe Keller, S. Mottola, S. Hviid
IGEP und DLR-PF

2. Baseline Sublimation Model of 67P
We predicted that the southern hemisphere would be much more strongly eroded and as a consequence dust particles would be migrating from there and cover horizontal planes on the north.
Hapi: area of minimal insolation
Erosion potential on the south is about 4 times larger than on the north hemisphere!
Water ice erosion due to (insolation) integrated over the whole orbit
Insolation, erosion, and morphology of comet 67P/Churyumov-Gerasimenko H. U. Keller, S. Mottola, B. Davidsson, S. E. Schröder, Y. Skorov, E. Kührt, et al. A&A (2015)
OSIRIS FTM - H. Uwe Keller

3. OSIRIS FTM - H. Uwe Keller
Dichotomy
Dichotomy of the back fall covered north and the consolidated very rough south with strong activity!
Cause: limited insolation on the north. Only 10 to 20% of water production during nothern summer
OSIRIS FTM - H. Uwe Keller

4. North-South Dichotomy
This abrupt change of morphology at the transition from northern weak to southern strong erosion supports the concept of global back fall

5. OSIRIS FTM - H. Uwe Keller
Gravitaitional potential
Hartley 2
Hapi is at the gravitational low and hence a sink for back fall material. The cavity near the north pole keeps it fresh for the next northern summer
See also Ian-Li Lai et al. subm.
OSIRIS FTM - H. Uwe Keller

6. OSIRIS FTM - H. Uwe Keller
Global back fall transport from south to north
ROLIS: back fall features over large surface area aligned south – north
This supports global back fall transport from south to north during southern summer
Icy grains can survive until forthcoming perihelion approach – early activity of Hapi
Mottola et al. (2015)
OSIRIS FTM - H. Uwe Keller

7. OSIRIS FTM - H. Uwe Keller
WAC image
Exposure time 2.4s
Irregularly shaped boulder rotates
Period ca. 10s
Size ca. 1 m
Apparent motion from image to image is caused by the spacecraft movement
OSIRIS FTM - H. Uwe Keller

8. OSIRIS FTM - H. Uwe Keller
WAC image
Exposure time 2.4s
Irregularly shaped boulder rotates
Period ca. 10s
Size ca. 1 m
Apparent motion from image to image is caused by the spacecraft movement
OSIRIS FTM - H. Uwe Keller

9. OSIRIS FTM - H. Uwe Keller
A Geologist’s Delight
Surface morphology and topography of 67P reflects complex erosion processes
collapsing cliffs
taluses
fracturing on all scales
dune-like features and “wind” tails around outcrops
And in addition seasonal sedimentation by back falling particles – dusty hail storms
In some areas back fall accumulates and quenches the erosion completely (sedimentation)
In first approximation the nucleus of 67P is essentially eroded on the south
The back fall on the north is “wet”, i. e. contains water ice and is active during northern summer
OSIRIS FTM - H. Uwe Keller

10. OSIRIS FTM - H. Uwe Keller
OSIRIS observes back-falling decimeter-sized chunks (Agarwal et al., submitted.).
Downward acceleration by rocket effect of active chunks
(See Agarwal et al.,later this session)
NAC, 2016-January-06ESA/Rosetta/MPS for the OSIRIS Team
OSIRIS FTM - H. Uwe Keller

11. Activity everywhere!
F18
(F22)
(F22)
(F22)
Activity everywhere – also from the back fall covered areas
=> back fall is “wet”, i. e. contains water ice
2015_05_11
OSIRIS FTM - H. Uwe Keller

12. Seasonal Dust Cover in Ma’at (latitude about 30° north)
Sept April 2015 (before equinox) March 2016 (before equinox)
NAC_ T Z_ID30_ _F22
NAC_ T Z_ID30_ _F82
NAC_ T Z_ID30_ _F22
Phase angle: 63.1 deg.
Phase angle: 54.7 deg.
Phase angle: 92.6 deg.
Thickness of dust cover can be estimated by photo clinometry: range decimeter to meter
Dust particles are “wet”, they sublime water and erode during northern summer
Hu et al. (2016), A&A, Submitted
OSIRIS FTM - H. Uwe Keller

13. OSIRIS FTM - H. Uwe Keller
ROSINA observations from 3.6 to 1.5 au pre-perihelion (Fougere et al. A&A 2016)
During northern summer only very little CO2 comes from the north. During southern summer H2O, CO2, and CO are well correlated (Fougere et al. submitted)
OSIRIS FTM - H. Uwe Keller

14. H2O and CO2 in February 2015 observed by VIRTIS
OSIRIS FTM - H. Uwe Keller
U. Finck private communication

15. OSIRIS FTM - H. Uwe Keller
Ground based observations
CN production rate of 67P (in molecules s-1) as a function of heliocentric distance (negative values indicate pre-perihelion data, positive post-perihelion). (Snodgrass et al. RSTA 2016)
OSIRIS FTM - H. Uwe Keller

16. OSIRIS FTM - H. Uwe Keller
Cliff in Ash Region
Overhang max. depth
= 15 m
Cliffs are well insolated
90 m
20 m
65 m
70 m
Compare observations of comets Borelly , Tempel 1, and Hartley 2
Groussin et al. (OSIRIS private communication) 
Back fall generally plays a role in the erosion process of comets
A few m thick layer of “dust (back fall)”
prevents erosion on the horizontal tops.
OSIRIS FTM - H. Uwe Keller

17. Thickness of the Layer(s)
Rough estimate:
RSI observed the total mass loss over perihelion: 0.2 % corresponding to a uniform layer d = 0.7 m (Sierks pr. com.)
If we then divide the surface in equal south and north hemispheres we can calculate the actual amount of material removed in the south, ds, taking thickness of the back fall on the northern hemisphere into account.
Let b be the fraction of material eroded on the south that makes it to the north
Let f be the sublimation efficiency on the north relative to that of the south
Then ds = 2 d / (1 - b + f)
OSIRIS FTM - H. Uwe Keller

18. Compositional Dichotomy of the Coma
During northern summer composition measurements in the coma are not representative for the nucleus proper
Skin depth for water sublimation is mm to cm
Skin depth for CO2 is in the range of cm to dm, CO even higher
Back fall particles of cm to dm size can maintain water ice but are depleted from CO2 and CO, and other volatiles
During northern summer the H2O/(CO2, CO, HCN) ratio is overestimated and vice versa during southern summer. We need to integrate over the orbit
Compositional results from flyby missions and ground based observations may need re-interpretation
OSIRIS FTM - H. Uwe Keller

19. Coma Observations and Homogeneity
The activity pattern of 67P is very complex and varying with heliocentric distance and most important season
Around perihelion (southern summer) Rosetta observed the pristine (original) nucleus
A considerable mass fraction is eroded into large particles (cm to dm) that themselves maintain water sublimation but are too warm to sustain CO2 or CO
During northern summer the H2O/(CO2, CO, HCN) ratio is overestimated and vice versa during southern summer. We need to integrate over the orbit
With the present orbital geometry erosion of the nucleus takes essentially place on the south
Models based on lifting dust by water ice sublimation will not work for large icy particles
Many of the previous ground based and s/c observations will need re-interpretation
Top Rosetta Rendezvous result!
OSIRIS FTM - H. Uwe Keller