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

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) 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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

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. 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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) 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

Activity everywhere! 21.00.01.669 F18 18.59.18.754 (F22) 19.59.18.781 (F22) 20.59.18.781 (F22) Activity everywhere – also from the back fall covered areas => back fall is “wet”, i. e. contains water ice 2015_05_11 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

Seasonal Dust Cover in Ma’at (latitude about 30° north) Sept. 2014 April 2015 (before equinox) March 2016 (before equinox) NAC_2014-09-20T11.53.25.342Z_ID30_1397549500_F22 NAC_2015-03-28T16.12.49.393Z_ID30_1397549000_F82 NAC_2016-03-14T01.00.13.130Z_ID30_1397549000_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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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) 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

H2O and CO2 in February 2015 observed by VIRTIS 3. 11. 2016 OSIRIS FTM - H. Uwe Keller U. Finck private communication

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) 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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. 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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) 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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 3. 11. 2016 OSIRIS FTM - H. Uwe Keller

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! 3. 11. 2016 OSIRIS FTM - H. Uwe Keller