Rosetta/OSIRIS Morphology

Jutzi Model

Region Description Head Neck Body Hatmehit Well-defined depression in the head region that appears to be filled with fine-grained smooth material overlain by a talus. We use the topography of the edge of the depression to define the region. Ma’at Main dust-covered region on the head. Similar to Ash. Smooth deposits showing ripple-like structures, a possible sign of mobilization. Sharp outcrops of underlying material are usually observed. Maftet Rough terrain, lineated, and bouldered with scattered patches of debris neighboring the Hatmehit, Nut and Serqet regions. Many small irregular depressions and pits which give the appearance of lifted blocks/chunks of material and possible fluidized volatile activity. Bastet Rough and heavily lineated region with minimal bouldering. Possibly part of the basal unit. Borders Hathor and requires anaglyph of the region to identify topographic differences. Separated from Ma’at by showing only limited debris cover. Nut Small depression between the Serqet ridge and the Ma’at/Hatmehit region. Heavily bouldered. Possible result of erosion of Serqet. Identifiable mostly through topography. Serqet Small region encompassing a sharp ridge and a flat and smooth plain with few boulders. The Serqet/Ma’at boundary is gradual. The Serqet/Anuket boundary is clearly defined by topography. The Serqet/Nut boundary is where the plain finishes and the surface texture changes from smooth to bouldered. Hathor A complex region opposite the Seth region on the body and hanging over the Hapi region. Un-mantled and heavily lineated in two dimensions. Shows signs of detachment. Lighting makes the lineated appearance appear very uniform but other viewing angles show that it is rough. Anuket A complex unit that is separated from Hathor by a ridge. Parallel lineations evident on Hathor are limited to absent. It is separated from Ma’at and Serqet by topography being somewhat depressed with respect to these. Neck Hapi Narrow region connecting the head and body of the comet. Currently the most active region and site of regular jet activity although exact source not defined. Smooth dusty-looking material along with dispersed large boulders that may have slumped from the head or body regions. Body Imhotep Geologically the most complex region on C-G. Extremely smooth, probably recently re-surfaced, yet bouldered region enclosed by horizontally bedded and vertically jointed ridges. There is strong evidence of mass-wasting all around the smooth areas as well as the presence of conical structures and pits that are possibly the result of mechanisms similar to mud volcanism. Ash Main dust-covered region of the body. Smooth deposits possibly a few meters thick. Similar to Ma’at region. Contains the only currently unambiguously identified impact crater on the surface of the comet. Khepry Rough- and bright-looking unit neighboring the Imhotep, Aten and Aker regions. Moderately lineated. Possibly an exposure of the comet’s basal unit. Rough by comparison with Aker although transition to it is gradual. Aten Well-defined depression between the Imhotep, Ash and Khepry regions. Not covered by airfall with no mantling material and hence clearly separable from Ash. Aker Dark-toned unit with a mixed degree of roughness. Lineated and showing tectonic-like features. Possibly a reworked section of the Khepry region. Several small smooth areas (ponded dust) evident. Babi Transitional region that grades smoothly into Ash and Seth regions in terms of debris cover. Neighbors the Aten and Aker regions and displays exposures of brittle mantling material at its contact with the Aten depression. Atum Highly complex region. Minimal bouldering but several small depressions showing some lineation. Irregular complex mounds also seen. Borders the Anubis unit with ill-defined margins in places. Anubis Smooth region very similar to Imhotep and, possibly, Hapi. Some scattered boulders possibly a result of mass-wasting. Smooth deposits appear faulted/folded in some regions and display linear features similar to those observed in Imhotep. Cones seen in Imhotep are absent. Apis Flat, smooth, and lineated unit showing irregular and polygonal lineations. Significant topographic change with respect to Atum and Imhotep. Seth Ubiquitous circular, semi-circular, and quasi-circular features. Strong evidence of collapse with active pits. Sharp topographic contact with Anubis. May underlie Hapi.

Anubis-Anuket

Apis-Atum-Ash

WAC Anuket-Ma’at

Aten

Serqet-Nut-Seth

Anuket-Hathor

Hatmehit-Maftet

Imhotep

Babi

Aker-Khepry

5 Basic Types Dust-Covered Mantle (Ash, Ma’at) Exposed Mantle (Seth) Large Scale Depressions (Hatmehit, Aten, Nut) Smooth Terrains (Hapi, Anubis, Imhotep) Consolidated surfaces (Hathor, Anuket, Aker, Apis, Khepry, Bastet, Maftet) More subtle differences within each category.

Early Image

Bright Material on the Surface It looks like dust on the surface here. Comparable to the surface to the left. But note the bright material on the floor nearby. This material is also bright independent of illumination conditions.

Another image of the bright materials

Examples

Calculation Cheng et al. looked at this in 2013 using Greeley and Iversen. (Ip pointed this out to me last Thursday). The Greeley and Iversen formulation is rather complex but the key point is that there is a minimum in velocity space. In other words, a U-shaped curve with a specific minimum corresponding to the grain size most easily entrained by the wind is evident. This is also seen in Greeley and Iversen’s data for which they used a wind tunnel. (see next slide)

Mars Fits I have re-calculated this for Mars (as a cross-check) and applied to the comet using cometary parameters. They are a lot of assumptions in here of course. Turns out that there is a plug and chug formula by Shao and Lu.

Parameters needed Particle bulk density = 350 kg/m3 g = 1.55 x 10-4 m/s2 d = 100 micron Gas pressure (the BIG unknown) = 0.03 Pa Computed for an active region at 2 AU. Gas temperature = 200 K

Result/Conclusion For the given parameter set you need a gas velocity of (only) 335 m/s. This is comparable to gas flow velocities seen in most fluid dynamics calculations (including our recent DSMC studies with Wing and the Taiwan group) even close to the surface. This implies that “wind”-driven motion is feasible for what appear to be sensible parameters. Hence, there are many uncertainties but this deserves to be taken very seriously.

Blocks - Movement Pedro and Anne-Therese have pointed out that we seem to have blocks of material that have moved.

Aten – Mantling Lost? Aten depression shows no evidence of mantling layer. You don’t need a DTM to see that it is a major depression. Aten is surrounded on 3 sides by mantled material but within it there is no evidence of mantled material. It is not the only place where we see evidence of large scale loss of the mantling layer but possibly the largest.

Overburden Pressure To remove a chunk completely or move it you have to overcome the overburden pressure. 𝑝= 𝑚𝑔 𝐴 = 𝜌 𝑑 𝑔 𝑚=𝜌𝐴𝑑 Let us assume that we can build this pressure sub-surface. Using g = 1.55 10-4 m/s2, ρ =350 kg/m3, and d = 50 m, we get just 2.7 Pa. This applies IF the pressure is applied uniformly at the base of the chunk. If not you require a higher pressure locally (and assume that the chunk can maintain its structure under the uneven load) 𝑝= 𝑚𝑔 𝐴 𝑠 𝐴 𝑒 = 𝜌 𝑑 𝑔 𝐴 𝑠 𝐴 𝑒 Note that Ekkehard calculated a tensile strength from overhangs of >12 Pa and clearly the local gas pressure cannot greatly exceed this value.

Pressure Requirements Use the Huebner et al. (2006) constants from his Table B1. Compute the vapour pressures as a function of temperature. Identify which temperature reaches the required pressure. If CO2 is the driving volatile then you need a temperature of at least 118 K at a depth of 50 m below the surface to produce sufficient pressure (>2.7 Pa) – which doesn’t sound unrealistic. If it is CO, the temperature has to be greater than 42 K. These temperatures are lower limits for the proposed mechanism but the exponential increase in equilibrium vapour pressure means that reduction in the area of a sub-surface pocket can be compensated by temperature increases of just a few degrees (unless we look at extremes). 𝐴=𝜋 𝑟 2 𝑝= 𝐴=𝜋 𝑟 2

Anubis