Analysis and characterisation of the Aswan *possible* cliff collapse M. Pajola, N. Oklay, M. Massironi, F. Scholten, F. Preusker, S. Höfner, M. Hofmann, S. Fornasier, C. Feller, J. B. Vincent, A. Lucchetti, M. R. El-Maarry, G. Naletto, H. Sierks, N. Thomas & the entire OSIRIS Team

We are answering to the Referee’s points.. ..almost ready to resubmit the work!

January 11, 2016 => when I sent you the email

January 11, 2016 => when I sent you the email

But then, actually, Sonia and Clement pointed out that: Looking at the color sequence taken on 19 july 2015 (Rosetta was at 178 km from the comet), it seems that the bright ‘collapsing’ cliff was already there, so => 26 DAYS before the perihelion passage (Aug 13). The bright (5-6 times than sourrounding regions) cliff in Aswan is already there!

But then, actually, Sonia and Clement pointed out that: Looking at an image taken on 15 july 2015 the bright ‘collapsing’ cliff was already there, so => 30 DAYS before the perihelion passage (Aug 13).

So when did it *fall* down??? But then, actually, Sonia and Clement pointed out that: WHILE on an image of 17 June the bright cliff is not there! So when did it *fall* down???

Thanks to Jean Baptiste => OUTBURSTS DATABASE The best candidate is a NAVCAM ourburst which happened on 10 July 2015. The best estimate for the source is lat=74, lon=220. This is right on the cliff, but there is about 30 degree uncertainty on the position.

Now, thanks to Frank Scholten and the DLR: A quantitative analysis of the possible collapse

115 m (from SHAP4S shape model) 115 m 12 m 70 m 70 m

Like the one Olivier showed days ago? An overhang? Like the one Olivier showed days ago?

Metrics of potential cliff collapse: Top length: 70 m View and measures from SHAP4S shape model 50 m Overall cliff height 140 m 70 m Metrics of potential cliff collapse: Top length: 70 m Mean top depth: 12 m Vertical height: 50 m Uncertainty of width measures: 10% or worse Rough estimate of the total volume of collapsed material (assuming a decrease of the depth from the top to the bottom, from 12 to 0 m, inspired by its overhang-like appearance in the shape model): 70 m x 50 m x 12 m / 2 = 21000 m3 Uncertainty of this volume estimate: ~30% => 15000 m3 - 30000 m3

Indentification/interpretation are definitely vague … View and measures from SHAP4S shape model Indentification/interpretation are definitely vague … => For better identification, understanding, and more accurate quantitative measurements, we need additional/better views of this feature! 50 m Overall cliff height 140 m 70 m Metrics of potential cliff collapse: Top length: 70 m Mean top depth: 12 m Vertical height: 50 m Uncertainty of width measures: 10% or worse Rough estimate of the total volume of collapsed material (assuming a decrease of the depth from the top to the bottom, from 12 to 0 m, inspired by its overhang-like appearance in the shape model): 70 m x 50 m x 12 m / 2 = 21000 m3 Uncertainty of this volume estimate: ~30% => 15000 m3 - 30000 m3

There is always a bright patch! I prepared an RGB There is always a bright patch!

Now thanks to Sonia and Clement: Aswan cliff spectrophotometry I/F is not corrected for illumination conditions Flattening of the slope beyond 600 nm consistent with previous observations of bright and water ice richer regions (ex. Hapi)

Now thanks to Sonia and Clement: Aswan cliff spectrophotometry Spectrophotometry indicates the usual spectrally red cometary behavior at wavelength shorter than 600 nm, than a smooth spectrum between 600-800 nm, and an increasing reflectance/spec slope beyond 930 nm. Potential explanation: absorption band in the 800-900 region or coma (dust) contamination in the nucleus reflectance. The overall spectral behaviour is similar to what usually observed on Hapi and consistent with ah higher aboundance of water ice.

Aswan cliff spectrophotometry Now thanks to Nilda: Aswan cliff spectrophotometry Methods & Outlook Lommel-Seeliger photometric correction applied to co-registered images. It looks like the spectral properties changed since 19-07-2015 (See Sonia’s presentation)

Reference image: NAC_2015-07-19T03. 37. 42. 179Z_IDB0_1397549005_F27 Reference image: NAC_2015-07-19T03.37.42.179Z_IDB0_1397549005_F27.IMG Phase Angle: 89° Spatial resolution: 3.3m/px R:IR, G: Orange, B: FUV

The bright patch has lower spectral slope than its vicinity, consistent with the spectral properties of active regions, but according to activity threshold, this patch is not active. We will make the direct comparison with the mean spectra of Hapi and water icy patches for a solid conclusion.

Comparison with Sonia’s presentation 3x3 box at similar locations to theirs We are consistent but the selection matters if you compare these plots with the ones in the previous slide. 5x5 box at similar locations to theirs

Reference image: NAC_2015-12-26T17. 04. 40. 384Z_IDB0_1397549001_F28 Reference image: NAC_2015-12-26T17.04.40.384Z_IDB0_1397549001_F28.IMG Phase Angle: 89° Spatial resolution: 1.4m/px R:IR, G: Orange, B: FUV

They are not active according to activity thresholds The mean spectra of the rois are more similar to avg surface. Aging (loosing volatiles, mixing with dust). We have the gas contribution back in here. In Blue, Hydra and Fe2O3. We should investigate what is changing here. Probably the observations are done at different time of the day. They are not active according to activity thresholds

What about the possible boulders that are below the cliff?

If we have the possibility to obtain 2-3 images at 50 cm/pixel covering this area as done in Sept 2014:

If we have the possibility to obtain 2-3 images at 50 cm/pixel covering this area as done in Sept 2014: We could perform the boulder size-frequency distribution of the material that fell down, and see if it is steeper than the two previous values or not 

The Aswan cliff collapse would be a really great test case for: Maurizio’s work on boulder size-frequency distributions; JB’s work on fractured cliffs as the source of cometary jets; Sebastian’s work on Thermophysics of fractures; Mark’s work on the biggest possible dimensions of boulders, once the height of a cliff is known; Matteo’s work on layers on 67P. The Composition Group => huge chance to observe spectral changes

In addition to this => an image perpendicular to the cliff would provide a look at the interior of the nucleus: Is it outgassing/sublimating? Does it now change with time as it evolves?

Thanks to the OSIRIS Team for providing this great opportunity! The Aswan cliff collapse is an incredible opportunity to test the great science we are doing on 67P! Thanks to the OSIRIS Team for providing this great opportunity!