The Agilkia boulders/pebbles size–frequency distributions: OSIRIS and ROLIS joint observations of 67P surface

Abstract

By using the images acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) and ROLIS (ROsetta Lander Imaging System) cameras, we derive the sizefrequency distribution (SFD) of cometary pebbles and boulders covering the size range 0.05-30.0 m on the Agilkia landing site. The global SFD measured on OSIRIS images, reflects the different properties of the multiple morphological units present on Agilkia, combined with selection effects related to lifting, transport and redeposition. Contrarily, the different ROLIS SFD derived on the smooth and rough units may be related to their different regolith thickness present on Agilkia. In the thicker, smoother layer, ROLIS mainly measures the SFD of the airfall population which almost completely obliterates the signature of underlying boulders up to a size of the order of 1 m. This is well matched by the power-law index derived analysing coma particles identified by the grain analyser Grain Impact Analyser and Dust Accumulator. This result confirms the important blanketing dynamism of Agilkia. The steeper SFD observed in rough terrains from 0.4 to 2 m could point out intrinsic differences between northern and southern dust size distributions, or it may suggest that the underlying boulders 'peek through' the thinner airfall layer in the rough terrain, thereby producing the observed excess in the decimetre size range. Eventually, the OSIRIS SFD performed on the Philae landing unit may be due to water sublimation from a static population of boulders, affecting smaller boulders before the bigger ones, thus shallowing the original SFD.

Surface Morphology of Comets and Associated Evolutionary Processes: A Review of Rosetta’s Observations of 67P/Churyumov–Gerasimenko

“…10A), as well as local SFD studies of sizes ≥2 m (Pajola et al 2016c, Fig. 10B) down to even the decimeter and centimeter scales (Pajola et al 2016a(Pajola et al , 2017a. The numerous studies focused on boulders suggest that their SFD can be indicative of their formation processes and how the surface of 67P evolves.…”

The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse

“…By considering the minimum three-pixels sampling rule, that minimizes the likelihood of misidentifications of what we are detecting [43], we set the lowest measurable boulder size at 1.5 m. The constant presence of shadows next to the boulders (the observations were performed with phase angles varying from 47¡ to 77¡), allowed us to identify even smaller boulders (2 pixels diameter, ∼1 m). However, as indicated in [20], we did not include these smaller populations in the cumulative size-frequency distribution (SFD) because they do not represent a complete dataset for such small sizes, as demonstrated by the clear roll-over below 1.5 m. Like [44][45][46], we considered a ÒboulderÓ (we underline that this terminology is not meant to imply any structural similarity to the boulders normally seen on Earth, but when we identified a feature with the mentioned characteristics, we inferred that it was a boulder) to be a positive relief detectable in various images obtained with different observation geometries, with a constant elongated shadow (if the phase angle is greater than 0¡). Furthermore, the boulder needs to appear detached from the ground on which it stands.…”

Multidisciplinary analysis of the Hapi region located on Comet 67P/Churyumov–Gerasimenko

“…In order to obtain the cumulative boulder size-frequency distribution per square kilometre we used the corresponding area computed from the 3D shape model of Hapi. Subsequently, in all log-log plots we fitted a regression line to the data to derive the power-law index of each size distribution, while the error bars for each value indicate the root of the cumulative number of counting boulders, following Michikami et al (2008) and Pajola et al (2016a).…”