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
\nImaging System) and ROLIS (ROsetta Lander Imaging System) cameras, we derive the size¿
\nfrequency distribution (SFD) of cometary pebbles and boulders covering the size range 0.05¿
\n30.0 m on the Agilkia landing site. The global SFD measured on OSIRIS images, reflects the
\ndifferent properties of the multiple morphological units present on Agilkia, combined with
\nselection effects related to lifting, transport and redeposition. Contrarily, the different ROLIS
\nSFD derived on the smooth and rough units may be related to their different regolith thickness
\npresent on Agilkia. In the thicker, smoother layer, ROLIS mainly measures the SFD of the
\nairfall population which almost completely obliterates the signature of underlying boulders up
\nto a size of the order of 1 m. This is well matched by the power-law index derived analysing
\ncoma particles identified by the grain analyser Grain Impact Analyser and Dust Accumulator.
\nThis result confirms the important blanketing dynamism of Agilkia. The steeper SFD observed
\nin rough terrains from 0.4 to 2 m could point out intrinsic differences between northern and
\nsouthern dust size distributions, or it may suggest that the underlying boulders `peek through¿
\nthe thinner airfall layer in the rough terrain, thereby producing the observed excess in the
\ndecimetre size range. Eventually, the OSIRIS SFD performed on the Philae landing unit may
\nbe due to water sublimation from a static population of boulders, affecting smaller boulders
\nbefore the bigger ones, thus shallowing the original SFD.