The pebbles/boulders size distributions on Sais: Rosetta’s final landing site on comet 67P/Churyumov–Gerasimenko

Abstract

By using the imagery acquired by the Optical, Spectroscopic, and Infrared Remote Imaging System Wide-Angle Camera (OSIRIS WAC), we prepare a high-resolution morphological map of the Rosetta Sais final landing site, characterized by an outcropping consolidated terrain unit, a coarse boulder deposit and a fine particle deposit. Thanks to the 0.014 m resolution images, we derive the pebbles/boulders size-frequency distribution (SFD) of the area in the size range of 0.07-0.70 m. Sais' SFD is best fitted with a two-segment differential power law: the first segment is in the range 0.07-0.26 m, with an index of −1.7 ± 0.1, while the second is in the range 0.26-0.50 m, with an index of −4.2 +0.4/−0.8. The 'knee' of the SFD, located at 0.26 m, is evident both in the coarse and fine deposits. When compared to the Agilkia Rosetta Lander Imaging System images, Sais surface is almost entirely free of the ubiquitous, cm-sized debris blanket observed by Philae. None the less, a similar SFD behaviour of Agilkia, with a steeper distribution above ∼0.3 m, and a flatter trend below that, is observed. The activity evolution of 67P along its orbit provides a coherent scenario of how these deposits were formed. Indeed, different lift pressure values occurring on the two locations and at different heliocentric distances explain the presence of the cm-sized debris blanket on Agilkia observed at 3.0 au inbound. Contrarily, Sais activity after 2.1 au outbound has almost completely eroded the fine deposits fallen during perihelion, resulting in an almost dust-free surface observed at 3.8 au.

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

“…Given the large number of particles with sizes smaller than 0.7 cm, such a break in the SFD could be a real result of intrinsic surface processes that deplete the smallest sizes when they form. Nevertheless, a more likely possibility is that such rollover may be the result of an observation bias, where fewer smaller particles are identified, in a similar fashion to the pixel resolution limit when pebbles are identified on surfaces (e.g., Pajola et al, 2017).…”

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

“…(c) The absorbed flux on the five locations based on Models B and C of Keller et al (2015b). ice sublimation and dust ejection due to 'self-cleaning' (Pajola et al 2017c;Fulle et al 2019) reduces both the deposits' thickness and the amount of small-size boulders, resulting in the shallower trend that we observe with the SFD analysis.…”

“…Following the same approach used in Pajola et al (2015) and Pajola et al (2017c), and given the wide statistics of boulders that we have on Hapi, we decided to derive the boulder SFD from localized areas that are characterized by homogeneous geological settings. We then made use of the geological map presented in Fig.…”

Diurnal variation of dust and gas production in comet 67P/Churyumov-Gerasimenko at the inbound equinox as seen by OSIRIS and VIRTIS-M on board Rosetta

“…A layer of decimetre-sized dust particles (cp. Pajola et al 2017) would thereafter have a ten times higher permeability than a layer of the same thickness of centimetre-sized particles. A higher gas permeability of the dust layer would result in a reduced pressure build-up and thus reduced dust production.…”