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

Maurizio Pajola (1) , Alice Lucchetti (2) , Marco Fulle (3) , Stefano Mottola (4) , Maximilian Hamm (4) , Vania da Deppo (5) , Luca Penasa (6, 7) , Gabor Kovacs (8) , Matteo Massironi (7) , Xian Shi (8) , Cécilia Tubiana (8) , Carsten Güttler (8) , Nilda Oklay (4) , Jean-Baptiste Vincent (4) , Imre Toth (9, 10) , Björn Davidsson (11) , Giampiero Naletto (12, 6, 5) , Holger Sierks (8) , Cesare Barbieri (6, 13) , Philippe L. Lamy (9) , Rafael Rodrigo (14, 15) , Detlef Koschny (16) , Hans Rickman (17, 18) , Horst Uwe Keller (5, 19) , Jessica Agarwal (8) , Michael F. A'Hearn (20) , Maria Antonella Barucci (21) , Jean-Loup Bertaux (22) , Ivano Bertini (6) , Gabriele Cremonese (2) , Stefano Debei (23) , Mariolino de Cecco (24) , Jakob Deller (8) , Mohamed Ramy El Maarry (25, 26) , Sonia Fornasier (21) , Elisa Frattin (2, 13) , Adeline Gicquel (8, 11) , Olivier Groussin (9) , Pedro J. Gutiérrez (27) , Sebastien Höfner (8) , Marc Hofmann (8) , Stubbe F. Hviid (4) , Wing-Huen Ip (28, 29) , Laurent Jorda (9) , Jörg Knollenberg (4) , Jörg-Rainer Kramm (8) , Ekkehard Kührt (4) , Michael Küppers (30) , Luisa M. Lara (31) , Monica Lazzarin (13) , José J. Lopez Moreno (27) , Francesco Marzari (13) , Harald Michalik (32) , Frank Preusker (4) , Frank Scholten (4) , Nicolas Thomas (33, 26)

1 ARC - NASA Ames Research Center
2 OAPD - INAF - Osservatorio Astronomico di Padova
3 OAT - INAF - Osservatorio Astronomico di Trieste
4 DLR Institut für Planetenforschung
5 IFN - CNR Institute for Photonics and Nanotechnologies
6 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
7 Dipartimento di Geoscienze [Padova]
8 MPS - Max-Planck-Institut für Sonnensystemforschung
9 LAM - Laboratoire d'Astrophysique de Marseille
10 Konkoly Observatory
11 JPL - Jet Propulsion Laboratory
12 DEI - Department of Information Engineering [Padova]
13 Dipartimento di Fisica e Astronomia "Galileo Galilei"
14 CAB - Centro de Astrobiologia [Madrid]
15 ISSI - International Space Science Institute [Bern]
16 ESTEC - European Space Research and Technology Centre
17 Department of Physics and Astronomy [Uppsala]
18 CBK - Space Research Centre of Polish Academy of Sciences
19 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
20 Department of Astronomy [College Park]
21 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
22 PLANETO - LATMOS
23 Department of Industrial Engineering [Padova]
24 University of Trento [Trento]
25 LASP - Laboratory for Atmospheric and Space Physics [Boulder]
26 Physikalisches Institut [Bern]
27 IAA - Instituto de Astrofísica de Andalucía
28 SSI - Space Science Institute [Macau]
29 Institute of Space Science [Taiwan]
30 Operations Department (ESAC)
31 UPM - Universidad Politécnica de Madrid
32 Institut für Datentechnik und Kommunikationsnetze
33 CSH - Center for Space and Habitability

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.

Domains: Earth and Planetary Astrophysics [astro-ph.EP]; Solar and Stellar Astrophysics [astro-ph.SR]

Maurizio Pajola, Alice Lucchetti, Marco Fulle, Stefano Mottola, Maximilian Hamm, et al.. The pebbles/boulders size distributions on Sais: Rosetta’s final landing site on comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, 2017, 471 (1), pp.680 - 689. ⟨10.1093/mnras/stx1620⟩. ⟨insu-01574968⟩