Birch, S.

Cambianica, P.

Hayes, A.

Oklay, N.

Vincent, Jean-Baptiste

Zacny, K.

Institute of Transport Research:Publications

Bouncing boulders on comet 67PJean-Baptiste Vincent (1), Samuel Birch (2), Alex Hayes (2), Kris Zacny (3), Nilda Oklay (1), Pamela Cambianica (4)(1) DLR Institute for Planetary Research, Berlin, Germany (jean-baptiste.vincent@dlr.de), (2) Cornell Center for Astrophysicsand Planetary Science, Cornell University, Ithaca, NY, USA, (3) Honeybee Robotics Ltd., (4) Center of Studies and Activitiesfor Space (CISAS) "G. Colombo", University of Padova, Padova, ItalyBoulders are ubiquitous on comet 67P [1]. Mostare found in taluses and probably created by rockfallevents from nearby cliffs; isolated boulders are some-times detected in smooth terrains, partially embeddedin airfall deposits.While escorting the comet for over two years aroundperihelion, ESA’s Rosetta observed many changes onthe surface in relation to these boulders [2], for in-stance: burial (partial or total); uncovering (partial ortotal); displacement over a short distance (slide, roll);large distance transport (jump, fall); fragmentation.These changes provide natural experiments fromwhich we can derive the mechanical properties of thecometary material associated to boulders and their sur-rounding terrains, with only few free parameters. Forinstance, we can use reliable estimations of mass andgravity from other studies ([3] + [4]) to derive the im-pact speed and calculate the compressive strength of afalling block depending on its survival, potential frag-mentation, size of impact print in the regolith...Such approach has been used with Rosetta’s lander.Biele et al (2015) derived the mechanical properties ofthe soil in the Agilkia region, where Philae toucheddown and bounced on 12 November 2014 [5]. Frommeasures of the velocity change and the size of foot-prints on the surface, they calculated that Agilkia’s sur-face is rather soft: a thick layer (20 cm) of dust withlow compressive strength (1 kPa).Studying the displacement of boulders across sev-eral regions of 67P allows us to repeat this experimentwith different terrains and test a wider range of physi-cal parameters. We can for example derive shear stressfrom rock slides, or regolith cohesion from craters cre-ated at various impact speeds. This is particularlyrelevant for future cometary missions like CAESAR(NASA New Frontiers proposal) which aim to acquireand return a cometary sample, and therefore need ac-curate determination of mechanical properties of thesurface in potential regions of interest.We will show at this conference the first results ofour analysis of several surface changes related to boul-ders, combining observations and numerical model-ing. We focus particularly on the case of bouncingobjects, such as the one presented in Figure 1. Here,this object of about 10 m diameter appears to havefallen from a nearby cliff and bounced several timeson the regolith without breaking. We use this observa-tion, and other similar events, to derive the cohesionand compressive strength of both consolidated (boul-der) and loose material (regolith) in several regions ofthe nucleus.Figure 1: Example of a 10 m boulder which bouncedseveral times on 67P’s regolith, observed by theOSIRIS narrow angle camera on Rosetta.References[1] M. Pajola et al, Size-frequency distribution of boulders≥7 m on comet 67P, A&A (2015)[2] J.-B. Vincent et al, Meter scale changes on comet 67P,EPSC (2018)[3] M. Pätzold et al, A homogeneous nucleus for comet 67Pfrom its gravity field, Nature (2016)[4] F. Preusker et al, The global meter-level shape model ofcomet 67P, A&A (2017)[5] J. Biele et al, The landing(s) of Philae and infer-ences about comet surface mechanical properties, Sci-ence (2017)AcknowledgementsOSIRIS was built by a consortium led by theMaxPlanck-Institut für Sonnensystemforschung, Göt-tingen, Germany, in collaboration with CISAS,University of Padova, Italy, the Laboratoired’Astrophysique de Marseille, France, the Insti-tuto de Astrofìsica de Andalucia, CSIC, Granada,Spain, the Scientific Support Office of the EuropeanSpace Agency, Noordwijk, The Netherlands, theInstituto Nacional de Técnica Aeroespacial, Madrid,Spain, the Universidad Politéchnica de Madrid, Spain,the Department of Physics and Astronomy of UppsalaUniversity, Sweden, and the Institut für Datentechnikund Kommunikationsnetze der Technischen Univer-sität Braunschweig, Germany. The support of thenational funding agencies of Germany (DLR), France(CNES), Italy (ASI), Spain (MEC), Sweden (SNSB)and the ESA Technical Directorate is gratefullyacknowledged.

Bouncing boulders on comet 67P

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Bouncing boulders on comet 67P

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