Geophysical Research Letters
Supporting Information for
Fractures on comet 67P/Churyumov-Gerasimenko observed by Rosetta/OSIRIS
M.R. El-Maarry1, N. Thomas1, A. Gracia-Berná1, R. Marschall1, A-T. Auger2, O. Groussin2, S.
Mottola3, M. Pajola4, M. Massironi4,5, S. Marchi6, S. Höfner7, F. Preusker3, F. Scholten3, L.,
Jorda2, E. Kührt3, H. U. Keller8,3, H. Sierks7, M. F. A'Hearn9, C. Barbieri10, M. A. Barucci11, J-L.
Bertaux12, I. Bertini4, G. Cremonese13, V. Da Deppo14, B. Davidsson15, S. Debei16, M. De
Cecco17, J. Deller7, C. Güttler7, S. Fornasier11, M. Fulle18, P. J. Gutierrez19, M. Hofmann7, S. F.
Hviid3, W-H. Ip20, J. Knollenberg3, D. Koschny21, G. Kovacs7, J.-R. Kramm7, M. Küppers22, P. L.
Lamy2, L. M. Lara19, M. Lazzarin10, J. J. Lopez Moreno19, F. Marzari10, H. Michalik23, G.
Naletto24,4,14, N. Oklay7, A. Pommerol1, H. Rickman15,25, R. Rodrigo26,
27
, C. Tubiana7, J-B.
Vincent7.
1Physikalisches
2Aix
Institut, Sidlerstr. 5, University of Bern, CH-3012 Bern, Switzerland
Marseille Université, CNRS, Laboratoire d'Astrophysique de Marseille, 13388 Marseille, France.
3Deutsches
4Centro
Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, 12489 Berlin, Germany
di Ateneo di Studi ed Attivitá Spaziali, "Giuseppe Colombo" (CISAS), University of Padova, 35131, Padova,
Italy.
5Dipartimento
6Solar
di Geoscienze, University of Padova, via G. Gradenigo 6, 35131 Padova , Italy.
System Exploration Research, Virtual Institute, Southwest Research Institute, cBoulder, Colorado 80302, USA.
7Max-Planck-Institut
8Institut
für Sonnensystemforschung, 37077 Göttingen, Germany.
für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, 38106
Braunschweig, Germany.
9Department
of Astronomy, University of Maryland, College Park, MD, 20742-2421, USA.
10Department
11LESIA,
Obs. de Paris, CNRS, Universite Pierre et Marie Curie, Univ. Paris-Diderot, 92195, Meudon, France.
12LATMOS,
13INAF
of Physics and Astronomy, University of Padova, 35122 Padova, Italy.
CNRS/UVSQ/IPSL, 78280, Guyancourt, France.
- Osservatorio Astronomico, vicolo dell’Osservatorio 5, 35122 Padova, Italy.
1
14CNR-IFN
UOS Padova LUXOR, 35131 Padova , Italy.
15Department
of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden.
16Department
of Industrial Engineering, University of Padova, 35131 Padova, Italy.
17Department
of Industrial Engineering, University of Trento, 38123 Trento, Italy.
18INAF
- Osservatorio Astronomico di Trieste, 34014 Trieste, Italy.
19Instituto
de Astrofísica de Andalucía (CSIC), 18008 Granada, Spain.
20Graduate
Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan.
21Scientific
Support Office, European Space Agency, 2201, Noordwijk, The Netherlands.
22Operations
23Institut
Department, European Space Astronomy Centre/ESA, 28691 Villanueva de la Cañada (Madrid), Spain.
für Datentechnik und Kommunikationsnetze der TU Braunschweig, 38106 Braunschweig, Germany.
24Department
25PAS
of Information Engineering, University of Padova, 35131, Padova, Italy.
Space Research Center, Bartycka 18A, PL-00716 Warszawa, Poland.
26International
27Centro
Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland.
de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain.
Contents of this file
Text S1 to S2
Figures S1 to S2
Introduction
The supporting information section includes three main contents:
1) A supplementary text to section 2 (methods and datasets) explaining how the spatial
resolution for the images displaying fractures was estimated using derived shape
models and 3D visualization tools.
2) A supplementary text to section 4 (potential formation mechanisms) that includes
additional information regarding thermal insolation weathering and the effects of
thermal gradients versus diurnal temperature range.
3) Two supporting figures for the fractures in the Hathor region (Fig. S1), and a regional
context view of the polygonal fractures in the Apis region.
2
Text S1.
Generally, OSIRIS images have a nominal spatial resolution calculated from the distance
between the OSIRIS camera (i.e., the Rosetta spacecraft) and the gravitational center of the
comet. In most cases, accounting for an average radius of 2 km [see Sierks et al., 2015] for the
comet’s irregular shape is enough to yield a first-order approximation of the spatial resolution
at the surface. However, for this study, an accurate estimate of the image resolution is needed
at the exact location of the fractures in order to assess properly their size-scale. Therefore, we
extracted the specific spatial resolution for our features of interest making use of the most
recent shape models for the comet with a typical vertical resolution of meter to sub-meter scale
using stereophotogrammetry [Preusker et al. 2012, and references therein] and
stereophotoclinometry [Gaskell et al. 2008] techniques. Using the shape model together with
orbit information from the SPICE kernels [Acton, 1996] and a 3D visualization engine
[Goldstone, 2011], OSIRIS's position, orientation and frustum are calculated, resulting in a
simulated image of the real one. Next, rays are cast from the camera position, passing through
each pixel in the virtual plane of the frustum to the shape model in order to calculate their
intersection points. Finally, the vector distance from OSIRIS's position and a particular
intersection point (i.e. the feature of interest in the image) yields an accurate spatial resolution.
3
Text S2.
Thermal insolation weathering comprises two main processes: thermal shock and thermal
fatigue. In this study we follow Hall and Thorn’s definitions of shock and fatigue where thermal
shock is defined as “a single stress event whereby sudden (large) changes in temperature
produce fractures because of the resulting stresses exceeding the capacity of the rock to adjust
other than through instantaneous failure”, whereas thermal fatigue is defined as being
produced by “temperatures that lead to repeated stresses (often far) below the normally
determined strength of the material involved” [Hall and Thorn, 2014].
Many studies suggest a “canonical” thermal gradient of 2 K min-1 as being required for
thermal shocking of materials (i.e., creating thermal-induced strain through the formation of
micro-cracks) [e.g., Hall 1999; Hall and André, 2001]. However, this estimate has been
challenged recently by a number of studies [e.g., Boelhouwers and Jonsson, 2013], which
suggest that this value should simply vary depending on the investigated material.
Nonetheless, our estimated high gradients of 15 K min-1 suggest that cometary surface
materials may indeed be thermally-shocked.
On the other hand, a number of studies suggest that the diurnal temperature range is a more
significant factor in thermo-mechanical weathering, which can operate at thermal gradients
even less than the 2 K min-1 as long as thermal cycling occurs thereby highlighting the larger
role of thermal fatigue in thermo-mechanical weathering [e.g., Viles et al., 2010; Molaro and
Byrne, 2012; Molaro et al., 2015]. Following such studies, we consider the diurnal temperature
range to be a stronger indicator of thermal fatigue than thermal gradients. However, we
consider the development of micro-cracks (probably through thermal shock) to be a
prerequisite for the onset of thermal fatigue. In fact, we note that our approach appear to be in
agreement with results from the Viles et al (2010)’s study. In their study, a certain group of
tested samples “group 1” (baseline olivine-bearing basalt, no salt, no pre-stressing) shows no
cracks or reduced strength after thermal cycling because it was not exposed to conditions of
thermal shock (the pre-stressing procedure of heating to 300° C and quenching), which would
have initiated the micro-fractures that could later grow through thermal fatigue. As such, we
consider both thermal gradients and diurnal temperature ranges as requirements for thermal
insolation weathering.
4
Figure S1. (Top) NAC image showing the complete mapping of strata and fractures on the cliff
of Hathor. (Below) Corresponding histogram. Image ID: NAC_2014-0807T20.37.34.564Z_ID30_1397549300_F22
5
Figure S2. [a] NAC cropped image showing a wider perspective of the view shown in Fig. 2a.
The box shows the location of [b]. [b] Close-up showing the smooth mantling unit that appears
to have been covering the fractured surface of the Apis region. Arrows show the location of
regions where the fractured surface appears to have been exhumed from below the smooth
mantle unit. Also visible are a group of small pits [A] in the mantle material, which is a feature
similarly observed in the dusty coatings of the Ma’at region and we interpret it to be a possible
sign of sublimation. Image
ID: NAC_2015-02-14T10.35.40.393Z_ID00_1397549000_F82.
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