On asteroids, fractures develop due to stresses driven by diurnal temperature variations at spatial scales ranging from sub-millimetres to metres. However, the timescales of such rock fracturing by thermal fatigue are poorly constrained by observations. Here we analyse images of the asteroid (101955) Bennu obtained by the Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) mission and show that metre-scale fractures on the boulders exposed at the surface have a preferential meridional orientation, consistent with cracking induced by diurnal temperature variations. Using an analytical model of fracture propagation, we suggest that fractures the length of those on Bennu’s boulders can be produced in 104–105 years. This is a comparable or shorter timescale than mass movement processes that act to expose fresh surfaces and reorient boulders and any preferential direction signature. We propose that boulder surface fracturing happens rapidly compared with the lifetime in near-Earth space of Bennu and other carbonaceous asteroids. The damage due to this space-weathering process has consequences for the material properties of these asteroids, with implications for the preservation of the primordial signature acquired during the accretional phases in the protoplanetary disk of our solar system.
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Raw and calibrated OCAMS images and OLA ancillary data are available via the Planetary Data System of NASA at sbn.psi.edu/pds/resource/orex/: refs. 49,50. The OLA v20 shape model is available via the Small Body Mapping Tool at sbmt.jhuapl.edu/. The image mosaics used in this study are available from refs. 20,41. Mosaic-FB3, mosaic-FB1, their respective .wcs files for SAOImageDS9 and the fracture files (SAOImageDS9 regions file) are available at https://doi.org/10.5281/zenodo.6373668. Source data are provided with this paper.
Fracture mapping was performed using the astronomical image display and visualization tool ‘SAOImageDS9’ software, which is available at cfa.harvard.edu/saoimageds9, and also using the XPA messaging system (http://hea-www.harvard.edu/RD/xpa/index.html). Codes to map fractures, calculate and plot their orientations and compute the fracture propagations are available at https://doi.org/10.5281/zenodo.6373668. The latter dataset also includes imaging mosaics FB1 and FB3 from their respective references.
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This material is based on work supported by NASA under contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx team for making the encounter with Bennu possible, to C. Wolner for editorial help and to the OPAL infrastructure of the Observatoire de la Côte d’Azur (CRIMSON) for providing computational resources and support. M.D. and C.A. acknowledge the French space agency CNES and support from ANR ‘ORIGINS’ (ANR-18-CE31-0014). M.P. was supported for this research by the Italian Space Agency (ASI) under the ASI-INAF agreement number 2017-37-H.0. S.R.S. was supported by grant number 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program. We thank D. Durda for useful comments that improved our manuscript.
The authors declare no competing interests.
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As Fig. 1 but without the red poly-lines resulting from fracture mapping.
Extended Data Fig. 2 Global fracture distribution on the mosaic of images obtained on 2019- 03-21 by OSIRIS-REx.
This mosaic shows the surface of Bennu between 0 and 360o of longitude and -70o and 70o of latitude from ref. 20. The red lines represent the fractures that we mapped. The blue horizontal lines at latitudes of -50o, -15o, 15o, and 50o mark the boundaries of the southern mid-latitude, equatorial, and northern mid-latitude bands for which the directions and the abundance of the fractures are reported in this study (see Figs. 2, 3 and Extended Data Fig. 6).
Extended Data Fig. 3 Verification of fracture identification by higher-spatial- resolution images and independent mapping.
Fractures (red) mapped on Mosaic-FB3 (a) and (b) on a higher-resolution (4 cm pixel−1) mosaic from the Reconnaissance-A flyover of Osprey5 (October 12, 2019; phase angle of 41o). The longer fracture on the largest boulder mapped in (a) is confirmed in (b). Small fractures (red arrows) in (b) cannot be seen in (a) and thus were not mapped. In (c) and (d) green rectangles delimit two of the seven areas of the independent fracture mapping. Yellow arrows indicate fractures mapped in both mapping. Red arrows indicate fractures only identified in the nominal mapping.
Extended Data Fig. 5 Windrose diagram of fractures’ azimuth in low- and high-boulder density regions and where a second fracture identification was performed.
In (a), black- and grey-lined rectangles show, respectively, the regions of high and low boulder density, according to Fig. 2 of ref. 15 where the corresponding windrose diagrams of fractures’ azimuth where calculated (displayed on the right side of each area). (b) Map of the seven rectangular regions within which a second co-author performed an independent fracture identification and mapping. The windrose diagram of the azimuth of the fractures mapped in all areas is overlaid on top of the background map.
(a) Image of a portion of a boulder located at 348o of longitude and -60o of latitude showing an impact crater and radial fractures (not mapped on Mosaic-FB3, because outside the -50o − 50o latitude range). Image ID is ocams20190328t203728s625 pol. (b) Depth vs. distance from crater centre profile, along the dashed line of panel (a) from point A to B, obtained from OLA scan 4252 using the method of ref. 27.
The green boulder rests on Bennu’s equator and is partially buried in regolith (grey area). Its z-axis is along Bennu’s radius. All surfaces of the boulder are free to expand and contract in response to thermo-mechanical forcing. There is no thermal contact between the regolith and the boulder, a fair assumption as L, D ≫ ls, the latter being the typical distance of propagation of diurnal temperature variations. The fracture is represented by the shaded surface, which penetrates the boulder, from its surface downward (z+) towards the centre of Bennu, and laterally along the x+ and x- directions.
The evolution as a function of time is displayed at different epochs for a set of surface fractures with an initially microscopic length distribution (curve at t = 0 kyr). The vertical slope of the t = 50 kyr and t = 100 kyr curves are very similar to the one measured for Bennu’s boulder fractures and displayed in Fig. 3. The thick-solid curve shows the size distribution of the boulders hosting the modelled fractures.
Azimuth of fractures and fracture segments.
Sorted lengths of fractures and fracture segments.
V-band normal reflectance of boulders and boulders with fractures.
Number of fractures per histogram bin of the independent fracture mapping.
Number of fractures and fracture segments per histogram bin in different latitudinal bands corrected for the number of boulders counted in the respective latitudinal band.
Height of the crater as a function of distance from its centre.
Simulated length of fractures using our model for different times and length of their hosting boulders.
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Delbo, M., Walsh, K.J., Matonti, C. et al. Alignment of fractures on Bennu’s boulders indicative of rapid asteroid surface evolution. Nat. Geosci. 15, 453–457 (2022). https://doi.org/10.1038/s41561-022-00940-3
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