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Alignment of fractures on Bennu’s boulders indicative of rapid asteroid surface evolution

Abstract

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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Fig. 1: Examples of different fracture morphologies and fracture mapping.
Fig. 2: Windrose histograms of the azimuthal direction of fractures and fracture segments.
Fig. 3: Cumulative distribution of the length of fractures and fracture segments.

Data availability

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.

Code availability

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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Acknowledgements

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.

Author information

Authors and Affiliations

Authors

Contributions

M.D. led the project, the interpretation of the results and the development of the manuscript. M.D., M.P. and M.M.A.A. mapped fractures; K.J.W. compared the independent fracture mapping. C.M. contributed to the interpretation of fractures. J.W. and M.D. developed the thermal-fatigue fracture-mechanics model, which was discussed and improved by J.L.M. D.N.D.G., C.B. and D.R.G. provided image processing support. M.D. and K.J.W. wrote codes for the analysis. C.A., R-L.B., H.C.C., C.A.B., D.N.D.G., D.R.G., J.L.M., B.R., S.R.S. and K.J.W. provided support in the interpretation of spacecraft imagery. C.A. and R-L.B. helped with comparison against impact-driven fractures. K.J.W. and B.R. contributed to the data interpretation during the OSIRIS-REx Regolith Development Working Group (RDWG) and Image Processing Working Group (IPWG) meetings and to design of the observations and data acquisition. K.J.W. and D.N.D.G. led the RDWG and the IPWG, respectively. D.S.L. made this study possible as the principal investigator of the OSIRIS-REx mission and contributed to the discussion of the results. M.D., C.M. and K.J.W. wrote the manuscript with contribution of text, figures, equations and discussions from all co-authors.

Corresponding author

Correspondence to Marco Delbo.

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The authors declare no competing interests.

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Primary handling editors: Tamara Goldin and James Super, in collaboration with the Nature Geoscience team. Nature Geoscience thanks Daniel Durda, Jean-Baptiste Vincent and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Examples of different fracture morphologies.

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. 4 Histogram of the V-band normal reflectance of boulders and boulders with fractures.

The boulders’ V-band normal reflectance is from ref. 7. The similarity of the distributions indicates that fracturing occurs in both of Bennu’s two main boulder populations (low-reflectance and high-reflectance)7.

Source data

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.

Source data

Extended Data Fig. 6 Windrose histograms of the azimuthal direction of fractures and fracture segments normalised to the number of boulders.

As for Fig. 2 with the difference that here all diagrams are normalised to the number of boulders catalogued7 in the corresponding latitude band.

Source data

Extended Data Fig. 7 A benched impact crater on boulder.

(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.

Source data

Extended Data Fig. 8 Model of a boulder with fracture.

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.

Extended Data Fig. 9 Modelling of the evolution of a length distribution of fractures.

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.

Source data

Extended Data Table 1 Estimation of the model uncertainties. Here we report a lower and an upper bound for each model parameter and the corresponding value of the time (\({{{\rm{U}}}}-{{{{\rm{B}}}}}_{{{{\rm{f}}}}}^{{{{\rm{t}}}}}\)) to create a ax = Lx fracture

Source data

Source Data Fig. 2

Azimuth of fractures and fracture segments.

Source Data Fig. 3

Sorted lengths of fractures and fracture segments.

Source Data Extended Data Fig. 4

V-band normal reflectance of boulders and boulders with fractures.

Source Data Extended Data Fig. 5

Number of fractures per histogram bin of the independent fracture mapping.

Source Data Extended Data Fig. 6

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.

Source Data Extended Data Fig. 7

Height of the crater as a function of distance from its centre.

Source Data Extended Data Fig. 9

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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