The impactor-to-crater size scaling relationships that enable estimates of planetary surface ages rely on an accurate formulation of impactor–target physics. An armouring regime, specific to rubble-pile surfaces, has been proposed to occur when an impactor is comparable in diameter to a target surface particle (for example, a boulder). Armouring is proposed to reduce crater diameter, or prevent crater formation in the asteroid surface, at small crater diameters. Here, using measurements of 1,560 craters on the rubble-pile asteroid (101955) Bennu, we show that the boulder population controls a transition from crater formation to armouring at crater diameters ~2–3 m, below which crater formation in the bulk surface is increasingly rare. By combining estimates of impactor flux with the armouring scaling relationship, we find that Bennu’s crater retention age (surface age derived from crater abundance) spans from 1.6–2.2 Myr for craters less than a few meters to ~10–65 Myr for craters >100 m in diameter, reducing the maximum surface age by a factor of >15 relative to previous estimates. The range of crater retention ages, together with latitudinal variations in large-crater spatial density, indicate that ongoing resurfacing processes render the surface many times younger than the bulk asteroid.
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Inferring interiors and structural history of top-shaped asteroids from external properties of asteroid (101955) Bennu
Nature Communications Open Access 06 August 2022
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OCAMS images from the Approach, Orbital A, and Detailed Survey mission phases64, and OLA data from the Orbital B mission phase65, are available via the Planetary Data System (https://sbn.psi.edu/pds/resource/orex/). The crater measurements (diameter, latitude and longitude of centres) will be available with this publication via FigShare.
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This material is based upon work supported by NASA under contracts NNG12FD66C and NNM10AA11C issued through the New Frontiers Program. The OLA build and Canadian science support were provided by a contract with the Canadian Space Agency. We are grateful to the entire OSIRIS-REx team for making the encounter with Bennu possible. The efforts of CosmoQuest’s Bennu mappers contributed to the measurement of Bennu’s crater population. A complete list of their names is available at CosmoQuest.org/Bennu/credits. S.R.S. acknowledges support from NASA grant no. 80NSSC18K0226 as part of the NASA OSIRIS-REx Participating Scientist Program. P.M. acknowledges funding from the French space agency CNES, from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 870377 (project NEO-MAPP) and from the Academies of Excellence of the IDEX JEDI of Université Côte d’Azur. M.P. was supported for this research by the Italian Space Agency (ASI) under ASI-INAF agreement no. 2017-37-H.0.
The authors declare no competing interests.
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A 141-m-diameter crater on Bennu, centered at 55.7°N and 62.7°E. White arrows in both frames indicate the crater location. a, PolyCam image 20181202t050321s325_pol_iofl2pan projected onto the shape model. b, The shape model only, colorized by facet radius. The images collected during the spacecraft’s final approach (acquired in early December of 2018), in conjunction with the shape model, were used to identify and measure large craters on Bennu.
a, The global SFD of Bennu in differential format. The black data points are all craters, normalized by Bennu’s global surface area. The green points are a population that includes all craters within ±60° latitude, and craters > 5 m diameter for higher latitudes. The difference is a small vertical shift in the differential values for craters < 5 m, the nature of the fishhook shape is unchanged. b, The differential crater populations for three distinct slope/latitude regions (Scheeres et al. 2019) on Bennu: the gray data are for craters < -20°S, the black data are for craters >+20°N, and the blue data are for craters within ±20° latitude. The fishhook shape is present in all three.
Extended Data Fig. 3 A flow chart of our process for generating simulated crater populations for a given modeled age, and definition of the diameter bins used to calculate fits between the observed and modeled crater populations.
a, A flow chart of our process for generating simulated crater populations for a given modeled age: (i) generate an impactor by sampling a diameter-based power-law SFD for either an MBA or NEA flux model; (ii) assign that impactor an impact speed based on the flux model; (iii) generate a boulder by sampling an area-based power-law SFD; (iv) apply TS2018 scaling for that combination of impactor and boulder; (v) repeat steps (i)–(iv) for N impactors predicted for the model age; (vi) repeat the generation of N impactors 100 times to sample the variability at a given age. b,c, Definition of the diameter bins used to calculate fits between the observed and modeled crater populations. The plot (b) labels small diameter bins with a number and large diameter bins with a letter. The table (c) lists the diameter bins used to define the bin sets used to compare observations with the model. For example, the small-crater bin set 5 uses diameter bins 2-8, and the large-crater bin set 4 uses diameter bins A-D. The labels a0 and a1 refer to the largest bin diameters that can occur in the older ages within the simulation data, which are larger than any of the observed craters on Bennu.
Extended Data Fig. 4 Comparison between observed and modeled crater populations for Bennu’s craters and an NEA flux.
a, residuals for all combinations of bin sets and modeled ages for small craters. The residuals are in the same form as the differential SFD, which is N/(dD A), where N is the number of craters in diameter range dD, and A is the surface area used for normalization, so are in units of #/(km km2). There is a consistent minimum across all bin sets. b, the specific residuals for bin set 1, which consists of diameter bin 4 (Extended Data Fig. 4). The black point is the median residual for the 100 runs, the purple line spans the 25-75% range, the light-green line spans the 5-95% range, and the gray line spans the 1-99% range. c, d, e, and f are the same as b for bin sets 2, 5, 8, and 10, respectively. We use the 1-99% minimum residuals across the modeled ages to bound the possible ranges for the small-crater retention age. Bin set 1 establishes the maximum age at 2.2 Myr; bin set 2 establishes the minimum age at 1.6 Myr. g, residuals for all combinations of bin sets and NEA modeled ages for large craters. h, the specific residuals for bin set 2, which consists of diameter bins a0, a1, A and B (Extended Data Fig. 4). i, j, and k are the same as h for bin sets 3, 4, and 5, respectively. We use the 1-99% minimum residuals across the modeled ages to bound the possible ranges for the small-crater retention age.
Extended Data Fig. 5 Crater diameter as a function of impactor diameter for a 30 cm diameter target boulder and 18.5 km/s impact speed.
Unlike strength and gravity scaling, which would be linear in this log-log plot, the TS2018 scaling transitions from an armoring regime at small sizes to a gravity regime at large sizes. There are two curves in the plot, one for the two gravitational end members present on Bennu (pole, light green, and equator, black), the resulting crater diameter is the same until ~200 m diameter. There’s only one candidate impact feature on Bennu larger than this size, and because we calculate our ages against the binned differential data, the small difference in size for this one crater would not change its host bin. Thus, the variation in surface acceleration on Bennu does not change the outcome of our simulations.
Extended Data Fig. 6 Comparison between observed and modeled crater populations for the large craters and an MBA flux.
a, residuals for all combinations of bin sets and modeled ages. The residuals are in the same form as the differential SFD, as described in Extended Data Fig. 5. b, the specific residuals for bin set 2, which consists of diameter bins a0, a1, A and B (Extended Data Fig. 4). Colors are as in Extended Data Fig. 5. c, d, and e are the same as b for bin sets 3, 4, and 5, respectively. We use the 1-99% minimum residuals across the modeled ages to bound the possible ranges for the small-crater retention age. The broad minimum in bin set 2 establishes the maximum age at 65 Myr; the two smallest residuals at 45 Myr and 60 Myr correspond to two cases, one for each of those ages, when there were no modeled craters larger than the observed craters, and the number of modeled craters in bin A match the observations. Bin set 5 establishes the minimum age at 10 Myr. f and g are the differential versions of the observed crater SFD and TS2018 model fits (see also Fig.1 and Fig. 4) for the 10 Myr and 65 Myr (minimum and maximum ages, respectively) with the 99% minimum residuals between observations and model results. The black data are the measured differential crater SFD of Bennu, the purple data are the median results of 100 runs, and the gray band is the 99% range of the 100 runs. Across this age span the range of the 100 modeled outcomes encompasses the variability in density seen in the three largest diameter bins.
A comparison between the fractional area covered by craters ≥ 20 m diameter (black line, left-hand axis) and the median surface slope (green line, right-hand axis) as a function of latitude. Unlike the small craters, which are not correlated with slope and latitude (Extended Data Figure 2), the abundance of larger craters is correlated with latitude and slope.
Extended Data Fig. 8 Modeled craters for a 2.6 Myr NEA age, and comparison with Ryugu and Itokawa craters.
a, This plot layers additional data sets over the style of presentation in Fig. 4. As in that figure, the black data are the measured differential crater SFD of Bennu, the purple data are the median results of 100 runs for a 2.6 Myr NEA flux model, the gray band represents the 99% range of the simulation results. Here we also include the results of one run, showing a specific case of the TS2018 scaling (green data), 100 Pa strength scaling (blue data), and gravity scaling (orange data); these data illustrate the different resulting crater populations for the three scaling laws given the same impactors. Unlike TS2018 scaling, strength- and gravity scaling continue to produce smaller craters given smaller impactors. However, above the fishhook diameter range, the strength data fall in the range of the TS2018 results, indicating that cratering strengths ≤100 Pa give comparable results to TS2018 for diameters larger than the armoring regime. TS2018 scaling approaches gravity scaling at the largest diameters on Bennu. Given that Bennu has been dynamically decoupled from the main belt for 2.6 Myr, TS2018 indicates that all craters 5 m diameter could have been formed by NEA impactors. b, c, and d are a comparison between Bennu’s crater population (black) with Ryugu (gray) and Itokawa (brown) in cumulative, differential, and R-format, respectively.
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Bierhaus, E.B., Trang, D., Daly, R.T. et al. Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface. Nat. Geosci. 15, 440–446 (2022). https://doi.org/10.1038/s41561-022-00914-5
Nature Geoscience (2022)
Nature Geoscience (2022)
Inferring interiors and structural history of top-shaped asteroids from external properties of asteroid (101955) Bennu
Nature Communications (2022)