Space missions1,2 and thermal infrared observations3 have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact4,5. Laboratory experiments6 and impact models4, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids4. Here we report that thermal fatigue7,8,9, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids10.
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This work was supported by the French Agence National de la Recherche (ANR) SHOCKS, the BQR of the Observatoire de la Côte d’Azur (OCA), the University of Nice-Sophia Antipolis, the Laboratory GeoAzur and the French National Program of Planetology (PNP). We benefited from discussions with K. J. Walsh and W. F. Bottke. A. Morbidelli helped with the dynamical model. M. Moumni, J.-M. Hiver and G. Thomas helped with the experiments, X-ray tomography and early data analysis. G.L. conducted part of his work as an INSU-CNRS delegate. S.M. acknowledges support from NASA SSERVI. The comments of S. Byrne improved this work fundamentally. Computations and data analysis were done on the CRIMSON cluster at OCA.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Protocol of the thermal fatigue laboratory experiments.
We place samples of Murchison and Sahara 97210 ∼1 cm in size in a climatic chamber where the air temperature is forced to follow cycles between 250 and 440 K with a period of 2.2 h. The air is anhydrous and at a pressure of 1 bar. Meteorites are analysed by X-ray computed tomography before the temperature cycles begin (t0), after 76 cycles (t1) and after 407 cycles (t2). From the scans, we measure (Methods) the increases in the volume and length of cracks as functions of the number of temperature cycles.
Extended Data Figure 2 Diurnal surface temperature excursions on asteroids as a function of the distance to the Sun.
Temperatures are calculated at the equator of a spherical asteroid by means of an asteroid thermophysical model15 (Methods), assuming the thermal properties (Extended Data Table 1) of a carbonaceous chondrite (the CM2 Cold-Bokkevel) and that of an ordinary chondrite (the H5 Cronstad). The asteroid rotation period is set to 6 h. The bolometric albedo is assumed to equal 0.02 for the carbonaceous chondrite and 0.1 for the ordinary chondrite.
Extended Data Figure 3 Volume growth of individual cracks as a function of the number of temperature cycles.
a, Murchison; b, Sahara 97210. Statistical errors are 1σ and are in general smaller than the plot symbols. The crack volume is measured by the procedure described in Methods. Cracks are labelled with the value of the initial slice of the corresponding volume of interest. If different volumes of interest are defined with the same initial tomographic slice, the last letter of the crack label is used to identify the crack (for example Z387a, Z378b, Z378c).
Extended Data Figure 4 Length growth of individual cracks as a function of the number of temperature cycles.
a, Murchison; b, Sahara 97210. Statistical errors are 1σ. The crack length is measured by the procedure described in Methods. See Extended Data Fig. 3 for crack labelling.
Extended Data Figure 5 No crack growth with no temperature cycles.
Volume growth of individual cracks in a specimen of Murchison that was transported from the climatic chamber to the computed tomography scanner without temperature cycling. Statistical errors are 1σ. The crack volume is measured by the procedure described in Methods. We performed three computed tomography scans: scan no. 0 (the label on the x axis) was obtained on the sample of the meteorite as it was received; scan no. 1 was carried out after the meteorite was transported from the computed tomography scanner to the climatic chamber and back; and scan no. 2 was obtained after a second transportation of the meteorite.
Extended Data Figure 6 Schematics of our micromechanical model.
a, Flow chart; b, schematic of the two-scale representation (Methods). ∂V is the surface of a body of volume V. A microscopic spherical inclusion, centred at the macroscopic point x is embedded in an infinite, effectively homogenized matrix. A general microscopic material point is located at a distance y measured from the centre of its nearest spherical inclusion located at x. The spherical inclusions of radius rc are located at the vertices of a cubic lattice with lattice parameter .
Extended Data Figure 7 Comparison of theoretical and measured crack growth.
Crack size is shown as a function of the number of temperature cycles predicted by our model and compared with experimental data for two particular cracks. Main plot: pre-existing cracks of 0.76 mm for Murchison and 0.41 mm for Sahara 97210 originating at the surface of the samples progressively propagate through the respective meteorites. The crosses indicate that the crack has reached a length equal to the meteorite diameter; therefore, full fragmentation occurs. Inset: comparison of the model with the length growth measured in our experiments for the same two cracks.
Extended Data Figure 8 Schematic of the surface layer of rocks modelled in this work.
The schematic is inspired by Fig. 3 of ref. 7. The layer is composed of rocks with different sizes. The rocks are subjected to spatial and temporal temperature gradients at the surface of the asteroid due to changes in the diurnal solar heating. The bottom of the rock layer is assumed to be in contact with bedrock that is deep enough to have a constant temperature. Temperatures are calculated assuming a uniform medium (no rock boundaries). This is probably a conservative approximation, because the presence of voids between rocks may enhance the temperature gradients owing to a reduction in the thermal conductivity. When the model starts, all rocks have a surface crack of the same length (30 μm), represented by the thin vertical line. It is very likely that cracks of this size, similar to the grain size of meteorites, are present in asteroidal material. Crack growth is from top to bottom. Crack growth is modelled until the time (survival time) the crack reaches the diameter of the rock (through-crack), and the rock is broken into two pieces. Rock survival times are shown in Fig. 1.
Extended Data Figure 9 Sensitivity of the model results to variations in Paris’s law parameters.
The stars indicate model results for the nominal values of the parameters given in Extended Data Table 1. a, Number of temperature cycles until fragmentation for a rock 1 cm in diameter as a function of the Paris’s law exponent, n. b, Discrepancy between the model and the experimental crack growth, defined by equation (4) in Methods, as a function of the Paris’s law exponent. The model–experiment discrepancy is of the order of 20% for the nominal values of the C and n. A 100% discrepancy would be clearly visible in Extended Data Fig. 7. c, Same as a, but here the number of cycles until rock fragmentation is plotted as a function of the Paris’s law factor C. d, Same as b, but here the discrepancy between the model and the experimental crack growth is plotted as a function of the Paris’s law factor C.
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Delbo, M., Libourel, G., Wilkerson, J. et al. Thermal fatigue as the origin of regolith on small asteroids. Nature 508, 233–236 (2014). https://doi.org/10.1038/nature13153
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