Fast boulder fracturing by thermal fatigue detected on stony asteroids

Abstract

Spacecraft observations revealed that rocks on carbonaceous asteroids, which constitute the most numerous class by composition, can develop millimeter-to-meter-scale fractures due to thermal stresses. However, signatures of this process on the second-most populous group of asteroids, the S-complex, have been poorly constrained. Here, we report observations of boulders' fractures on Dimorphos, which is the moonlet of the S-complex asteroid (65803) Didymos, the target of NASA's Double Asteroid Redirection Test (DART) planetary defense mission. We show that the size-frequency distribution and orientation of the mapped fractures are consistent with formation through thermal fatigue. The fractures' preferential orientation supports that these have originated in situ on Dimorphos boulders and not on Didymos boulders later transferred to Dimorphos. Based on our model of the fracture propagation, we propose that thermal fatigue on rocks exposed on the surface of S-type asteroids can form shallow, horizontally propagating fractures in much shorter timescales (100 kyr) than in the direction normal to the boulder surface (order of Myrs). The presence of boulder fields affected by thermal fracturing on near-Earth asteroid surfaces may contribute to an enhancement in the ejected mass and momentum from kinetic impactors when deflecting asteroids.

Fig. 1. Dimorphos boulders’ fractures.

a Dimorphos high-resolution mosaic created with the final 10 DRACO images from 11.447 s to 0.856 s before the impact (pixel scale ranging between 34.9 cm/px and 2.6 cm/px), where the pink box shows the area covered by the image (dart_0401930049_43695_01_iof) analyzed in this work; b Close up of the image (dart_0401930049_43695_01_iof) acquired 1.818 s before the impact with a pixel scale of 5.5 cm where boulders’ fractures are visible and identifiable; c same as b with 54 mapped fractured on different boulders dimension where the modeled boulders are outlined by ellipses. The largest boulder in the scene (6.62 m across), Atabaque Saxum, displays 6 fractures on its surface (Supplementary Fig. 1).

Fig. 2. Rose diagram and cumulative distribution of fractures’ length.

a Rose diagram showing the NW-SE preferred orientation of Dimorphos boulders’ fractures; b The fractures’ length distribution is well-fitted by an exponential function of the form N = N_oe^bL, where N is the fracture cumulative number, and b is a factor that scales the exponent of the function. We derived that the exponential fit is valid for fracture lengths larger than 0.77 m. Source data are provided as a Source Data file.

Fig. 3. Atabaque’s diurnal temperature evolution profile for different thermal inertia values.

a The area impacted by DART on Dimorphos at periapsis. b Same area at apoapsis. The temperature at a given time is calculated as the average of the nodal temperatures across Atabaque Saxums’s surface.

Fig. 4. Atabaque Saxum temperature map variation.

a, c, and e show Atabaque’s morning (8 AM), afternoon (4 PM), and midnight (12 AM) temperatures at periapsis, while b, d, and f are the same but for apoapsis.

Fig. 5. Orbital variation of heliocentric distance and of Atabaque’s average diurnal temperature and ∆T.

a The heliocentric orbit period of the Didymos system is 770 days. The open circles indicate the position at which the thermophysical simulation is conducted. The illumination geometry (i.e., the Sun’s position with respect to the system) was sampled every 70 days from periapsis along the system’s heliocentric orbit using a SPICE kernel. b The average diurnal temperature and ∆T variations over the heliocentric orbit.

Fig. 6. The thermal fractures on Dimorphos’ boulders are shallow.

Time to break up t_B,z (in units of million years, Myr) as a function of rock size D. These results are obtained by scaling the results for ordinary chondrites from ref. ^,. to the rotation period (P) and orbit of the Didymos system (“Methods” section). We adopt the material properties in Table 1.

Fig. 7. The thermal fractures on Dimorphos’ boulders may have developed in less than 100,000 years.

a Cumulative distribution of the length a_x of the horizontally propagating fractures in a boulder of thermal inertia 370 J s^−1/2 m⁻² K⁻¹ and b of thermal inertia 1000 J s^−1/2 m⁻² K⁻¹. The fractures’ initial size-frequency distribution resembles that of microcracks observed in meteorites and evolved to become similar in slope to that of the host boulders and the measured fractures in 10–100 kyr. This time is likely an upper limit because we neglect microscopic stress between elements of the rocks and the effect of eclipses (“Methods” section). The black curves correspond to the average of propagation at aphelion and perihelion with a 3:1 weight. We adopt the material properties in Table 1.