The Physical Properties of Carbonaceous Asteroids
Daniel Britt
University of Central Florida
The rendezvous and sampling of asteroids 162173 Ryugu and 101955 Bennu have opened a new window on carbonaceous
asteroid physical properties. Both objects have the structure of low-density rubble piles [1], with surfaces that are extremelyrough, covered in starkly angular blocks and boulders, and depleted in fine particle sizes and dust [2]. The returned Ryugu
material is similar to CI carbonaceous chondrites [3] and Bennu is likely either CI or CM-like materials. The surface of Ryugu
exhibits very low thermal conductivity and high porosity [4]. What does this imply for the link between the physical propertiesof carbonaceous asteroids and hydrated carbonaceous meteorites?
The mineralogy these objects are dominated by phyllosilicates, primarily serpentines and saponite [3]. For Ryugu theserpentines are the magnesium-rich endmember similar to CI mineralogies. For Bennu the serpentine mineralogy is still
unknown but may be the iron-rich endmember Cronstedtite. One major characteristic of serpentines is a strong negative
excursion in the coefficient of linear thermal expansion in the range of 200°-250° K [5]. This effect is shown in Figure 1 for
five CM carbonaceous chondrites. Since the carbonaceous materials are basically conglomerates that include some very
dissimilar minerals, this negative thermal expansion excursion in serpentines may be key to understanding their surface
properties as they evolve in the inner solar system. Cobbles and boulders exposed on the surfaces of carbonaceous asteroids
will undergo extreme temperature excursions as they rotate into the Sun and then are exposed to deep space. All the minerals
in the conglomerates will expand and contract according to their thermal expansion coefficients. For most minerals theexpansion is a linear, positive function of temperature. However, for carbonaceous chondrites the serpentines and otherphyllosilicates will expand, contract, and then expand again in a course of their heating and subsequent cooling. This willproduce an extreme energetic effect on the mineral grain boundaries, essentially pushing them apart, contracting to open space,
and then pushing apart again. The overall effect will be to weaken the conglomerate and induce increasing porosity into what
appear to be coherent cobbles/boulders. This will leave the surface covered with boulders and cobbles that are extremely
friable, porous, and weak. Because the effect is limited to the penetration depth of the rotational thermal pulse, it may be that
only the surface materials are significantly weakened by this process. Cobbles buried at some modest depth and do not undergo
temperature excursions may retain significantly more coherence and strength than the surface material.
One important observation is the lack of small particle size and dusty materials in the surface regolith. With the stresses
induced by the thermal excursions, it would be expected that at least some of the boulder and cobble material would
disaggregate to form a dusty regolith. However, the surface
appears much like a “desert pavement” where active
processes extract the fine materials from the surface andarmor it with particle sizes too large to be moved by theactive processes. Active ejection processes have been
observed on Bennu with rocks as large as 10 cm being
ejected. Because of the low gravity, fine particle sizes arelikely to be entrained in the solar wind once ejected,
resulting in a depletion of fines on the surface. Below thesurface on both Ryugu and Bennu sampling activities
revealed horizons more enriched in fine materials [1].

Figure 1. Thermal Expansion Coefficient for CM carbonaceous chondrites

References[1] Sugita S. et al 2019. Science 364: 252 [2] Michikami T. et al. 2019. Icarus 331: 179. [3] Yokoyama T. et al. 2022. Science
10.1126/science.abn7850 [4] Gott M. et al. 2019 Nature Astronomy 3(11): 971. [5] Opeil C. et al. 2020. MAPS
10.1111/maps.13556. [6] Lauretta D. et al. 2019 Science 366:6470.