Evidence of Permafrost Processes in C-asteroid Regoliths
M. Zolensky1, T. Nakamura2, T. Mikouchi3, H. Yurimoto4, T. Noguchi5, R. Okazaki6, H. Yabuta7, H. Naraoka6, K. Sakamoto8,
S. Tachibana3, S. Watanabe9, and Y. Tsuda81
ARES, NASA Johnson Space Center, Houston, TX 77058, USA (michael.e.zolensky@nasa.gov); 2Tohoku University, Miyagi
980-8578, Japan; 3The University of Tokyo, Tokyo 113-0033, Japan; 4Hokkaido University, Sapporo 060-0819, Japan; 5KyotoUniversity, Kyoto 606-8502, Japan; 6Kyushu University, Fukuoka 819-0395, Japan; 7Hiroshima University, HigashiHiroshima 739-8526, Japan; 8ISAS/JAXA, Sagamihara 252-5210, Japan; 9Nagoya University, Nagoya 467-8501, Japan.
Introduction: C-complex asteroids have suffered a wide variety of physical processing, some of which are not very obvious.
Carbonaceous chondrites experienced and recorded a very wide range of chemical and physical processing in both nebular andasteroidal settings [1-7], resulting in the following textures: brecciation, flattened chondrules and foliation (CV and CM
chondrites in particular), carbonate and phyllosilicate veins (CI, CM, CV3 dark inclusions), local alignment of matrix
phyllosilicates (CR2, CI, CM), and shearing (mylonitization) around lithic fragments. Many of the extensively alteredcarbonaceous chondrites contain rounded to elliptical aggregates of phyllosilicates, carbonates, spinels (chromite andmagnetite), Fe-Ni sulfides, and embayed olivines and pyroxenes, generally ascribed to impact shock or static burial pressures[6,7]. However, it is probable that even in the wettest regions of an asteroid dry periods were experienced during the periodic
breaching of an icy surficial rind [8], which could have occurred during impacts or "volcanic" venting of gas and heat from theinterior (this assumes internal heating). Thus, there should have been multiple wet-dry cycles experienced by these regolith or
immediately subsurface materials. We have previously suggested that all the deformation features mentioned above wouldhave arisen naturally from cycles of wet-dry and, more critically, freeze-thaw environmental conditions (permafrost) inasteroid regoliths [5]. Here we detail two related textures observed in CI chondrites, including asteroid Ryugu samples(essentially CI chondrite), consistent with permafrost processes.
Permafrost Processes in Regoliths: It is well-known to soil scientists that conditions of radically alternating humidity canhave important morphologic and petrologic consequences [9]. Grains and lithic clasts can become rotated, crushed and drawn
out into linear features (shearing). Porosity (including contraction and shearing cracks) and other bulk physical properties willvary in a dramatic manner. Easily altered materials will be dissolved while more resistant materials will be pulverized andmixed into matrix. These effects would be most pronounced for the C1-2 lithologies where the swelling clay saponite can befound in abundance, including the CI chondrite meteorites and asteroid Ryugu samples. Another important process to beconsidered is periodic growth and melting of ice crystals in the regolith [10-11]. The positive molal volume change during
crystallization of water will induce oriented microfabrics to develop in the regolith, normal to the direction of ice crystal
growth. Thus, masses of platy grains (in this case phyllosilicates, especially saponite flakes) will develop a pronounced
compaction and preferred alignment. This process will recur for each freezing episode. Since the orientation of the growing ice
mass will vary for each succeeding generation of growth, the eventual result will be to impart a particular, invasive, regolith
fabric consisting of sheets of aligned clays for each generation, which will appear as anastomosing strings of phyllosilicateswith roughly aligned basal directions in polished or thin sections (Figure 1). Our study has revealed that such textures arecommon in the CI chondrites.
Ice crystal growth could explain the preferred alignment of saponite observed in some Ryugu samples (see Figure 6b of[12]). However, a more likely explanation for this particular texture is mobilization of saponite flakes during wet, water
saturated episodes, and deposition of these flakes onto the surface of underlying rock fragments under the force of gravity.
These features are termed “silt caps”, and these are a common feature of permafrost soils [11,13]. Of course, a majordifference between permafrost processes on earth as compared to asteroids is the greatly reduced gravity on the latter.
However, mass wasting is usually observed on small bodies including asteroids (including Ryugu and Bennu) and comet
nuclei [14,15], so gravity-driven processes operated on the Ryugu parent body. Growth and collapse of subsurface asteroidal
icicles would also impart cyclical changes in bulk regolith porosity, induce rotation and movement of crystals and lithic
fragments through frost heaving, and consequent shearing [11]. This process could also account, to some degree, for theflattened chondrules observed in some carbonaceous chondrites (especially CMs).
Conclusions: We therefore suggest that cyclical, indigenous environmental processes, rather than impact gardening, could be
responsible for some of the late-stage petrologic characteristics of wet carbonaceous chondrites, and Ryugu samples. We notethat this suggestion has also recently been proposed by another group [16]. Bulk petrographic features of additional Ryugu
samples and carbonaceous chondrites should be investigated more systematically in order to test this idea, especially samples
rich in clays such as saponite.
References: [1] Moskovitz et al. (2013) Icarus 224, 24; [2] Vilas (2008) Ap. J. 135, 1101–1105; [3] Lazaro et al. (2012) A&A549, L2; [4] Zolensky et al. (2022) MAPS 57, MAPS.13909; [5] Zolensky (1995) Meteoritics 30, 606-607; [6] Sneyd et al.
(1988) Meteoritics 23, 139-149; [7] Scott et al. (1992) GCA 56, 4281-4293; [8] DuFresne and Anders (1962) GCA 26, 10851114; [9] Nahon (1991) Introduction to the Petrology of Soils and Chemical Weathering, pp. 122-133; [10] Van Vliet and Fox(2018) Frost Action. In Interpretation of Micromorphological Features of Soils and Regoliths, 2nd Ed., Elsevier; [11] Zhang etal. (2021) The Influence Mechanism of Freeze-Thaw on Soil Erosion: A Review. Water 13, 1010; [12] Nakamura T. et al.,(2022) Science 377, 10.1126/science.abn8671; [13] Menzies and Ellwanger (2011) Boreas 40, 271-288; [14] Jawin et al.

(2020) JGR Planets 125, Issue 8; [15] Steckloff and Samarasinha (2018) Icarus 312, 172-180; [16] Nakamura E. et al. (2022)
Proceedings of the Japan Academy, Series B 98, 227-282.

Figure 1. Ivuna CI chondrite, in cross polars. Two views of the same area in different rotation angles showing two distinct
directions of phyllosilicate (yellow stringers) preferential orientations (arrows). Views measure 2 mm in longest dimension.