Effects of Diurnal Temperature Cycling on the Surface of the Moon and Asteroids

EFFECTS OF DIURNAL TEMPERATURE CYCLING ON THE SURFACE OF THE MOON AND ASTEROIDS. M. Patzek1, O. Rüsch1,2, J. L. Molaro3, and B. Gundlach1, 1Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D48149 Münster. [email protected], 2Space Exploration Institute, Fbg de l'Hôpital 68, 2000 Neuchâtel, Schweiz. 3Planetary Science Institute, 1700 East Fort Lowell, Suite 106 Tucson, AZ, Introduction: Understanding regolith evolution on airless planetary bodies requires analysis across scales from meters to microns. Thermal fatigue, driven by diurnal temperature variations, generates complex stress fields [1,2] that, along with meteoroid impacts [e.g., 3], contribute to regolith formation. While laboratory experiments have shown thermal cycling (250-440K) can induce grain-scale cracking [2,4], recent observations of blocky surfaces and anomalous thermal behavior on small bodies [e.g., 58] highlight gaps in our understanding. To improve upon previous studies [9], we developed a new experimental approach able to explore lower temperature ranges (<200K), the role of compositional heterogeneity, and high-vacuum conditions to eliminate atmospheric effects. This study presents a laboratory experiment designed to investigate these effects for the lunar anorthosite breccia NWA 11273 and basaltic eucrite NWA 11050. In a second step, we extended this study towards (carbonaceous) chondrite materials to study the effects of diurnal temperature variations on Sand C-type asteroids at ~1 AU. Experimental Setup: To simulate airless planetary surface conditions, we conducted experiments within an evacuated cryostat cooled with liquid nitrogen minimizing surface water adsorption. We used up to ~10x10x10 mm³ meteorite cubes for the eucrite and lunar samples, which were mounted on a copper cold finger (Fig. 1). A 100W cartridge heater on the cold finger provided controlled heating up to 475K. Temperature monitoring, conducted at the cold finger's top and bottom, and within a monitor sample (El Hammami, H5; Fig. 1), confirmed successful heat conduction and accurate temperatures. Results and Discussion: The eucrite and lunar anorthosite breccia have been cycled between 175 K and 375 K in the evacuated cryostat with a ramping rate of ~2 K/min. The investigations via low vacuum SEM after 10, 20, 50, 100, and 400 total cycles revealed different types of changes on the sample surfaces: Smaller flakes that move from run to run and eventually are lost or relocated (e.g., Fig. 1b-e) and cracks forming and increasing in their length and width. While the lunar anorthosite breccia NWA 11273 shows prominent micro-flaking (<5 to ~30 µm), the eucrite NWA 11050 forms extensive cracks during thermal cycling. The enhanced micro-flaking activity on the lunar anorthosite breccia may be attributed to the higher abundance of glass and feldspar, which is a result of its geologic history, i.e., mature, reworked, and compacted regolith breccia versus basaltic rock. Implications: The obtained experimental results indicate, that the type of rock and its mineralogy is of fundamental importance for the type of fragments and soil, which can form through thermal fatigue. While solid rocks such as magmatic or metamorphic rocks react to thermal fatigue with the formation of cracks as shown by the basaltic eucrite NWA 11050, mature rocks which are highly reworked in the regolith of planetary bodies (i.e., the Moon) contribute to fine-grained, tenth-of-μm-sized soil as shown by the micro-flaking observed on the sample. References: [1] Ravaji B. et al. (2019) JGR, 124(12), 3304-3328. [2] Molaro J. L. et al. (2017) Icarus 294 247-261. [3] Hörz F. et al. (2020) Planet. Space Sci 194, 105105. [4] Molaro J. L. and Byrne S., (2012) JGR, 117, E10. [5] Gundlach B. and Blum J. (2013) Icarus, 223(1), 479-492. [6] Watanabe S. et al. (2019) Science, 364(6437), 268-272. [7] DellaGiustina et al. (2019) Nat. Astr., 3(4), 341-351. [8] Grott M. et al. (2019) Nat. Astr., 3(11), 971-976. [9] Delbo M. et al. (2014) Nature, 508(7495), 233-236. Fig. 1: a) Experimental design with meteorite samples sitting ontop of the cold finger. be) low vacuum SEM bakscscattered electron image showing and area, where a micro -flake is forming and ultimately detachin g through cycling (0 cycles to 400 cycles).