Thermal fatigue is considered a primary mechanism driving rock disaggregation and regolith production on the surface of airless planetary bodies, acting over millions to billions of diurnal temperature cycles. However, its long-term effectiveness has never been experimentally tested. In particular, the Kaiser effect suggests that no additional damage is expected to accumulate when a material is subjected to repeated loading at the same maximum stress level. Here, we use real-time acoustic emission (AE) monitoring to track fracture activity during 100 repeated thermal cycles of ΔT = 190 °C on three meteorites of different petrological type; CM2 Aguas Zarcas, CV3 Allende, and H3-5 Oum Dreyga. The CM2 meteorite exhibited early onset of fracturing activity as indicated by increased AE activity, whereas the CV3 and H3-5 samples showed low activity, with AE occurring predominantly during the later cycles, but ceasing towards the end. These findings suggest that the surfaces of Ch/Cgh-type asteroids, represented by CM2 material, are likely to be more susceptible to thermally driven regolith production, but this response is progressively limited due to threshold-controlled fracturing, whereas S-type bodies, linked to ordinary chondrites, may experience comparatively limited thermal fatigue under similar conditions. CV3 material, analogous to certain anhydrous C-complex/K-type asteroids, likewise appears relatively resistant to damage accumulation across repeated thermal cycles. We propose that thermal fatigue is a threshold-controlled process rather than an indefinitely acting mechanism.

Recent models of the near-Earth asteroid population show that asteroids must be super-catastrophically destroyed when they evolve to orbits with perihelion passages well inside of Mercury’s orbit. The heliocentric distances at which the disruptions typically occur are tens of solar radii, which is too far from the Sun for asteroids to be destroyed by sublimation and tidal disruption. The typical disruption distance also appears to be larger for darker asteroids. Here, by carrying out irradiance experiments in vacuum that replicate the conditions in the near-Sun environment, we show that CI meteorite simulants are destroyed within minutes when exposed to the level of solar irradiance encountered at heliocentric distances of up to about 0.2 au. Our results provide an explanation for the scarcity of dark, carbonaceous asteroids with perihelion distances less than 0.2 au, and for the observed mass-loss rate of the asteroid-like object 322P/SOHO 1 assuming its composition is similar to CI carbonaceous chondrites.

The behaviour of granular materials such as soils has been well studied on Earth and various models have been proposed to describe this behaviour under a range of conditions, including the application of penetrometry for identifying geotechnical parameters in situ. Properties other than bearing strength such as solid-composition and grain size of these materials are usually determined beforehand by, for example, the use of core samples or sieve analysis. The behaviour of granular materials and the operation of penetrometers in microgravity are much less understood. Core samples and sieve analysis are not available or practical for space missions to small bodies or other planets in the solar system because of mass and power constraints and the mechanical complexity involved. Penetrometers are small, comparatively lightweight instruments suitable as lander payloads. They can be used to study properties of surface regolith such as grain size and penetrometers were a staple payload during the early days of planetary exploration on the Moon, a body with significant gravity. Penetrometers have however never been used on minor bodies and their effectiveness in microgravity is unclear. This work examines to what extent penetrometers can be used to identify grain size distributions in granular samples, both in Earth gravity and microgravity environments. Our penetrometry experiments showed that identification of grain sizes is possible in 1 g, however in microgravity grain size information will require careful post-processing because of high noise levels. We also analysed the effect of penetrometer tip shape and penetration velocity in both gravity regimes. Penetrometers may, with the addition of complementary measurements, be effective tools for constraining regolith grain size below the surface, albeit with some difficulty in microgravity.

The surface of Mercury is exposed to extreme diurnal thermal variations caused by the high intensity of solar radiation, the radiative loss due to the planet’s lack of atmosphere, its eccentricity and its 3:2 spin - orbit resonance. This work presents an experimental study on terrestrial rocks used as Mercury analogues subjected to hermean conditions. We simulate the power density of a planetary surface at Mercury’s perihelion distance of 0.31 au using the Space and High-Irradiance Near-Sun Simulator (SHINeS) at Luleå University of Technology. The reflectance spectra were acquired in the visible and near-infrared wavelength range for every sample before and after irradiation. Permanent spectral changes are observed in all samples towards the longer wavelengths in the visible spectrum after only one thermal cycle. Darkening is evident in both the visible and near-infrared spectrum ranges, combined with reddening in the visible-to-near-infrared region in most of our samples. We propose that darker samples like boninite, basalt, and diorite are more likely to experience spectral changes due to their low albedo. 

Understanding the properties of planetary regolith is important for unravelling the origin and evolution of bodies in our solar system. On Earth, properties of regolith are identified in the laboratory through a variety of methods such as coring samples, shear tests, and triaxial tests. In space missions, these laboratory methods cannot be used, and one must rely on in-situ measurements for identifying soil mechanical properties. Penetrometers are perfect for this due to their small size, light weight and simple use. Penetrometers have been used on Earth to identify soil shear strength properties such as cohesion and angle of internal friction. However, results are highly dependent on the experimentally obtained data and the types of soil that are tested. In this paper, we use a model developed by Kang et al., that links penetrometry data to geotechnically relevant quantities such as the angle of internal friction, test this model using our own laboratory experiments, and compare this to known values as well as triaxial test results. We identified that for silica glass beads the effects of grain size and tip shape are very pronounced, with similar values of internal friction found in other sources and our triaxial tests. We observed that penetrometry can indicate similar angles of internal friction comparable to laboratory values for more irregular and angular sample materials such as the Phobos simulant used to prepare for JAXA's MMX mission. Our results suggest that a penetrometer could be used as a ‘poor man's triaxial test’ in planetary exploration.

Characterising the mechanical properties of minor bodies is essential for understanding their origin and evolution. Past missions such as Hayabusa2 have landed on asteroids to sample and discover what these bodies are made of. However, there has been conflicting evidence and reports into the physical properties of the granular surface material of these bodies. With future missions such as Japan Aerospace eXploration Agency’s Martian Moons eXploration mission landing on Phobos, the understanding and identification of these physical properties is crucial to maximising the scientific output from these missions. Penetrometry, the determination of the reaction force that an object experiences as it penetrates a surface, can help to understand the essential properties of regolith, such as grain size, porosity and cohesion. Results of penetrometry experiments are largely analysed based on empirical models, which presents us with a challenge if we want to apply them to understand granular materials on asteroid surfaces because gravity cannot be eliminated in the laboratory. Hence, it is essential to verify penetrometry as a method and validate penetrometry instrument designs in microgravity. For this purpose, we conducted a microgravity experiment onboard a parabolic flight campaign. Our experiment tested the use of penetrometry in asteroid-analogue environments by investigating samples with varying properties, such as grain size distribution and shape, and then compared to 1 g experiments to understand the role microgravity plays. The experiment provided a substantial database for future analysis. This paper will focus on the design of the experiment and the parabolic flight campaign in which the experiments were conducted. The design decisions and the variables adjusted during the experiment will be discussed, evaluating how these influenced the campaign and its outcomes. We will also provide a snapshot of preliminary results of the data captured during this experiment. For example, we show the effect of cohesion on penetrometer reaction force, with more cohesive materials providing larger reaction forces nearly of the same magnitude of their 1 g counterparts. We also show that penetrometer tip shapes provide different reaction forces and that flat tips provide the largest reaction force compared to the others. The influence of penetration velocity will be investigated further with the aid of theoretical models. Early indications from the results seen so far are promising for future analyses and will provide key information for the analysis of penetrometry data on future missions.

We investigate the sublimation behaviour of brine-saturated Martian analogue materials in laboratory experiments and how compositional variations affect the development of the sample surfaces and subsurfaces. As a regolith analogue for our experiments, we used granular Saddleback basalt with the three grain size distributions 212–500 m, 212–2000 m, and 1000–2000 m. Our brine was composed of ferric sulphate, magnesium sulphate, and magnesium chloride. The samples were placed in a environmental chamber and exposed to a temperature of 243 K and a CO2 atmosphere of 5 mbar for several days. The samples were then insolated at 590 W m−2 with a solar simulator, representing the insolation rate at Mars whilst changes in surface morphology were monitored in vertical and lateral directions by two cameras. No explicit salt efflorescence could be produced on the surface. However, we found a stratification characterised by four layers developed within the samples. Beneath (i) a solidified crust at the surface, (ii) a dry, unconsolidated layer formed, partially characterised by white magnesium salt precipitates. A (iii) third layer was characterised by yellowish hydrated ferric sulphate precipitates, which filled considerable parts of the pore space between the regolith analogue grains. The (iv) fourth layer still contained water in the form of ice crystals and moisture. The spectral and mechanical properties of the sample surfaces were analysed after the sublimation experiment using reflectance spectroscopy in the visual and near/mid-infrared spectral range and cone penetration testing. The spectral comparisons of the uppermost crust with the non-saline regolith analogue material displayed a distinct spectral blue slope, indicating the presence of hydrated ferric sulfates. The strength of the upper crusts varied from marginally consolidated to strongly cemented depending on grain size and salt content. Our observations indicate that extensive salt crusts on the Martian surface are less likely to have been formed by brines present in the subsurface. Due to high evaporation rates, significant amounts of dissolved salts already precipitated below the surface of Mars, potentially leading to the formation of underground duricrusts.

In planetary science, the statistical properties of spatial distributions are frequently examined to understand the formation and evolution of a body's surface. The surfaces of the asteroids directly explored by spacecraft are covered with numerous boulders and/or regolith particles. However, the spatial distribution of these boulders has not been statistically studied, although much statistical research has been done on the spatial distributions of craters. Thus, it is not known whether the spatial distribution of boulders on asteroids explored by spacecraft is random or not. Squyres et al. (1997) developed a simple model of crater formation and obliteration based on several assumptions, but some of their assumptions do not hold for boulders. In this study, we construct a simple model of the spatial distribution of boulders by verifying some assumptions, and investigate the effect of various assumptions and parameter variations on the model results. From these quantitative calculations, we investigate the spatial distribution of boulders on the asteroids Eros, Ryugu, and Itokawa. Our quantitative results show that boulders on Eros are spatially clustered at the 95 % confidence level. On the other hand, on Ryugu and Itokawa, decameter-sized boulders are spatially less clustered, while meter-sized small boulders are spatially clustered, all at the 95 % confidence level. This suggests that the clustered spatial distribution of small boulders on Ryugu and Itokawa can be explained by their migration.

The surfaces of sub-kilometer-sized asteroids directly explored by spacecraft, such as Itokawa, Ryugu and Bennu, are covered with blocks and/or regolith particles, whose shapes are considered clues to understanding their formation and evolution on the asteroid's surface. Ryugu particles returned by the Hayabusa2 mission are likely fragments resulting from impacts because their shapes resemble impact fragments from laboratory experiments. However, there is a lack of laboratory impact experiments examining the shapes of fragments in carbonaceous chondrites, thought to originate from carbonaceous asteroids such as Ryugu and Bennu. The measured sizes of Ryugu particles are in the mm and sub-mm range, similar to the sizes of chondrules. Also, carbonaceous chondrites are generally structurally weaker than the basalts and granites often used in previous laboratory impact experiments. Differences in the strength of the chondrules and matrix might affect the overall strength of the meteorite. In this study, as a first step towards a better understanding of impact fragment shapes in carbonaceous chondrites, we conducted impact experiments on the carbonaceous meteorite Allende (CV3). A spherical alumina projectile with 1.0 mm and a glass projectile with 0.80 mm in diameter were fired into 1–2 cm-sized Allende targets at nominal impact velocities of 2.0 and 4.0 km/s, respectively. To investigate the correlation between the chondrules (typically sub-mm in size) and the shapes of fine fragments, we measured the shape distributions of sub-mm impact fragments using X-ray microtomography. We observed several impact fracture surfaces along the chondrule boundaries. In addition, these fragments tended to be rounder than fragments from previous impact experiments. However, because the total number of these fragments is relatively small, the fragments were found to have the same overall shape distribution as previous laboratory impact fragments, Itokawa particles and Ryugu particles. This may imply that impact fragment shapes are independent of the bulk material strength. These findings will be useful for understanding the formation process of regolith layers on asteroid surfaces, Itokawa particles, Ryugu particles, and Bennu particles.

Snow ice (SI) forms from freezing wet snow, known as slush, and contributes to the thickness of level and brash ice. However, the mechanism of snow-slush-snow ice transformation has not been extensively investigated to date, despite the difference in the freezing rate of slush in comparison with water is important for estimating the ice thickness. In this study, we examined the growth of initial congelation ice (CI) and snow ice (SI) in a fresh water tank exposed to outdoor weather conditions in Luleå, northern Sweden. The tank of size 1.8 × 0.65 × 1.2 m in length, width and height was divided into two compartments to facilitate the simultaneous growth of CI and SI. A total of 12 experiments were conducted in the years 2021 and 2022. The transformation from slush to snow ice was achieved by submerging various amounts of snow in the compartments. It was observed that approximately 35% of the initial snow transformed into SI. Snow ice grew 4 mm°C-0.5 d-0.5 faster than congelation ice. The CI growth under SI was 1 mm°C-0.5 d-0.5 slower than the CI growth under CI. This study provides valuable insights for modelling snow-slush-snow ice transformation and designing future laboratory-scale experiments.