Constraining the parameter space of comet simulation experiments

Our interpretation of the data returned by Rosetta and other cometary missions is based on the predictions of theoretical models and the results of laboratory experiments. For example, Kossacki et al. (2015) showed that 67P's surface hardness reported by Spohn et al. (2015) can be explained by sintering. The present work supports Rosetta's observations by investigating the hardening process of the near-surface layers and the change in surface morphology during insolation. In order to create as simple an analogue as possible our sample consists of pure, porous H2O ice and carbon black particles. The observations suggest that translucence of the near-surface ice is important for enabling subsurface hardening. As an end product of our experiments we also obtained carbon agglomerates with some residual strength

UK Mars research and priorities in the Aurora programme

John Bridges and Axel Hagermann summarize an RAS Special Discussion Meeting in January 2011, which looked at the prospects for the UK exploring Mars

Speed of sound in nitrogen as a function of temperature and pressure

Speed of sound measurements in nitrogen by Younglove and McCarty [J. Chem. Thermodynam. 12, 1121–1128 (1980)] are revisited and an empirical polynomial equation for the speed of sound is derived. The polynomial coefficients differ from those given by Wong and Wu [J. Acoust. Soc. Am. 102, 650–651 (1997)] with the result that discrepancies between predicted and measured values at low temperatures are reduced. The maximal error over the complete temperature and pressure range from 80 to 350 K and 0.031 to 0.709 MPa is reduced from 5.38% to 0.78%

Boulder sizes and shapes on asteroids: A comparative study of Eros, Itokawa and Ryugu

In order to understand the geological evolution of asteroids Eros, Itokawa and Ryugu and their collisional history, previous studies investigated boulder size distributions on their surfaces. However, quantitative comparison of these size distributions is hampered by numerous differences between these studies regarding the definition of a boulder's size, measuring technique and the fitting method to determine the power-index of the boulder size distributions. We provide a consistent and coherent model of boulder size distributions by remeasuring the boulders on the entire surfaces of Eros and Itokawa using the Small Body Mapping Tool (SBMT) and combining our observations with the Ryugu data of Michikami et al. (2019). We derived power-indices of the boulder size distributions of −3.25 ± 0.14 for Eros, −3.05 ± 0.14 for Itokawa and −2.65 ± 0.05 for Ryugu. The asteroid with the highest number density of boulders ≥ 5 m turns out to be Ryugu, not Itokawa, as suggested by an earlier study. We show that the appearance of the boulders tends towards more elongated shapes as the size of an asteroid decreases, which can be explained by differences in asteroid gravity and boulder friction angles. Our quantitative observational results indicate that boulder migration preferentially affects smaller boulders, and tends to occur on larger asteroids

Hardness and Yield Strength of CO2 Ice Under Martian Temperature Conditions

Although ice fracturing and deformation is key to understanding some of the landforms encountered in the high-latitude regions on Mars and on other icy bodies in the solar system, little is known about the mechanical characteristics of CO2 ice. We have measured the hardness of solid CO2 ice directly in the laboratory with a Leeb hardness tester and calculated the corresponding yield strength. We have also measured the hardness of water ice by the same method, confirming previous work. Our results indicate that CO2 ice is slightly weaker, ranging between Leeb ∼200 and 400 (∼10 and 30 MPa yield strength, assuming only plastic deformation and no strain hardening during the experiment), for typical Martian temperatures. Our results can be used for models of CO2 ice rupture (depending on the deformation timescales) explaining surface processes on Mars and solar system icy bodies

Stability of Subsurface Carbon Dioxide Ice over the Obliquity Cycle

Carbon dioxide (CO2) ice does not remain stable at the surface of Mars for long periods of time over the obliquity cycle. We use the UK version of the LMD Mars Global Circulation Model (MGCM) [1] with a newly integrated subsurface scheme to investigate how the timescales for the stability of CO2 ice are affected by overlying regolith at different obliquities within the range expected for Mars over the last 4 Myrs [2

Oblique impact cratering experiments in brittle targets: Implications for elliptical craters on the Moon

Most impact craters observed on planetary bodies are the results of oblique impacts of meteoroids. To date, however, there have only been very few laboratory oblique impact experiments for analogue targets relevant to the surfaces of extraterrestrial bodies. In particular, there is a lack of laboratory oblique impact experiments into brittle targets with a material strength on the order of 1 MPa, with the exception of ice. A strength on the order of 1 MPa is considered to be the corresponding material strength for the formation of craters in the 100 m size range on the Moon. Impact craters are elliptical if the meteoroid's trajectory is below a certain threshold angle of incidence, and it is known that the threshold angle depends largely on the material strength. Therefore, we examined the threshold angle required to produce elliptical craters in laboratory impact experiments into brittle targets. This work aims to constrain current interpretations of lunar elliptical craters and pit craters with sizes below a hundred meters. We produced mortar targets with compressive strength of 3.2 MPa. A spherical nylon projectile (diameter 7.14 mm) was shot into the target surface at a nominal velocity of 2.3 km/s, with an impact angle of 5°‐90° from horizontal. The threshold angle of this experiment ranges from 15° to 20°. We confirmed that our experimental data agree with previous empirical equations in terms of the cratering efficiency and the threshold impact angle. In addition, in order to simulate the relatively large lunar pit craters related to underground cavities, we conducted a second series of experiments under similar impact conditions using targets with an underground rectangular cavity. Size and outline of craters that created a hole are similar to those of craters without a hole. Moreover, when observed from an oblique angle, a crater with a hole has a topography that resembles the lunar pit craters. The relation between the impact velocity of meteoroids on the Moon and the probability of elliptical crater formation was investigated based on our experimental results and an existing empirical equation. The results suggest a distinct possibility that most craters in the 100 m size range on the Moon, given their elliptical shape, originated as secondary craters. © 2016 The Author

Planetary heat flow from shallow subsurface measurements: Mars

Planetary heat flow probes measure heat flow (depth-resolved temperature and thermal conductivity) to provide insight into the internal state of a planet. The probes have been utilized extensively on Earth, twice on the Moon, and once on the Surface of comet 67P-CG. Mars is an important target for heat flow measurement as heat flow is a critical parameter in Martian thermal history models. Earlier studies indicate that Martian planetary heat flow can be accessed at 5 m below the surface in dry regolith monitored over at least one Martian year. A one Martian year monitoring period is necessary because, in the shallow subsurface, heat flow from the interior is superposed with time varying heat flow contributions, primarily due to insolation. Given that a heat flow probe may not achieve its target depth or monitoring period, this study investigates how the depth (2–5 m), duration (0–1 Martian year) and quality of measurements influence the accuracy of planetary heat flow. An inverse model is used to show that, in the preceding scenarios, the accuracy of planetary heat flow directly estimated from depth-dependent thermal conductivity with 10–20% precision errors, temperatures with 50–100 mK precision errors and modelling uncertainties up to 500 mK, can, on average, be improved by a factor of 27 with optimization to 13%. Accuracies increase with sensor penetration depth and regolith monitoring period. Heat flow optimized from instantaneous measurements or those with the shortest regolith monitoring periods have increased accuracy where the frequency and amplitude of the temperature variation are lowest. The inverse model is based on the Function Specification Inversion method. This study demonstrates that a solution subspace can be identified within a space of uncertainties modelled for the temperature measurements and planetary heat flow: the subspace is defined by a constant log-ratio of their respective standard deviations. Optimized heat flow estimates display reduced correlation with increasing temperature precision and systematic conductivity errors, with the constraint of other known model parameters. Consequently, the model permits upper bounds to be placed on the conductivity estimate without conductivity optimization, as heat flows are optimized to a limiting value with increasing systematic conductivity errors for any given parameter set. Overall, the results demonstrate a 52% chance of achieving a direct heat flow estimate accurate to within 40%, with the same being 82% after optimization

A Simple Way of Simulating Insolation on a Rotating Body with a Commercial Solar Simulator

The surfaces of all solid bodies in the solar system, planets, moons, comets and asteroids, experience short-term temporal variations of solar irradiation which depend on their respective spin rates. These so-called insolation cycles affect temperature variations, climate, photosynthesis in plants, etc. Hence, experimental reproduction of these cycles is important for space analogue simulations. In this short note we describe a simple, low-cost method to simulate diurnal cycles in the laboratory using a type of commercial solar simulator commonly used for experimental investigation in planetary science