Mechanical Properties of Cometary Surfaces

Mechanical properties, in particular, strength (tensile, shear, compressive) and porosity, are important parameters for understanding the evolution and activity of comets. However, they are notoriously difficult to measure. Unfortunately, neither Deep Impact nor other comet observations prior to Rosetta provided firm data on the strength of cometary material. This changed with the Rosetta mission and its detailed close observation data and with the landing(s) of Philae in 2014. There are already many articles and reviews in the literature that derive or compile many different strength values from various Rosetta and Philae data. In this paper, we attempt to provide an overview of the available direct and indirect data; we focus on comet Churyumov–Gerasimenko/67P but include a discussion on the Deep Impact strength results. As a prerequisite, we start by giving precise definitions of ‘strength’, discuss soil mechanics based on the Mohr–Coulomb ‘law’ of micro-gravity, and discuss bulk density and porosity, sintering, and the physics of the strength of a cohesive granular medium. We proceed by discussing the scaling of strength with the size and strain rate, which is needed to understand the observational data. We show how measured elastic properties and thermal (conductivity) data can be correlated with strength. Finally, a singular very high strength value is reviewed as well as some particularly small-strength values inferred from the bouncing motion of Philae, data from its collisions with the surface of the comet, and scratch marks it left, allegedly, on the surface close to its final resting site. The synthesis is presented as an overview figure of the tensile and compressive strength of cometary matter as a function of the size scale; conclusions about the size dependence and apparent natural variability of strength are drawn

Design and Analysis of a LWIR Detector Electronics for Space Applications

µBolometers based on a-Si technology provide a marketable performance while at the same time requiring only moderate resources in terms of cooling requirements. The PICO1024 is a high resolution (1024x768 pixels) infrared image sensor for thermography applications. It makes use of LYNRED latest, state of the art, 17 µm pixel pitch technology. The PICO1024 sensor is a 2-dimensional detector array sensitive to infrared radiation in the long wave spectral range (LWIR) from 8 to 14 m. It uses microbolometer technology to convert infrared radiation into electronic signal for use in thermal imaging cameras. Starting from an analysis of the Capacitance Trans-Impedance Amplifier (CTIA) the front-end electronics architecture is described and analyzed. This includes transient and AC Spice simulations as well as an analysis of the readout signal chain noise performance. The paper also presents a possible replacement of key components (ADC, opamp) by low power, high performance space qualified IEEE parts. An instrument concept presents the implementation of the PICO1024 into a thermal infrared multi-spectral imager for an Io mission

Numerical simulation of Ryugu’s thermophysical properties using the discrete element method

We utilized the LIGGGHTS program package to apply the discrete element method and investigate the heat flow through a particle bed with uniform size distribution. Our aim was to compare the numerical simulations with Sakatani's thermal conductivity model. This study was conducted to establish a benchmark in our understanding of the program package and the Sakatani model

LRAD – A Radiometer for the Lunar South Ppole Hopper µNOVA

The Lunar Prospector discovery of areas of neutron suppression and the subsequent interpretation of hydrogen enrichment near the lunar poles brought the possibility of volatile resources sequestered at the poles to the forefront of the lunar science community. Subsequently the LCROSS experiment showed that water ice is present within at least one permanently shadowed region (PSR) near the south pole and that it can be stable over geological timescales inside permanently shadowed regions (PSRs). Analysis of UV observations gathered by the Lunar Reconnaissance Orbiter Diviner instrument are consistent with the presence of surface frost in some PSRs with temperatures below 110 K. Further, the depth-to-diameter ratios of simple craters as determined from Lunar Orbiter Laser Altimeter (LOLA) altimetric measurements indicate that deposits of water ice in these cold traps may be up to 50 m thick. Moreover, water ice may be present in small PSRs at scales down to, and below 10 meters [5]. Such small-scale cold traps could significantly increase water inventory estimates and eventually simplify extraction

Results from a Comparison of Approximate Analytical Solutions with a Detailed Numerical Inversion Analysis to Determine the Thermal Conductivity of the Regolith at the Mars InSight Landing Site Using Data from HP3 Heating Experiments

A direct measurement of the regolith thermal conductivity at the Mars InSight landing site (4.50°, 132.62°E) was made by heating experiments using the physical properties package (HP3) of the Mars InSight mission. Temperature and time data from these heating experiments, after removal of background temperature variations, were analyzed using a finite element model for which Monte Carlo simulations were run varying regolith thermal conductivity, density, thermal contact conductance between the probe and the regolith to determine parameter combinations that best fit the heating curve. In terms of simulating details of heating experiment this data reduction and numerical inversion is as complete as possible within the current constraints of the experiment. However, no information was included in the model concerning regolith thermal conductivity variations radial to the probe caused during penetration of the probe

In-Situ Radiometric Investigation of Phobos using the MMX Rover’s miniRAD Instrument.

The JAXA MMX sample return mission to the martian moons will deliver a rover to the surface of Phobos that will investigate the landing area using its navigation cameras (NavCams), its regolith facing cameras (WheelCams), its Raman spectrometer (RAX), as well as its mid infrared radiometer (miniRAD). The distance that can be travelled by the Rover depends on the yet unknown terrain properties, but is estimated to range from a few meters to hundreds of meters. The rover and its instruments will operate on the surface of Phobos for at least 100 days

LRAD – The Radiometer for the Lunar South Pole Hopper μNOVA.

Permanently shadowed regions (PSRs) are present at the Lunar poles. The surface temperature inside a PSR can remain below 110K, low enough fow stable water ice to exist in vacuum. The presence of ater ice at the Lunar Poles has been indicated by remote sensing bservations, in particular by the Lunar Prospector, LCROSS, and LRO missions. As part of the NASA Commerical Lunar Payload Service program, the Nova-C lander will be build and operated by the US-based company Intuitive Machines. Nova-C will land near the Lunar south pole for in-situ inverstigations of potential volatiles in the regolith. Nova-C will carry the S.P. Hopper to the Moon which will in a sequence of short flights land within a PSR inside Marston crater (informal name) close to a potential landing site for the Artemis Astronauts [7]. The Hopper will map the lunar surface with two cameras and a themal infrared radiometer (LRAD)

Phobos Regolith Simulant for MMX Mission: Spectral Measurement for Remote Target Identification and Deconvolution System Training

The two natural satellites of Mars, Phobos and Deimos are both important targets for scientific investigation. The JAXA mission Martian Moons eXplorer (MMX) is designed to explore Phobos and Deimos, with a launch date scheduled for 2024. The MMX spacecraft will observe both Martian moons and will land on one of them (Phobos, most likely), to collect a sample and bring it back to Earth. The designs of both the landing and sampling devices depend largely on the surface properties of the target body and on how its surface is reacting to an external action in the low gravity conditions of the target. The Landing Operation Working Team (LOWT) of MMX started analyzing previous observations and theoretical/experimental considerations to better understand the nature of Phobos surface material, developing a Phobos regolith simulant material for the MMX mission [1]. At the Institute for Planetary Research of the German aerospace Center (DLR) in Berlin we performed a spectral characterization of the Phobos regolith simulant. Those data will be used to train an Artificial Neural Network (NN) to produce a system that could rapidly classify data during the mission and for endmember decomposition

Thermal fracturing on comets: Applications to 67P/Churyumov-Gerasimenko

We simulate the stresses induced by temperature changes in a putative hard layer near the surface of comet 67P/Churyumov-Gerasimenko with a thermo-viscoelastic model. Such a layer could be formed by the recondensation or sintering of water ice (and dust grains), as suggested by laboratory experiments and computer simulations, and would explain the high compressive strength encountered by experiments on board the Philae lander. Changes in temperature from seasonal insolation variation penetrate into the comet’s surface to depths controlled by the thermal inertia, causing the material to expand and contract. Modelling this with a Maxwellian viscoelastic response on a spherical nucleus, we show that a hard, icy layer with similar properties to Martian permafrost will experience high stresses: up to tens of MPa, which exceed its material strength (a few MPa), down to depths of centimetres to a metre. The stress distribution with latitude is confirmed qualitatively when taking into account the comet’s complex shape but neglecting thermal inertia. Stress is found to be comparable to the material strength everywhere for sufficient thermal inertia (≳ 50 J m−2 K−1 s−1∕2) and ice content (≳ 45% at the equator). In this case, stresses penetrate to a typical depth of ~0.25 m, consistent with the detection of metre-scale thermal contraction crack polygons all over the comet. Thermal fracturing may be an important erosion process on cometary surfaces which breaks down material and weakens cliffs