Yann Brouet

Surface charging of thick porous water ice layers relevant for ion sputtering experiments

Abstract We use a laboratory facility to study the sputtering properties of centimeter-thick porous water ice subjected to the bombardment of ions and electrons to better understand the formation of exospheres of the icy moons of Jupiter. Our ice samples are as similar as possible to the expected moon surfaces but surface charging of the samples during ion irradiation may distort the experimental results. We therefore monitor the time scales for charging and discharging of the samples when subjected to a beam of ions. These experiments allow us to derive an electric conductivity of deep porous ice layers. The results imply that electron irradiation and sputtering play a non-negligible role for certain plasma conditions at the icy moons of Jupiter. The observed ion sputtering yields from our ice samples are similar to previous experiments where compact ice films were sputtered off a micro-balance.

Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko

Permittivity measurements on porous samples of volcanic origin have been performed in the 0.05–190 GHz range under labo- ratory conditions in support of the Rosetta mission to comet 67P/Churyumov–Gerasimenko, specifically with the MIRO radiometric experiment and CONSERT radar experiment. Methods. The samples were split into several subsamples with different size ranges covering a few μm to 500 μm. Bulk densities of the subsamples were estimated to be in the 800 to 1500 kg/m3 range. The porosities were in the range of 48% to 65%. From 50 MHz to 6 GHz and at 190 GHz, permittivity has been determined with a coaxial cell and with a quasi-optical bench, respectively. Results. Without taking into account the volume-scattering effect at 190 GHz, the real part of the permittivity, normalized by the bulk density, is in the range of 2.1 to 2.6. The results suggest that the real part of the permittivity of an ice-free dust mantle covering the nucleus is in the 1.5−2.2 range at 190 GHz. From these values, a lower limit for the absorption length for the millimeter receiver of MIRO has been estimated to be between 0.6 and 2 cm, in agreement with results obtained from MIRO in September 2014. At frequencies of interest for CONSERT experiment, the real part of the permittivity of a suspected ice-free dust mantle should be below 2.2. It may be in the range of 1.2 to 1.7 for the nucleus, in agreement with first CONSERT results, taking into account a mean tem- perature of 110 K and different values for the dust-to-ice volumetric ratio. Estimations of contributions of the different parameters to the permittivity variation may indicate that the porosity is the main parameter.

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 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.

Characterization of the permittivity of controlled porous water ice-dust mixtures to support the radar exploration of icy bodies

The internal properties of porous and icy bodies in the Solar System can be investigated by ground-penetrating radars (GPRs), like the CONSERT instrument onboard the Rosetta spacecraft which has sounded the interior of the nucleus of comet 67P/C-G. Accurate constraints on the permittivity of icy media are needed for the interpretation of the data. We report novel permittivity measurements performed on water ice samples and icy mixtures with porosities in the 31–91% range. The measurements have been performed between 50 MHz and 2 GHz with a coaxial cell on a total of 38 samples with a good reproducibility. We used controlled procedures to produce fine-grained and coarse-grained ice samples with a mean diameter of 4.5 μm and 67 μm, respectively, and to prepare icy mixtures. The JSC-1A Lunar regolith simulant was used as the dust component in the mixtures. The results are focused on the real part e′ of the permittivity, which constrains the phase velocity of the radio waves in low-loss media. The values of e′ show a non-dispersive behavior and are within the range of 1.1 to 2.7. They decrease with the increasing porosity Φ according to E(1 − Φ), with E equal to about 3.13 for pure water ice, and in the 3.8–7.5 range for ice-dust mixtures with a dust-to-ice volumetric ratio in the 0.1–2.8 range, respectively. These measurements are also relevant for radiometers operating in the millimeter-submillimeter domains, as suggested by the non-dispersive behavior of the mixtures and of the pure components.

CORRELATION OF THE VISIBLE AND RADAR STRATIGRAPHIC RECORDS OF MARS’ NPLD

Introduction: A long-standing problem in Mars Polar Science is the interpretation of the stratigraphic record preserved in Mars’ icy North Polar Layered Deposits (NPLD) [1] (Fig. 1a), whose accumulation patterns of ice and dust have long been thought to associate with recent climatic changes forced by variations of the planet’s astronomical parameters [2,3]. The internal layering of the NPLD is visible in exposures within a series of spiraling troughs that dissect the NPLD dome (Fig. 1a,b). Studies have relied on remote images of these troughs to map the stratigraphy [4–8] and search for a connection between NPLD accumulation and astronomical forcing [9–12]. Sub-surface radar sounding has also proved invaluable in observing the internal structure of the deposits. The SHARAD instrument detects changes in dielectric properties with depth. As these vary for beds with different amounts of dust, layering is observed as radar reflections [13]. The optical and radar-based stratigraphies have predominantly been studied in isolation. In terrestrial climate science, orbital climate forcing was ultimately confirmed by the correlation of various datasets [14], suggesting that this integration is necessary to unlock a complete climate record for the NPLD. In general, both radar and optical layers are assumed to result from varying amounts of dust impurities in water ice [15] (assuming negligible and uniform porosity). Christian et al. [16] attempted the first quantitative correlation and found a general agreement between properties of radar reflectors and visible layers, such as bed geometry and wavelength. This result provided evidence that the same physical quantity (potentially dust fraction) controls the formation of both radar reflectors and protruding strata. However, the unique correlation of a particular radar reflector with one visible layer or layer-packet remains an open problem. Here, we present our approach to this correlation by modeling the SHARAD propagation through layered media whose electrical properties are based on topographic profiles of bed exposures. These are extracted from Digital Terrain Models (DTMs) made with images from the HiRISE instrument. The objective is to test the hypothesis that highly protruding ‘marker beds’ have sufficient dielectric contrast with the neighboring beds to create radar reflections, thus associating individual reflectors to visible layers. These profiles can inform orbitallyforced accumulation models [17,18], that could unlock the temporal climate record of the NPLD.

Tensile strength of 67P/Churyumov-Gerasimenko nucleus material from overhangs

We directly measured twenty overhanging cliffs on the surface of comet 67P/Churyumov-Gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support them against collapse under the comets gravity. We find extremely low strengths of around 1 Pa or less (1 to 5 Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features andthe implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67P nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties either with size over the 10-100 m range studied here or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km).

Thermal inertia and roughness of the nucleus of comet 67P/Churyumov-Gerasimenko from MIRO and VIRTIS observations

Aims. Using data from the Rosetta mission to comet 67P/Churyumov-Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal inertia (TI) and roughness in several regions on the largest lobe of the nucleus. Methods. We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014. Results. The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK