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

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 dis- charging 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.Comment: arXiv admin note: text overlap with arXiv:1509.0400

Near-infrared reflectance spectroscopy of sublimating salty ice analogues. Implications for icy moons

The composition of the surface of the Galilean icy moons has been debated since the Galileo mission. Several chemistries have been proposed to explain the composition of the non-icy component of the moon's surfaces, notably, sulphuric acid hydrates and magnesium and sodium sulphates. More recently, magnesium and sodium chlorides have been proposed to explain features observed in ground-based observations. We have considered four salts (NaCl, Na2SO4, MgSO4 and MgCl2) with various concentrations, to produce salty ice analogues. Granular particles were produced by a flash-freezing procedure. Additionally, compact slabs of salty ices were produced by a slow crystallisation of salty liquid solution. These two methods mimic the end-members (plumes and slow ice block formation) for producing hydrated salty ices on the surface of icy moons such as Europa and Ganymede. We have monitored the near-infrared (NIR) evolution of our salty ices during sublimation, revealing differences between the granular particles and the slabs. The slabs formed a higher amount of hydrates and the most highly hydrated compounds. Granular ices must be formed from a more concentrated salty solution to increase the amount of hydrates within the ice particles. The sublimation of salty ices removed all excess water ice efficiently, but the dehydration of the salts was not observed. The final spectra of the slabs were most flattened around 1.5 and 2.0 μm, especially for the Na2SO4, MgCl2 and MgSO4, suggesting the presence of stable, highly hydrated compounds. We find that Na2SO4, MgCl2 and MgSO4 are most compatible with the non-icy component at the surface of the icy moons as observed by the NIMS instrument on Galileo and by ground-based observations

Experimental simulation to analyse geomorphological properties of cometary surfaces with outgassing volatiles

We want to study the development of cometary and other outgassing surfaces of airless planetary objects with analog laboratory experiments on Earth. The focus is on the evolution of different morphologies, taking into account the composition of the sample material and variable insolation flux. Our aim is to understand how different cometary materials interact and how the appearance of cracks and boulders develop during insolation and outgassing

TEMPus VoLA: The timed Epstein multi-pressure vessel at low accelerations.

The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro-gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle-gas interaction, (ii) the drag coefficients of dust aggregates with variable particle-gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of dust particle packing fraction, and Knudsen, Stokes, and Reynolds numbers. The mechanisms investigated are also relevant for our understanding of the emission of dust from active surfaces, such as cometary nuclei, and new experimental data will help interpreting previous datasets (Rosetta) and prepare future spacecraft observations (Comet Interceptor). We report on the performance of the experiments, which has been tested over the course of multiple flight campaigns. The project is now ready to benefit from additional flight campaigns, to cover a wide parameter space. The outcome will be a comprehensive framework to test models and numerical recipes for studying collective dust particle aerodynamics under space-like conditions

Yearly and seasonal variations of low albedo surfaces on Mars in the OMEGA/MEx dataset: Constraints on aerosols properties and dust deposits

The time variations of spectral properties of dark martian surface features are investigated using the OMEGA near-IR dataset. The analyzed period covers two Mars years, spanning from early 2004 to early 2008 (includes the 2007 global dust event). Radiative transfer modeling indicates that the apparent albedo variations of low to mid-latitude dark regions are consistent with those produced by the varying optical depth of atmospheric dust as measured simultaneously from the ground by the Mars Exploration Rovers. We observe only a few significant albedo changes that can be attributed to surface phenomena. They are small-scaled and located at the boundaries between bright and dark regions. We then investigate the variations of the mean particle size of aerosols using the evolution of the observed dark region spectra between 1 and 2.5 {\mu}m. Overall, we find that the observed changes in the spectral slope are consistent with a mean particle size of aerosols varying with time between 1 and 2 {\mu}m. Observations with different solar zenith angles make it possible to characterize the aerosol layer at different altitudes, revealing a decrease of the particle size of aerosols as altitude increases

Cometary dust analogues for physics experiments

The CoPhyLab (Cometary Physics Laboratory) project is designed to study the physics of comets through a series of earth-based experiments. For these experiments, a dust analogue was created with physical properties comparable to those of the non-volatile dust found on comets. This "CoPhyLab dust" is planned to be mixed with water and CO$_2$ ice and placed under cometary conditions in vacuum chambers to study the physical processes taking place on the nuclei of comets. In order to develop this dust analogue, we mixed two components representative for the non-volatile materials present in cometary nuclei. We chose silica dust as representative for the mineral phase and charcoal for the organic phase, which also acts as a darkening agent. In this paper, we provide an overview of known cometary analogues before presenting measurements of eight physical properties of different mixtures of the two materials and a comparison of these measurements with known cometary values. The physical properties of interest are: particle size, density, gas permeability, spectrophotometry, mechanical, thermal and electrical properties. We found that the analogue dust that matches the highest number of physical properties of cometary materials consists of a mixture of either 60\%/40\% or 70\%/30\% of silica dust/charcoal by mass. These best-fit dust analogue will be used in future CoPhyLab experiments

A snapshot full-Stokes spectropolarimeter for detecting life on Earth

Instrumentatio

A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues

The Rosetta spacecraft made a rendezvous with comet 67P/Churyumov-Gerasimenko (67P) in 2014 August, soon after the Philae module landed on the small lobe of the nucleus on 2014 November 12. The CONSERT instrument, onboard Rosetta and Philae, sounded the upper part of the interior of 67P with radiowaves at 90 MHz and determined an average of the real part of the permittivity (hereafter ) equal to about 1.27. The SESAME-PP instrument, onboard Philae, sounded the near-surface of the small lobe in the 400–800 Hz range and determined a lower limit of equal to 2.45. We use a semi-empirical formula obtained from measurements of performed in the laboratory at 243 K on water ice and ice-basaltic dust mixtures, with a controlled porosity in the 31–91 per cent range and a dust-to-ice volumetric ratio in the 0.1– 2.8 range, to interpret the results of the two instruments, taking into account the temperature and frequency dependences. A graphical method is proposed to derive ranges of porosity and dust-mass fraction from a value of derived from observations. The non-dispersive behaviour of below 175 K, allows us to compare the values of obtained by CONSERT and SESAME-PP. We show that the porosity of the small lobe of 67P increases with depth. Based on new measurements of analogues of complex extraterrestrial organic matter, the so-called tholins, we also suggest that, for the dust component in the cometary material, the presence of silicates has more effect on than organic materials

Targeting and image acquisition of Martian surface features with TGO/CaSSIS

CaSSIS is a high-resolution visual telescope onboard the ExoMars Trace Gas Orbiter. The mission started the primary science phase in April 2018. The relatively small single image footprint (typically 40 km × 9.5 km) when compared to the total surface area of Mars demands that images should be targeted and target selection is key for the science return. This paper describes the science planning concept set around the target selection, and the process followed in order to generate the CaSSIS commands. The tools used are described as well as all the iterations and teams involved. Finally, special cases and the handling of contingencies are discussed. The procedures may serve as a guideline for future high-resolution instruments on missions to planetary objects