Context. The ROSINA instrument on board the Rosetta spacecraft measured, among others, the outgassing of noble gases from comet 67P/Churyumov- Gerasimenko. The interpretation of this dataset and unravelling underlying desorption mechanisms requires detailed laboratory studies. Aims. We aim to improve our understanding of the desorption patterns, trapping, and fractionation of noble gases released from the H₂O:CO₂-dominated ice of comet 67P. Methods. In the laboratory, ice films of neon, argon, krypton, or xenon (Ne, Ar, Kr, and Xe) mixed in CO₂:H₂O were prepared at 15 K. Temperature-programmed desorption mass spectrometry is employed to analyse the desorption behaviour of the noble gases. Mass spectrometric ROSINA data of 67P were analysed to determine the fraction of argon associated with CO₂ and H₂O, respectively. Results. CO₂ has a significant effect on noble gas desorption behaviour, resulting in the co-release of noble gases with CO₂, decreasing the amount of noble gas trapped within water, shifting the pure phase noble gas peak desorption temperature to lower temperatures, and prolonging the trapping of neon. These effects are linked to competition for binding sites in the water ice and the formation of crystalline CO₂. Desorption energies of the pure phase noble gas release were determined and found to be higher than those previously reported in the literature. Enhancement of the Ar/Kr and Ar/Xe ratios are at best 40% and not significantly influenced by the addition of CO₂. Analysis of ROSINA mass spectrometric data shows that the fraction of argon associated with H₂O is 0.53 ± 0.30, which cannot be explained by our laboratory results. Conclusions. Multicomponent ice mixtures affect the desorption behaviour of volatiles compared to simple binary mixtures and experiments on realistic cometary ice analogues are vital to understanding comet outgassing.

Context. Comets are considered to be remnants from the formation of the Solar System. ESA s Rosetta mission targeted comet 67P/Churyumov-Gerasimenko and was able to record high-quality data on its chemical composition and outgassing behaviour, including low abundances of N2 that are observed to be correlated with H2O and CO2 in approximately a 63:37 ratio. Aims. In this work, the thermal desorption behaviour of N2 in H2O:CO2 ices was studied in the laboratory to investigate the co-desorption behaviour of N2 within the two most abundant cometary ices in 67P and to derive desorbing fractions in different temperature regimes. Methods. H2O:CO2:N2 ices of various ratios were prepared in a gas mixing system and co-deposited at 15 K onto a copper sample holder. Sublimation of the ice was measured using temperature programmed desorption mass spectrometry. Quantitative values were derived for the fraction of N2 co-desorbing with CO2 and H2O respectively. To validate the results, H2O:CO2:13CO ices were prepared as well. Results. The experiments show that the co-desorption of N2 with CO2 in H2O:CO2:N2 ices depends on the bulk amount of CO2 present in the ice. The fraction of N2 trapped in H2O reduces as more N2 and CO2 are added to the mixture. CO behaves qualitatively similar to N2, but more CO is found to co-desorb with CO2. To reproduce the ratio of N2 desorbing with H2O over that of CO2 (N2(H2O)/N2(CO2)), our ice analogues need to contain =15% CO2, while 67P contains =7.5% CO2. Large fractions of N2 can be removed from the ice due to heating up to 70 K, but for ice that most closely resembles that of 67P, the loss fraction of pure phase N2 is expected to be =20%. Therefore, N2 is suggested to be a minor carrier of nitrogen in the comet.

We present a study of an impacting descent probe that increases the science return of spacecraft orbiting or passing an atmosphere-less planetary bodies of the solar system, such as the Galilean moons of Jupiter. The descent probe is a carry-on small spacecraft (<100 kg), to be deployed by the mother spacecraft, that brings itself onto a collisional trajectory with the targeted planetary body in a simple manner. A possible science payload includes instruments for surface imaging, characterisation of the neutral exosphere, and magnetic field and plasma measurement near the target body down to very low-altitudes (~1 km), during the probe’s fast (~km/s) descent to the surface until impact. The science goals and the concept of operation are discussed with particular reference to Europa, including options for flying through water plumes and after-impact retrieval of very-low altitude science data. All in all, it is demonstrated how the descent probe has the potential to provide a high science return to a mission at a low extra level of complexity, engineering effort, and risk. This study builds upon earlier studies for a Callisto Descent Probe for the former Europa-Jupiter System Mission of ESA and NASA, and extends them with a detailed assessment of a descent probe designed to be an additional science payload for the NASA Europa Mission.

The stabilization of rock slabs by means of anchoring systems (e.g. rockbolts or cables) is a complex problem involving not only geometrical issues, but also mechanical issues. In particular, the main points to be considered are (i) the degradation of mechanical properties of the block-bedrock interface, (ii) the dependence of the block-anchor interaction loads on the block displacements and (iii) the technical/economical optimization of the stabilizing system. In addition, whilst conventional design approaches usually tackle the problem by adopting a "force-based" method, the most recent design approaches require a "performance-based" analysis of the system, respecting both Ultimate Limit State (ULS) and Serviceability Limit States (SLS), capable of modelling even the passive behaviour of the stabilizing system. From a theoretical point of view, this implies that neither traditional limit equilibrium methods nor hybrid methods can be adopted in the design, since a displacement based design approach taking into account the block-anchor coupling is rather required. The interaction forces in fact directly depend on the block displacement field, which, on its turn, depends on the supporting forces given by the anchoring system. Moreover, although the behaviour of the block is usually modelled by adopting a small displacement scheme, the mechanical response of the anchor in the neighborhood of the sliding plane has to be modelled by means of a large displacement scheme, accounting even second order effects. Traditional finite element or finite difference 3D numerical analyses implementing such aspects are often too demanding (both from a computational and an economical point of view) to be considered actual cost-effective design tools for the engineers, especially during the early stages of the design process. In this paper a simplified displacement based procedure for analyzing the stability against sliding of an unstable rock block is presented, by comparing the performances of a rockbolt ad of a cable. The mechanical behaviour of the anchor is reproduced by means of a finite element approach and its transversal and axial interaction with the rock is modelled by means of non-linear springs, lumping the interaction forces in each node. An explicit finite difference scheme is implemented to solve the motion equation of the rock block. Several examples and numerical analyses are shown in order to highlight the importance of second order effects and to clarify the role of the bending stiffness of the anchor. Such approach is particularly useful for the optimization of the intervention measure. © 2016 Associazione Geotecnica Italiana, Rome, Italy.