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Gas flow through layers of porous materials plays a crucial role in technical applications, geology, petrochemistry and space sciences (e.g. fuel cells, catalysis, shale gas production and outgassing of volatiles from comets). In many applications the Knudsen regime is predominant, where the pore size is small compared to the mean free path between intermolecular collisions. In this context common parameters to describe the gas percolation through layers of porous media are the probability of gas molecule transmission and the Knudsen diffusion coefficient of the medium. We show how probabilistic considerations on layer partitions lead to the analytical description of the permeability of a porous medium to gas flow as a function of layer thickness. The derivations are made on the preconditions that the reflection at pore surfaces is diffuse and that the pore structure is homogenous on a scale much larger than the pore size. By applying a bi-hemispherical Maxwell distribution, relations between the layer transmission probability, the half-transmission depth and the Knudsen diffusion coefficient are obtained. For packings of spheres, expressions of these parameters in terms of porosity and grain size are derived and compared with former standard models. A verification of the derived equations is given by means of numerical simulations, also giving evidence that our analytical model for sphere packing is more accurate than the former classical models.

Comet observation and analysis is an area where amateur observers can make a significant contribution. Their observations allow regular monitoring of comets, alerting the professional community to interesting events, and providing raw data to supplement professional data. The links between the professional and amateur communities are therefore very important. Comet analysis is challenging. Encouraging the community to agree consistent methodologies, and parameters for analysis, will result in a more robust data set for monitoring and analysis. The last comet workshop was held in 2015 and the comet community welcomed the opportunity to meet again. The main aims and objectives of the workshop were agreed, following consultation, to be: To foster stronger working relationships and cooperation within the professional and amateur comet community, based on a shared understanding of the challenges and opportunities. To take stock of where cometary science stands post-Rosetta and how Pro-Am observations fit into potential future research. To draw together the various strands of work currently going on within the community, particularly on coordination, techniques, standards and archiving and agree the way forward. To consider how best to encourage, and equip, more people to become involved in the study of comets, whether directly through observation (including access to the Europlanet Telescope Network), or through analysis of online data sources. To explore how cometary science can be used in outreach and education. It was also agreed that speakers should include a wide range of amateurs, students and professional astronomers, and that allowing ample time for panel-led discussions was very important. The panels should include specialists and society representatives. An accessible location was chosen, the Stefanik Observatory in Prague, along with online access. We will report on the outcomes from the hybrid workshop and the proposed next steps. Acknowledgements The workshop has been organised in cooperation with Europlanet 2024 Research Infrastructure (RI), the British Astronomical Association, Planetum Prague, and the Czech cometary community SMPH. Europlanet 2024 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871149.

The success of the Rosetta mission to comet 67P/Churyumov–Gerasimenko has revolutionized our view of comets, while opening a plethora of new questions. In order to find the answers to them and harness the full potential of the new data, an international consortium named “Cophylab – Comet Physics Laboratory” (Cophylab.space) was launched in 2018. In this project several experiment campaigns were initialized to study cometary properties in a controlled environment. The idea was to isolate individual properties and processes in dedicated laboratory experiments. One of the experiment campaigns was designed to characterize gas flow properties of dry porous materials in a first step, with the aim of developing a model that improves our understanding of the outgassing of comets.Before the initial model is extended to consider the sublimation of volatile components, it needs to be validated by alternative methods, such as numerical simulations. For this purpose, we chose the finite element method, to test the combination of the Darcy and Knudsen flow model, which was used in the preceding study.Our approach was to use the results of the experiment as input in the simulations and compare the output with the measurements. This comparison confirmed the validity of the model and its assumptions. In particular, the sample is assumed to be homogenous and isotropic on a macroscopic scale, so that it can be described by a set of averaged parameters. While this description is relatively accurate for samples with well-defined grain shapes (e.g. spherical glass beads), significant discrepancies occur for inhomogeneous materials such as lunar, Asteroid or Martian analogues.We investigated various aspects that were initially neglected in the evaluation of the measurements, such as channel building in the sample, boundary effects and non-ideal geometry of the experimental setup, which will be complemented by inhomogeneities that occur naturally in random close packing or ballistic deposition samples. Furthermore, we assessed the models range of applicability through a thorough review of the different flow regimes encountered in the measurements. Our findings indicate that boundary effects, as well as non-ideal geometry have a significant influence particularly in samples with larger grains. For finer grained samples on the other hand, inhomogeneities are the most probable cause for discrepancies. The grain size also plays an important role regarding the flow regime and its corresponding parameters.The work for this study was performed in the framework of a master’s thesis, as part of the Cophylab project, which is funded by the D-A-CH program (DFG GU1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36)

A profound understanding of gas flow in porous media is of great interest for various technological and scientific fields. Its investigation by laboratory measurements, however, poses several challenges. In particular, the determination of macroscopic flow parameters from pressure and gas flow measurements is prone to various error influences, some of which are very difficult to analyze experimentally. Computer simulations are a solution in this context as they facilitate modifications of the underlying geometry and boundary conditions in a flexible way. Here we present a simulation framework for the analysis of a recent experiment for determining the Knudsen diffusion coefficient and viscous permeability of various porous granular materials. By combining the finite element method with analytical models and other numerical methods, we were able to identify previously neglected physical effects that increase the uncertainty of the measurements. In particular, the porosity increase due to finite sample dimensions, in a layer of about a grain diameter thickness near the container wall, creates a deviation of the measured pressure gradient. This deviation amounts to ca. 5% for a sample width of about 100 grains and a porosity of 0.5, and is indirectly proportional to the porosity. The second most prominent error source, the sample support sieve, causes a slight constriction of the flow volume. Simulations of this effect show an error around 4%–7%, dependent on the grain size. Based on these findings we recommend an overall sample dimension of 100 grains or larger. As an example of failures of the sample homogeneity, we elaborate how channels through the sample influence the flow properties. Respective suggestions for keeping all discussed effects negligible are discussed in detail. Our methodology demonstrates how the combination of finite element computations with analytical representations of the involved macroscopic parameters can assess the validity and accuracy of laboratory experiments.

To validate the physical model used to describe gas flow through porous, granular media in recent experiments [1], we conducted a finite element analysis (FEA). The main parameters of a sample, such as gas permeability, Knudsen diffusion coefficient and porosity were determined from the experiment and used as an input for the simulations, with the aim of replicating the pressure evolution (measured quantity) depending on external gas flow. While the results matched well for samples with large and spherical grains, the deviations were significant for materials with small and irregularly shaped grains. An improvement of the model could be achieved with complementary simulation methods and a more detailed analysis of the samples. 1. Introduction In the past decades, multiple cometary space missions (e.g. Deep Impact and Rosetta) have provided us with better data and understanding of the solar system’s primordial objects. The most recent project is a cooperation of six European institutions, called “Cometary Physics Laboratory (CoPhyLab)” and has already produced a plethora of results (see webpage https://cophylab.space/index.php?id=27). In this work we investigated gas flow through porous media with a simulation approach, in order to compare it with the experiment and validate its physical model. 2. Experiment The basis and motivation of this work was the experiment performed by [1] (see Figure 1), where the gas flux through a set of artificial and natural porous materials was analysed. To describe the flux J [mol∙m-2∙s-1] of dry air in the given pressure range, a combination of viscous flow and free molecular diffusion was chosen as the physical model: The gas permeability B­0 (viscous flow) and Knudsen diffusion coefficient DK (molecular diffusion) are the main parameters which describe a sample’s ability to allow gas to flow through it. They were determined by measuring the steady state pressure difference across the inlet and outlet of the sample for different gas flux levels. The porosity of the materials was measured through packing experiments. 3. Simulation Since the physical model describes the gas flow on a macroscopic level, with the main parameters being determined as average values across the whole sample, a FEA is the ideal method. The simulations were computed with the software “COMSOL Multiphysics®”, in which the sample was modelled as a cylinder (see Figure 2). The built-in physics module “Darcy’s Law” was selected to provide the governing equations and all input parameters were taken from the measurements (sample dimensions, B0, DK, porosity, viscosity etc.). In stationary studies, the pressure at the sample outlet and the inward mass flux of the measurements were defined as boundary conditions for the simulation, with the pressure in the rest of the sample being the free variable. For transient studies, the empty volumes of the vacuum chamber, as well as the pressure dependent performance characteristic of the pump were taken into consideration to be able to compare the dynamic process. Furthermore, variation studies of the main parameters were carried out to investigate their individual influence on the pressure evolution. Boundary effects such as the facilitation of gas flow due to lower porosity at the container wall or opening of channels in the sample, or impediment due to the holding sieve, were also analysed by adapting the material parameters for the respective boundary layers. 4. Results For the comparison of stationary simulation results with those of the experiment, the measured pressure at the inlet face of the sample was taken as a boundary condition and for transient simulations also the outlet face pressure (see Figure 3 for an exemplary comparison of results). The deviations were relatively small for synthetic material with large grain sizes (i.e. glass beads with a diameter above 200 µm). However, for smaller grain sizes and natural materials the discrepancies were quite large (up to 35%) and featured no apparent correlation regarding the amount of gas flow. Regarding parameter variation studies, it is important to note that the creation of physical samples where only one parameter varies is very difficult, because many of the parameters are directly or indirectly coupled with each other. Nevertheless, the results indicated that in regimes of low B0 and DK, as is the case for small grain size samples, a variation of either parameter has a large influence on the pressure evolution. This is particularly important when considering measurement and regression errors. When boundary layers were added for the analysis of boundary effects (higher permeability and diffusion at the cylinder wall or in a channel), most of the gas flowed through it and the pressure difference across the sample was minimal. Deviations between simulations with and without the holding sieve, manifested themselves as temporary spikes when the gas flow was increased, with steady state differences below 2%. 5. Summary and Outlook The results indicate that the applied model is well consistent with the measurements of homogenous samples (e.g. synthetic materials with large, spherical grains). However, in the case of small and irregularly shaped grains, the material’s anisotropy could explain the discrepancies. To improve the existing model, it is necessary to analyse the physical samples on a smaller scale and better understand the interdependencies of the governing parameters. This can be achieved by creating virtual samples and using alternative simulation methods (i.e. Monte Carlo), which work on a microscopic scale. 6. Acknowledgements This work was carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (GU 1620/3-1 and BL298/26-1/SNF 200021E 177964/FWF I 3730-N36). References [1] M. Schweighart, W. Macher, G. Kargl, B. Gundlach, H. L. Capelo, Viscous and Knudsen gas flow through dry porous cometary analogue material, Monthly Notices of the Royal Astronomical Society, 2021;, stab934, https://doi.org/10.1093/mnras/stab934

Since February 2020, the Horizon 2020 research infrastructure project Europlanet 2024 (EPN 2024 RI; website: bit.ly/2Z1XA8u) is serving the planetary sciences community with diverse schemes and funding opportunities. As part of this project, the Europlanet Telescope Network (website: bit.ly/37SCiyj), a collaboration between small telescope facilities in Europe and beyond, kick-started in June of the same year with the goal to provide coordinated observations, support for space missions, and to better integrate amateur astronomers into planetary sciences. Since then, the network is providing access for professional and citizen scientists to 16 different observatories ranging from 40cm to 2m in size (see figure and list below). While the network plans to draw in additional facilities and to build new collaborations, scientists and amateurs can apply through a rolling Call for Observations (website: bit.ly/2Br5LDt) to visit those facilities that are already participating in the network and have their expenses reimbursed for the time needed to make their observations. This also comprises the possibility of remote observations since the network includes 14 robotic or remotely operated facilities. Any of the short applications to the call (2 to 3 pages in size) will be reviewed anonymously by the Science Advisory Panel on a bi-monthly basis which should assure a relatively fast reaction to potential observational alerts. Even though the focus of the network is on planetary sciences and exoplanets, the project does also accept applications covering other scientific areas. The first proposals granted, therefore, cover a diverse scientific field, ranging from asteroids to exoplanets, and the observation of variable nebulae. This presentation will review the Europlanet Telescope Network and the facilities involved, how researchers and amateur astronomers can apply, and will also give examples on the first observations performed at the network. Besides coordinated observations at the Europlanet Telescope Network, this project also supports the integration of amateur astronomers into planetary sciences through the organization of dedicated workshops and the provision of a newly developed web platform for the issuing of observational alerts and the organization of coordinated campaigns. While we will overview these activities in the second part of our presentation, we will also give an outlook onto the upcoming years and on the support that we are yet planning to provide for amateur astronomers and the planetary sciences community. The telescopes in the network: 1. Pic du Midi Observatory, France: 1.06-m telescope2. Moletai Astronomical Observatory, Lithuania: 1.65-m and 35/51-cm telescopes3. Kryoneri Observatory, Greece: 1.2-m telescope4. Skalnate Pleso Observatory, Slovakia: 1.3-m and 61-cm telescopes5. Faulkes Telescope Project: two 2-m, nine 1-m, and ten 40-cm robotic telescopes6. Tartu Observatory, Estonia: 1.5-m and 60-cm telescopes, and 30-cm robotic telescope7. Danish 1.54-m telescope at La Silla Observatory, Chile: 1.54-m telescope8. Beacon Observatory, UK: 42-cm telescope9. Observatorie del Teide, Spain: 82-cm and 45-cm telescopes10. Calar Alto Observatory, Spain: 1.23-m telescope11. Lisnyky Observation Station, Ukraine: 70-cm telescope12. Chuguev Observatory, Ukraine: 70-cm telescope13. Terskol Peak Observatory, Ukraine: 2-m and 60-cm telescopes14. Konkoly Observatory, Hungary: 1-m and 80-cm telescopes15. Ussuriysk Astrophysical Observatory, Russia: 25-cm and 50-cm telescopes16. Rozhen Observatory, Bulgaria: 2-m, 60-cm, and 50/70-cm telescopes Further information on the different facilities can be found at bit.ly/2Br5LDt. Acknowledgements. Europlanet 2024 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871149.

CoPhyLabLaboratory experiments are of major importance to understand the activity of comets and to support future space missions. However, past comet simulation experiments were performed under the assumption that comets are mainly composed of water ice with only a limited amount of dust. In the past years, however, the Rosetta mission has shown that cometary nuclei consist primarily of dust and less volatile materials are present than previously thought. Hence, it is high time to set up a new series of laboratory experiments with the aim to investigate the physics of realistic cometary analogue materials. This task is currently addressed by the CoPhyLab (Comet Physics Laboratory) which is a joint project among different partner institutions. This laboratory aims at studying the physics of cometary analogue materials. This task is approached by firstinvestigating isolated physical properties in so-called small experiments (S experiments). In a next step, the experiment’s complexity is increased step-by-step by either adding further components to the sample, or by studying several physical properties under different conditions (large experiments, which will be performed in the L chamber). S1: the tensile strength of organic materialsThe knowledge of the tensile strength of the cometary surface is of key importance to better understand the activity of comets. The tensile strength determines the strain required to detach material from the surface. As organic materials are ubiquitous in space, they could have played an important role during the planet formation process and are most likely incorporated into cometary nuclei. This S experiment campaign provides new measurements on the tensile strength of various granular organic materials. These materials are investigated by the Brazilian Disc Test and the measured values are normalised to a grain size of one micrometer and a volume filling factor of 0.5 for better comparability. The experiments show that the tensile strength of organic materials ranges over four orders of magnitude. Graphite and paraffin have much higher tensile strengths values compared to silica, whereas the tensile strength of coals is very low. This work demonstrates that organic materials are not generally stickier than silicates, or water ice. S2: gas permeability of analogue materials The cometary nucleus is made of water ice, organics and silicateous dust and the ice is trapped inside the matrix of non-volatiles. Hence, the evolving gas has to stream away from its originating region inside the surface layers towards the surface. This work package has the aim to investigate the gas transport mechanisms through porous cometary analogue materials. Therefore, gas flow measurements are performed to investigate the permeability of several materials, which are chosen to mimic cometary surface properties. With these measurements, the gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained. These simulants are tested with respect to different filling heights, packing properties and grain shapes. The gas flow experiments show that the grain size distribution and the packing density of the samples are primarily influencing the permeability of the sample. S3: thermal conductivity of analogue materialsMeasurements of the thermal properties of analogue materials are essential in interpreting remote sensing data and the findings of in-situ instruments. The thermal properties of the subsurface layers determine the surface temperature of asteroids and comets. The temperature stratification inside planetary object is a key parameter to understand the processing of their interior. This experiment campaign is dedicated to measure the thermal properties of analogue materials. In preparation for these measurements we have set up a small vacuum chamber equipped with an infrared camera and temperature sensors. The samples are illuminated for a short duration by a laser. We then compare the measured temperature profiles with the predictions of a thermophysical model to determine the thermal conductivity of the samples. S4: ejection of materialWhen comets approach the Sun, the sublimation pressure will be reached inside the material. If the tensile strength is exceeded by the evolving pressure, the particles can be ejected from the cometary surface and are accelerated. However, the details of the dust dynamics close to the surface are not understood in detail. The idea of this S experiment campaign is to develop an experimental routine to track ejected particles from a sample composed of granular water ice. Therefore, we recorded the power of the illumination, the temperature of the sample and measured the particle trajectories with an high speed camera. Furthermore, the experiments are also simulated by a thermophysical model. Our experiments show that samples composed of pure granular water ice can eject water-ice particles by the pressure build up of water vapour in their interior. Compressed samples posses an higher activity level (ejection events per second) compared to uncompressed samples. The ejected particles have a non-zero initial velocity which is most probably caused by a very fast acceleration of the particles before the first data point is recorded by the camera. The L chamberThe core of this project is the realisation of a comet simulation chamber which will be capable to utilise multiple instruments to monitor and measure the sample properties before, during and after the experiment campaigns. This chamber will be used to perform long duration experiments at low temperatures and low pressures. At this stage (end of June, 2020), the chamber is already installed in place and is vacuum tight, the cooling shield is assembled and the sample carrier cart as well as the self-made glove box are ready to use. The next steps comprise the integration of the cooling shield and the main cooling system. We foresee to run the first experiments in approximately six weeks from now. During the EPSC conference we will provide a technical overview of the chamber and we will present the first experiments performed in the L chamber. AcknowledgementsThis work was carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (GU 1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36). DB and JB thank the Deutsches Zentrum f\"ur Luft- und Raumfahrt for support under grant 50WM1846.

Over the last few decades, our picture of comets has been continuously changing and growing due to several successful space missions, as well as cometary simulation projects in the laboratory (e.g. KOSI 1987-1992, CoPhyLab 2018 - 2021). This work aims for a better understanding of the gas transport through a porous cometary surface layer. Therefore, gas flow measurements have been performed in our laboratory to investigate the permeability of several analogue materials, which have been chosen to mimic cometary surface properties.For the first measurements, which we are reporting here, only dry materials, free of volatiles have been selected, to isolate the gas transport from gas production inside the materials. They include glass beads made of soda lime glass, which are sieved into separate fractions to obtain distinct grain size ranges from 45 µm up to 4.3 mm. The Mars simulant JSC-Mars 1 is used in the experiments, as well as JSC-1 as a lunar soil simulant. Furthermore, an Asteroid analogue material named UCF/DSI-CI-2 from the Exolith Lab in Florida is also used. A quartz sand called UK4 mined at a local quarry in Graz is investigated as well. In a further step, a sample is created by mixing different grain size fractions of the glass beads replicating the grain size distribution of the Asteroid simulant.The materials are also treated on a shaking table in order to obtain the packing properties of the samples. For the gas flow experiments a cylindrical sample container, with 4 cm diameter, is filled with the sample (30 mm in height) and placed inside of the vacuum chamber at the interface of two separate volumes. Four pressure sensors covering different pressure ranges monitor the gas pressure in the two volumes. A vacuum pump in the lower volume removes the gas from the chamber and through a gas inlet a defined flow of the test gas (compressed air) is inserted into the upper volume. Due to this set-up, the gas flow can only pass through the sample material. To avoid particle fluidisation and thus a texture change in the sample the gas flow is intentionally directed downwards through the sample. The gas flow is controlled by regulators from 0.15 mg/s up to 19.2 mg/s. Via the measured pressure difference between the upper and lower volume, in equilibrium flow, the gas permeability and the Knudsen diffusion coefficient of the sample material are obtained. The gas flow experiments show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. From the analysis of the permeability measurements it is clearly visible that the larger the grains the bigger the permeability. The measured permeability values range from 10-13 to 10-8 m². This work is part of the CoPhyLab project funded by the D-A-CH programme (DFG GU1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36).

The detection and characterization of lunar resources, including water ice, is a key area of interest for a new generation of lunar missions. The electrical properties of water ice in the extremely low frequency (ELF) range support its detection by means of in-situ permittivity measurements. A new type of miniaturized subsurface permittivity sensor is presented, which is under development for the PROSPECT package on the Luna-27 lander mission. Here, the sensor concept is described, key design features are presented and possible operations modes are introduced. The expected accuracy for measurements of the relative permittivity along the borehole is ~10-15% and depends mainly on the accuracy of borehole geometry models. Laboratory test results from a prototype sensor on a water ice / simulant mixture at cryogenic temperatures are presented, demonstrating the capability to detect water ice at 125 K. The improved capabilities of the flight design are discussed, operational modes are explained and alternative electrode configurations for lunar landers and rovers are proposed.

Banerdt, William B. et al., NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude M = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indicate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately M = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding M = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Planetary sciences is an exciting part of astronomy, and even though space missions are nowadays providing high-quality data from remote and in-situ measurements of the planets, asteroids and comets, ground-based observations are still yielding exciting results by providing crucial data sets for a better understanding of the diverse bodies in the Solar System. The discovery of thousands of exoplanetary systems during the last few decades further enhances the need for ground-based observations, collaboration between small telescope facilities and for strengthening the contribution of amateur astronomers to planetary sciences. Access to the Europlanet Telescope Network As part of the recently launched Europlanet 2024 Research Infrastructure (EPN 2024 RI; https://bit.ly/2Z1XA8u), a new collaboration between telescopes around the world has been started for providing coordinated observations and rapid responses in support of space missions and in following-up of new events. The so-called Europlanet Telescope Network (https://bit.ly/37SCiyj) will thereby provide professional scientists and amateur astronomers with access to an initial set of 16 telescope facilities (see Fig. 1) up to 2 m in diameter. While the network plans to draw in additional facilities and to build new collaborations, particularly in regions that are currently under-represented in planetary sciences, scientists and amateurs can now apply to visit those facilities that are already participating in the network and have their expenses reimbursed for the time needed to make their observations. This also comprises the possibility of remote observations since the network includes some robotic or remotely operated facilities. The Call for Observations (see website: https://bit.ly/2Br5LDt) runs from June 2020 until the end of 2023, and is coordinated through EPN 2024 RI, which is funded by the European Commission’s Horizon 2020 program. Fig. 1: The Europlanet Telescope Network (see also Section The Telescopes). Supporting Amateur Astronomers Besides fostering collaboration and providing access to small telescope facilities, the Europlanet Telescope Network also plans to support the integration of amateur astronomers into planetary sciences, since their observations can be of crucial importance for several scientific areas. Planetary atmospheres, for instance, is a field that benefits intensively from amateur astronomers. Their regular observations at high-spatial resolution are used by researchers to perform detailed studies of the atmosphere dynamics of Jupiter and Saturn and surveys of atmospheric activity on Neptune. They also provide excellent coverage of dust storms on Mars and complementary observations of Venus to space missions that observe over different viewing geometries. The accumulated observing time provided by amateurs makes it likely that unexpected events (such as convective storms on Jupiter and Saturn; impacts on Jupiter) are discovered by amateurs faster than by orbiting spacecraft or regular ground-based observation programs. Many amateurs supply their data to public data repositories that cover multiple planets like ALPO-Japan (http://alpo-j.sakura.ne.jp) or PVOL (http://pvol2.ehu.es/pvol2) on a daily basis, resulting in extremely fast follow-ups of interesting events. Space missions like NASA’s Juno keep excellent interactions with amateur astronomers that provide a large amount of observations to the mission through a dedicated website (https://www.missionjuno.swri.edu/junocam) used by Juno’s scientists to target some of their high-resolution observations. Advanced asteroid studies by using the GaiaGOSA web service (gaiagosa.eu) is another important example on how amateur observers can help planetary scientists. The photometric observations of asteroids provided by amateurs are essential for the determination of rotation periods. Data gathered from a few oppositions allow to determine detailed physical parameters that are crucial for determining the nature of these particular objects such as spin axis orientation, shape, and size, but also for verifying our understanding of the history of the Solar System. Recently, the data gathered by amateur observers helped to determine more precise shapes and volumes of 13 asteroids (Podlewska-Gaca et al., 2020, A&A, id.A11; see also Fig. 2) and to reveal the true nature of asteroid (10) Hygiea, which turned out to be a perfect candidate for a dwarf planet rather than an asteroid (Vernazza et al., 2020, Nature Astr., 136-141). Fig. 2: The shape of (68) Leto asteroid determined with help of GaiaGOSA observers. The Telescopes (Fig. 1): Pic du Midi Observatory, France: 1.06-m telescope Moletai Astronomical Observatory, Lithuania: 1.65-m and 35/51-cm telescopes Kryoneri Observatory, Greece: 1.2-m telescope Skalnate Pleso Observatory, Slovakia: 1.3-m and 61-cm telescopes Faulkes Telescope Project: two 2-m, nine 1-m, and ten 40-cm robotic telescopes Tartu Observatory, Estonia: 1.5-m and 60-cm telescopes, and 30-cm robotic telescope Danish 1.54-m telescope at La Silla Observatory, Chile: 1.54-m telescope Beacon Observatory, UK: 42-cm telescope Observatorie del Teide, Spain: 82-cm and 45-cm telescopes Calar Alto Observatory, Spain: 1.23-m telescope Lisnyky Observation Station, Ukraine: 70-cm telescope Chuguev Observatory, Ukraine: 70-cm telescope Terskol Peak Observatory, Ukraine: 2-m and 60-cm telescopes Konkoly Observatory, Hungary: 1-m and 80-cm telescopes Ussuriysk Astrophysical Observatory, Russia: 25-cm and 50-cm telescopes Rozhen Observatory, Bulgaria: 2-m, 60-cm, and 50/70-cm telescopes Further information available at https://bit.ly/2Br5LDt. Acknowledgements: Europlanet 2024 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871149.

The NASA InSight Mission is expected to be launched in 2018 after the initial date of the launch in 2016 had to be postponed due to technical problems. The major tasks of the InSight Mission will be to investigate the appearance of tectonic activity, as well as to obtain more accurate information on the interior structure of Mars and the detection of the inner heat flow that is created by the core. Therefor, a seismometer (SEIS) and a heat flow probe (HP3) will be placed by the InSight lander onto the surface of the planet. The HP3 will penetrate 3 - 5 metres deep into the ground of Mars to avoid the influence of the solar radiation on the heat flow measurement and to allow measurement of the temperature gradient using thermal sensors on the trailing cable at different depths. The penetration process will be used additionally to investigate the mechanical behaviour of the soil by measuring the displacement of the probe after a set of strokes. The dynamics of the hammering mechanism and its penetration performance is dependent on the soil behaviour. Therefore, a coupled simulation of the hammer mechanism and the soil response is used to investigate the behaviour of the soil and the performance of the HP3. The simulation is done by the use of the Discrete Element Method (DEM) which allows to simulate both the hammering and penetration in one model, where the coupling is done by velocity and force exchange. The simulation model is used to investigate the densification of the soil due to the dynamic penetration process, to determine the displacement of the probe and to investigate the influence of confining boundaries, which are unavoidable in laboratory tests. The use of the DEM allows to investigate the behaviour at the particle level, so that the simulations provide insights in the soil behaviour at micro-scale, e.g. the force chains transmitting the applied load to the soil or the track of the particles during the penetration.

The NASA InSight mission will provide an opportunity for soil investigations using the penetration data of the heat flow probe built by the German Aerospace Center DLR. The Heat flow and Physical Properties Probe (HP^3) will penetrate 3 to 5 meter into the Martian subsurface to investigate the planetary heat flow. The measurement of the penetration rate during the insertion of the HP3 will be used to determine the physical properties of the soil at the landing site. For this purpose, numerical simulations of the penetration process were performed to get a better understanding of the soil properties influencing the penetration performance of HP^3. A pile driving model has been developed considering all masses of the hammering mechanism of HP^3. By cumulative application of individual stroke cycles it is now able to describe the penetration of the Mole into the Martian soil as a function of time, assuming that the soil parameters of the material through which it penetrates are known. We are using calibrated materials similar to those expected to be encountered by the InSight/HP^3 Mole when it will be operated on the surface of Mars after the landing of the InSight spacecraft. We consider various possible scenarios, among them a more or less homogeneous material down to a depth of 3–5 m as well as a layered ground, consisting of layers with different soil parameters. Finally we describe some experimental tests performed with the latest prototype of the InSight Mole at DLR Bremen and compare the measured penetration performance in sand with our modeling results. Furthermore, results from a 3D DEM simulation are presented to get a better understanding of the soil response.

The thermal properties of the surface and subsurface layers of planets and planetary objects yield important information that allows us to better understand the thermal evolution of the body itself and its interactions with the environment. Various planetary bodies of our Solar System are covered by so-called regolith, a granular and porous material. On such planetary bodies the dominant heat transfer mechanism is heat conduction via IR radiation and contact points between particles. In this case the energy balance is mainly controlled by the effective thermal conductivity of the top surface layers, that can be directly measured by thermal conductivity probes. A traditionally used method for measuring the thermal conductivity of solid materials is the needle-probe method. Such probes consist of thin steel needles with an embedded heating wire and temperature sensors. For the evaluation of the thermal conductivity of a specific material the temperature change with time is determined by heating a resistance wire with a well-defined electrical current flowing through it and simultaneously measuring the temperature increase inside the probe over a certain time. For thin needle probes with a large length-to-diameter ratio it is mathematically easy to derive the thermal conductivity, while this is not so straightforward for more rugged probes with a larger diameter and thus a smaller length-to-diameter ratio. Due to the geometry of the standard thin needle probes they are mechanically weak and subject to bending when driven into a soil. Therefore, using them for planetary missions can be problematic. In this paper the thermal conductivity values determined by measurements with two non-ideal, ruggedized thermal conductivity sensors, which only differ in length, are compared to each other. Since the theory describing the temperature response of non-ideal sensors is highly complicated, those sensors were calibrated with an ideal reference sensor in various solid and granular materials. The calibration procedure and the results are described in this work.

Several planetary lander missions conducted in the past and planned for the near future have instruments on board, which are dedicated to the determination of various material properties, among them mechanical properties of the surface like material strength and penetration resistance. In this paper two instruments are considered in more detail: (i) the MUPUS penetrator, a device aboard the Lander Philae of ESA׳s Rosetta mission, and (ii) the Mole HP 3 , which is part of the payload of NASA׳s next Discovery mission InSight , due for landing on Mars in 2016. Both devices are driven by hammering mechanisms designed to work under low or micro-gravity conditions and blaze themselves a trail into the subsurface of their respective target bodies. Naturally the speed with which this process takes place and if penetration is possible at all depends on the mechanical properties of the soil. However, a quantitative evaluation of soil mechanical parameters from measured depth-versus-time data is not a straightforward task. In this paper we apply an old technique, originally developed for modelling the driving of a pile into the ground, to describe the performance of penetrators and Moles developed for planetary applications. The numerical pile driving model of Smith (1962) is scaled and adapted for this purpose and used to predict the penetration behaviour of these instruments in dependence of their internal construction and the properties of the soil they are driven in. The model computes the permanent set of the surrounding soil in response to one hammer blow cycle as well as the oscillations and waves excited inside the devices and in the surrounding soil. Both the penetration resistance of the tip and the resistance caused by friction of the penetrator along the cylindrical side wall are calculated. By comparing the modelling results with previous laboratory measurements it is demonstrated that the models produce realistic results and can be used both as tools for proper design of future penetration devices and for the evaluation of soil-mechanical properties from measured penetration data. The most sensitive soil parameter controlling penetration progress per stroke is the ultimate static bearing capacity of the ground. Although the method used for our calculations is essentially the same as that applied by Smith (1962) for describing the behaviour of long and heavy piles in geotechnical engineering, it is found that the penetration process for such small and light weight systems takes place in a quite different way. While for the former secondary oscillations are irrelevant for the determination of the permanent set caused by one stroke, in the latter the penetration in response to one hammer stroke takes place in many small increments, until a stable depth is reached. Our model describes the full dynamic behaviour of the system over an arbitrary time span and has the potential to be adapted to systems with other geometrical dimensions and masses, as long as the diameter/length ratio of the probe remains reasonably small.

Context. The Microwave Instrument for the Rosetta Orbiter (MIRO) was one of the key instruments of the Rosetta mission, which acquired a wealth of data, in particular as the orbiter moved in the close environment of comet 67P/Churyumov-Gerasimenko (August 2014–September 2016). It was the only instrument of the Rosetta payload that was able to measure temperatures in the near-subsurface layers of the cometary nucleus down to a depth of some centimeters. This range is most relevant for understanding the mechanisms of cometary activity. Aims. We simulate the 3D temperature distribution for two selected regions that were observed by MIRO in March 2015 when the comet was at a distance of about 2 au from the Sun. The importance of a full 3D treatment for a realistic subsurface temperature distribution and the thermal heat balance in the uppermost subsurface is investigated in comparison with analogous 1D simulations. Methods. For this purpose, we developed a numerical heat transfer model of the surface as well as the near-subsurface regions. It enabled us to solve the heat transfer equation in the subsurface volume with appropriate radiation boundary conditions taken into account. The comparison with 1D simulations was made on the basis of the same solar irradiation history. Results. Although the temperature gradient is predominantly normal to the comet surface, we still find that tangential flows may be responsible for local temperature differences of up to 30 K (a few Kelvin on the average) in the uppermost subsurface layers. From the results of the 3D simulations, we calculated the MIRO antenna temperature. A comparison with the actual measurements shows good agreement for the MIRO submillimeter channel, but there is a notable discrepancy for the millimeter channel. This last assessment is not related to the use of the 3D model; potential causes are discussed in some detail with a view to future studies.

In our current understanding, active cometary nuclei comprise a volatile-depleted outer crust covering a mixture of dust and ices. During each perihelion passage the thermal wave penetrates the crust and sublimates a portion of these ices, which then escape the nucleus, dragging with them dust particles that replenish the coma and dust tail. The flux of released gases is likely to vary as a complex function of solar distance, nucleus structure, spin rate, etc. It has been previously hypothesised that at some point a fluidised state could occur, in which the gas drag is approximately equal to the weight of overlying dust and ice grains. This state is well understood and used in industrial processes where extensive mixing of the gas and solid components is desired. The literature on fluidisation under reduced gravity and pressure conditions is here reviewed and published relations used to predict the conditions under which fluidisation could occur in the near-surface of a cometary nucleus. The general trend is that the minimum fluidisation velocity decreases with reducing gravity but increases with reducing pressure. However, true fluidisation is no longer possible in free molecular flow, when the gas viscosity is meaningless. As a result, fluidisation is unlikely to be driven by H2O sublimation below a dust mantle, where pressures are too low to allow such flow. Requisite pressures could, however, be achieved at the crystalline/amorphous phase change boundary, which could support fluidisation if the overlying material is granular.

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

Icy surfaces like the polar caps of Mars, comets, Edgeworth–Kuiper belt objects or the surface areas of many moons in the outer Solar System behave different than rock and soil surfaces when irradiated by solar light. The latter ones absorb and reflect incoming solar radiation immediately at the surface. In contrast, ices are partially transparent in the visible spectral range and opaque in the infrared. Due to this fact it is possible for the solar radiation to reach a certain depth and increase the temperature of the sub-surface layers directly. This internal temperature rise is called “solid-state greenhouse effect,” in analogy to the classical greenhouse effect in an atmosphere. It may play an important role in the energy balance of various icy bodies in the Solar System. Within the scope of a project conducted at the Space Research Institute of the Austrian Academy of Sciences in Graz the solid-state greenhouse effect was investigated experimentally and theoretically. A number of experiments with diverse materials, focussing mainly on layered samples with a surface cover consisting of transparent H 2 O-ice, were performed. The samples were irradiated under cryo-vacuum conditions by a solar simulator. The temperature distributions inside the samples were measured and compared with the results of numerical model calculations. We found that the predicted sub-surface temperature maximum is very clearly measurable in glass beads samples with various particle size distributions, but can also be detected in transparent compact surface ice layers. However, in the latter case it is less distinct than originally expected. Measuring the effect by laboratory methods turned out to be a difficult task due to the shallow depth where the temperature maximum occurs.

European Space Agencies fifth cornerstone mission BepiColombo includes a ‘Surface Element’ to land a scientific payload on the surface of Mercury. The current strawman payload includes a heat flow and physical properties package (HP3), focussing on key thermal and mechanical properties of the near-surface material (down to a depth of 2– 5 m ) and the measurement of heat flow from Mercury's interior, an important constraining parameter for models of the planet's interior and evolution. We present here an overview of the HP3 experiment package and its possible accommodation in a self-inserting ‘mole’ device. A mole is considered to be the most appropriate deployment method for HP3, at least in the currently-assumed case of an airbag-assisted soft landing architecture for the Mercury Surface Element.

The main aspect of this paper is to provide a synthesis between two major lines of development in the understanding of mass and heat transfer in a volatile porous medium. The first one is a macroscopic approach, where the medium is considered as a continuum, and heat and mass transfer equations are solved under appropriate boundary conditions for temperature and gas pressure (G. Steiner and N. I. Komle 1991, Planet. Space Sci. 39 , 507–513; Y. Mekler et al. 1990, Astrophys. J. 356 , 682–686; S. J. Espinasse et al. 1991, Icarus 92 , 350–365), while the second one is a kinetic model, calculating gas flow in tubes under the assumption of a known temperature distribution (N. I. Komle and G. Dettleff 1991, Icarus 89 , 73–84; Yu. V. Skorov et al. 1999, Icarus 140 , 173–188). We review briefly the main aspects of this previous work, and subsequently present a combined consistent model, which uses a macroscopic heat transfer equation, but kinetic solutions for the gas flow. This new model was implemented as a numerical code and its performance is demonstrated by a couple of example calculations. The main advantage of the new model in comparison to the macroscopic approach is the fact that it avoids specifying a boundary condition for gas pressure at the surface, because the emitted gas flux is found at any time with the aid of the kinetic calculation. The local balance of sublimation and condensation in the interior of the porous ice can be calculated more consistently than is possible by macroscopic models only, because surface pressure and density develop in a “natural” way and no external boundary condition for the pressure must be imposed. We consider the development of temperature distribution and gas flux in ice samples in response to surface irradiation. Both pure ice and ice covered by a dust mantle are studied. The results are compared with corresponding solutions obtained on the basis of a macroscopic model, and differences are discussed in detail. Finally, experimental data obtained from previous comet simulation experiments (KOSI and related laboratory experiments) are reconsidered. In particular, temperature profiles and gas fluxes from the KOSI-9 experiment (E. Grun et al. , 1993, J. Geophys. Res. 98 , 15,091–15,104) are interpreted in terms of the kinetic approach.

The first — and possibly deepest — in situ science measurements on the 46P/Wirtanen nucleus will be made by two sensors of the Rosetta Lander's MUPUS experiment. A piezoelectric shock accelerometer (ANC-M) and a resistance temperature sensor (ANC-T) will be mounted in the Lander's harpoon anchor. This will be shot into the surface at about 60 m s −1 on touchdown, reaching a final depth of between a few centimetres and about 2.5 m, depending on the hardness of the ground and the maximum available cable length. Early indications of the strength of the surface material and any distinct layers should prove valuable to subsequent depth-sensitive investigations, including the MUPUS thermal probe, seismic sounding experiments, the sampling drill and composition analyses of the extracted material. Interpretation of the ANC-M data will help to constrain models of the formation and evolution of the material found at the landing site and document the mechanical and structural context of nearby sampled material. We report on the results of recent test shots performed with a prototype anchor into several porous materials: two types of glass foam, H2O ice and CO2 ice. With the help of data from direct shear tests and quasi-static penetration tests, we interpret the processed deceleration data using a cavity-expansion penetration model. Layers of distinctly different strengths can be detected and located, and the deceleration profiles are in reasonable agreement with the profiles obtained by quasi-static tests. The anchor projectile's long sharp tip tends to smear out the boundaries, however. In applying the penetration model we found that the coefficient of sliding friction and the target's volumetric strain have a much stronger influence on the deceleration profile than the initial target density and angle of internal friction. Very small values of volumetric strain (corresponding to high ‘drag coefficient’) were required to fit deceleration profiles to the measured data for the glass foam, contrary to what we initially expected by inspecting the thin layer of crushed material around the walls of the penetrated channel. We interpret this to mean that such brittle, porous materials as the glass foam (and perhaps highly porous, brittle, cryogenic ice) do not exhibit plastic deformation before failing completely by the crushing of cell walls. The decelerating forces are thus thought to be dominated by momentum transfer to the crushed material and by the crushing strength of the cellular microstructure, rather than by the force required to deform the target plastically. The cavity-expansion model seems to be well-suited to the ice shots, but for the brittle, cellular glass foam, alternative approaches, taking into account the material's microstructure, are needed. As a first step in this direction, a microstructural model linking textural properties of the material (pore and grain size, and relative contact area between grains) is applied to the glass foam data, obtained from quasi-static penetration tests and from direct shear strength tests. It is demonstrated that the dependence of strength on porosity can be well represented by the model suggested. A microstructural model for sintered ices, relating strength properties to porosity and thermal properties, would be useful for interpretation of MUPUS ANC-M data in the context of other physical properties measurements. The work presented here may also have some relevance to the design of future comet landers or penetrators. The harpoon anchor/penetrometer approach could be employed on other minor body landing missions, while the modelling technique is similar in many ways to that appropriate for other penetrometers/penetrators.

In the years 2011–2013 the ESA mission Rosetta will explore the short period comet 46P/Wirtanen. The aims of the mission include investigation of the physical and chemical properties of the cometary nucleus and also the evolutionary processes of comets. It is planned to land a small probe on the surface of the comet, carrying a multitude of sensors devoted to in situ investigation of the material at the landing site. On touchdown at the nucleus, an anchoring harpoon will be fired into the surface to avoid a rebound of the lander and to supply a reaction force against mechanical operations such as sample drilling or instrument platform motion. The anchor should also prevent an ejection of the lander due to gas drag from sublimating volatiles when the comet becomes more active closer to the Sun. In this paper, we report on the development of one of the sensors of the MUPUS instrument aboard the Rosetta Lander, the MUPUS ANC-M (mechanical properties) sensor. Its purpose is to measure the deceleration of the anchor harpoon during penetration into the cometary soil. First the test facilities at the Max-Planck-Institute for Extraterrestrial Physics in Garching, Germany, are briefly described. Subsequently, we analyse several accelerometer signals obtained from test shots into various target materials. A procedure for signal reduction is described and possible errors that may be superimposed on the true acceleration or deceleration of the anchor are discussed in depth, with emphasis on the occurrence of zero line offsets in the signals. Finally, the influence of high-frequency resonant oscillations of the anchor body on the signals is discussed and difficulties faced when trying to derive grain sizes of granular target materials are considered. It is concluded that with the sampling rates used in this and several other space experiments currently under way or under development a reasonable resolution of strength distribution in soil layers can be achieved, but conclusions concerning grain size distribution would probably demand much higher sampling rates.

In recent years the exploration of planetary bodies by surface probes has entered a new phase of interest. The mechanical properties of the near-surface layers of cometary and planetary bodies, including their strength, texture and layering, are important parameters needed both for a proper physical understanding of these bodies and for the design of lander missions. The strength properties of such a surface can be determined either by measuring the penetration resistance encountered by a slowly penetrating tip (quasi-static penetrometry), or by impacting the surface with an artificial body whose mechanical properties and impact velocity are known. At low speeds (up to about 300 m s −1 ), it is often feasible to measure the force or deceleration during penetration (‘impact’ or ‘dynamic’ penetrometry). If the event is better described in terms of hypervelocity impact cratering than penetration, analysis of the resulting crater is performed instead. Other uses of artificial impacts include the generation of seismic waves to aid the calibration of seismometers and the formation of a crater and / or ejecta for analysis or sampling of the target body. Such methods feature in many recent, current and forthcoming planetary missions. Examples include: the Surface Science Package of the Huygens probe, the anchoring harpoon of the Rosetta Lander , various Mars penetrators and landers, NASA's Deep Impact mission and ESA's planned Mercury cornerstone mission BepiColombo . A short overview of missions (past, present, future) featuring penetrometry experiments or artificial impacts is given. The theory of impact penetration and methods to interpret deceleration profiles in terms of material strength are discussed and applied to data sets obtained from test shots performed with the Rosetta Lander anchoring harpoon.

Owing to the low surface gravity of the Rosetta target comet 46P/Wirtanen, a means of anchoring the Rosetta Lander to the cometary surface will be necessary. This task can be accomplished by firing an anchor into the cometary soil immediately after touchdown to prevent a rebound of the spacecraft from the surface or subsequent ejection by other forces, and to allow for mechanical activities (drilling, etc.) at the landing site. The rationale for anchoring is examined, based on estimates of the main forces likely to act on the spacecraft after landing. We report on the development of an anchoring device using a pyrotechnic gas generator as a power source and an instrumented anchor. In addition to the anchoring function, which is the primary purpose of this system, the integration of acceleration and temperature sensors into the tip offers the possibility to determine some important material properties of the cometary surface layer. The accelerometer is designed to measure the deceleration history of the projectile and is thus expected to give information on how the material properties (in particular strength) change within the penetrated layer(s), while the temperature sensor will measure temperature variations at the depth at which the anchor finally comes to rest. As the mechanical properties of the material are not known, it is difficult to predict the final depth of the anchor with any great certainty, but it may well be greater than that reached by any other of the lander's instruments. The instrumented anchor will be part of the MUPUS experiment, selected to form part of the Rosetta Lander payload. We report on results of laboratory simulations of anchor penetration performed at the Institut fur Weltraumforschung, Graz, and compare these with models of projectile penetration. The value of the results expected from the penetrometry experiment in the context of an improved understanding of cometary processes is discussed.

The simulation of cometary surface conditions by laboratory experiments has proven to be an efficient way towards a better understanding of physical phenomena at and below the surface of a comet nucleus (Grun et al., Geophys. Res. Lett. 18, 245–248, 1991). A question which has not yet been answered by the comet simulation (KOSI) experiments performed so far is the influence of organic matter on the physical properties of the sublimation residua. Therefore, a number of experiments performed in a small vacuum chamber cooled by liquid nitrogen are reported on, which were dedicated to study the influence of organics on the thermal properties of a cometary analogue sample. Using aliphatic hydrocarbons of low volatility (paraffins) as model substances for the organic compounds, the formation of a several centimetres thick cohesive residuum in response to heating of the sample was observed. In one of the experiments the evolution of an originally homogeneous multi-component sample (containing water ice, organics, and minerals) to a residuum containing (finally) only minerals and organics was followed. During this evolution the thermal properties changed dramatically. The heat conductivity of the cohesive residuum was found to be at least an order of magnitude larger than the typical value for a loose dust mantle containing no organic material. Thus the evolution of comets with the same thermal history containing a considerable amount of organics might be quite different from that of a comet consisting only of ices and minerals.

In this erratum we acknowledge EASCO as one of the inspirational mission concepts that helped the development of our original mission concept OSCAR.

The aim of this work is to improve the method of calculating the thermoconductivity of porous ices and porous icy/rocky mixtures. The main interest are planetological applications, mostly for the physics of comets as well as icy satellites. However, the results could—in modified form—also be applied to the Earth's glaciers. The present work is an extension of an existing model of thermoconductivity of porous ice given by Steiner and Komle (Planet. Space Sci. 39, 507, 1991), which takes into account energy transport by gas and by contact points between grains, but with the simplifying assumption that the structure of the material remains unchanged during thermal evolution. Now the sintering of ice grains, which leads to a continuous growth of the grain-to-grain contact area (Hertz factor) is incorporated into the thermal evolution model. Example calculations illustrating the effect of grain sintering on thermal evolution in different temperature ranges are performed, using initial and boundary conditions suitable to the geometry used in the ‘comet simulation’ experiments described in Komle et al. (Planet. Space Sci. 39, 515, 1991; Planet. Space Sci. 40, 1311, 1992). It was found that the grain sintering process, which was commonly neglected in previous studies of the thermoconductivity of ice, may lead to significant changes of the matrix conductivity on time scales of hours to days. Therefore it should also play an important role in the thermal history of comet nuclei and thus affect the gas and dust emission activity of comets.

International audience; The Huygens probe landed on the then unknown surface of Titan in January 2005. A small, protruding penetrometer, part of the Surface Science Package (SSP), was pushed into the surface material measuring the mechanical resistance of the ground as the probe impacted the landing site. We present laboratory penetrometry into room temperature surface analogue materials using a replica penetrometer to investigate further the nature of Titan's surface and examine the sensor's capabilities. The results are then compared to the flight instrument's signature and suggest the Titan surface substrate material consists of sand-sized particles with a mean grain size ∼2 mm. A possible thin 7 mm coating with mechanical properties similar to terrestrial snow may overlie this substrate, although due to the limited data we are unable to detect any further layering or grading within the near-surface material. The unusual weakening with depth of the signature returned from Titan has, to date, only been reproduced using a damp sand target that becomes progressively wetter with depth, and supports the suggestion that the surface may consist of a damp and cohesive material with interstitial liquid contained between its grains. Comparison with terrestrial analogues highlights the unusual nature of the landing site material.

Regolith and dust cover the surfaces of the Solar Systems solid bodies, and thus constitute the visible surface of these objects. The topmost layers also interact with space or the atmosphere in the case of Mars, Venus and Titan. Surface probes have been proposed, studied and flown to some of these worlds. Landers and some of the mechanisms they carry, e.g. sampling devices, drills and subsurface probes (“moles”) will interact with the porous surface layer. The absence of true extraterrestrial test materials in ample quantities restricts experiments to the use of soil or regolith analogue materials. Several standardized soil simulants have been developed and produced and are commonly used for a variety of laboratory experiments. In this paper we intend to give an overview of some of the most important soil simulants, and describe experiments (penetrometry, thermal conductivity, aeolian transport, goniometry, spectroscopy and exobiology) made in various European laboratory facilities.

One possibility to explore the subsurface layers of icy bodies is to use a probe with a “hot tip", which is able to penetrate ice layers by melting. Such probes have been built and used in the past for the exploration of terrestrial polar ice sheets and may also become useful tools to explore other icy layers in the Solar System. Examples for such layers are the polar areas of Mars or the icy crust of Jupiter’s moon Europa. However, while on Earth a heated probe launched into an ice sheet always causes melting with subsequent refreezing, the behaviour of such a probe in a low pressure environment is quite different. We report on the results of some experiments with a simple “melting probe" prototype with two different kinds of hot tips in a vacuum environment. For one of the tips the probe moved into two types of ice samples: (i) compact water ice and (ii) porous water ice with a snow (firn) like texture. It was also found that the penetration behaviour was basically different for the two sample types even when the same kind of tip was used. While in the porous sample the ice was only subliming, the phase changes occurring during the interaction of the tip with the compact ice are much more complex. Here alternating phases of melting and sublimation occur. The absence of the liquid phase has severe consequences on the performance of a “melting probe" under vacuum conditions: In this environment we find a high thermal resistance between the probe surface and the underlying ice. Therefore, only a low percentage of the heat that is generated in the tip is used to melt or sublime the ice, the bulk of the power is transferred towards the rear end of the probe. This is particularly a problem in the initial phases of an ice penetration experiment, when the probe has not yet penetrated the ice over its whole length. In the compact ice sample, phases could be observed, where a high enough gas pressure had built up locally underneath the probe, so that melting becomes possible. Only during these melting periods the thermal contact between the probe and the ice is good and in consequence the melting probe works effectively.

[1] We report on thermal conductivity measurements performed on glass spheres of different grain sizes under varying pressure conditions ranging from 10−5 up to 1000 hPa. Glass spheres of 0.1 up to 4.3 mm were used as an analogue for the coarse-grained fractions of planetary regolith. From the obtained conductivity versus pressure data, sample pore sizes were derived and compared to estimated pore sizes. An increasing difference between derived and estimated pore size with increasing grain size was found. The behavior of the granular matter with decreasing pressure was analyzed by estimating the Knudsen number for the given system. The results indicate a high variability of the effective thermal conductivity for Martian conditions. Furthermore, the results imply that the thermal conductivity reaches a grain size-dependent, but pressure-independent, value for pressures below 0.01 hPa. For vacuum conditions a linear relation between grain size and effective thermal conductivity was found. Additionally, a mixture was analyzed, which showed a stronger decrease with gas pressure compared to the single-sized samples. From the pore size derived for the mixture an “effective” grain size composed of weighted mean of the mixture components was determined.

The upcoming lunar lander missions, for example Chang’e 2 from CNSA and several mission proposals and studies currently under consideration at NASA (e.g. Neal et al., ROSES 2006 Proposal to NASA, 2006), ESA (e.g. Hufenbach, European Workshop on Lunar Landers, ESTEC, Noordwijk, The Netherlands, 2005; Foing, EPSC Abstracts, vol 2, EPSC2007-A-00422, European Planetary Science Congress, Potsdam, Germany, 2007) and JAXA, Japan (Matsumoto et al., Acta Astronautica, 59:68–76, 2006) offer new possibilities to measure the thermal properties of the lunar regolith and to determine the global lunar heat flow more accurately than it is hitherto known. Both properties are of high importance for the understanding of the lunar structure and the evolution of the Moon–Earth system. In this paper we present some work on new thermal sensors to be used for in situ investigations of the lunar soil in combination with novel drilling techniques applicable for the lunar regolith. Such systems may preferably be mounted on mobile stations like the lunar rover currently built for the Chinese Chang’e 2 mission. A general description of a presently tested prototype of the lunar rover is given and mounting possibilities for a drilling system and thermal sensors are shown. Then we discuss some options for thermal sensors and drills and how they could be combined into one compact instrument. Subsequently a tube-like sensor suitable for measuring the thermal conductivity of the material surrounding a borehole is described in more detail. Finally the performance of such a tube-shaped sensor when applied in a lunar borehole is investigated by thermal modelling and compared with the behaviour of a more conventional needle-shaped sensor.

MUPUS, the multi purpose sensor package onboard the Rosetta lander Philae, will measure the energy balance and the physical parameters in the near-surface layers – up to about 30 cm depth- of the nucleus of Rosetta’s target comet Churyumov-Gerasimenko. Moreover it will monitor changes in these parameters over time as the comet approaches the sun. Among the parameters studied are the density, the porosity, cohesion, the thermal diffusivity and conductivity, and temperature. The data should increase our knowledge of how comets work, and how the coma gases form. The data may also be used to constrain the microstructure of the nucleus material. Changes with time of physical properties will reveal timescales and possibly the nature of processes that modify the material close to the surface. Thereby, the data will indicate how pristine cometary matter sampled and analysed by other experiments on Philae really is.

It is well-known that the permanent terrestrial ice sheets (glaciers and polar caps) contain a lot of information about the recent geological history and in particular about climatic changes. Extrapolating this fact to other ice sheets in the solar system (e.g. the Mars polar regions, the icy moons of the outer planets, etc.), we may expect a similar wealth of information. To obtain this information it is possible to drill holes or melt the ice by a heated probe, which in this way is able to penetrate the surface and investigate the deeper layers in situ. In the latter case the driving agent is the heating power and the weight of the probe. In this paper we consider the application of such “melting probes” for exploring the structure of ice sheets in extraterrestrial environments. We describe several laboratory experiments with simple melting probes performed under cryo-vacuum conditions and compare the results with tests in a terrestrial environment. The experiments revealed that under space conditions the downward motion of a heated probe in an ice sheet is characterized by intermittent periods of sublimation and melting of the surrounding ice, sometimes interrupted by periods where a part of the probe's outer surface is frozen to the surrounding ice. This leads to a temporary blocking of the probe's downward motion. A similar situation can occur when the trailing tether is frozen in behind the probe. During the periods of ice sublimation the penetration process is significantly more power consuming, due to the large difference between the latent heat of sublimation and the latent heat of melting for water ice.

Ultraviolet (UV) radiation can act on putative organic/biological matter at the Martian surface in several ways. Only absorbed, but not transmitted or reflected, radiation energy can be photo-chemically effective. The most important biological UV effects are due to photochemical reactions in nucleic acids, DNA or RNA, which constitute the genetic material of all cellular organisms and viruses. Protein or lipid effects generally play a minor role, but they are also relevant in some cases. UV radiation can induce wavelengths-specific types of DNA damage. At the same time it can also induce the photo-reversion reaction of a UV induced DNA photoproduct of nucleic acid bases, the pyrimidine dimers. Intense UVB and UVC radiation, experienced on early Earth and present-day Mars, has been revealed to be harmful to all organisms, including extremophile bacteria and spores. Moreover, the formation of oxidants, catalytically produced in the Martian environment through UV irradiation, may be responsible for the destruction of organic matter on Mars. Following this, more laboratory simulations are vital in order to investigate and understand UV effects on organic matter in the case of Mars. We have designed a radiation apparatus that simulates the anticipated Martian UV surface spectrum between 200 and 400 nm (UVC–UVA). The system comprises a UV enhanced xenon arc lamp, special filter-sets and mirrors to simulate the effects of the Martian atmospheric column and dust loading. We describe the technical setup and performance of the system and discuss its uses for different applications. The design is focused on portability, therefore, the Mars-UV simulator represents a device for several different Mars simulation facilities with specific emphasis on Mars research topics.

The surface of Saturn's largest satellite —Titan — is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion1. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system2. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package3, 4. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

[1] This paper discusses test results obtained in both laboratory and terrestrial environment conditions for the “Multipurpose Sensors for Surface and Sub-Surface Science” Thermal Probe (MUPUS-TP), which has been developed for the European Space Agency Rosetta cometary rendezvous mission. The probe is intended to provide in situ long-term observations of the thermal evolution of the comet nucleus and will measure a thermal conductivity profile with time in the top 30 cm of the comet nucleus. The basic operating principles of the probe are briefly described, including typical test results gathered in terrestrial snow and soil. The tests in snow provide verification of the probe as a useful tool for monitoring the metamorphism of snow on the Earth. The tests in soil are intended to demonstrate the probe's suitability as an alternative to other methods of energy measurement currently practiced in soil physics research. The tests of the probe in the natural environment of the Earth provide a demonstration of the behavior of the instrument in the presence of complex energy exchange processes before it is used on the comet.

Thermal and mechanical properties of cometary ices are closely associated with each other. Both are largely determined by the texture (porosity, grain size distribution, grade of sintering) of the material. The surface probe of the Rosetta mission to comet 46P/Wirtanen (Rosetta Lander) will for the first time measure these thermo-mechanical key parameters in situ, using a hammering device, a drill, and anchors to be shot into the ground by pyrotechnical means. Several of these components are associated to the experiment MU PUS (MU lti-PU rpose-S ensors for surface and subsurface science). The development of this instrument has now reached a mature state, as the flight model is already delivered and integrated with the spacecraft. We describe the main aspects of the experiment, outline the evaluation methods, and show representative results from test measurements.