Norbert I. Kömle

The landing(s) of Philae and inferences about comet surface mechanical properties

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.

Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko

Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 J m−2 K−1s-1/2. The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.

Line heat-source measurements of the thermal conductivity of porous H2O ice, CO2 ice and mineral powders under space conditions

Abstract Measurements of the thermal conductivity of porous loose mineral, porous H 2 O ice and porous CO 2 ice samples under low temperatures (77 K T −4 Pa p 5 Pa) are reported. The samples were selected to cover the end members of possible comet nucleus compositions and the ambient conditions were chosen to investigate the samples under space conditions. A transient technique is used for the measurements which is well suited for in situ application. The method is based on the line heat-source technique: a thin internally heated cylindrical sensor is inserted into the sample material. The thermal conductivity is deduced from the observed temperature rise in the sensor and the heating power applied. Depending on sensor dimensions, single experiment runs may be completed within a few minutes. The method proved to be accurate, fast and well suited for an application in the laboratory as well as in situ , e.g. on future comet nucleus or Mars missions. A thermal probe (MUPUS-PEN) which employs the experimental technique discussed here has been proposed for the ROSETTA surface science package “RoLand”. The thermal conductivity of the loose dunite sample is studied as a function of gas pressure. At low pressures, it is almost constant and close to 0.03 W m −1 K −1 . At atmospheric pressure, the thermal conductivity is about one order of magnitude higher. Both domains are linked by a pressure region with a strong pressure dependency of the thermal conductivity. Three porous water ice samples with different pore sizes have been investigated. The results are in agreement with theoretical predictions (e.g. Steiner et al. , 1991) and reveal a strong increase of the thermal conductivity at temperatures close to the sublimation temperature of water ice (≈ 200 K in vacuo ). The increase is due to heat transport by pore filling vapour which is more effective in samples with large pore radii. The measured matrix conductivity is close to 0.02 W m −1 K −1 , while maximum values for the effective (= matrix + vapour) thermal conductivity at high temperatures exceed 0.25 W m −1 K −1 . Similar results are obtained for one porous CO 2 ice sample.

Temperature evolution of porous ice samples covered by a dust mantle

Abstract The existence of nonvolatile dust mantles covering the ices of cometary nuclei is suggested both from the behavior of many cometary lightcurves and from the direct observations of Comet Halleys nucleus by the Giotto and VEGA spacecraft in March 1986. In this paper we present a systematic study which sheds some light on the evolution of such dust-ice systems when they are irradiated by the sun. Hereby we assume that the dust mantle as well as the underlying ice are grainy, porous structures permeable to gases. It is found that both the surface temperature of the dust mantle and the temperature at the dust-ice interface are strong functions of the average pore radius of the materials and of the thermal conductivity of the mantle. For moderately insulating dust mantles (coal dust) with grain sizes in the micrometer range, high ice temperatures (up to the triple point of water ice) may be reached in response to irradiation with one solar constant. If the dust mantles are extremely bad heat conductors (assuming radiative heat transfer only) the ice temperature remains below 200 K in all cases considered. An increase of the dust mantle thickness may result either in a higher or in a lower ice temperature, depending upon the parameters chosen.

Using the anchoring device of a comet lander to determine surface mechanical properties

Abstract 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 landers 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.

Sublimation coefficient of water ice under simulated cometary-like conditions

Abstract In papers dealing with evolution of cometary nuclei it is commonly assumed that the coefficients of sublimation α s and condensation α c of vapour are both equal to one. The experimental investigation of ice samples under simulated cometary-like conditions (Kossacki, K.J., Komle, N.I., Leliwa-Kopystynski, J., Kargl, G., 1997. Thermal and structural evolution of cometary subsurface layer: selfconsistent model and experimental verification. Icarus 128, 127–144) suggests, however, that the sublimation flux calculated with the Hertz–Knudsen formula and the above assumption is nearly an order of magnitude too high. This may imply that actual values of α s for the ice/dust sample used in these experiments are of the order of 0.1. A similar conclusion can be drawn for α c from the results of various experiments concerning growth of ice crystals from the vapour phase and their sublimation (Lamb, D., Scott, W.D., 1972. Linear growth rates of ice crystals grown from the vapor phase. Journal of Crystal Growth 12, 21–31; Beckmann, W., Lacmann, R., 1982. Interface kinetics of growth and evaporation of ice II. Journal of Crystal Growth 58, 433–442; Sei, T., Gonda, T., 1989. The growth mechanism and the habit change of ice crystals growing from the vapour phase. Journal of Crystal Growth 94, 697–707). The exact values of both of these coefficients depend on various parameters such as temperature, concentration of surface impurities and deviation of the vapour pressure from that of the phase equilibrium. In this work the temperature dependence of the sublimation and condensation coefficients is discussed and an appropriate formula is proposed to fit the experimental results. This new formulation is then used to analyse the implications for the thermal conductivity of a porous cometary-like ice and the rate of vapour flux from a cometary nucleus.

Thermal properties of cometary ices and sublimation residua including organics

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.

Astrobiology with haloarchaea from Permo-Triassic rock salt

Several viable halophilic archaebacteria were isolated previously from rock salt of Permo-Triassic age in an Austrian salt mine; one of these strains was the first to be recognized as a novel species from subterranean halite and was designated Halococcus salifodinae . The halophilic microorganisms have apparently survived in the salt sediments over extremely long periods of time. Halobacteria could therefore be suitable model organisms for exploring the possibility of long-term survival of microbes on other planets, in particular, since extraterrestrial halite has been detected in meteorites and is assumed to be present in the subsurface ocean on Europa. Our efforts are directed at the identification of the microbial content of ancient rock salt and the development of procedures for the investigation of the halobacterial response to extreme environmental conditions. Using modified culture media, further halophilic strains were isolated from freshly blasted rock salt and bore cores; in addition, growth of several haloarchaea was substantially improved. Molecular methods indicated the presence of at least 12 different 16S rRNA gene species in a sample of Alpine rock salt, but these strains have not been cultured yet. The exploration of Mars is a target of space missions in the 21st century; therefore, testing the survival of haloarchaea under conditions comparable to present-day Mars, using a simulation chamber, was begun. Preliminary results with Halococcus and Halobacterium species suggested at least tenfold higher survival rates when cells were kept in liquid brines than under dry conditions; staining of cells with the LIVE–DEAD kit, which discriminates between damaged and intact membranes, corroborated these data.

Heating and evaporation of icy particles in the vicinity of Comets

Abstract Although the detailed structure of the surface layers of a cometary nucleus is still unknown, there are good reasons that favor a porous and fragile ice/dust mixture. Such a nucleus will most probably emit ice particles which are blown off from the surface by sublimating gases. Their sizes may range from meter-sized boulders down to micro- and submicrometer-sized particles. In this paper we present a model which allows to calculate the “history” of ice particles with radii up to 100 μm released from a cometary nucleus. Temperatures and sublimation rates of both pure ice particles and ice particles polluted by a dark, nonvolatile material are computed. It is found that the black body approximation of the solar UV-radiation noticeably shortens the lifetimes of large (≥10 μm) particles. Moreover, the use of different formulae for the vapor pressure of ice found in the literature may affect the lifetime calculations by a factor of 10. The sublimation minimum predicted by Patashnik and Rupprecht (1975) for pure ice particles at a size of about 20 μm is confirmed. However, even small amounts of pollution in the ice tend to smear out this minimum and make it disappear which leads to much shorter particle lifetimes.

A model of the thermal conductivity of porous water ice at low gas pressures

Abstract This study is concerned with energy transfer in porous water ice at large Knudsen numbers of the sublimated gas. We present a formula for the effective thermal conductivity, λeff, which is found to be valid for a wide range of material parameters, in particular also for high porosity materials, where other expressions for λeff published in the literature lose their validity. It is found that above ∼ 190 K λeff increases strongly with temperature and depends mainly on the average grain size, while at low temperatures it is primarily controlled by the value of the Hertz-factor. The influence of the porosity is relatively weak. The reliability of our expression for λeff has been checked by comparing computed temperature profiles with the temperature recordings of comet simulation experiments. The predicted temperatures are in good agreement with the experimental results. Finally, our model is applied to a typical surface layer of a cometary nucleus. A strong temperature dependence of the effective thermal conductivity in cometary water ice is predicted.