G. Kargl

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturns largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titans atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. 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.

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.

The influence of grain sintering on the thermoconductivity of porous ice

Abstract 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 Earths 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.

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.

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.

A heat flow and physical properties package for the surface of Mercury

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 Mercurys interior, an important constraining parameter for models of the planets 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.

Accelerometry measurements using the Rosetta Lander's anchoring harpoon: experimental set-up, data reduction and signal analysis

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.

Experimental studies of the cratering process in porous ice targets

Abstract The electrothermal accelerator of the Fachgebiet Raumfahrttechnik at the Technical University of Munich was used to perform impact experiments on porous ice targets. The projectiles were Nylon cylinders of 25 mg mass, accelerated to velocities between 0.9 an 3.8 km/s. The targets were produced by blowing water into LN 2 using an airbrush gun as used in the target production of the KOSI experiments. The diameter and depth of the resulting craters were measured, as well as the mass loss of the target. The ejected crater mass is consistent with the ejected mass of craters on compact ice. The volume is larger by the ratio of the densities of the targets.

Determination of physical properties of planetary sub-surface layers by artificial impacts and penetrometry

Abstract 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, NASAs Deep Impact mission and ESAs 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.