Klaus J. Seidensticker

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.

DEVELOPMENT OF A DUST MANTLE ON THE SURFACE OF AN INSOLATED ICE-DUST MIXTURE : RESULTS FROM THE KOSI-9 EXPERIMENT

Astronomical observations indicate that formation and destruction of dust mantles on cometary nuclei may be the cause for erratic and systematic variations of cometary activity, i.e. emission of dust. A laboratory experiment (KOSI-9) has been performed to study the evolution of a dust mantle on top of a sublimating ice-dust mixture in vacuum. A sample consisting of water ice with a 10% (by weight) admixture of olivine grains has been insolated in three periods at variable intensities from 200 to 1900 W/m2. Both increasing surface temperature of the sample and decreasing gas and particle emissions indicated the formation of a dust mantle during the first period. During the second insolation period after the gas flux had reached a critical value of a few 1021 water molecules m−2 s−1, avalanches of mantle material occurred on the inclined sample surface, broke up the mantle locally, and opened up a fresh icy surface. Enhanced ice and dust particle emission resumed for some time from these spots. A large number of the emitted dust particles were of a fluffy aggregate structure, i.e., they had large cross section to mass ratios compared to compact particles. During the third period the critical gas flux was not reached and no enhanced dust and ice emission was observed. A dry dust mantle of a few millimeters thickness developed during the course of the experiment. Consequences of these findings for cometary scenarios are discussed.

Philae's first days on the comet

On 12 November 2014, Philae landed on the surface of comet 67P/Churyumov-Gerasimenko (67P), making an almost 30-year dream a reality. The pioneering flybys of 1P/Halley in 1986 revealed that despite being made primarily of ice, it was covered in highly absorbing carbonrich molecules. What is their composition? When did they form, and through which chemical routes? Might they have constituted prebiotic molecules necessary for life? At a larger scale, what can one learn from comets that has relevance to the evolution of the solar system and planets? ![Figure][1] 12 NOVEMBER 2014: PHILAE LANDED ON THE NUCLEUS OF COMET 67P CREDIT: ESA/ROSETTA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA To address such questions, the Rosetta mission sought to perform a broad range of in-depth structural, physical, and chemical measurements from remote, in situ, and landed vantages. The candidate payload opened for a competitive selection included an instrumented Surface Science Platform (SSP). The initial two that were selected later merged into what is known as Philae, instrumented by 10 principal investigators selected by the SSP providers. The Philae platform and payloads were developed and operated by a highly integrated consortium of institutes, agencies, and industries. Philaes scientific objectives were to provide ground-truth information and complement remote measurements performed from the Rosetta orbiter ( Science 347 , 23 January 2015) and to offer a self-standing suite of in situ measurements never before performed on a comet. This issue presents a first set of results acquired aboard Philae in the first 63 hours after it separated from Rosetta, descended, initially touched down on the comet at the site known as Agilkia, and finally came to rest at the site known as Abydos. The release and descent happened as planned, precisely documented by imaging (Mottola et al. ), ranging (Kofman et al. ), thermal mapping (Spohn et al. ), and the evolution of the magnetic properties (Auster et al. ). The prospect of landing on such an alien body, at 515 million km from Earth and 3 astronomical units (AU) from the Sun, was far more challenging than imagined. The unexpected bounce at touchdown required a major reshuffling and adaptation of the first sequence of science operations. It also provided the opportunity for additional measurements, whereas the bouncing and traversing constrained the mechanical (Biele et al. ) and magnetic properties of the surface. ROLIS imagery at its highest resolution (1 cm per pixel) showed the surface of the comet near Agilkia to be dominated by the presence of granular material free of any dust deposits (Mottola et al. ). Regolith mobilization processes appear to be involved with the formation of these features. Once Philae came to rest at Abydos, the revised first science sequence began. CIVA panoramic images characterized the surrounding cometary material down to the millimeter scale and the attitude of Philae at rest (Bibring et al. ). The MUPUS package measured and constrained the thermal and mechanical properties of the near-surface material of the comet surface at Abydos (Spohn et al. ), indicating that the near-surface layers consist of a hard dust-rich sintered ice, possibly covered by a thin dust layer. The CONSERT bistatic radar provided an opportunity to investigate the comets internal structure (Kofman et al. ). The upper “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. The average permittivity provides ranges of the volumetric dust/ice ratio and the internal porosity. The dust component may be comparable, from the dielectric properties, to that of carbonaceous chondritic meteorites. COSAC and Ptolemy independently measured the composition of the volatile constituents of the grains lifted at touchdown and of the species outgassed at the final landing site (Goesmann et al. and Wright et al. ). The grains are primarily made of carbon-rich species in a complex suite of molecules, including precursors to some biomolecules and other compounds never before identified in comets. Taken together, these first measurements performed at the surface of 67P profoundly modify our view of comets. 67P is nonmagnetized on a scale of less than a meter, with its surface layers composed of both sintered ices, which are hard in nature, and fluffy grains and pebbles of organic materials, possible remnants from the era of comet formation itself. Although it remains to be seen whether these observations hold true for all comets, the discoveries made by Philae—including these initial results—will continue to shape our view of the history of the solar system. [1]: pending:yes

Energy analysis of porous water ice under space-simulated conditions: results from the KOSI-8 experiment

Abstract A sublimation experiment (KOSI-8) was performed on a pure, porous water ice sample under conditions of low pressure and temperature and insolation by an artificial sun. Temperatures within the sample, gas flux from the surface, mass loss of the sample during the experiment and the irradiation input were measured for a detailed analysis of energy flow. A main feature of the analysis was the development of a convex temperature profile along the middle axis of the sample as a consequence of heat transfer by water vapor flow. The amount of the energy transport by the water was about 40% of the total heat flux available for heating the sample. To interpret the unusually high temperatures measured a few millimeters below the surface, it is assumed that the radiative energy input penetrates about 3 mm into the sample (solid-state greenhouse effect). To trace the flow of water vapor during the experiment, the top 10 mm of the ice was enriched with 10% HDO. Ice samples for isotopic analyses were taken from the post-insolation surface and from various depths. Isotopic enrichment was observed only at the surface, corresponding to a 2.3% admixture of recondensed vapor from the initially 10-mm-thick surface layer, to a depth of 19 mm below the original surface. No isotopic enrichment could be detected below a crust that had formed, i.e. 57 mm below the original surface. During the experiment an average of about 16 mm of the ice eroded from the surface. The weight of the sample was continuously monitored and a total mass loss of 1.75 ± 0.05 kg was measured between start and end of insolation. A comparison of the power balance of a pure, porous ice sample and an ice/dust sample shows that in either case the net flux of energy available for sublimation and warming of the sample is less than 20% of the insolation. In the case of a pure ice sample, most of the irradiation is reflected due to the high albedo; in the case of an ice/dust sample, thermal reradiation of the sample is very high because of the high surface temperatures, ≈ 350 K.

Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko - As constrained by the Permittivity Probe SESAME-PP/Philae/Rosetta

Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment−Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be >2.45 ± 0.20, which is consistent with a porosity <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer.

Temperature evolution and vapour pressure build-up in porous ices

Abstract We report on a new series of comet simulation experiments performed in the “Small Chamber” at the DLR Koln. Different types of non-volatile materials have been used to simulate the effect of dust mantles on heating and vapour pressure of an underlying porous ice sample. In all experiments the flow resistance of the mantle material caused a measurable pressure build-up inside the sample and a temperature rise larger than would be expected from a freely sublimating icy surface exposed to the same energy input. Comparison of experiments with different mantle grain sizes revealed similar vapour pressures and ice temperatures. This result is probably caused by the fact that the smaller (and lighter) particles, even if they are not blown away, are redistributed by the gas stream and form pores much larger than the original grain size. The flow resistance is then determined by the flow resistance of the whole structure, not by the grain size of the original particles. Theoretical estimates and model calculations reveal that deviations from the equilibrium vapour pressure should be small inside porous water ice. Only within a distance of about three pore radii from a sublimating surface can major deviations occur. The experiments utilizing pure (white) ice confirmed this prediction: reasonable agreement between the theoretically expected saturation vapour pressure and the measured gas pressure inside the sample was found. Admixture of a darkening agent (solution of carbon and an organic constituent) in even very small amounts to the water out of which the porous ice is produced, caused, however, a significant reduction of the vapour pressure inside the pores.

CASSE — The ROSETTA Lander Comet Acoustic Surface Sounding Experiment — status of some aspects, the technical realisation and laboratory simulations

Abstract In 2003 the ROSETTA space mission to Comet 46P/Wirtanen will be launched by the European Space Agency (ESA). On board of the spacecraft there will be a Lander platform, which opens for the first time the possibility to carry out “in situ” measurements on a comet surface. The ROSETTA Lander experiment CASSE aims to investigate the outermost surface of the comet by means of transmitting, receiving and even passively monitoring acoustic waves in a frequency range from a few hundred to several kilohertz. In particular, by transmitting and recording compression and shear waves, the mechanical properties of the cometary surface will be investigated. To guarantee sufficient ground contact in any foreseeable surface topography, and in any composition of the material to be encountered, e.g. dust, sand, ice and mixtures of these materials, the acoustic transmitters (actuators), stacked piezoceramics, and triaxial commercial piezoelectric accelerometers as receivers, will be integrated in each of the three feet of the ROSETTA Lander. The Lander feet thus act as antennas for the transmitters and receivers. By switching different actuators and accelerometers, an analysis of the cometary surface material will become possible by direct foot to foot transmission of the acoustic signals. Further, an in-depth sounding will allow the detection of layered structures or embedded local inhomogeneities. The ground contact of the Lander will be strengthened by a harpoon providing a fixation force up to at least 5 N per foot. This paper describes the first design of the Lander feet incorporating piezoceramic composite stacks as sound transmitters and receivers, and the experiments to test their efficiency. In the first part of this paper, the results of acoustic wave propagation experiments in ice, sand and olivine dust, performed in the laboratory are reported. These experiments verify that a signal strength of at least 20 dB above noise can be expected for acoustic wave propagation in cometary analogue materials. The second part of the paper deals with the outdoor experiments, performed with sensors integrated into the Lander feet prototypes and placed in a sand filled construction container. The experiments serve to estimate the minimal range of detectability of the sounding signals.

The Philae lander mission and science overview

The Philae lander accomplished the first soft landing and the first scientific experiments of a human-made spacecraft on the surface of a comet. Planned, expected and unexpected activities and events happened during the descent, the touch-downs, the hopping across and the stay and operations on the surface. The key results were obtained during 12–14 November 2014, at 3 AU from the Sun, during the 63 h long period of the descent and of the first science sequence on the surface. Thereafter, Philae went into hibernation, waking up again in late April 2015 with subsequent communication periods with Earth (via the orbiter), too short to enable new scientific activities. The science return of the mission comes from eight of the 10 instruments on-board and focuses on morphological, thermal, mechanical and electrical properties of the surface as well as on the surface composition. It allows a first characterization of the local environment of the touch-down and landing sites. Unique conclusions on the organics in the cometary material, the nucleus interior, the comet formation and evolution became available through measurements of the Philae lander in the context of the Rosetta mission. This article is part of the themed issue ‘Cometary science after Rosetta’.

Dust Impact Monitor (SESAME-DIM) on board Rosetta/Philae: Millimetric particle flux at comet 67P/Churyumov-Gerasimenko

Context. The Philae lander of the Rosetta mission, aimed at the in situ investigation of comet 67P/Churyumov-Gerasimenko, was deployed to the surface of the comet nucleus on 12 November 2014 at 2.99 AU heliocentric distance. The Dust Impact Monitor (DIM) as part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) on the lander employed piezoelectric detectors to detect the submillimetre- and millimetre-sized dust and ice particles emitted from the nucleus. Aims. We determine the upper limit of the ambient flux of particles in the measurement range of DIM based on the measurements performed with the instrument during Philae’s descent to its nominal landing site Agilkia at distances of about 22 km, 18 km, and 5 km from the nucleus barycentre and at the final landing site Abydos. Methods. The geometric factor of the DIM sensor was calculated assuming an isotropic ambient flux of the submillimetre- and millimetre-sized particles. For the measurement intervals when no particles were detected the maximum true impact rate was calculated by assuming Poisson distribution of the impacts, and it was given as the detection limit at a 95% confidence level. The shading by the comet environment at Abydos was estimated by simulating the pattern of illumination on Philae and consequently the topography around the lander. Results. Based on measurements performed with DIM, the upper limit of the flux of particles in the measurement range of the instrument was of the order of 10-8−10-7 m-2 s-1 sr-1 during descent. The upper limit of the ambient flux of the submillimetre- and millimetre-sized dust and ice particles at Abydos was estimated to be 1.6 × 10-9 m-2 s-1 sr-1 on 13 and 14 November 2014. A correction factor of roughly 1/3 for the field of view of the sensors was calculated based on an analysis of the pattern of illumination on Philae. Conclusions. Considering particle speeds below escape velocity, the upper limit for the volume density of particles in the measurement range of DIM was constrained to 10-11 m-3−10-12 m-3. Results of the calculations performed with the GIPSI tool on the expected particle fluxes during the descent of Philae were compatible with the non-detection of compact particles by the DIM instrument.