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

Spohn, Tilman; Knollenberg, J.; Ball, Andrew; Banaszkiewicz, M.; Benkhoff, J.; Grott, M.; Grygorczuk, J.; Hüttig, Christian; Hagermann, A.; Kargl, Günter; Kaufmann, E.; Kömle, Norbert I.; Kührt, Ekkehard; Kossacki, Konrad J.; Marczewski, W.; Pelivan, Ivanka; Schrödter, R.; Seiferlin, K.

doi:10.1126/science.aab0464

Cited by 178 publications

(144 citation statements)

References 28 publications

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“…C.1). Hence, it is stipulated here that the dust cover is icy (however slightly) from some shallow depths (De Sanctis et al 2015;Spohn et al 2015;Shi et al 2016a). The thermal model is hereafter referred to as the "dust mantle" model.…”

Section: Modeled Erosion Of Water Icementioning

confidence: 99%

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

Shi

Sierks

et al. 2017

A&A

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Context. Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P/Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology. Aims. We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission. Methods. Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes. Results. Surface changes discussed here were concentrated at mid-latitudes, between about 20• N and 40• N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0.5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night. Conclusions. The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or/and redeposited over other areas of the nucleus.

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Section: Modeled Erosion Of Water Icementioning

confidence: 99%

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

Shi

Sierks

et al. 2017

A&A

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“…Abydos is a geologically and thermally distinct site when compared to Agilkia. Abydos was almost permanently in shadow with low diurnal surface temperatures ranging from 90 K to 130 K and has a hard sintered surface with limited regolith (Spohn et al 2015). The lower temperature likely prohibited sublimation of volatile organic species which thus could not diffuse into the COSAC mass spectrometer during all later six sniffing mode measurements.…”

Section: Discussionmentioning

confidence: 99%

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

Krüger

Goesmann

Giri

et al. 2017

A&A

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Context. The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae is equipped with two gas analyzers: The Cometary Sampling and Composition experiment (COSAC) and the gas chromatograph and mass spectrometer Ptolemy. Aims. COSAC is designed for in situ analysis of organic molecules on 67P while Ptolemy is optimised to measure ratios of stable isotopes. Methods. On 12 to 14 November 2014 both instruments measured the organic composition of the comet nucleus material through seven measurements in sniffing mode during Philae's hopping and at its final landing site Abydos. We compare the temporal evolution of intensities of several ion species identified by both mass spectrometers. For COSAC this is the first analysis of the temporal behaviour of the measured ion species. Results. All ion species showed the highest intensities in the first spectra measured by both instruments about 20 to 30 minutes after Philae's first touchdown at Agilkia, and a decay during the six consecutive measurements at Abydos. Both instruments measured a nearly identical decay of the water peak (m/z 18), and also CO (m/z 28) behaved similarly. In the COSAC measurements the peak at m/z 44 decays much slower than all the other ion species, including the water peak. In particular, the m/z 44 peak decays much slower in the COSAC measurements than in the Ptolemy data. This supports our earlier interpretation that COSAC for the first time analyzed a regolith sample from a cometary nucleus in situ, while Ptolemy measured cometary gas from the ambient coma. The m/z 44 peak measured by COSAC was likely dominated by organic species, whereas the peak measured by Ptolemy was interpreted to be mostly due to CO 2 . Ion species heavier than m/z 30 tend to decay somewhat slower in the COSAC measurements than in the Ptolemy data, which may be related to differences in the exhaust designs between both instruments.

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“…This technique was used by the Huygens landing on Titan (Lorenz et al, 1994;Zarnecki et al, 2005). Impact penetrometry is particularly valuable when visiting a body for the first time (Jones, 1971;Spohn et al, 2015). A limitation of the technique is that it performs a 'single point' measurement that may not be representative of the region around the lander.…”

Section: Regolith Structure and Microstructure Measured By A Landing mentioning

confidence: 99%

Detection of structure in asteroid analogue materials and Titan’s regolith by a landing spacecraft

Paton

Green

Ball

et al. 2016

Advances in Space Research

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We compare measurements made by two impact penetrometers of different sizes and with different tip shapes to further understand penetrometer design for performing pentrometry on an asteroid. To this end we re-visit the interpretation of data from the Huygens' penetrometer, ACC-E, that impacted Titan's surface. In addition we investigate the potential of a spacecraft fitted with a penetrometer to bounce using a test rig, built at The Open University (UK).Analysis of ACC-E laboratory data, obtained from impacts into ∼4 mm diameter gravel, was found to produce an unusual decrease in resistance with depth (force-depth gradient) which was also seen in the Huygens' ACC-E data from Titan and originally interpreted as a wet or moist sand. The downward trend could also be reproduced in a hybrid Discrete Element Model (DEM) if it was assumed that the near surface particles are more readily * Corresponding author mobilised than those deeper in the target. With regard to penetrometer design penetration resistance was found to be sensitive to the ratio of particle to tip diameter. A clear trend was observed with a conical tip penetrometer, X-PEN, of decreasing force-depth gradients with increasing particle sizes most likely due to a transformation from a bulk displacement of material by the penetrating tip to more local interactions. ACC-E, which has a hemispherical tip, was found to produce a wider range of force-depth gradients than X-PEN, which had a conical tip, possibly due to difficulties dislodging jammed particles. Both penetrometers were able to determine particle diameter and mass after post-processing of the data.Laboratory simulations of landings with the test rig suggest that a large impact penetrometer under certain circumstances could absorb a significant amount of the elastic energy of the spacecraft possibly aiding landing. Alternatively a small impact penetrometer would allow the spacecraft to bounce freely off the surface to make a measurement at another location.

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

Cited by 178 publications

References 28 publications

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

Detection of structure in asteroid analogue materials and Titan’s regolith by a landing spacecraft

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