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Register a new userTensile Strength of 67P/Churyumov-Gerasimenko Nucleus Material from Overhangs
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We directly measure twenty overhanging cliffs on the surface of comet 67P/Churyumov-Gerasimenko extracted from the latest shape model and estimate the minimum tensile strengths needed to support them against collapse under the comets gravity. We find extremely low strengths of around one Pa or less (one to five Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features and implied previous collapse of another, suggests that they are prone to failure and that true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of 67Ps nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties with size, over the $sim10-100$ m range studied here, or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small (tens of km) body.
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We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface, using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies and mechanical considerations. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20 deg) are covered by a fine material and contain a few large ($>$10 m) and isolated boulders, ii) intermediate-slope terrains (20-45 deg) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from $<$1 m to 10 m for the majority of them, and iii) high-slope terrains (45-90 deg) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3-15 Pa (upper limit of 150 Pa), 4-30 Pa for the shear strength of fine surface materials and boulders, and 30-150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.
We present the results of the photometric and spectrophotometric properties of the 67P/Churyumov-Gerasimenko nucleus derived with the OSIRIS instrument during the closest fly-by over the comet, which took place on 14 th February 2015 at a distance of {~} 6 km from the surface. Several images covering the 0{deg}-33{deg} phase angle range were acquired, and the spatial resolution achieved was 11 cm/pxl. The flown-by region is located on the big lobe of the comet, near the borders of the Ash, Apis and Imhotep regions. Our analysis shows that this region features local heterogeneities at the decimetre scale. We observed difference of reflectance up to 40{%} between bright spots and sombre regions, and spectral slope variations up to 50{%}. The spectral reddening effect observed globally on the comet surface by Fornasier et al. (2015) is also observed locally on this region, but with a less steep behaviour. We note that numerous metre-sized boulders, which exhibit a smaller opposition effect, also appear spectrally redder than their surroundings. In this region, we found no evidence linking observed bright spots to exposed water-ice-rich material. We fitted our dataset using the Hapke 2008 photometric model. The region overflown is globally as dark as the whole nucleus (geometric albedo of 6.8{%}) and it has a high porosity value in the uppermost-layers (86{%}). These results of the photometric analysis at a decimetre scale indicate that the photometric properties of the flown-by region are similar to those previously found for the whole nucleus.
The OSIRIS/NAC camera onboard the Rosetta spacecraft acquired approximately 27000 images of comet 67PChuryumov-Gerasimenko at spatial scales down to a few centimeters. Numerous sequences of images separated by a few minutes suitable to stereo reconstruction allowed producing anaglyphs. They offer three-dimensional views complementary to other technics as a tool to understand the topography of the nucleus. Each one is documented by a set of 17 parameters. Over 1820 anaglyphs are available on a dedicated website at https://rosetta-3dcomet.cnes.fr
The Rosetta space probe accompanied comet 67P/Churyumov-Gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. This work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. We use observations acquired with the OSIRIS/NAC camera during July-October 2015, that is, close to perihelion. More than 200 jets of different intensities were identified directly on the nucleus. Some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the vivible-to-near-infrared region. We attribute this spectral behavior to icy grains mixed with the ejected dust. Some of the jets have an extremely short lifetime. They appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. We also report a resolved dust plume observed in May 2016 at a resolution of 55 cm/pixel, which allowed us to estimate an optical depth of $sim$0.65 and an ejected mass of $sim$ 2200 kg. We present the results on the location, duration, and colors of active sources on the nucleus of 67P from the medium-resolution (i.e., 6-10 m/pixel) images acquired close to perihelion passage. The observed jets are mainly located close to boundaries between different morphological regions. Jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. This study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered by illumination conditions. Faint jets or outbursts are not associated with a particular terrain type or morphology.
Dust is an important constituent in cometary comae; its analysis is one of the major objectives of ESAs Rosetta mission to comet 67P/Churyumov-Gerasimenko (C-G). Several instruments aboard Rosetta are dedicated to studying various aspects of dust in the cometary coma, all of which require a certain level of exposure to dust to achieve their goals. At the same time, impacts of dust particles can constitute a hazard to the spacecraft. To conciliate the demands of dust collection instruments and spacecraft safety, it is desirable to assess the dust environment in the coma even before the arrival of Rosetta. We describe the present status of modelling the dust coma of 67P/C-G and predict the speed and flux of dust in the coma, the dust fluence on a spacecraft along sample trajectories, and the radiation environment in the coma. The model will need to be refined when more details of the coma are revealed by observations. An overview of astronomical observations of 67P/C-G is given and model parameters are derived from these data where possible. For quantities not yet measured for 67P/C-G, we use values obtained for other comets. One of the most important and most controversial parameters is the dust mass distribution. We summarise the mass distribution functions derived from the in-situ measurements at comet 1P/Halley in 1986. For 67P/C-G, constraining the mass distribution is currently only possible by the analysis of astronomical images. We find that the results from such analyses are at present rather heterogeneous, and we identify a need to find a model that is reconcilable with all available observations.
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