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تسجيل مستخدم جديدAre fractured cliffs the source of cometary dust jets ? Insights from OSIRIS/Rosetta at 67P
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Dust jets, i.e. fuzzy collimated streams of cometary material arising from the nucleus, have been observed in-situ on all comets since the Giotto mission flew by comet 1P/Halley in 1986. Yet their formation mechanism remains unknown. Several solutions have been proposed, from localized physical mechanisms on the surface/sub-surface (see review in Belton (2010)) to purely dynamical processes involving the focusing of gas flows by the local topography (Crifo et al. 2002). While the latter seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller features (a few meters wide) that connect directly to the nucleus surface. We monitored these jets at high resolution and over several months to understand what are the physical processes driving their formation, and how this affects the surface. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets, and linked them precisely to their sources on the nucleus. Results.We show here observational evidence that the Northern hemisphere jets of comet 67P arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features, and therefore the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.
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The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images citep{Bertini17}. This function show a deep minimum at phase angles near 100$^circ$, and a strong backscattering enhancement. These two properties cannot
be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r gtrsim$10 $mu$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70% range.
Before Rosetta, the space missions Giotto and Stardust shaped our view on cometary dust, supported by plentiful data from Earth based observations and interplanetary dust particles collected in the Earths atmosphere. The Rosetta mission at comet 67P/
Churyumov-Gerasimenko was equipped with a multitude of instruments designed to study cometary dust. While an abundant amount of data was presented in several individual papers, many focused on a dedicated measurement or topic. Different instruments, methods, and data sources provide different measurement parameters and potentially introduce different biases. This can be an advantage if the complementary aspect of such a complex data set can be exploited. However, it also poses a challenge in the comparison of results in the first place. The aim of this work therefore is to summarise dust results from Rosetta and before. We establish a simple classification as a common framework for inter-comparison. This classification is based on a dust particles structure, porosity, and strength as well as its size. Depending on the instrumentation, these are not direct measurement parameters but we chose them as they were the most reliable to derive our model. The proposed classification already proved helpful in the Rosetta dust community and we propose to take it into consideration also beyond. In this manner we hope to better identify synergies between different instruments and methods in the future.
We investigated Wosret, a region located on the small lobe of the 67P/Churyumov-Gerasimenko comet subject to strong heating during the perihelion passage. This region includes Abydos, the final landing site of the Philae lander. We analyzed high-reso
lution images of the Wosret region acquired between 2015 and 2016 by the OSIRIS instrument on board the Rosetta spacecraft. We observed a few morphological changes in Wosret, related to local dust coating removal with an estimated depth of $sim$ 1 m, along with the formation of a cavity measuring 30 m in length and 6.5 m in depth, for a total estimated mass loss of 1.2 $times$ 10$^6$ kg. The spectrophotometry of the region is typical of medium-red regions of comet 67P, with spectral slope values of 15-16 %/(100 nm) in pre-perihelion data acquired at phase angle 60$^o$. Wosret has a spectral phase reddening of 0.0546 $times 10^{-4}$ nm$^{-1} deg^{-1}$, which is about a factor of 2 lower than what was determined for the nucleus northern hemisphere regions, possibly indicating a reduced surface micro-roughness due to the lack of widespread dust coating. A few tiny bright spots are observed. Morphological features such as goosebumps or clods are widely present and larger in size than similar features located in the big lobe. Compared to Anhur and Khonsu, two southern hemisphere regions in the big lobe which are also exposed to high insolation during perihelion, Wosret exhibits fewer exposed volatiles and less morphological variations due to activity events. Our analysis indicates that the small lobe has different physical and mechanical properties than the big one and a lower volatile content, at least in its uppermost layers. These results support the hypothesis that comet 67P originated from the merging of two distinct bodies in the early Solar System.
The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of scientific experiments designed to complete the most detailed study o
f a comet ever attempted are onboard Rosetta. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The disk-averaged phase function of the nucleus of 67P shows a strong opposition surge with a G parameter value of -0.13$pm$0.01 in the HG system formalism and an absolute magnitude $H_v(1,1,0)$ = 15.74$pm$0.02 mag. The integrated spectrophotometry in 20 filters covering the 250-1000 nm wavelength range shows a red spectral behavior, without clear absorption bands except for a potential absorption centered at $sim$ 290 nm that is possibly due to SO$_2$ ice. The nucleus shows strong phase reddening, with disk-averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3$^{circ}$--54$^{circ}$ phase angle range. The geometric albedo of the comet is 6.5$pm$0.2% at 649 nm, with local variations of up to $sim$ 16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700-750 nm that is associated with coma emissions.
The Southern hemisphere of the 67P/Churyumov-Gerasimenko comet has become visible from Rosetta only since March 2015. It was illuminated during the perihelion passage and therefore it contains the regions that experienced the strongest heating and er
osion rate, thus exposing the subsurface most pristine material. In this work we investigate, thanks to the OSIRIS images, the geomorphology, the spectrophotometry and some transient events of two Southern hemisphere regions: Anhur and part of Bes. Bes is dominated by outcropping consolidated terrain covered with fine particle deposits, while Anhur appears strongly eroded with elongated canyon-like structures, scarp retreats, different kinds of deposits, and degraded sequences of strata indicating a pervasive layering. We discovered a new 140 m long and 10 m high scarp formed in the Anhur/Bes boundary during/after the perihelion passage, close to the area where exposed CO$_2$ and H$_2$O ices were previously detected. Several jets have been observed originating from these regions, including the strong perihelion outburst, an active pit, and a faint optically thick dust plume. We identify several areas with a relatively bluer slope (i.e. a lower spectral slope value) than their surroundings, indicating a surface composition enriched with some water ice. These spectrally bluer areas are observed especially in talus and gravitational accumulation deposits where freshly exposed material had fallen from nearby scarps and cliffs. The investigated regions become spectrally redder beyond 2 au outbound when the dust mantle became thicker, masking the underlying ice-rich layers.
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