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تسجيل مستخدم جديدMass loading at 67P/Churyumov-Gerasimenko: a case study
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We study the dynamics of the interaction between the solar wind ions and a partially ionized atmosphere around a comet, at a distance of 2.88 AU from the sun during a period of low nucleus activity. Comparing particle data and mag- netic field data for a case study, we highlight the prime role of the solar wind electric field in the cometary ion dynamics. Cometary ion and solar wind proton flow directions evolve in a correlated manner, as expected from the theory of mass loading. We find that the main component of the acceler- ated cometary ion flow direction is along the anti-sunward direction, and not along the convective electric field direc- tion. This is interpreted as the effect of an anti-sunward polarisation electric field adding up to the solar wind con- vective electric field.
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Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the S
olar System. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared and the study of chondritic porous interplanetary dust particles that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection. Here we report in situ measurements of dust particles at comet 67P/Churyumov-Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles. The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release.
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 mas
s spectrometer Ptolemy. On 12 to 14 November 2014 both instruments measured the organic composition of the comet nucleus material through seven measurements in sniffing mode during Philaes 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. All ion species showed the highest intensities in the first spectra measured by both instruments about 20 to 30 minutes after Philaes 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.
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.
The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experim
ent (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by sub-millimeter and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. The DIM sensor measures dynamical data like flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. DIM was operated during three mission phases of Philae at the comet: (1) Before Philaes separation from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected. (2) During Philaes descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in-situ dust detector. (3) At Philaes final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae, or due to shading by obstacles close to Philae, or both. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately $250, mathrm{kg,m^{-3}}$. The particle could have been lifted off from the comets surface by sublimating water ice.
Cometary outbursts offer a valuable window into the composition of comet nuclei with their forceful ejection of dust and volatiles in explosive events, revealing the interior components of the comet. Understanding how different types of outbursts inf
luence the dust properties and volatile abundances to better interpret what signatures can be attributed to primordial composition and what features are the result of processing is an important task best undertaken with a multi-instrument approach. The European Space Agency textit{Rosetta} mission to 67P/Churyumov-Gerasimenko carried a suite of instruments capable of carrying out this task in the near-nucleus coma with unprecedented spatial and spectral resolution. In this work we discuss two outbursts that occurred November 7 2015 and were observed by three instruments on board: the Alice ultraviolet spectrograph, the Visual Infrared and Thermal Imaging Spectrometer (VIRTIS), and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). Together the observations show that mixed gas and dust outbursts can have different spectral signatures representative of their initiating mechanisms, with the first outburst showing indicators of a cliff collapse origin and the second more representative of fresh volatiles being exposed via a deepening fracture. This analysis opens up the possibility of remote spectral classification of cometary outbursts with future work.
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