A post-Rosetta understanding of polarimetric observations of comets (original) (raw)
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The phase function and density of the dust observed at comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2018
The OSIRIS camera onboard Rosetta measured the phase function of both the coma dust and the nucleus. The two functions have a very different slope versus the phase angle. Here, we show that the nucleus phase function should be adopted to convert the brightness to the size of dust particles larger than 2.5 mm only. This makes the dust bursts observed close to Rosetta by OSIRIS, occurring about every hour, consistent with the fragmentation on impact with Rosetta of parent particles, whose flux agrees with the dust flux observed by GIADA. OSIRIS also measured the antisunward acceleration of the fragments, thus providing the first direct measurement of the solar radiation force acting on the dust fragments and thus of their bulk density, excluding any measurable rocket effect by the ice sublimation from the dust. The obtained particle density distribution has a peak matching the bulk density of most COSIMA particles, and represents a subset of the density distribution measured by GIADA. This implies a bias in the elemental abundances measured by COSIMA, which thus are consistent with the 67P dust mass fractions inferred by GIADA, i.e. (38 ± 8) per cent of hydrocarbons versus the (62 ± 8) per cent of sulphides and silicates.
Monthly Notices of the Royal Astronomical Society, 2016
The Rosetta spacecraft made a rendezvous with comet 67P/Churyumov-Gerasimenko (67P) in 2014 August, soon after the Philae module landed on the small lobe of the nucleus on 2014 November 12. The CONSERT instrument, onboard Rosetta and Philae, sounded the upper part of the interior of 67P with radiowaves at 90 MHz and determined an average of the real part of the permittivity (hereafter) equal to about 1.27. The SESAME-PP instrument, onboard Philae, sounded the near-surface of the small lobe in the 400-800 Hz range and determined a lower limit of equal to 2.45. We use a semi-empirical formula obtained from measurements of performed in the laboratory at 243 K on water ice and ice-basaltic dust mixtures, with a controlled porosity in the 31-91 per cent range and a dust-to-ice volumetric ratio in the 0.1-2.8 range, to interpret the results of the two instruments, taking into account the temperature and frequency dependences. A graphical method is proposed to derive ranges of porosity and dust-mass fraction from a value of derived from observations. The non-dispersive behaviour of below 175 K, allows us to compare the values of obtained by CONSERT and SESAME-PP. We show that the porosity of the small lobe of 67P increases with depth. Based on new measurements of analogues of complex extraterrestrial organic matter, the so-called tholins, we also suggest that, for the dust component in the cometary material, the presence of silicates has more effect on than organic materials.
Astronomy & Astrophysics, 2019
Context.The dust-brightness phase curves that have been measured by the OSIRIS cameras on board the Rosetta spacecraft within the coma of comet 67P/Churyumov-Gerasimenko (67P) present a remarkable flattened u-shape.Aims.Our goal is to compare these phase curves with those of tentatively analog dust samples to assess the key dust properties that might induce this shape.Methods.Light-scattering measurements have been made with the PROGRA2 instrument in the laboratory and in microgravity conditions on samples of different physical properties and compositions that are likely to be representative of cometary dust particles.Results.We find that the brightness phase curves of a series of interplanetary dust analogs that have been recently developed (to fit the polarimetric properties of the inner zodiacal cloud and their changes with heliocentric distance) are quite comparable to those of 67P. Key dust properties seem to be related to the composition and the porosity.Conclusions.We conclud...
Probing the internal structure of the nuclei of comets
Planetary and Space Science, 2009
There is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m À3 for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov-Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov-Gerasimenko, thanks to the on-board CONSERT experiment. r
Spatial variations of brightness, colour and polarization of dust in comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2017
We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.
Characteristics of cometary dust in the innermost coma derived from polarimetry by Giotto
Monthly Notices of the Royal Astronomical Society, 2013
We interpret the polarimetric measurements of Comets 1P/Halley and 26P/Grigg-Skjellerup carried out by the Giotto space probe using light-scattering properties of model agglomerate particles having different structure and composition. Our simulations suggest that the overall low linear polarization registered in the innermost coma can be interpreted in terms of a high abundance of Mg-rich silicate particles and depletion of carbonaceous materials. Variation of the polarization throughout the innermost coma could be explained either through a significant variation in the size distribution or a slight variation of refractive index of cometary dust;
Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition
Astronomy and Astrophysics, 2010
Context. Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Methods. Recent imaging polarimetric observations were conducted at the Haute-Provence observatory (France) on 2009 March 17−19 at 35 • phase angle and at IUCAA Girawali observatory (India) on 2008 December 25−27 at 36 • phase angle and on 2009 April 30-May 1 at 29 • phase angle. With the imaging technique, the intensity and linear polarization variations are studied through the various coma regions. These observations are compared to other cometary data (e.g. Jupiter family comets) and to numerical and experimental simulations.
Unexpected and significant findings in comet 67P/Churyumov–Gerasimenko: an interdisciplinary view
Monthly Notices of the Royal Astronomical Society, 2016
ESA's Rosetta Mission has followed Comet 67P/Churyumov-Gerasimenko from 3.6 au inbound to 3.6 au outbound. Many results are largely unexpected, as compared to previous models based on in situ and ground-based observations of Jupiter-family comets. The main topics discussed in this review are (1) the importance of the large concavities characterizing the 67P nucleus, that, (2) coupled to the nucleus obliquity, make seasons an unexpectedly important source of many phenomena observed in this and probably in most comets; (3) the mostly uniform distribution of ices over the nucleus surface; (4) the high dust-to-water mass ratio, which implies that much of the nucleus mass is in the form of minerals partly coming from the inner proto-solar nebula, thus making 67P very porous and less hydrated than primitive CI chondrites. 67P nucleus may have never experienced any collision at speeds larger than 1 m s −1 .
Density and Charge of Pristine Fluffy Particles from Comet 67P/CHURYUMOV–GERASIMENKO
The Astrophysical Journal, 2015
The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA's Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov-Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to <1 kg m −3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds <1 m s −1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.
Recent polarimetric observations of comet 67 P/Churyumov-Gerasimenko
Proceedings of the International Astronomical Union, 2009
Remote observations of solar light scattered by dust in comet 67P/Churyumov-Gerasimenko coma are of major importance to assess the properties of the dust and thus to prepare the rendezvous of the Rosetta spacecraft with comet 67P/Churyumov-Gerasimenko. We present polarimetric data obtained from India in December 2008 and France in March 2009. Compared with previous observations of this comet and of other Jupiter family comets, they confirm that it is dust-poor, although it may exhibit outbursts leading to the ejection of dust particles from its subsurface, especially after its perihelion passage.