A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues (original) (raw)
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Variations in cometary dust composition from Giotto to Rosetta, clues to their formation mechanisms
Monthly Notices of the Royal Astronomical Society, 2016
This paper reviews the current knowledge on the composition of cometary dust (ice, minerals and organics) in order to constrain their origin and formation mechanisms. Comets have been investigated by astronomical observations, space missions (Giotto to Rosetta), and by the analysis of cometary dust particles collected on Earth, chondritic porous interplanetary dust particles (CP-IDPs) and ultracarbonaceous Antarctic micrometeorites (UCAMMs). Most ices detected in the dense phases of the interstellar medium (ISM) have been identified in cometary volatiles. However, differences also suggest that cometary ices cannot be completely inherited from the ISM. Cometary minerals are dominated by crystalline Mg-rich silicates, Fe sulphides and glassy phases including GEMS (glass with embedded metals and sulphides). The crystalline nature and refractory composition of a significant fraction of the minerals in comets imply a high temperature formation/processing close to the proto-Sun, resetting a possible presolar signature of these phases. These minerals were further transported up to the external regions of the disc and incorporated in comet nuclei. Cometary matter contains a low abundance of isotopically anomalous minerals directly inherited from the presolar cloud. At least two different kinds of organic matter are found in dust of cometary origin, with low or high nitrogen content. N-poor organic matter is also observed in primitive interplanetary materials (like carbonaceous chondrites) and its origin is debated. The N-rich organic matter is only observed in CP-IDPs and UCAMMs and can be formed by Galactic cosmic ray irradiation of N 2-and CH 4-rich icy surface at large heliocentric distance beyond a 'nitrogen snow line'.
Composition of cometary dust: The case against silicates
Astrophysics and Space Science, 1975
It is argued that the infrared emission including 10 and 18/~ features observed in recent con'Lets is unlikely to be due to silicates. The vaporization temperature of the emitting material 500 K is consistent with emission by crystalline polyformaldehyde, There ]has been a significant advance in recent years of our knowledge of the chemical composition of the volatile ('icy') component of comets, which is ultimately responsible for the gaseous emission in the coma and plasma tail. At least in a majority of comets this component seems to be dominated by H20 ice, with other species trapped as impurities in the H20 lattice to form clathrates (Delsemme, 1973; Mendis and Ip, 1975). Part of the reflected solar radiation in the coma and dust tail is also probably due to the scattering by small icy grains stripped from the cometary nucleus by the evaporating gases (Delsemme and Wenger, 1970; lp and Mendis, 1974). There is, however, also a much less volatile ('dust') component capable of achieving high temperatures (>500 K) (Ney, 1974). The chemical composition of this component is virtually unknown. While evidence from meteor streams associated with comets (e.g. fl-Taurids) as well as Sun-grazing comets (e.g. Ikea-Seki, 1965 VIII) indicate that alkali metals as well as metals from the iron group are present in the cometary dust grains, broad infrared spectral features at 10 # and 18 r seen in the recent comets Bennett (1969i), Kohoutek (1973f), and Bradfield (1974b) have been widely attributed to silicates, as in the case of the interstellar and circumstellar dust (Icarus, 1974). An alternative identification of these two spectral features with formaldehyde polymers ((H2CO)n) have, however, been suggested recently in the case of both the interstellar grains (Wickramasinghe, 1974, 1975), as well as the comets (Vafiysek and Wickramasinghe, 1975). It seems difficult, with the present observations, to discriminate between these two candidates, spectroscopically. We will, therefore, examine the cometary data bearing mainly on the physical nature of the grains with a view of choosing between these two
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 .
Science (New York, N.Y.), 2015
The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ(-1)), and the broad absorption feature in the 2.9-to-3.6-micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.
Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta
Monthly Notices of the Royal Astronomical Society
Cometary ices are rich in CO 2 , CO and organic volatile compounds, but the carbon content of cometary dust was only measured for the Oort Cloud comet 1P/Halley, during its flyby in 1986. The COmetary Secondary Ion Mass Analyzer (COSIMA)/Rosetta mass spectrometer analysed dust particles with sizes ranging from 50 to 1000 μm, collected over 2 yr, from 67P/Churyumov-Gerasimenko (67P), a Jupiter family comet. Here, we report 67P dust composition focusing on the elements C and O. It has a high carbon content (atomic C/Si = 5.5 +1.4 −1.2 on average) close to the solar value and comparable to the 1P/Halley data. From COSIMA measurements, we conclude that 67P particles are made of nearly 50 per cent organic matter in mass, mixed with mineral phases that are mostly anhydrous. The whole composition, rich in carbon and non-hydrated minerals, points to a primitive matter that likely preserved its initial characteristics since the comet accretion in the outer regions of the protoplanetary disc.
Astronomy & Astrophysics
Context. Because comets are part of the most primitive bodies of our solar system, establishing their chemical composition and comparing them to other astrophysical bodies gives new constraints on the formation and evolution of organic matter throughout the solar system. For two years, the time-of-flight secondary ion mass spectrometer COmetary Secondary Ion Mass Analyzer (COSIMA) on board the Rosetta orbiter performed in situ analyses of the dust particles ejected from comet 67P/Churyumov-Gerasimenko (67P). Aims. The aim is to determine the H/C elemental ratio of the refractory organic component contained in cometary particles of 67P. Methods. We analyzed terrestrial and extraterrestrial calibration samples using the COSIMA ground-reference model. Exploiting these calibration samples, we provide calibration lines in both positive and negative ion registration modes. Thus, we are now able to measure the cometary H/C elemental ratio. Results. The mean H/C value is 1.04 ± 0.16 based o...
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.