Cometary Dust

Abstract

This review presents our understanding of cometary dust at the end of 2017. For decades, insight about the dust ejected by nuclei of comets had stemmed from remote observations from Earth or Earth's orbit, and from flybys, including the samples of dust returned to Earth for laboratory studies by the Stardust return capsule. The long-duration Rosetta mission has recently provided a huge and unique amount of data, obtained using numerous instruments, including innovative dust instruments, over a wide range of distances from the Sun and from the nucleus. The diverse approaches available to study dust in comets, together with the related theoretical and experimental studies, provide evidence of the composition and physical properties of dust particles, e.g., the presence of a large fraction of carbon in macromolecules, and of aggregates on a wide range of scales. The results have opened vivid discussions on the variety of dust-release processes and on the diversity of dust properties in comets, as well as on the formation of cometary dust, and on its presence in the near-Earth interplanetary medium. These discussions stress the significance of future explorations as a way to decipher the formation and evolution of our Solar System.

Keywords: Aggregates; Comet formation; Comets; Comets: coma, nucleus, trail; Comets: individual: 1P/Halley, 9P/Tempel 1, 67P/Churyumov-Gerasimenko, 81P/Wild 2, C/1995 O1 Hale-Bopp; Cosmic dust; Dust; Jupiter-family comets; Organics; Origin of life; Rosetta; Solar System formation; Solar nebula; Stardust.

Fig. 1

Rosetta with labelled instruments, including the lander before its release. The dedicated dust analysing instruments COSIMA, GIADA and MIDAS, as well as the OSIRIS camera system, the VIRTIS VIS/NIR imaging spectrometer and the CONSERT antenna are located on the panel of Rosetta that is mainly oriented towards the cometary nucleus. After ESA/ATG medialab.

Fig. 2

Comet 67P/C-G dust elemental ratios relative to Fe and to the CI chondrite composition [Lodders 2010] compared to 1P/Halleys dust and 81P/Wild 2s dust collected in Stardust aerogel. The values correspond to the relative ratios (E/Fe)comet / (E/Fe)CI. The N/C atomic ratio has been measured by Fray et al. (2017). The error bars for 67P/C-G data come from the uncertainties on the determination of Relative Sensitivity Factors (RSFs) uncertainties and can be amplified by successive normalizations. For instance, the uncertainty displayed for C is the addition of uncertainty on the RSF for calculation of C/Si, plus uncertainty for Si/Fe, plus uncertainty on C/Fe in CI-type chondrite. Therefore, error bars can be “artificially” enhanced due to successive normalizations to compare all elements on a same plot (this explains why error bars on N are so large in this plot, compared to the actual value measured for N/C = 0.035 ± 0.011 [Bardyn et al. 2017].

Fig. 3

Composition of 67P/C-G dust particles. (a) Relative abundance of elements in atom numbers for 67P/C-G dust particles. In this chart, H abundance is not directly measured and the atomic ratio H/C=1 is assumed. Elements under the label “other” include mainly N, Na, Si, and Mn, which relative abundances have been measured with COSIMA, and elements such as S and Ni, not yet quantified with COSIMA, but assumed to have a Solar abundance. (b) Tentative repartition between a mineral and an organic component assuming that elements such as C, N, H and some of the O are the constitutive elements of the organic phase, while the remaining O and other elements are included in the mineral phase. For the oxygen, it is assumed as an upper limit that minerals can be at most with a SiO₄ stoechiometry, the remaining O being incorporated in the organic phase [Bardyn et al. 2017]. (c) Relative abundance in volume of mineral and organic components in 67P/C-G dust particles. This is calculated considering that a relative density for minerals of 3, and a relative density of 1 for organic molecules.

Fig. 4

Left panel: A typical compact particle collected with MIDAS, extending beyond the field of view and exhibiting a well-defined boundary. The surface features at smaller scales are interpreted as individual units composing the dust. Right panel: A large fluffy particle (also extending beyond the field of view of MIDAS) that is interpreted to represent the remains of a fractal aggregate with fractal dimension D_f = 1.7 ± 0.1 that compacted on collection [Mannel et al. 2016]. Another possibility, the rebound of a larger dust particle bouncing on the target and depositing a surface layer, is less likely due to the structural differences found in experimental tests [Ellerbroek et al. 2017]. ESA/Rosetta/IWF for the MIDAS team IWF/ESA/LATMOS/Universiteit Leiden/Universität Wien

Fig. 5

Morphology of particles collected by COSIMA, using the classification of Y. Langevin et al., 2016; 5.a: “compact” particle exhibiting well-defined boundary; 5.b “glued cluster”, with sub-components apparently linked together by a matrix.; 5.c “rubble pile”, with a central mound surrounded by an apron of smaller sub-components; 5.d: “shattered cluster”, with widely distributed components and a size to height ratio smaller than 10. Such particles present morphological similarities with the largest particles collected by MIDAS (Fig. 4), which are 10 times smaller.

Fig. 6

Mass and cross-section measurements of compact particles detected by GIADA from August 2014 to September 2016 (the error bars refer to 1σ standard error of the 271 GDS+IS measurements). The data are compared with the trends of prolate and oblate ellipsoids of aspect ratio of 10 (dotted lines) and 5 (dashed lines), respectively, and with dust bulk densities of Fe sulphides, ρ₁ = 4600 kg m⁻³ (upper lines), and of hydrocarbons, ρ₂ = 1200 kg m⁻³ (lower lines). The GDS signal saturates at a cross section > 10⁻⁶ m². Particles with cross-sections < 2 10⁻⁸ m² (and most of those with mass < 10 ⁻⁸ kg) were too small and fast to be detected by GDS. The flux at masses > 2 10⁻⁷ kg was very low during the entire mission due to the spacecraft safety constraints. All these facts explain why all data are distributed in a mostly horizontal cloud. The particles close to the upper dotted and dashed lines have an almost zero porosity, and are compact aggregates of minerals condensed in the inner protoplanetary disc (they are too large to have an interstellar origin [Brownlee 2014]). The particles at right of the lower dotted and dashed lines have both a large porosity and a composition dominated by hydrocarbons. The particles in between are a porous mixture of minerals and hydrocarbons.

Fig. 7

Power index of the differential size distribution of 67P/C-G. Dotted lines: power index before the 2015 equinox. Continuous lines: power index after the 2015 equinox, up to just after the 2015 perihelion. The ranges along the x axis show the instrument sensitivity, the ranges along the y axis are given by the uncertainty of the power index. Stars: COSIMA data [Merouane et al. 2017]. Diamonds: GIADA data [Rotundi et al. 2015] [Fulle et al. 2016c]. Triangles: OSIRIS observations of single particles in the coma [Rotundi et al. 2015] [Fulle et al. 2016c] [Ott et al. 2017]. Dotted diamond: OSIRIS observations of pebbles in Sais [Pajola et al. 2017a]. Dotted square: ROLIS observations of pebbles in Agilkia [Pajola et al. 2017a]. Squares: Ground-based observations, tail models [Moreno et al. 2017]. Crosses: Ground-based observations, trail models [Moreno et al. 2017].

Fig. 8

Instruments PROGRA² [Hadamcik et al. 2009a] and CODULAB [Muñoz et al. 2011], used to measure, through experimental studies in the laboratory or under microgravity conditions, the light scattering properties of various analogues of cometary dust in suspension.

Fig. 9

From left to right, ordered by increasing sizes: Electron micrograph of stratospheric IDP (Particle L2021A1, about 13 × 10 μm, credit NASA) ; Electron micrograph of Antarctic micrometeorite DC02-03-53 collected near the Concordia station (size about 30 × 40 μm, [Duprat et al. 2007]); Optical image of the giant cluster IDP U2-20GCA (scale bar 100 μm) [Messenger et al. 2015]).

Fig. 10

Maximum equilibrium temperatures for particles entering the Earth atmosphere at 30 km s⁻¹. The horizontal line corresponds to a typical temperature of sublimation of 2100 K (adapted from [Levasseur-Regourd and Lasue 2011]).