Synthesis of the Morphological Description of Cometary Dust at Comet 67P

TL;DR

This paper synthesizes and classifies data on cometary dust from Rosetta and previous missions, establishing a common framework based on structure, porosity, strength, and size to facilitate comparison and integration of diverse measurements.

Contribution

It introduces a simple, reliable classification framework for cometary dust properties, aiding inter-comparison of diverse datasets from multiple instruments and missions.

Findings

Proposed a classification based on structure, porosity, strength, and size.

Validated the framework's usefulness within the Rosetta dust community.

Facilitates future synergy and comparison across different datasets.

Abstract

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 Earth's 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 particle's 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.

Full text

1 Introduction

When comets become active, they release gas and dust, where the latter is then carried away by the gas to form the cometary coma. The detailed physical processes of the dust release from the surface are not well known. However, given that cometary material is known to exhibit a very low strength (AttreeEtal:2018; GroussinEtal:2015) and processes take place under the extremely low cometary gravity (SierksEtal:2015), the required forces for the dust lift-off are likely gentle. This is the mechanism by which a comet – formed 4.57 billion years ago – slowly decomposes back into its building blocks. The level of the primitiveness of these dust particles with respect to their formation time can be debated but it is clear that they still carry clues to the early formation of comets and our solar system. It must be the ultimate goal of cometary dust studies – whether from Earth or by space missions – to interpret results from this viewpoint and aim to decipher these clues.

It was the purpose of Rosetta, “ESA’s Mission to the Origin of the Solar System” (SchulzEtal:2009), to provide the data in support of this goal. Three instruments on Rosetta were exclusively dedicated to the study of dust in the coma of comet 67P/Churyumov-Gerasimenko. But several other instruments from the suite of 11 instruments on-board Rosetta and 10 instruments on the lander Philae were equally suited and successful in the study of cometary dust. Results of these dust studies are presented in Sects. 3.1 to LABEL:sect:Philae.

Rosetta-era studies of cometary dust are standing on the shoulders of space missions like Giotto and Vega at 1P/Halley and Stardust at 81P/Wild 2. Giotto was equipped with two dedicated dust instruments, the Dust Impact Detector System (DID) and the Particle Impact Analyzer (PIA), providing the first in-situ data of cometary dust shortly after its release from a comet. Additionally, the Optical Probe Experiment (OPE) retrieved local dust brightness and polarisation. The Vega spacecraft were equipped with the dust mass spectrometer PUMA (on Vega 1; KisselKrueger:1987; KruegerEtal:1991) and the dust particle detectors SP-1 and SP-2 (on Vega 1 and 2, respectively; ReinhardBattrick:1986), all in-situ dust instruments. The Stardust spacecraft, during its flyby at comet Wild 2, collected dust particles to bring them to Earth for detailed analysis (Stardust results are described further in Sect. LABEL:sect:Stardust). Additional in-situ information was provided by Stardust’s Dust Flux Monitor Instrument (DFMI; TuzzolinoEtAl:2004) and Cometary and Interstellar Dust Analyzer (CIDA; KisselEtal:2004).

Aspects of cometary dust can also be studied from Earth: Telescope observations can for instance determine levels of activity and the morphology of the large scale dust tails and trails, thus dynamics of dust particles; photo-polarimetric studies allow the interpretation of the dust particles’ structures. Cometary dust particles, once lifted from a comet, can travel through the solar system and eventually cross the Earth orbit. These are collected in the Earth stratosphere as interplanetary dust particles (IDPs) or on the Earth surface as micrometeorites (MMs). These aspects will all be summarised and discussed in Sects. LABEL:sect:IDP and LABEL:sect:EarthObs.

The main goal of this work is to summarise Rosetta results on cometary dust and make these comparable among themselves but also with studies outside Rosetta. Here we focus on the morphology and structure of cometary dust particles; for the composition and mineralogy, the reader is referred to EngrandEtal:2016, ZolenskyEtal:2006 and others. Many results were published by the different Rosetta instrument teams and due to the nature of the complementary instruments, measurement parameters are different and not directly comparable. We will in Sect. 2 establish a clear language and classification for dust particles of different morphologies. This is not a new definition but we try to summarise the consensus of the community and then rigorously stick to it. Based on this, we will in Sect. 3 summarise results from Rosetta, Stardust, and Earth-based observations. As a key result, these are summarised in Table LABEL:tab:table and Fig. LABEL:fig:OverviewPlot. These are compared and discussed in Sect. LABEL:sect:discussion and a short conclusion is presented in Sect. LABEL:sect:conclusion.

This work is the result of a series of workshops and discussions, involving the largest part of the Rosetta dust community. It is clear that due to the extent of data still being interpreted, this can only be a first step in a concerted understanding of Rosetta data in particular and cometary dust in general. However, the work is ongoing and we aim to continue combining our results in the spirit of this paper.

2 Classification

2.1 General Nomenclature

Different communities or even different scientists tend to use slightly different nomenclatures. This work is a large collaborative effort and the aim is to form a broad agreement (or at least identify disagreements). It is therefore critical to be as explicit and precise as possible, which is why we provide here the used and agreed nomenclature. We intend to keep consistence with the nomenclature used in the Stardust (e.g., brownlee_comet_2006) as well as planetesimal formation (e.g., DominikEtal:2007) communities.

A grain is the smallest component we consider in this study. It is a solid particle with a tensile strength (typically ¿ MPa) that is larger than forces acting in its environment. A grain is likely irregular in shape but homogeneous in composition. It is the constituent that forms the aggregates and agglomerates defined below. Grains were created by condensation, either in the Solar System’s protoplanetary disk or earlier in the interstellar medium or AGB-star outflows (e.g., AlexanderEtal:2007). We do not specify the grain’s material in this definition.

The term monomer is often used in this context and must not be mistaken with the definition of a monomer molecule. In the dust community, monomer is used synonymously with grain, often in theoretic works. We thus propose to keep this term but restrain its use to spherical or elliptical grains or their mathematical description.

We use the term (dense) aggregate for an intimate assemblage of grains, rigidly joined together and not readily dispersed. Such dense aggregates might look like grains from the outside but in fact contain different mineralogical components (grains) in the inside. The smallest components observed in the Stardust sample were these aggregates (brownlee_comet_2006).

A (porous) agglomerate is constituted of grains or dense aggregates. The binding forces are much smaller than the grains’ or aggregates’ inner binding forces such that agglomerates are easily dispersed. Agglomerates are the particles that are expected to form through dust agglomeration in the early protoplanetary disk (DominikEtal:2007).

The terms aggregate and agglomerate are often used synonymously, describing what we define here as agglomerate. However, we see the need to formally discriminate between these two. The precise distinction is not always consistent in the literature (e.g., NicholsEtal:2002; Walter:2013) and we choose the definition that is prevalently used in the community whenever the two are discriminated at all. We propose to address them as dense aggregate and porous agglomerates wherever the precise wording is important (as dense we consider porosities $<10\%$, see below).

Furthermore, we distinguish the case of a fractal agglomerate, which is showing a fractal and dendritic nature, implying a very high porosity of typically $>99\%$. For these, the fractal dimension $D_{\mathrm{f}}$ defines the relation between mass $m$ and size $r$ as $m\propto r^{D_{\mathrm{f}}}$ (e.g., review by Blum:2006). In our case, the relevant fractal agglomerates have $D_{\mathrm{f}}<3$ and are typically in the range 1.5 – 2.5. This is consistent with particles formed by cluster-cluster agglomeration (Blum:2006).

Finally, we use the term particle as a generic term for any unspecified dust particle. This can be anything from a monomer to an agglomerate and implies that the nature of a particle is not further known or considered.

2.2 Structure and Porosity Classification

Based on the general nomenclature above, we further refine the description of the structures and porosities of dust particles. Besides a particle’s size, the porosity and structure are parameters which are to some degree accessible for Rosetta’s dust instruments and are a focus of this work. We introduce three groups, which will prove to be useful in terms of categorizing Rosetta dust observations summarised below. Each group comprises physical properties and a structure, which can explain these properties. Specifically, the three discriminating properties chosen here are (a) porosity, (b) structure, and (c) strength. Various structures can be possible within these groups, which are illustrated in Fig. 1 and explained below one by one. Also, each of these structures will be compared to examples in nature, laboratory, or theory in Figs. 2 – 6.

The group solid describes particles with (a) a porosity ¡ 10 %, that are (b) consolidated and (c) exhibit a high strength similar to rock. Particles that fall into this group are the grains and dense aggregates described in Sect. 2.1, as well as chondrules or calcium aluminium rich inclusions (CAIs). The tensile strength should be in the MPa range and higher, which is only the case for solid particles of low porosity. The latter is chosen to be ¡ 10 %, to be much smaller than the random close packing of a granular medium ($\sim 40$ %, OnodaLiniger:1990), clearly discriminating between compressed agglomerates. The most reasonable mechanism to create these low porosities for cometary particles is thermal processing, i.e., compaction through (partial) melting or vapour transport.

We identify two structures that fall into this group. Irregular grains and spherical monomers (SOLID_1 in Fig. 1) are the smallest. Examples of irregular grains used in laboratory analogue experiments are shown in Fig. 2 (left). Many different materials have been used in laboratory experiments, while the examples here are from diamond (top left, PoppeEtal:2000a) and Forsterite (bottom left, TamanaiEtal:2006). Spherical monomers can easily be formed in the laboratory from super saturated gas or liquid phases and are also used for analogue experiments in astrophysics. All three examples in Fig. 2 (right) are from SiO2 but with different size distributions (top to bottom: PoppeEtal:2000a; ColangeliEtal:2003; BrissetEtal:2017). Monomers in nature are not perfectly spherical and exhibit surface roughness.

If grains form a dense aggregate, we expect a morphology like SOLID_2, which is an idealized (simplified) Stardust particle. Figure 3 shows three thin sections of solid aggregates from Stardust (top: Brownlee:2014; bottom left: brownlee_comet_2006). Also some interplanetary dust particles (IDPs) collected in the Earth’s stratosphere in the NASA Cosmic Dust Catalog111Example images of interplanetary dust particles in this work are from the NASA Cosmic Dust Catalog Volume 15 from 1997 (see e.g. Brownlee:1985; Brownlee:2016). resemble this morphology, an example is presented in Fig. 3 (L2021B6, bottom right).

The structures following hereafter are based on agglomerated particles from the solid group. SOLID_1 are drawn grey (e.g., silicates) or blue (e.g., ices) to make clear that the composition can be varying. However, we want to leave the shape and composition of the constituent grains open on that scale and therefore assume that the agglomerates below can form out of any of those grains in any mixed state. The composition of many agglomerates in Stardust and Rosetta is known but fall outside the scope of this paper.

The second class, group fluffy, describes agglomerates, which (a) have a very high porosity (¿ 95 %). These (b) are likely fractal and dendritic agglomerates, the only reasonable explanation for extreme porosities, and (c) show a very low strength (Pa range). A visualized example is FLUFFY_1 in Fig. 1. Fractal agglomerates are very well known from the literature, in particular in the context of early planet formation (Blum:2006). The examples in Fig. 4 (left) show fractal agglomerates from SiO2, grown under laboratory conditions. The top one is a small agglomerate out of 1.9 $\mu$m monomers from HeimEtal:1999 while the lower one is significantly larger and has a fractal dimension of $D_{\mathrm{f}}\approx 1.8$ (not measured for this specific agglomerate but for similar ones formed under the same condition, Blum:2004). The example on the top right is a fractal agglomerate formed in a computer simulation by ballistic cluster-cluster agglomeration, consists of 8192 monomers and exhibits a fractal dimension of 1.99 (WadaEtal:2008). The bottom right example is from the Rosetta/MIDAS experiment and will be discussed in detail in Sect. 3.1.

Finally, the group porous collects the remaining parameter range with particles of (a) porosities between 10 and 95 %. These are (b) considered as loosely bound agglomerates with (c) an intermediate but rather low strength, typically in the order of 1 Pa to 100 kPa. Laboratory analogue experiments demonstrated that in the case of silicates, this depends only mildly on composition (BlumEtal:2006). Due to their higher stickiness in collisions (GundlachBlum:2015), ice agglomerates may form easier in the solar nebula. However, their intrinsic cohesion (tensile strength) is very similar to that of silicates (GundlachEtal:2018a) as long as the temperatures are low. For temperatures above $\sim 140$ K, $\mu$m-sized ice particles start to sinter on timescales shorter than $\sim 10^{5}$ s so that for cometary nuclei close to the ice-evaporation front, the mechanical strength might be increased (GundlachEtal:2018b). Sintering can also occur for silicates (Poppe:2003) and organics (KouchiEtal:2002).

Figure 1 provides two examples for this group: POROUS_1 is a van-der-Waals agglomerate with a rather homogeneous structure, bound by surface forces. Similar agglomerates are studied in laboratories and computer simulations. In Fig. 5, we present an SEM image of a loose agglomerate consisting of 0.5 $\mu$m solid Zirconium silicate particle (top left, BlumMuench:1993), an IDP from the NASA Cosmic Dust Catalog (L2021A1, bottom left), an x-ray tomography reconstructed cut though an agglomerate from SiO2 monomers (top right, KotheEtal:2013), and an agglomerate used for numeric simulations (bottom right, WadaEtal:2011).

The sub-structure might not be as homogeneous and the second example for the porous group in Fig. 1 (POROUS_2) represents a cluster consisting of smaller agglomerates with voids in-between. Such hierarchic agglomerates were produced in laboratory experiments as shown in Fig. 6 (top left). This is a back-light illuminated agglomerate, grown from smaller (100 $\mu$m) agglomerates under microgravity conditions (BrissetEtal:2016). The other two examples are from Rosetta/COSIMA (centre) and Rosetta/MIDAS (right) and will be explained in detail in Sect. 3.1 and LABEL:sect:COSIMA. A hierarchic agglomerate structure can formally be described as being fractal if the agglomerate consists of hierarchically structured (self-similar) sub-agglomerates. In the case of POROUS_2 we assume either one single sub-agglomerate size or few cascades of sub-structures, such that all requirements for the porous group are still fulfilled. The strength of these clusters is significantly smaller than POROUS_1 structures if the number of contacts is small at sub-structure boundaries.

It should be noted that a classification by these three groups is not always unambiguous. Structure, porosity, and strength have a likely but non-mandatory correlation. It can therefore be possible that a studied particle shares properties of more than one group such that a classification is not easily possible. This is in particular the case when also the structure shows properties of different groups as in the mixed cases in Fig. 7.

POROUS_SOLID_1 is a particle from the solid group, mantled with an agglomerate layer. An example for such an agglomerate is the common picture of a mantled chondrule. A polished cross-section of a rimmed chondrule by MetzlerEtal:1992 is shown on the top left of Fig. 8 and an isolated chondrule analogue by BeitzEtal:2012 is shown in the top right (inset with different coating technique). From a density measurement, one would interpret this particle as a member of the solid group, while the outer appearance (e.g., light scattering) would cloak it as a member of the porous group. The Stardust particle T57 Febo (brownlee_comet_2006, also Fig. 8 bottom left) is another mixed case POROUS_SOLID_2. Depending on the ratio between solid and mixed component, the group would be ambiguous. Finally, a solid particle with an attached fractal structure as depicted in FLUFFY_SOLID_1 was observed in IDPs (NASA Cosmic Dust Catalog, L2021A7, Fig. 8 bottom right) and shares reflection and density properties from the solid and fluffy group.

Occurring ambiguities will further be discussed in Sec. 3 wherever they occur.

3 State of Knowledge

In this section we summarise the knowledge on cometary dust with a focus on Rosetta.

Sections 3.1 through LABEL:sect:Philae focus on Rosetta dust instruments. These are all different and thus complementary in nature: The Micro-Imaging Dust Analysis System (MIDAS; see RiedlerEtal:2007) collected dust particles and determined their shape and structure with an atomic force microscope. It was thus an in-situ instrument with an imaging method. The same is the case for the Cometary Secondary Ion Mass Analyser (COSIMA; see KisselEtal:2007), where collected particles were studied with a microscope and with a secondary ion mass spectrometer. The Grain Impact Analyser and Dust Accumulator (GIADA; see ColangeliEtal:2007) was another in-situ instrument but without an imaging method. Particles were instead crossing a laser curtain and their size and speed was determined from the signal of scattered light (Grain Detection System; GDS). The particles then collided on the Impact Sensor (IS) where their momentum (thus mass if velocity is known) could be measured if they carried enough momentum. The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS; see KellerEtal:2007), consisting of a narrow- and a wide-angle camera, could observe – within the inner coma – individual (but still unresolved) dust particles as well as a diffuse signal from a large ensemble of undistinguishable particles. In either case, the interpretation requires an assumption of the light scattering properties. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS; see CoradiniEtal:2007) could spectrally resolve the diffuse coma to infer colour and temperature of the unresolved dust. As OSIRIS and GIADA above, also VIRTIS relies on the scattered light and model assumptions on scattering properties. On the surface of comet 67P, the lander Philae also studied the dust and we put the focus here on the dust monitor DIM as part of Philae/SESAME (SeidenstickerEtal:2007) as well as the down-looking camera Philae/ROLIS (MottolaEtal:2007) and the cameras for panoramic imaging Philae/CIVA (BibringEtal:2007).

Sections LABEL:sect:Stardust through LABEL:sect:EarthObs extend the picture beyond recent Rosetta findings. We will consider constraints from the large body of Earth based observation as well as studies of cometary dust in laboratories, in particular the samples brought back to Earth by Stardust.

The intention of this section is to summarise the individual results and compile them into a comparative table (Table LABEL:tab:table). While the summaries shall be descriptive and comprehensive, the resulting table is a simplification, which is intended as a framework and an aid to memory for cross comparison. While individual instrument groups have so far interpreted specific instrument result, we are here aiming – with all Rosetta instrument teams involved – to homogenize our understanding and nomenclatures. For a more general and complementary review of cometary dust with a focus on Rosetta, the reader is also referred to the article by LevasseurRegourdEtal:2018.

3.1 Rosetta/MIDAS

The MIDAS atomic force microscope revealed the surface structure of particles with nanometre resolution for 1 – 50 $\mu$m sized particles. All studied particles show surfaces with textures that can be interpreted as that of an agglomerate consisting of smaller subunits, which could again be of agglomerate structure (bentley_midas_2016).

One particle was pointed out to exhibit an extraordinarily loose packing of subunits, and a sophisticated structural analysis was conducted (Fig. 4, bottom right; bentley_midas_2016; mannel_fractal_2016). Subunits range from 0.58 to 2.57 $\mu$m with an average of 1.48 $\mu$m (bentley_midas_2016), while it cannot be excluded that these subunits are again agglomerates with subunit sizes less than about 500 nm. The particle is expected to have compacted during collection so that its image can be interpreted as a projection of the original structure onto the target plane (Fig. 4, bottom right) which was determined to be fractal with a fractal dimension $D_{\mathrm{f}}=1.7\pm 0.1$