Comet dust

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Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin. When the Earth passes through a comet dust trail, it can produce a meteor shower.

Contents

Physical characteristics

Size

The majority of dust from comet activity is sub-micrometer [1] to roughly micrometer in size. [2] [3] However, this fraction is short-lived, as radiation pressure causes them to blow out of the Solar System [4] [5] or spiral inwards. [6] [7]

The next size class is large, "fluffy" [4] [5] or "cluster-type" [8] aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed [9] as the porous aggregates tend to fracture [10] or compact. [8] [11] [12]

Larger particles are micrometeoroids, [13] [14] not dust. [15] [16] In the absence of a definition from the IAU, [17] [18] groups devised their own definitions of dust: smaller than 100 micrometers, [19] 50, [20] 40, [21] 30, [22] and 20 microns, [23] and <10 μm. [24] [25] [26] [16] Some of these dust/micrometeorite definitions are approximate or ambiguous, [27] [28] [29] some overlapping or self-conflicting. [30] [23] [22]

The IAU released a formal statement in 2017. Meteoroids are 30 micrometers to 1 meter, dust is smaller, and the term "micrometeoroid" is discouraged (though not micrometeorite). [31] The IMO noted the new definition, [32] but still displays a prior definition on their site. [33] The Meteoritical Society site retains its prior definition, 0.001 cm. [34] The AMS has posted no rigorous definition. [35] [36]

Composition

Dust is generally chondritic in composition. Its monomers contain mafic silicates, such as olivine and pyroxene. [37] Silicates are rich in high-condensation temperature forsterite and enstatite. [27] As these condense quickly, they tend to form very small particles, not merging droplets.

As with chondritic meteoroids, particles contain Fe(Ni) sulfide [38] [39] and GEMS (glass with embedded metal and sulfides) [38]

Various amounts of organics (CHON) are present. [40] [41] [42] Though organics are cosmically abundant, and were widely predicted to exist in comets, they are spectrally indistinct in most telescopes. Organics were only confirmed via mass spectrometry during the Halley flybys. [43] [44] Some organics are in the form of PAHs (Polycyclic Aromatic Hydrocarbons). [45] [19] [46] [47] [48]

Very small inclusions of presolar grains (PSGs) may be found. [27] [48]

Dust and comet origin

Microscopic view of comet dust particle Comet dust microscopic photo.jpg
Microscopic view of comet dust particle

The models for the origin of comets are: [49]

  1. the interstellar model,
  2. the Solar System model,
  3. primordial rubble piles,
  4. aggregation of planetesimals in the dust disk around the Uranus–Neptune region,
  5. cold shells of material swept out by the protostellar wind.

Bulk properties of the comet dust such as density as well as the chemical composition can distinguish between the models. For example, the isotopic ratios of comet and of interstellar dust are very similar, indicating a common origin.

The 1) interstellar model says that ices formed on dust grains in the dense cloud that preceded the Sun. The mix of ice and dust then aggregated into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in the 1970s. [50] [51]

In the 2) Solar System model, the ices that formed in the interstellar cloud first vaporized as part of the accretion disk of gas and dust around the protosun. The vaporized ices later resolidified and assembled into comets. So the comets in this model would have a different composition than those comets that were made directly from interstellar ice.

The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter was forming.

Stardust's discovery of crystalline silicates in the dust of comet Wild 2 implies that the dust formed above glass temperature (> 1000 K) in the inner disk region around a hot young star, and was radially mixed in the solar nebula from the inner regions a larger distance from the star or the dust particle condensed in the outflow of evolved red giants or supergiants. The composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars. [52]

A comet and its dust allow investigation of the Solar System beyond the main planetary orbits. Comets are distinguished by their orbits; long period comets have long elliptical orbits, randomly inclined to the plane of the Solar System, and with periods greater than 200 years. Short period comets are usually inclined less than 30 degrees to the plane of the Solar System, revolve around the Sun in the same counterclockwise direction as the planets orbit, and have periods less than 200 years.

A comet will experience a range of diverse conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the comet nucleus, beginning the formation of a dust layer. Near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely.

A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses on the order of meters are thought to have accumulated on the surfaces of short-period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid.

Further assemblages and bodies

Dust particles, aided by ices and organics, form "aggregates" [27] [38] [53] (less often, "agglomerates" [54] ) of 30 to hundreds of micrometers. These are fluffy, [19] [55] due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates. [56]

The next size category is pebbles, of millimeters to centimeters scale. [57] [58] [59] Pebbles were inferred at 103P/Hartley 2, [60] and imaged directly at 67P/Churyumov-Gerasimenko. [59] [57] Astrophysical use of the word "pebble" differs from its geological meaning. [61] In turn, the next-larger geological term, "cobble," has been skipped by Rosetta scientists. [62]

Even larger bodies are "boulders" (decimeter-scale and above) or "chunks." These are rarely seen in the coma, as gas pressure is often insufficient to lift them to significant altitude or escape velocity. [63] [64] [65]

The building blocks of comets are the putative cometesimals, [66] analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale, [67] boulder-scale, [68] or otherwise has been a key topic in Solar System and exoplanet research. [55] [69] [70] [71]

(Mis)Use of the term "dust"

At best, "dust" is a collective noun for the non-gas portion of the coma and tail(s). At worst, the term is an English usage, understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields. [72] The larger solids are more properly called "debris" [73] [74] [64] or, for all non-gases, the general "particles" [75] [76] [44] or "grains." [77] [56] [22]

Comet 2P/Encke

Encke is officially a dust-poor, gas-rich comet. [6] [78] [79] Encke actually emits most of its solid mass as meteoroids or "rocks," [6] not dust. ISO measured no infrared evidence of a classical cometary dust tail due to small particles. [80]

Related Research Articles

The zodiacal light is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in a roughly triangular shape along the zodiac, and appears with less intensity and visibility along the whole ecliptic as the zodiacal band. Zodiacal light spans the entire sky and contributes to the natural light of a clear and moonless night sky. A related phenomenon is gegenschein, sunlight backscattered from the interplanetary dust, appearing directly opposite to the Sun as a faint but slightly brighter oval glow.

<i>Rosetta</i> (spacecraft) European orbiter sent to study a comet

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<span class="mw-page-title-main">Micrometeoroid</span> Meteoroid with a mass of less than one gram

A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.

<span class="mw-page-title-main">67P/Churyumov–Gerasimenko</span> Periodic contact binary comet

67P/Churyumov–Gerasimenko is a Jupiter-family comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of 135,000 km/h. Churyumov–Gerasimenko is approximately 4.3 by 4.1 km at its longest and widest dimensions. It was first observed on photographic plates in 1969 by Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, after whom it is named. It most recently came to perihelion on 2 November 2021, and will next come to perihelion on 9 April 2028.

<i>Philae</i> (spacecraft) Robotic European Space Agency lander that accompanied the Rosetta spacecraft

Philae was a robotic European Space Agency lander that accompanied the Rosetta spacecraft until it separated to land on comet 67P/Churyumov–Gerasimenko, ten years and eight months after departing Earth. On 12 November 2014, Philae touched down on the comet, but it bounced when its anchoring harpoons failed to deploy and a thruster designed to hold the probe to the surface did not fire. After bouncing off the surface twice, Philae achieved the first-ever "soft" (nondestructive) landing on a comet nucleus, although the lander's final, uncontrolled touchdown left it in a non-optimal location and orientation.

<span class="mw-page-title-main">Coma (comet)</span> Cloud of gas or a trail around a comet or asteroid

The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes near the Sun in its highly elliptical orbit. As the comet warms, parts of it sublimate; this gives a comet a diffuse appearance when viewed through telescopes and distinguishes it from stars. The word coma comes from the Greek κόμη (kómē), which means "hair" and is the origin of the word comet itself.

<span class="mw-page-title-main">Micrometeorite</span> Meteoroid that survives Earths atmosphere

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<span class="mw-page-title-main">Accretion (astrophysics)</span> Accumulation of particles into a massive object by gravitationally attracting more matter

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<span class="mw-page-title-main">Comet nucleus</span> Central part of a comet

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Michael R. Combi, is a space science professor at the University of Michigan. Combi's focus is planetary astronomy, and he specializes in the detailed modeling of cometary comae. His model for the distribution of water molecules and associated byproducts has been invaluable in understanding a wide variety of coma observations. He also contributed to discoveries related to the interactions between solar winds and comet tails.

CI chondrites, also called C1 chondrites or Ivuna-type carbonaceous chondrites, are a group of rare carbonaceous chondrite, a type of stony meteorite. They are named after the Ivuna meteorite, the type specimen. CI chondrites have been recovered in France, Canada, India, and Tanzania. Their overall chemical composition closely resembles the elemental composition of the Sun, more so than any other type of meteorite.

Eberhard Grün is a German planetary scientist who specialized in cosmic dust research. He is an active emeritus at the Max Planck Institute for Nuclear Physics (MPIK), Heidelberg (Germany), research associate at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder (Colorado), and was a professor at the University of Heidelberg until his retirement in 2007. Eberhard Grün has had a leading role in international cosmic dust science for over 40 years.

The Micro-Imaging Dust Analysis System (MIDAS) is one of several instruments on the European Space Agency's Rosetta mission which studied in-situ the environment around the active comet 67P/Churyumov–Gerasimenko as it flew into the inner Solar System. MIDAS is an atomic force microscope (AFM) designed to collect dust particles emitted from the comet, and then scan them with a very sharp needle-like tip to determine their 3D structure, size and texture with very high resolution.

<span class="mw-page-title-main">Hanna von Hoerner</span> German astrophysicist and physicist (1942-2014)

Hanna von Hoerner was a German astrophysicist. She founded the company von Hoerner & Sulger which produces scientific instruments, notably cosmic dust analyzers used on space missions by the European Space Agency (ESA) and NASA.

DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science) is a planned mission to flyby the Geminids meteor shower parent body 3200 Phaethon, and sample dust originating from the "rock comet". The spacecraft is being developed by the Japanese space agency JAXA, and will demonstrate advanced technologies for future deep space exploration. As of 2023, DESTINY+ is planned to be launched in 2025.

CAESAR (spacecraft) Proposed sample-return mission to a comet

CAESAR is a sample-return mission concept to comet 67P/Churyumov–Gerasimenko. The mission was proposed in 2017 to NASA's New Frontiers program mission 4, and on 20 December 2017 it was one of two finalists selected for further concept development. On 27 June 2019, the other finalist, the Dragonfly mission, was chosen instead.

<span class="mw-page-title-main">Space dust measurement</span> Space dust measurements

Space dust measurement refers to the study of small particles of extraterrestrial material, known as micrometeoroids or interplanetary dust particles (IDPs), that are present in the Solar System. These particles are typically of micrometer to sub-millimeter size and are composed of a variety of materials including silicates, metals, and carbon compounds. The study of space dust is important as it provides insight into the composition and evolution of the Solar System, as well as the potential hazards posed by these particles to spacecraft and other space-borne assets. The measurement of space dust requires the use of advanced scientific techniques such as secondary ion mass spectrometry (SIMS), optical and atomic force microscopy (AFM), and laser-induced breakdown spectroscopy (LIBS) to accurately characterize the physical and chemical properties of these particles.

<span class="mw-page-title-main">Dust astronomy</span> Branch of astronomy

Dust astronomy is a subfield of astronomy that uses the information contained in individual cosmic dust particles ranging from their dynamical state to its isotopic, elemental, molecular, and mineralogical composition in order to obtain information on the astronomical objects occurring in outer space. Dust astronomy overlaps with the fields of Planetary science, Cosmochemistry, and Astrobiology.

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