Determining the origin of scoriaceous micrometeorites.

Micrometeorites are particles of extra-terrestrial material less than 2 mm across. These have been collected from a number of environments, including Antarctic blue ice and snow, deep sea sediments and even rooftops, and are the most abundant form of meteorite material available to scientists. Scoriaceous micrometeorites are micrometeorites dominated by micron-sized equant iron-rich olivine crystals within a glassy mesostasis, which is thought to be indicative of having undergone caused by superheating from friction with the Earth's atmosphere due to the orbital momentum of the asteroid, which is greater than that caused by simply falling.

In a paper published in the journal Geology on 17 August 2017, Matthew Genge and Martin Suttle of the Impact and Astromaterials Research Centre at Imperial College London, and the Earth Sciences Department at The Natural History Museum, and Matthias Van Ginneken of Earth System Science at the Vrije Universiteit Brussel, present the results of a study in which they examined scoriaceous micrometeorites in order to attempt to determine their origin and the stresses that they been subjected to.

Scoriaceous micrometeorites contain clusters of clusters of forsterite and enstatite crystals, which Genge et al. believe to have been formed by the fragmentation of larger crystals due to superheating, as other micrometeorites lack these, suggesting that the the original crystals cannot be fractured in this way by shockwaves passing through the minerals as the parent bodies fragment in the atmosphere, impact the ground, or undergo some process in space unrelated to their eventual decent to Earth. They suggest that the most likely cause of such fragmentation is thermal stress, as different minerals within the micrometeorite expand at different rates when heated.

Backscattered electron images of micrometeorites. (A) Highly vesicular scoriaceous micrometeorite containing a cluster of forsterite (FO) relicts and eskoliate (CR). (B) A scoriaceous micrometeorite with a well developed external magnetite rim and clusters of forsterite (FO). Small triangular shards of forsterite are present close to larger crystals. This particle is similar to a micro-porphyritic olivine cosmic spherule. An expanded inset shows a cluster of small enstatite crystals. (C) A scoriaceous micrometeorite containing abundant vesicles and a magnetite rim. Two areas of relicts occur, one enstatite (FE) exhibits abundant fractures partially infilled with mesostasis (MV). (D) A scoriaceous micrometeorite containing two clusters of forsterites (FO) consisting of numerous individual crystals. (E) An unmelted finegrained micrometeorite with an external igneous rim (IR) surrounding an unmelted core (UC). A forsterite relict (FO) is present that truncates the igneous rim ((f) shows expanded view) and contains numerous fractures partially infilled with melt and is surrounded by a magnetite rim (MR). Fracturing within the crystal is most abundant in the part closest to the surface of the particle. Scale bars are 50 μm, except in (f) where it is 5 μm. Genge et al. (2017).

Genge et al. were able to construct a model of the rate at which forsterite expands due to heating, and from this determine the temperature to which these minerals had been raised, and the difference in temperature across the mineral grain; suggesting that in some cases this temperature difference may be as much as 4000 K per μm, resulting in a high degree of shear stress, caused by different parts of the crystal expanding at different rates, and causing the crystal to shatter.

Such a high temperature difference within a micrometeorite requires some explanation, as most minerals conduct heat fairly well. Genge et al. theorise that this may have been caused by the parent bodies having been comprises of at least 5% phyllosilicates (sheet minerals such as micas, chlorite, serpentine, talc, and the clays), which conduct heat poorly, and which would have been destroyed by the very high temperatures implied. 

This in turn suggests that the original material from which these micrometeorites were derived was similar in composition to that of a CI1 or CM2 carbonaceous chondrite, meteorites with a high composition of phyllosilicates. CM2 chondrites have previously been shown to lose phyllosilicates due to dehydration (the loss of hydrogen and oxygen from the mineral as water) at high presures, suggesting that scoriaceous micrometeorites may be formed specifically from the fragmentation of such chondrites, rather than being the products of any meteorite raised to the correct temperature.

About half of all micrometeorite-sized particles entering the Earth's atmosphere are thought to be derived from members of the Veritas Asteroid Family (a group of asteroids in the Outer Main Belt thought to have formed about 8.5 million years ago by the break-up of a large parent-body), with much of the remaining material derived from the Koronis Family Asteroids (a group of asteroids in the Central Main Belt thought to have formed by the collision of two large bodies about two billion years ago), and a small contribution from the Themis Family (a group of asteroids in the Outer Main Belt thought to be the main source of carbonaceous chondrites).

The Koronis Family Asteroids is thought to produce only ordinary chondritic material, while both the Veritas and Themis families are thought to produce carbonaceous chondritic material. Genge et al. suggest that the likely carbonaceous origin of scoriaceous micrometeorites implies that these are most likely to have originated from the Veritas and Themis asteroid groups.

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The origin of Comet P/2006 VW₁₃₉.

Comet P/2006 VW₁₃₉ is a Main-Belt Comet, one of a recently discovered group of objects, which inhabit the main Asteroid Belt, but which produce a tail similar to that of a comet when at their closest to the Sun. Asteroids and comets have traditionally been viewed as very different types of objects; asteroids being rocky objects of the inner Solar System, Comets being icy objects from the outer system that occasionally get thrown into the inner system, melting as they do so (strictly speaking sublimating, the solid ices turning directly to gasses in the vacuum of space) and generating snowy tails that point away from the Sun. 

Main-Belt comets do not fit into either of these neat categories, they inhabit the Main Asteroid Belt, but have eccentric orbits (i.e. orbits that are not perfect circles centered on the Sun) and when they are closest to the Sun produce comet-lie tails, suggesting that they are primarily icy objects, similar to comets.

The orbit of P/2006 VW₁₃₉. Green squares are sightings; (a) 30 August 2011, (b) 5 November 2011, (c) 12-14 November 2011, (d) 22 November-4 December 2011, (e) 16-19 December 2011, (f)  7 January 2012. (A) Represents the aphelion, the point in the orbit where the orbiting object is furthest from the Sun. (P) Represents the perihelion, the point in the orbit closest to the Sun. Also shown are the orbits of Mercury, Venus, Earth, Mars and Jupiter. The scale is in Astronomical Units (AU), one AU being equal to the average distance between the Earth and the Sun. Hsieh et al. (2012).

In a paper published on the arXiv database at Cornell University Library on 22 May 2012, and also due to be published in the Monthly Notices of the Royal Astronomical Society, Bojan Novaković of the Department of Astronomy at the Faculty of Mathematics at the University of Belgrade, Henry Hsieh of the Institute for Astronomy at the University of Hawaii, and Alberto Cellino of the INAF–Osservatorio Astronomico di Torino, discuss the origin of P/2006 VW₁₃₉, and its relationship to other objects in similar orbits.

Novaković et al. compared the orbit of P/2006 VW₁₃₉ to that of 24 other objects in similar orbits, calculating their orbits backwards to look for a common origin. 19 of these objects were found to be in stable orbits; the remaining 5 were in unstable orbits, and therefore unlikely to have been there very long. These objects are most likely to have been shifted into their current orbits in the recent past (though this might mean a hundred thousand years or so) by encounters with other objects, and will most probably be shifted out of their current orbits within the next hundred thousand years or so. Of the remaining 19 objects, 11 could be traced backwards to a likely common origin with P/2006 VW₁₃₉, about 7.5 million years ago, in the breakup of a larger body, probably as the result of a collision. 

Furthermore the parent body probably belonged to a group of asteroids known as the Themis Family, which are thought to have originated in the breakup of a larger body about 2.5 billion years ago. This suggests that both the objects of the 'P/2006 VW₁₃₉ Family' and the Themis Family may also be substantially comet-like in composition, i.e. icy rather than rocky in composition.

See also Asteroid 2012 KP24 passes Earth at a distance of 51 000 km, Fragments of 22 April meteor found in California, Fireball over Nevada and California, The Lyrid Meteors and Asteroid 2012 DA14 may pass within 21 000 km of the Earth.

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