Sunday 4 May 2014

The Chelyabinsk Meteorite and its implications for the origin of the Baptistina Asteroid Family.

The Chelyabinsk Meteorite detonated in the atmosphere over the southern Russia on 15 February 2013 with an equivalent energy to 500 kilotons of TNT. From the size of the explosion it is estimated to have been an object with an equivalent diameter of 17-20 m (i.e. if it had been a perfect sphere it would have been 17-20 m in diameter). Using video footage of the meteorite entering the atmosphere to project its trajectory backwards, it has been calculated that the meteorite originated in the inner part of the Main Asteroid Belt, close to the V₆ resonance (a point within the Inner Main Asteroid Belt where astroids reach a 6:1 resonance with Saturn - completing six orbits for every one orbit of Saturn - a point at which asteroids slowly have their orbits elongated by the tidal influence of the planet, until they are thrown into a new orbit, often one that involves crossing the orbit of Mars and the other inner planets of the Solar System), and further suggested that its orbit would have been very similar to that of the Q-type Near Earth Asteroid (86039) 1999 NC43 (Q-type asteroids are thought to be compositionally similar to ordinary chondrite meteorites, but this is based upon remote sensing of the spectral properties of the asteroids, rather than studies of their mineralogy in the lab, as occurs with meteorites). 

It has also been suggested that the Chelyabinsk Meteorite may have been part of the same population of Near Earth Asteroids as 2011 EO40, and noted that the airburst took place 16 hours before the closest approach to the Earth of the 30 m Near Earth Asteroid 2012 DA14 (though this had a completely different trajectory, ruling apparently out any connection). Ultimately all such attempts to link the Chelyabinsk Meteorite to particular parent bodies are, at best, highly speculative, but this does not mean that the meteorite has nothing to tell us about asteroids in the Inner Solar System.

In a paper published on the arXiv database at Cornell University Library on 26 April 2014, a team of scientists led by Vishnu Reddy of the Planetary Science Institute in Tucson Arizona, examine the spectral properties of the Chelyabinsk Meteorite, and discuss the implications of these findings for our understanding of asteroid families in the Inner Main Asteroid Belt.

Following the initial airburst several hundred fragments of meteorite were collected from across the Chelyabinsk region, including one 654 kg meteorite from the bottom of Lake Chebarkul. The Chelyabinsk Meteorite has shows three different lithologies, though these are essentially similar in composition and density. Firstly, there are clasts of fairly typical LL chondrite material (LL chondrite stands for Low iron, Low metal ordinary chondrite; thee are the least abundant type of ordinary chondrites). Then there is a fusion crust formed by the passage of the meteorite through the Earth. It also has a substantial component of shock-blackened impact melt material, thought to have been formed before its encounter with Earth. This shock blackened is essentially similar to the LL chondrite material in overall composition, containing forsteritic olivine, orthopyroxene, plagioclase, triolite, and traces of kamacite and chromite, but is has a higher proportion of triolite and kamacite and a lower plagioclase content. It appears much darker than the LL chondrite material due to the presence of fine grained metal sulphides (triolite) and metal particles (kamacite) in droplets, intragranular fillings, and veins. Such impact melts have been seen before, but are extremely rare, occurring in only about 0.5% of ordinary chondrites, so the large amount of such material available from the Chelyabinsk Meteorite provides new opportunities to study this material.

Sample of Chelyabinsk LL5 chondrite that was used in this study with the lighter LL5 chondrite clasts embedded in a matrix of shock blackened/impact melt material. Reddy et al. (2014).

LL chondrites have previously been linked to the Flora Asteroid Family in the Inner Main Asteroid Belt on a number of occasions, due to the apparent close match between the lithologies of the meteorites and the spectra of the asteroids. About 60% of Near Earth Asteroids appear to be linked to the Flora Asteroid Family (based upon spectral analysis), but only about 10% of meteorites recovered appear to be LL chondrites; the reason for this discrepancy is unclear. As the majority of the Flora Asteroid Family are close to the V₆ resonance in the Inner Main Asteroid Belt, it would be predicted that this family of asteroids would be particularly good at delivering bodies into Earth-crossing orbits, though it would appear that the parent bodies of the H and L chondrites (High iron ordinary chondrites, the most abundant form of meteorites, and Low iron ordinary chondrites, the second most common type of meteorites) are better at doing so.

The Flora Asteroid Family is one of two large known asteroid families in the Inner Main Asteroid Belt (the other being the Vesta Asteroid Family), accounting for 15-20% of all asteroids discovered prior to 2002. The family is named for the asteroid (8) Flora, which is approximately 140 km in diameter, and forms about 80% of the total mass of the asteroid family. The best studied of these objects is (951) Gaspra, which was visited by the Galileo Spacecraft in 1991.

Galileo image of (951) Gaspra. USGS/NASA/JPL.

Near Earth Asteroid (25143) Itokawa, which was visited and sampled by the Japan Aerospace Exploration Agency’s Hayabusa Spacecraft in 2005 has an LL chondrite lithology and a Flora Asteroid Family spectrum and apparent origin. Spectral analysis based studies of (8) Flora suggest that it has a similar composition to (25143) Itokawa. The Chelyabinsk Meteorite has slightly less iron in its olivine and pyroxene than either of these bodies, but falls comfortably within the range of both LL chondrites and Flora Family Asteroids as a whole, suggesting that the Chelyabinsk asteroid probably originated within the Flora Asteroid Family.

The Baptistina Asteroid Family was identified in 2005 based upon similarities of albedo (the amount of light reflected by an object) semi-major axis (average distance from the Sun), eccentricity (extent to which an object gets closer to and further away from the Sun during its orbit), and inclination to the plain of the Solar System. There appears to be an overlap between the Baptistina and Flora asteroids in terms of orbital properties, suggesting that the Baptistinas are a cluster within the Flora Asteroid Family, however while the Floras are mainly high-albedo S-type asteroids, the Baptistinas are predominantly low albedo C- or X-type asteroids.

The calculated orbit of (298) Baptistina. JPL Small Body Database Browser.

Studies of the Baptistina Asteroid Family suggest that these asteroids are also compositionally similar to LL chondrites, though with subdued olivine and pyroxene absorption bands (i.e. the asteroids either contain less of these minerals, or it is less detectable by spectrographic methods). In addition many Baptistina Asteroids, and more-or-less all of the smaller members of the family are X-type bodies, which have albedos of less than 15% (i.e. they reflect less than 15% of the light that reaches them), making it impossible to analyse their mineralogy with current methods. 

Reddy et al. compared the spectra of 10 Baptista Family Asteroids to the shock blackened material from the Chelyabinsk Meteorite. Based upon this they suggest that these objects could have an LL chondrite composition with a variable amount of shock blackened material, ranging from about 10% for 2001 FZ63 to 100% for 1998 FB147 (the Chelyabinsk Meteorite contains about 50% shock blackened material). This suggests that shock blackening similar to that seen in the Chelyabinsk Meteorite could be responsible for the low albedos of the Bapsistina Asteroid Family, suggesting that these objects originated from a collisional event involving a large Flora Family Asteroid, which resulted in substantial shock blackening of the surviving fragments.

The shock blackening in the Chelyabinsk Meteorite appears to have occurred due to melting and recrystallization of iron-sulphur minerals, at least partially derived from the iron-sulphur content of the olivine and plagioclase minerals in the original LL chondrite material. This would require a temperature of ~1261 K (988˚C) at a pressure of one atmosphere (i.e. the pressure on the surface of the Earth). It has previously been suggested that the Baptistina Asteroid Family originated in a collisional event that broke up a 170 km asteroid about between 90 and 160 million years ago (it was also suggested that one of the resultant fragments was responsible for the end-Cretaceous Extinction Event on Earth, but there is no evidence for this). 

However modelling such an impact event proved to be somewhat difficult. A 40 km object impacting a 170 km object at about 5 km per second would be sufficient to cause the breakup of the larger body (most collisions within the Asteroid Belt are thought to happen at about this speed), but would only produce about 0.00013 impactor masses worth of impact melt. Since a 40 km object would only have about 1% of the mass of the 170 km object, this would represent a very small proportion of the resultant debris, not enough to account for the degree of blackening seen in the Baptistina Family Asteroids, particularly as some of the resultant material would be ‘lost’ inside larger bodies formed from the accretion of several smaller bodies in the immediate aftermath of the collision (and therefore invisible to remote observations, having no effect on the overall albedo of the object).

Reddy et al. calculate that in order to produce the spectral properties seen in the Baptistina Family Asteroids, a collision would need to be large enough to shock darken at least 5% of the original material. This would require an impact at about 8-10 km per second, with a 40 km asteroid striking a highly porous 170 km asteroid target (although this is a minimum estimate; it may well require more than 5% shock blackened material to produce the reduced albedos, and therefore a correspondingly larger impact). An impact at a speed greater than 10 km per second would be needed to cause appreciable impact blackening to a non-porous 170 km body. While such events are thought to be extremely rare, and generally to involve bodies travelling on orbits highly inclined to the plain of the Solar System (rare in the Main Asteroid Belt, since such orbits generally originate when asteroids have close encounters with planets) such an event occurring once within the 70 million year window calculated for the possible origin of the Baptistina Asteroid Family is not inconceivable.

Reddy et al. do not go as far as proposing that the Chelyabinsk Meteorite originated as a member of the Baptistina Asteroid Family, though they do not rule it out either, since they judge that such a designation would be untestable guesswork. However they do believe that the information gleaned from the mineralogy of the meteorite can help us to understand that of the asteroids, and therefore better understand their origin.

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