Muawia Shaddad (center right) and Peter Jenniskens (center left) point towards a fragment of the Almahata Sitta meteorite, in the company of several students from the University of Khartoum. (Image: Peter Jenniskens/SETI institute).

Almahata Sitta arrived on Earth with a bang. It was the first recovered meteorite to be tracked in space (as Asteroid 2008 TC3) (see previous blog entry). But what sort of meteorite is it?

Almahata Sitta is classified as a ureilite, a strange name, which is taken from the village of Novo Urei in Central Russia, where a similar type of meteorite fell in 1886. Ureilites are the second largest group within a broad class of meteorites known as achondrites (literally meteorites that do not contain chondrules).

Thin section images of two Antarctic ureilites. ALHA77257 is composed of relatively coarse-grained olivine (~80%) and pyroxene (~15%). EET 83309 is a polymict ureilite consisting mainly of clumps of olivine crystals (up to 3 mm in diameter), with lesser amounts of pyroxene and a small amount of plagioclase. These coarser-grained crystals are enclosed in a fine-grained fragmental matrix.

Most ureilites have a coarse-grained texture and are mainly composed of two minerals, olivine and pyroxene, but also contain significant amounts of carbon. A small fraction of ureilites (about 10%) show evidence of having been pulverised by impact processes. They consist mainly of ureilite fragments, but also contain material from other asteroids and, as a result, are given the name polymict ureilites. Almahata Sitta is the first polymict ureilite fall; previous examples have all been finds.

To search for fragments of Almahata Sitta, students and staff of the University of Khartoum formed a line 1 km long. (Image: Peter Jenniskens/SETI institute).

Following initial recovery of the fragments of Almahata Sitta, an international scientific consortium was set up to study this exceptionally important meteorite.

A report giving the results of the initial scientific work on Almahata Sitta was published in the journal Nature in March 2009. This was followed by a session devoted to the meteorite at the annual Lunar and Planetary Science Conference, Houston, in March 2010. Finally, a special double issue of Meteoritics and Planetary Science, the official journal of the Meteoritical Society, was published at the end of 2010. This contains 19 research articles covering all aspects of the fall of Asteroid 2008 TC3 and subsequent studies of the Almahata Sitta meteorite.

As Peter Jenniskens and Muawia Shaddad point out in their introduction to this special issue, these studies reveal that Almahata Sitta is a unique and extraordinary meteorite. It is essentially a chaotic mixture of millimetre-sized rounded rock and mineral fragments set in a fine-grained porous matrix. The fragments in the meteorite not only come from the ureilite parent body, but also from a range of other asteroids, some of which were previously unknown types. Almahata Sitta also contains amino acids, although these are at a lower abundance than found in the more primitive meteorite class known as carbonaceous chondrites. This lower concentration probably reflects the high temperatures experienced by the ureilite parent body.

Compared to other ureilites, Almahata Sitta shows some unique characteristics and, hence, is described as being “anomalous”. These unusual features include its overall fine-grained texture and the presence of large carbon-rich aggregates.

Ureilites are widely believed to be samples of the mantle of a large asteroid, at least 200 kilometres in diameter, that was catastrophically disrupted early in the history of the Solar System. Shortly after this event, the debris from the collision was reassembled into a second generation asteroid. Almahata Sitta is a fragment of this later body.

So, Almahata Sitta is a witness to the chaos and anarchy that reigned in the early Solar System, a place in which a swarm of mini-planets formed rapidly, only to be destroyed shortly afterwards during catastrophic collisions with their near neighbours. The larger planets, including Earth, profited from the chaos by growing ever larger as they swept up the debris from the destruction of these smaller worlds. It is a process that continues today, albeit on a much, much smaller scale.