According to a team of Russian scientists reporting today at the Goldschmidt conference in Italy, the Chelyabinsk meteorite either collided with another body or came too close to the Sun before it fell to our planet.

The researchers from the Institute of Geology and Mineralogy in Novosibirsk have analyzed fragments of the meteorite, the main body of which fell to the bottom of the Chebarkul Lake near Chelyabinsk on February 15, 2013. Although all of the fragments are composed of the same minerals, the structure and texture of some fragments show that the meteorite had undergone an intensive melting process before it was subjected to extremely high temperatures on entering the Earth’s atmosphere.

“The meteorite which landed near Chelyabinsk is a type known as an LL5 chondrite and it’s fairly common for these to have undergone a melting process before they fall to Earth,” said Dr Victor Sharygin, who is a first author of the study (an abstract has been published in the Mineralogical Magazine).

“This almost certainly means that there was a collision between the Chelyabinsk meteorite and another body in the solar system or a near miss with the Sun.”

Based on their color and structure, the researchers have divided the meteorite fragments into three types: light, dark and intermediate.

The lighter fragments are the most commonly found, but the dark fragments are found in increasing numbers along the meteorite’s trajectory, with the greatest number found close to where it hit the Earth. The dark fragments include a large proportion of fine-grained material, and their structure, texture and mineral composition shows they were formed by a very intensive melting process, likely to have been either a collision with another body or proximity to the Sun. This material is distinct from the ‘fusion crust’ – the thin layer of material on the surface of the meteorite that melts, then solidifies, as it travels through the Earth’s atmosphere.

The fine-grained material of the dark fragments also differs from the other samples as it commonly contains spherical ‘bubbles’ which are either encrusted with perfect crystals of oxides, silicates and metal or filled with metal and sulfide.

Surprisingly, the scientists also found small quantities of platinum group elements in the meteorite’s fusion crust. They identify these elements as an alloy of osmium, iridium and platinum, but its presence is unusual as the fusion crust is formed over too short a time period for these elements to easily accumulate.

“Platinum group elements usually occur as trace elements dispersed in meteorite minerals, but we found them as a nanometer-sized mineral (100-200 nm) in a metal-sulfide globule in the fusion crust of the Chelyabinsk meteorite,” Dr Sharygin said.

“We think the appearance of this platinum group mineral in the fusion crust may be linked to compositional changes in metal-sulfide liquid during remelting and oxidation processes as the meteorite came into contact with atmospheric oxygen.”

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Bibliographic information: Sharygin V et al. 2013. Mineralogy of the Chelyabinsk meteorite, Russia. Mineralogical Magazine, 77 (5), p. 2189; doi: 10.1180/minmag.2013.077.5.19