by Acad. Erik GALIMOV in charge of the Vernadsky Institute of Geochemistry and Analytical Chemistry (RAS), Chairman of the Committee on Meteorites, RAS Presidium

This happened on the 15th of February of 2013: at 9 h 20 min local time a bolide burst into the atmosphere near the city of Chelyabinsk. The fireball exploded above the ground and caused significant damage to buildings. Many residents were injured by window glass fragments. The fall of this heavenly body was registered by private and service cameras that made it possible to trace the trajectory of the bolide and evaluate the parameters of the blast. Its geographical parameters: 54.86+0.05 °N and 62.20±0.15 °E. The energy yielded in the explosion is estimated at 440 kilotons of the TNT equivalent, that is it was twenty times as high as that of the A-bomb dropped over Hiroshima, Japan, in August 1945. The mass of this celestial body was about 10,000 tons, and it was 18 to 20 meters large; the velocity of its entry into the atmosphere, 20 km/s. Hitting the atmosphere, the bolide broke into fragments, with many splinters, mostly several inches large, reaching the ground surface.

Meteorites are the domain of the Committee on Meteorites affiliated with the Presidium (presiding body) of the Russian Academy of Sciences and working on the basis of the Vernadsky Institute of Geochemistry and Analytical Chemistry (GEO-CHE) in whose care is the larger part of the Russian collection of meteorites, one of the world's oldest and largest. This is a great cultural, scientific and historical heritage.

As caretaker our Committee is keeping the collection in order and replenishing it. Expert examination of new finds and their cataloguing is another facet of its work.

Understandably, we viewed the Chelyabinsk event as the zone of our responsibility: we were supposed to look for fragments of the bolide and identify them. As early as Monday, February 18, we started equipping a field party from among workers of the meteoritics laboratory of our research institute. I asked Governor Mikhail Yurevich to assist our field workers in their explorations at Chelyabinsk. The same day I got in touch with Vladimir Puchkov, federal minister for emergencies. He said his workers were already out there for on-site studies, and that his ministry would aid us in every possible way. Local services, I must say, were quite cooperative. The next day the field party under Dmitry Badyukov, deputy head of our meteoritics laboratory, arrived at Chelyabinsk to collect the meteorite's fragments and five days after, the first samples were at our research center.

Collecting samples was not as much difficult--small fragments riddled the snow cover. The landing trajectory was dotted with ice cones and splinters underneath. Many amateur seekers were likewise taking part in the search. Local television asked them to hand fragments they found to our people. There were a few cases like that, though. All told we collected about 3 kilograms of extraterrestrial matter.

We put several laboratories on the research, including the meteoritics laboratory headed by Dr. Mikhail Naz-arov. This is our head laboratory responsible for petro-

logical analysis. Taking part were also the central analytical laboratory under Prof. Vladimir Kolotov, deputy director of our Institute, and its subdivisions (sectors)--of microprobe analysis (Nataliya Kononkova) and of X-ray fluorescence analysis (Irma Roshchina) that did chemicomineralogical assays. The noble metals laboratory (with Dr. Irina Kubrakova in charge) analyzed the samples for siderophiles, chalcophiles and rare-earths. The laboratory of isotopes geochemistry (Prof. Yuri Kostitsyn) fulfilled very laborious research into Rb/Sr (⁸⁷Sr/⁸⁶Sr) and Sm/Nd (¹⁴³Nd/¹⁴⁴Nd) systems. The carbon geochemistry laboratory (Dr. Vyache-slav Sevastyanov) analyzed the isotope composition of carbon. The cosmochemistry laboratory (Viktor Alex-eyev) studied tracks of the meteorite's cosmic irradiation.

Our immediate objectives: 1) collect material; 2) identify the collected matter and classify it; 3) file a report to the Nomenclature Committee of the International Meteorite Society for cataloguing it and endorsing its name.

That was our mandatory program. Any meteorite is an object of scientific research: its age, origin and cosmic record are likewise of interest.

Meteorites are of different classes: siderites (iron meteorites or pallasites), stony-iron meteorites (sidero-lites), chondrites, achondites. Martian (SNC) and lunar meteorites are assigned to extraordinary groups. These are fragments of Martian and lunar rock dislodged from the surface of Mars and moon and landing eventually on earth.

Chondrites are the commonest group of meteorites broken down according to their chemical composition into several subgroups. Of most interest to us are coaly chondrites closest in their composition to primordial matter. Coaly chondrites of the Cl type (Ivuna, Orguel, etc.) are carbon- and water-rich, they contain organic matter, amino acids and other prebiological compounds. There are also interstellar dust particles antecedent to the formation of the sun. That is why such chondrites are regarded as the initial trigger chemistry of the solar system.

Other types of coaly chondrites like CV and CO are objects metamorphosed this way or that. Coaly chondrites are the most oxygenated type of meteoric matter

comprising carbonates, hydrates and FeO iron oxide. EH and EL chondrites containing iron in the metallic and sulfide form are the most reduced elements among chondrites. Yet ordinary chondrites are most common, divided into three groups: H--high-ferrous, L--low-ferrous and LL--very low-ferrous.

Microscopy shows particular chondrules quite well in the Chelyabinsk meteorite. Chondrules are one of the oldest mineral structures in the solar system. Tracer methods (isotope) allow to determine the age of the solar system with fairly good accuracy: 4 bln 568 mln years. Two billion years after the sun flared up, CAI (calcium-aluminum refractory inclusions) condensed in the vapor-gaseous nebula around it, to be followed by chondrules. They entered into the composition of primary solid objects (planetesimals) and then aggregated into bodies of astroidal dimensions. Chondrules were partly remelted in asteroids and acted upon by magma-tism and heat. Although differentiated for the most part, some of their matter remained actually intact.

Types of chondrites.

Collisions of asteroids resulted in their fragmentation and classes of meteorites known to us.

Chemical and petrological assays of its fragments allow to assign the Chelyabinsk meteorite to a definite class. The content of iron in its oxidized and reduced forms made it possible to assign the meteorite to the chemical LL (very low) type. The content of fayalite (Fe₂SiO₄) in olivine and ferrosilite (FeSiO₃) in pyroxene correlate with that. So, the meteorite belongs to the class of ordinary, common chondrites of the LL chemical group. The petrological type was the next parameter to be determined. There are seven types like that. The first is represented by the least modified substance. These are carbon- and water-rich chondrites. The second type comprises well-preserved chondrules which contain a hydrothermally changed substance fairly rich in carbon. The follow-up types--from the third on--reveal traces of temperature-induced metamorphism: 400 to 600 °C for the third type, 600 to 700 °C for the fourth, 700 to 750 °C for the fifth, 750 to 950 °C for the sixth and over 950 °C for the seventh type. The contours of chondrules become less defined with rising metamorphism, their matrix gets more crystalline and coarse, and the composition of basic minerals (olivines and pyroxene), more homogeneous, while the carbon content scales down. The conclusive evidence of the petrological analysis of the Chelyabinsk meteorite allows to assign it to the fifth petrological type.

Yet another parameter was classification according to impact. This type meteorites are broken down into six groups--from S1 that carries no traces of impact load to S5 and S6, the high-impact groups containing crushed glass and characterized by a well-developed mosaic structure and high-bar minerals like ringwoodite.

Mikhail Nazarov, our petrologist, assigns the Chelyabinsk to S4, the group of moderate impact meteorism whereby its matter was subjected to 25-35 GPa impact loads.

And last, meteorites are assessed according to their preservation. We distinguish between falls and finds. Falls are meteorites whose impact was observed and that were collected soon after. Finds are meteorites that were discovered by accident, sometimes years after, modified by the action of the atmosphere, water and microorganisms, that is subjected to weathering. Most valued are meteorites collected immediately after fall. The Chelyabinsk meteorite belongs to this category--its weathering index is equal to zero (WO).

In its olivine and pyroxene content "Chelyabinsk" is assigned to the group of LL chondrites.

So this meteorite was identified as a common chon-drite: LL5/S4-WO.

This identification, together with the name, "Chelyabinsk", was presented to the Nomenclature Committee of the International Meteorite Society.

The "Chelyabinsk" is interesting in many other ways. It has two phases: one, the light phase, is apparently its basic part. Together with well-preserved chondrules it comprises numerous tracers of crushing, fissures and cracks as well as veinlets filled in with hardened melt. The other phase is dark, fine-grained--it is the result of melt caused by the impact.

The table below sums up but a small part of data on different elements (lithophiles, siderophiles, chalco-philes); both phases are identical in practical terms, their composition is typical of LL type meteorites. Consequently, the dark melt was not a product of mag-matism and differentiation, it was indeed produced by an impact.

Table

Comparative content of some elements in two different phases of the "Chelyabinsk" meteorite and in LL type chondrites (μg/g)

The chemical-petrological analysis was complemented by an isotope analysis of carbon and a research into the Rb/Sr and Sm/Nd geochronological systems.

The content (c = 0.02 percent) and the isotope composition of carbon measured with a Delta-Plus mass spectrometer at the carbon geochemistry laboratory were found to be within a range typical of ordinary chondrites with somewhat different values of δ¹³C for the light phase (δ¹³C varied from -21.5 to -25.2 percent) and the dark phase (δ¹³C varied from -25.3 to -28.5 percent) as expected.

A study of the Sm/Nd systems by Yuri Kostitsyn showed that the meteorite had stood up to appreciable impacts before it crashed at Chelyabinsk. ¹⁴³Nd/¹⁴⁴Nd-Sm/Nd is isochronous, and this attests to an impact 300 mln years ago. Large impact melts in the dark phase must be a sequel to that event.

The Rb/Sr system is out of joint. This means there must have been at least yet another impact, less destructive, in the meteorite's subsequent record. As a result, the more sensitive ⁸⁷Sr/⁸⁶Sr--Rb/Sr system was disturbed, while the more stable Sm/Nd system stayed on.

That impact must have caused a considerable jointing of the meteor. Entering the earth's atmosphere, the bolide broke up. The surface of its contact with the atmosphere rose manifold, there was a dramatic energy release in the form of an explosion accompanied by a fireball and a Shockwave.

The available bits of evidence on the orbit of the Chelyabinsk meteorite indicate it is among the five or six thousand heavenly bodies singled into the AAA group (Apollos, Amors, Atons) the elliptical orbit of which crosses into the terrestrial orbit; hence a higher probability of their collision with the globe.