In the wake of Russia’s high profile meteor incident last week, Michael Babechuk explores the fascinating phenomena that are meteors, meteorites, and meteoroids.

In the early hours of February the 15th, 2013, a meteoroid entered the atmosphere above the Ural Mountains in Russia near Chelyabinsk, generating an intense explosive shockwave and fireball across the sky (known as a meteor). The shockwave resulted in significant building damage and injuries related primarily to shattered glass. The following links provide more detailed news coverage and some spectacular footage of the meteor:

The bright illuminating flash of the fireball and the trailing streak across the sky are the result of the intense amounts of kinetic energy from the meteoroid being converted to light and heat upon its passage through the atmosphere. Incandescent light is generated as the meteoroid undergoes frictional heating and reaches its melting point. Additional light is generated during the superheating of the air in the meteoroid’s path which causes atoms in the atmosphere to lose electrons and instantly recapture them. Melt and gas generated from the superheated part of the meteoroid is transferred to the trailing air stream – these molten liquid droplets rapidly cool and give rise to the streak which follows the meteoroid’s path. This process, known as ablation, reduces the mass of the meteoroid, but also protects it during its descent. During ablation, the deposition of a thin (generally a mm or less) fusion crust made from the melt usually occurs on the surface of the meteoroid. The friction from the air slows the meteoroid from its cosmic velocity prior to reaching the Earth (if it does at all). Often, the meteoroid will split up, resulting in several meteors following the same trajectory; the largest fragments travel furthest with the smaller ones forming an elliptical fall area behind them. Upon reaching the Earth’s surface, the meteoroid is known as a meteorite. Whether a meteoroid survives to be collected as a meteorite depends primarily on its incoming velocity, its size and composition, and the angle of the trajectory.

The audible shockwave is generated by the compression of air before the meteoroid(s) when it travels through the atmosphere at greater than supersonic speeds. The slower travel of sound waves relative to light creates a delay between the fireball and shockwave. In the case of Chelyabinsk, several were injured while moving towards windows to inspect the light generated from the fireball, not anticipating the imminent sonic boom.

The record of witnessed meteors extends as far back as 650 B.C. and collected meteorites were revered for their rarity and significance. An extraterrestrial origin for meteorites, however, wasn’t accepted until the late 18th century. Prior to this, explanations for meteorites included volcanic ejecta, lightning striking rocks or were religious based. In 1794, Ernst Florens Friedrich Chladni, a German physicist published his arguments for meteorites being extraterrestrial and correctly linked the fireballs with the interaction of a meteoroid and the atmosphere. His arguments went largely ignored until the early 19th century when the chemistry and mineralogy of meteorites was examined in more detail. It became evident that the meteorites contained minerals that are not common, not stable or even unknown to occur on the surface of the Earth. The scientific field of ‘meteoritics’ was born.

Most of the early documented meteorites were falls, that is, they were witnessed as meteors prior to collection. However, the increasing recognition and documentation of meteorites throughout the 20th century led to an increasing number of finds (meteorites not observed to fall). Many of the components of meteorites are susceptible to the forces of nature and as a consequence they can weather away after their fall. Not surprisingly, in the most recent years, deserts (e.g., New Mexico, USA, the deserts of North Africa and Australia) and Antarctica have become the largest source of meteorite finds. The dry climate and contrast of the landscape enhances the preservation and discovery potential of meteorites. Several meteorite expeditions to Antarctica have been led following the discovery of large “deposits” of meteorites. Meteorites trapped in the ice following their fall are transported by the gradual ice flow until the ice meets a barrier (e.g., a mountain) and they accumulate. Wind erosion then exposes the deposit of meteorites at the surface again. Over 20 000 meteorites have been found in Antarctica to date.

Our current knowledge of meteorites tells us that they are the remnants of the earliest formed solid bodies in the Solar System, with some meteoritic components having been dated to as old as 4568 million years before present (e.g, Amelin et al., 2010; Bouvier and Wadhwa, 2010). Compositionally, they are crudely classified into stony meteorites (formed mostly of silicate minerals), iron meteorites (formed mostly of metals), and stony-irons (a mixture of metal and silicates). The information that can be gained from meteorites drove the science of meteoritics throughout the 20th century and even still with more advanced instruments the precision to which we can date and understand meteorites continues to propel forward.

Some meteorite facts (from Meteorites – a Journey Through Space and Time by Alex Bevan and John De Laeter):

Meteorites are named after the place of their fall/find or the closest geographical feature

There are two (substantiated) historical reports of a meteorite hitting a person directly (both non-fatal), and two animal fatalities as a result of meteorite impact (one dog and one horse)

Most meteoroids enter the atmosphere at speeds of 15-70 km/s

The largest meteorite find to date is the Hoba meteorite, with a mass of roughly 60 metric tons. It is an iron meteorite found in 1920 in Namibia.

Terminology (from Meteorites and Their Parent Bodies, by Harry Y. McSween Jr.) Fall a recovered meteorite that was observed to fall Find a recovered meteorite that was not observed to fall Meteoroid a small object orbiting the Sun in the vicinity of the Earth Meteor a streak of light in the sky produced during transit of a meteoroid through the Earth’s atmosphere; also, the glowing meteoroid itself Meteorite extraterrestrial object that survives passage through the atmosphere and reaches the Earth’s surface as a recoverable object

Author:

Michael Babechuk

Ph.D. student, Department of Geology, Trinity College Dublin

References/Additional reading:

Amelin, Y., Kaltenbach, A., Iizuka, T., Stirling, C.H., Ireland, T.R., Petaev, M., Jacobsen, S.B., 2010. U-Pb chronology of the solar system’s oldest solids with variable 238U/235U. Earth and Planetary Science Letters 300, 343-350.

Bevan, A., and De Laeter, J. 2002. Meteorites – A Journey Through Space and Time. University of New South Wales Press, Australia.

Bouvier, A., Wadhwa, M., 2010. The age of the solar system redefined by the oldest Pb-Pb age of a meteoritic inclusion. Nature Geosciences 3, 637-641.

McSween, H.Y., Jr., 1999. Meteorites and Their Parent Bodies. Cambridge University Press, UK.

Norton, O.R., Chitwood, L.A., 2008. Field Guide to Meteors and Meteorites. Springer-Verlag London, UK.

Image courtesy of NASA – by artist Don Davis.

Interesting links:

http://www.bimsociety.org/index.php

http://epswww.unm.edu/iom/ident/index.html

http://www.meteoriticalsociety.org/

http://geology.cwru.edu/~ansmet/