March General Meeting Report
Speakers: ASV Members Paul Deany and Trevor Hand, on “Meteors and Meteorites”.
Meteor and Meteorite objects falling through the sky from outer space have acquired a unique category of their own, with many classifications and subgroups. A meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. While in space it is called a meteoroid. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.
Meteorites that are recovered after being observed as they transit the atmosphere or impact the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.
Meteorites have traditionally been divided into three broad categories: stony meteorites, rocks mainly composed of silicate minerals; iron meteorites, largely composed of metallic iron-nickel; and stony-iron meteorites that contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy
Meteorites are always named for the place where they were found, usually a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter (e.g., Allan Hills 84001 or Dimmitt (b)). Some meteorites have informal nicknames: the Sylacauga meteorite is sometimes called the "Hodges meteorite" after Ann Hodges, the woman who was struck by it; the Canyon Diablo meteorite, which formed Meteor Crater has dozens of these aliases. However, the single, official name designated by the Meteoritical Society is used by scientists, catalogers, and most collectors.
Paul Deany guided us through the recorded history of meteors, which, once past the millennia of superstition stages (see “Long History”), is surprisingly short. It really started only in 1794, when the German musician and physicist Ernst Chladni (1756-1827) published a small book On the Origin of Pallas Iron..., asserting that masses of iron and of rock actually do fall from the sky, producing fireballs when heated by friction with the air. He concluded that they must be cosmic objects because they seem to travel too fast for gravitational acceleration alone, perhaps debris ejected from planets by explosions or collisions. Reaction to the book ranged from skepticism to ridicule. How could there be rocks in space? Aside from the stars, planets, moons and comets, everyone knew that space itself was empty. Aristotle and Newton had said so. And yet Chladni was right. Today Chladni is regarded by many historians not only as the 'father of acoustics' (for his seminal experimental work on sound vibrations) but also as the founder of meteoritics.
Paul encouraged us to support the activities of the ASV Meteor Section and explained how to go about observing meteors (see Major Showers in 2009). He went on to extol the contributions amateur astronomers can now make to this new science in this, the International Year of Astronomy.
Trevor Hand then told the audience about Meteors and their impact on us. He spoke of the numerous meteorite falls and finds and listed the countries that have the highest recorded meteorite landings: Antarctica holds the record with 19,884. He presented specimens of the different classification groups: Stony Meteorites are the most common at 82%, Iron Meteorites (~95% Iron and ~4% Nickel) at 5%, and the Stony Iron, the rarest, with just 1%. Some meteors (if large and dense enough) do not burn up completely in the air and on collision with the ground cause impact craters. Sites like the 4000 year old Campo del Cielo in Argentina or Wolfe Creek Crater in Australia are classic examples and well-known to collectors of meteorites. Wolfe Creek Crater, measuring more than 870 metres across and 50 metres deep, is the second largest rimmed meteorite crater in the world. The largest is Meteor Crater (mentioned already above) in Arizona. Dating techniques establish Wolfe Creek Crater as the older, at around 300,000 years, while Meteor Crater is about 50,000 years old.
Energy from impacts can create extremely high temperatures. Surrounding material may be melted, shattered or ejected and scattered over large areas. Sand can be converted to glass, eg green Moldavite, white Libyan Dessert Glass or Tektites.(called “Australites” if found in Australia). An Impactor of 10km diameter for instance, weighing some 100 Teratons would create a crater some 180km diameter and 50 metres deep and throw up enough material to change our Earth’s climate for decades to come.
Following the identification of meteorites on Earth of Martian and Lunar origins, and meteorites having already been found on the Moon and on Mars, a meteorite is now defined as any natural object on the surface of any celestial body that has come from elsewhere in space.
The vote of appreciation was given by Perry Vlahos who, together with Barry Cleland, also made the presentation to both speakers of the traditional ASV Reds. Alfred Klink