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2 Meteorites Chapter 2 Meteorites Underlying every topic in planetary science are two basic properties of the solar system that are determined from the analysis of meteorites. First, meteorites give us our best estimate of the age of the solar system. The time at which the solar system formed provides a time frame for judging the signicance of many of the physical processes that aect the planets. Second, meteorites give us our best estimate of the initial composition of the solar system. Incredibly, the elemental abundances found in the oldest meteorites, the chondrites, match point for point with the elemental abundances found spectroscopically in the Sun’s photosphere. The study of meteorites, which is quite interdisciplinary, is central to the study of planetary science as a whole. 2.1 Falls and Finds A meteoroid is a small rock that is orbiting the Sun. A meteoroid that happens to fall into Earth’s atmosphere heats up and becomes an incandescent meteor. If a piece of a meteoroid reaches the ground it is called a meteorite. A meteorite is described as a fall or a nd depending on whether witnesses saw it enter the atmosphere or not. One of the oldest preserved falls occurred in 1492, the same year that Columbus discovered the New World, when a 127 kg stony meteorite landed in a wheat eld near the Alsatian (now French) town of Ensisheim. 2–1 2.1.1 The Allende Fall An extremely important fall occurred in Chihuahua, Mexico, in 1969, when a large meteor was observed to come into the atmosphere in several pieces. The rst piece was found near a house in the small village of Pueblito de Allende. Following standard practice, all of the meteorite fragments that were recovered from that fall are collectively named Allende. The Allende fall occurred just as the Apollo program was swinging into full gear, and it gave scientists who were preparing for the arrival of moon rocks an opportunity to practice on an extraterrestrial sample. Because it is such an old meteorite, and because there is plenty of it to go around, analysis of the Allende meteorite has taught us much about the early solar system. Allende is a member of an important class of stony meteorites called chondrites. They are so named because they contain chondrules (from the Greek word condros, meaning “grain”), which are primitive, spherical objects that condensed out of the proto- planetary nebula before being incorporated into the larger rock. Chondrules are puzzling features because their existence implies signicant heating event in the early solar nebula ( 0 to 10 Ma). The glassy texture of these igneous features implies heating to tempera- tures in excess of 1500C to 1900C, followed by rapid cooling, on a time scale of on the order of an hour. The mm-size of the chondrules indicates that they were distributed in the solar nebula, but the rate of cooling required to explain them rules out formation very close to the sun, where the solar nebula would have been much too warm. Many mechanisms for this ash heating event have been discussed, often relating the thermal or electromagnetic emissions from the variable, nascent sun. In the fall of 1999 scientists from the University of Dublin proposed that chondrules were produced by a gamma ray burst. Gamma ray bursts are rare and extremely poorly understood phenomena that may be related to ex- plosive end of life of supermassive stars. Another idea is that shock waves related to the formation of Jupiter provided the impetus for chondrule formation. It remains to be seen whether chondrule formation was a consequence of one of these low probability events. 2.1.2 The Tagish Lake Fall Another important fall occurrred in the Tagish Lake region in western Canada on January 18, 2000. The meteorite is a carbonaceous chondrite and it represents some of the most primitive soalr system materials ever recovered. Over 10,000 fragments were recovered, with more than 2000 fragments larger than a gram. Perhaps most importantly, approximately 0.85 kg was recovered in the days after the reball by an individual (Jim Brook) who had the good sense to bag the samples without touching them to minimize contamination. These samples were also kept frozen which will allow perhaps the best-yet characterization of volatiles and organics. Analysis of this meteorite is now underway. 2.1.3 Antarctic Finds Finds are more common than falls. Many meteorites are discovered serendipitously by hikers and by farmers plowing their elds. In recent years, the best source of meteorites has been the ice elds of Antarctica. In 1969, a group of Japanese geologists studying glaciers discovered nine meteorites laying on the bare ice near the Yamato Mountains in Queen 2–2 Maud Land. Meteorites are dark and easy to spot on ice. It turned out that these nine specimens were members of four dierent classes of meteorites. Since that time, thousands of meteorites have been recovered from Antarctica. The meteorites collect naturally at locations where the ice sheets, which ow several meters per year under their own weight, stagnate when they encounter mountain ranges. Wind erosion then ablates the top layers of the ice and, over time, a concentration of meteorites works its way to the top. Such ice is bluish in color and is easy to spot from the air, which allows scientists to plan ahead for the best places to look. A robot designed by Japanese scientists to search autonomously for meteorites made its rst nd in January, 2000. 2.2 Chemical Composition of a Rock Here we present the most common mineral types. These minerals occur in meteorites but also compose the crusts and mantles of the terrestrial planets, moons, and asteroids. Olivine is a silicate mineral rich in iron and magnesium, principally (Mg,Fe)2SiO4. Here magnesium and iron are in solid solution, which means that either element can occupy a given location in the cyrstal matrix. Olivine is the primary constituent of the Earth’s mantle and is found in igneous and metamorphic rocks. Pyroxene is any of a group of crystalline mineral silicates common in igneous and metamorphic rocks and containing two metallic oxides, as of magnesium, iron, calcium, or +2 sodium. A general formula is (Mg,Fe)SiO3, though Ca may also substitute. Pyroxene is a common mineral in the Earth’s oceanic crust. Plagioclase constitutes any of a common rock-forming series of triclinic feldspars, consisting of mixtures of sodium and calcium aluminum silicates that form a solid solution. The chemical formula is (K, Na, Ca)Al2Si2O8. Plagioclase feldspar is common in the oceanic and continental crust of Earth and in addition is the primary constituent of the lunar highlands. 2.3 Age of a Rock Rocks provide us with a means of measuring time in billions of years. The detailed study of the fossil record allows geologists and paleontologists to accurately determine the relative chronological order of the stratigraphic layers in Earth’s crust. Absolute ages are less accurately determined than relative ages, however. But since all rocks contain trace amounts of radioactive material absolute ages can be found by radiometric dating. 2.3.1 Poisson Probability Distribution and the Rate Equation Radioactive decay occurs because some of the mass of an atom is held in binding energy. (Recall that Einstein taught us that energy (E) is equivalent to mass (m) with the simple and elegant expression E = mc2, where c is the velocity of light.) If there is ”too much” binding energy (as determined by quantum mechanics), then the nucleus will decay spontaneously – by radioactive decay – or by an induced nuclear reaction (neutron bombardment) to a lower energy state. Decay can occur by three classes of mechanisms: - Alpha decay - escape of a He4 nucleus; the strong forces that bind nuclei dictates that it is dicult to escape from the potential well. particles are not very energetic 2–3 and cannot jump the potential well but Heisenbergs Uncertainty Principle tells us that a particle can sometimes exist outside the threshold due to uncertainty in position. - Beta decay - escape of an electron or positron by: (1) Electron emission (): for nuclides over-rich in neutrons (n p + e + ) (2)→Positron emission (+): for nuclides depleted in neutrons (p n + e+ + ) (3) Electron capture (ec): an S electron in K shell on an atom has→ nite probability of being inside nucleus. - Gamma decay - emission of gamma radiation that occurs when nuclei are in excited energy states. Newly-produced nuclei are not in the ground state and produce -rays. Nuclear binding energies are very large and nuclei are so small (order 1 angstrom = 8 10 cm) that radioactive decay rates are not signicantly aected by physical conditions on Earth such as pressure and temperature. However, half-lives can be changed slightly by changes in bonding energy. For example, solar wind studies have shown that radioactive beryllium decays at slightly dierent rates on the sun and Earth. Radioactive decay is governed by the Poisson probability distribution. Other Poisson processes include the number of impact craters that form on a planet’s surface over a given time interval, the waiting time for a subway train, the number of typos made by an experienced typist, or any other process that involves random events that occur infrequently. It is useful to review the derivation of the Poisson probability distribution because it shows where the concept of a “half-life” in radioactive decay originates. Also, we will be using the same method for calculating expected values from random processes introduced here when we study blackbody radiation in Chapter 4. We start rst with the binomial probability distribution.
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