- Reading_ Meteorites

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Harvard University *

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Geology

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Oct 30, 2023

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Meteorites Every few weeks a new meteorite is picked up in some country or other after falling from the sky as a bright fireball. These strange gifts from space are known from their speed of arrival, and from the angle at which they enter the upper atmosphere, to come from the asteroid belt. The asteroid belt is a zone in the solar system between the orbits of Mars and Jupiter, and it is occupied by numerous small rocky ‘mini-planets’ called asteroids. The largest asteroid of all, Ceres, is about 1000 km in diameter. Q: Where do meteorites originate? .space The great majority of meteorites are made from a very unusual kind of sandstone in which most of the sand grains are made from a green stony material. The green color is due to an abundance of olivine. Olivine is a mineral (i.e. a natural chemical compound) made from the elements magnesium, silicon, iron and oxygen. Mixed in with the green olivine-bearing grains are di±erent grains, perhaps 10% of the total, made from tiny bits of shiny, magnetic iron metal. These ‘sandstone’ meteorites are called chondritic meteorites. Figure 6 shows a sawn surface of a chondritic meteorite that fell at Dundrum near Cashel in County Tipperary in 1865. This meteorite has been kept ever since in the museum at the Department of Geology in Trinity College where it is on display. Q: What is olivine? What elements are present in Olivine? Green stony material, magnesium, silicon, iron and oxygen Q: Chondritic meteorites contain approximately what percent of magnetic iron? 10% The picture above is of the Dundrum chondritic meteorite showing a ﬂat surface that was cut by a diamond-tipped saw to expose the interior. Bright ﬂecks are grains of shiny iron metal. The grey material surrounding the metal grains is stony and includes an abundance of the mineral olivine. Meteorites have been eagerly investigated by scientists over the past two centuries, and a great deal is known about them. The chemical elements they contain have been analyzed carefully, and what is most remarkable is that the chemical composition of chondritic meteorites is almost exactly the same as the chemical composition of the sun when gasses like hydrogen and helium are not included in the comparison. The chemical composition of the sun has been inferred from studies of thin dark lines in the colored spectrum that appear when sunlight is passed through a glass prism. Each dark line corresponds to a particular chemical element, and the darker the line, the greater the amount of that element. Q: How does the composition of chondritic meteorites compare with the composition of the sun (assuming we ignore Hydrogen)? They are almost the same

The good chemical match between the sun and chondritic meteorites shows that both the sun and the asteroids (the source of meteorites) were made from the same original batch of material. By implication the Earth, which lies between the Sun and the asteroid belt, is believed to have been made from the same starting material. Q: Why is the comparison of the composition of chondritic meteorites with the composition of the sun important to what we know about the Earth? Its made the same materials This view fits neatly with our current understanding of the origin of the solar system. The sun and planets came into existence a little over 4.6 billion years ago when part of an enormous cloud of gas and dust, drifting in the Milky Way galaxy, became unstable and collapsed inwards on itself under the inﬂuence of its own gravity. The dust in the cloud had the same chemical composition as the sun and chondritic meteorites. Gravity brought most of the gas and dust together in a single central mass (the infant sun) but some of the gas and dust became spread out in a ﬂat disk rotating around the sun. The dust in this disk included an abundance of olivine-rich grains and bits of metal. The dust eventually clumped together to form larger objects and larger objects, culminating in the planets and asteroids, all orbiting the sun in the same plane and moving in the same direction. The gas (hydrogen and helium) was soon lost from the asteroids and the inner, terrestrial planets (Mercury, Venus, Earth and Mars) because their gravity was too feeble to hold on to it, but the gas did stick to Jupiter and the other icy-cold giant planets beyond. Q: Fill in the blanks with the following terms: asteroids, gravity, olivine-rich dust, planets, sun . An enormous cloud of gas and _________dust_________ _ collapses inwards on itself due to _gravity_______ . The collapse causes the dust to spin into a ﬂat disk with a central mass (our ___sun____ ). Many smaller masses, composed of the same material, begin to form larger objects such as the _____planets______ , ________astreoids_____ , and other objects. So the Earth probably began as a mixture of about 90% stony, olivine-rich material, and about 10% iron metal, just like that seen in chondritic meteorites. Soon after its formation the interior of the Earth is thought to have become very hot, so that the bits of iron melted and dribbled down towards the center of the planet to form a dense liquid iron core. The less dense olivine-rich material, with its iron removed, is believed to have remained behind as the Earth’s mantle. Geologists refer to a rock made largely from olivine as peridotite. Thus, the evidence in meteorites strongly suggests that the Earth’s mantle is made from peridotite, and that the core is made from iron. Q: Why did the iron separate from the olivine-like material in the Earth when it was young? It melts and its more dense Q: What is peridotite? Rock made largely from olivine Q: How is peridotite di±erent from Olivine? What caused the di±erence (during the formation of Earth)? Peridotite has less iron

The conclusion that the core is probably made from iron, and is molten, fits well with the Earth’s magnetism. Scientists have shown how convective ﬂowing motions within a molten iron core can produce the kind of strong magnetic field that the Earth has today. As for the mantle being made from peridotite, it is reassuring to note that the speed of P-waves through peridotite, measured in the laboratory, is 8 km/s, the same as the speed observed for seismic waves traveling in the mantle (i.e. below the Moho). Another piece of evidence for a peridotite mantle comes to light when the origin of the igneous rock basalt is considered. Q: Does the seismic evidence seem to confirm or refute the idea that the mantle is made of peridotite and the core has liquid iron? Explain your answer. Yes because the earth magnetic field starts with the core. Q: We will be learning about Earth’s magnetic field today. What does the last paragraph in the article suggest causes Earth’s magnetic field? Explain. Iron in the core

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