Arthur Ross Hall of
Meteorites
American Museum
of Natural History
Origins Section |
Origins (blue section) Planetary Puzzles (orange section) Impacts (green section) Theater |
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Abundance of the Elements Carbonaceous chondrites have elemental abundances nearly identical to those observed in the Sun, except for elements that occur primarily as gases. The elements were not separated, one from another, before the chondrites were formed. Carbon-rich (carbonaceous) chondrites are made of dust and small objects left over from the formation of the solar system. Most of the solar system is in the sun. The rest is in planets, moons, asteroids, and comets. Material in the early solar system accreted into larger and larger objects, called planetesimals. The carbonaceous chondrites are from planetesimals which were not large enough to ever melt. The carbonaceous chondrites most like the sun in composition do NOT contain chondrules, or high-temperature inclusions. They are made only of matrix, and their oxygen isotope signatures are like that of the Earth. |
Large colorful grains are the mineral olivine Mg2SiO4, and dark material in between is silicate liquid cooled quickly to make a glass. |
Free-floating Objects in Space Chondrules give "chondrites" their name, even though the chondrites with compositions most like the sun do not contain chondrules. The minerals in chondrules are rich in magnesium, silicon, and iron. Most chondrules were once partially or totally molten, and cooled to spherical shapes, like glassy beads, while they were freely floating in space. Then they... Refractory inclusions are rich in calcium, aluminum, and other elements which melt at very high temperatures, so they are also called "CAIs". They appear as white-colored objects in the Allende meteorite. Some are large, like the one shown at left. CAIs are the first objects which formed in our solar system. Not all CAIs were once molten. Many condensed directly from the gas to solid state. Matrix dust is the carbon-rich, extremely fine-grained material between chondrules and refractory inclusions in carbonaceous chondrites. The matrix contains nearly all the water molecules, carbon, and other volatile elements in chondrites. Many large organic molecules, such as amino acids, have been isolated from the matrix of carbon-rich chondrite meteorites. The minerals in matrix are thought to be dust which was accumulated in the meteorites as they grew into small, early planets. [Scientists have proposed a great many theories for how chondrules and CAIs formed. No single theory explains all the many things we know about them.] |
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Reading the Rocks Geologists cut rocks from Earth to discover what they are made of and how they formed. Meteoriticists do the same with meteorites. Slicing Meteoriticists cut meteorites into sections, then make polished, translucent thin sections which can be viewed through a microscope. Microbeam Analysis Using ion beam instruments, the chemical and isotopic compositions of tiny mineral grains can be learned. From these results, scientists deduce how the solar system formed. (THIS IS TOO COMPLICATED) Age Dating The ages of chondrules and refractory inclusions can be learned by chemical analysis, after they are separated from the rest of the rock. Their ages tell the sequence of events which occurred in the early solar system. |
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This diagram shows a fundamental difference between the types of carbonaceous
chondrites. It is central to all ideas about the origin of the solar system. What does it say? There are three stable, naturally occurring isotopes of oxygen: oxygen 16, 17, and 18. Since 16 is the most abundant, the ratios of 17 and 18 to 16 in meteorite samples are compared with the same ratios in ocean water, plotted where the dashed lines cross at zero. Oxygen isotopes are the key to distinguishing between meteorite types. All material on the Earth and moon falls along a single line (blue). Many carbonaceous chondrites, in contrast, retain minerals which have excesses of oxygen 16. Why? Scientists do not know. The GENESIS mission will sample the solar wind in 200_. Comparing the oxygen isotopes in the solar wind to those in meteorites will answer many questions. Although a number of theories have been proposed to explain these chemical differences, no one theory fits all the available facts. |
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LL very low metal content L low metal content H high metal content Enstatite Chondrites EL low metal content EH high metal content |
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What does this diagram say? Iron is a very important element in the cosmos. Iron can occur in an oxidized form, like rust, where it combines with oxygen. Iron can also occur in a reduced form, as metal or combined with sulfur. This diagram compares these two forms of iron, reduced and oxidized, and also shows the total amount of iron in each type of chondritic meteorite. Total iron: The diagonal dotted line corresponds to the total amount of iron in the sun, where iron occurs as vapor. Any pair of meteorites having the same ratio of total iron, compared to silicon, will fall on a diagonal line parallel to the one shown. Notice that the H (high iron) chondrites, and the carbon-rich chondrites, all fall on the line corresponding to the solar iron content. The L (low iron) and LL (very low iron) chondrites do not. Reduction of iron: This plot illustrates another important fact. The ordinary chondrites, and the enstatite chondrites, differ from the carbon-rich (carbonaceous) chondrites because their iron is less oxidized. The carbonaceous chondrites are most like the sun in their over-all compositions. The ordinary and the enstatite chondrites are differentiated. They are lower in oxygen content, and the L and LL types are deficient in iron, measured relative to silicon. Although a number of theories have been proposed to explain these chemical differences, no one theory fits all the available facts. |
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Sometimes, the pieces of a broken up (brecciated) meteorite are cemented together, as in Portales Valley. Melt rocks are simply meteorites which have been melted. Mesosiderites .... |