Meteorites: Types, Origins, and Recognition PDF

# Meteorites Sneha Raghuvanshi Assistant Professor Department of geology University of Lucknow-226007 raghuvanshi_snehalkouniv.ac.in ## What are Meteorites? A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survive...

# Meteorites Sneha Raghuvanshi Assistant Professor Department of geology University of Lucknow-226007 raghuvanshi_snehalkouniv.ac.in ## What are Meteorites? A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. * When the object enters the atmosphere, various factors like friction, pressure, and chemical interactions with the atmospheric gases cause it to heat up and radiate that energy. * It then becomes a meteor and forms a fireball, also known as a shooting star or falling star. * Meteorites vary greatly in size. For geologists, a bolide is a meteorite large enough to create a crater. * Meteorites have traditionally been divided into three broad categories: * Stony meteorites are rocks, mainly composed of silicate minerals. * Iron meteorites that are largely composed of metallic iron-nickel. * Stony-iron meteorites that contain large amounts of both metallic and rocky material. ## Meteoroid: A small asteroid from microns to 1 meter. * In the void of space, meteoroids travel at great speeds. They can enter Earth’s atmosphere at speeds from 39 600 km/h up to 259 200 km/h. * As a meteoroid travels through the atmosphere, it meets resistance from the air. * This resistance is from a force called friction. But an even more important force comes from the compression of the air in front of the moving meteoroid. * The compression of air increases the pressure of the air. The high pressure increases the temperature of the air. This burns material on the meteor’s surface, creating a visible light. ## What’s the difference? * **Meteor:** A meteoroid that enters Earth’s atmosphere and burns up. * **Meteorite:** A meteoroid, especially one that has hit Earth’s surface. * The largest meteorite found on Earth is the Hoba meteorite discovered in Namibia in 1920. The Hoba meteorite weighs roughly 54,000 kilograms (119,000 pounds). * **Asteroid:** A rocky object that orbits the sun and has an average size between a meteoroid and a planet. ## Asteroid Facts * The vast majority of meteorites are fragments of shattered asteroids. Asteroids are rocky bodies found mostly in the asteroid belt, between Mars and Jupiter. * Jupiter is the largest planet in our solar system, and its gravity is very strong. * Asteroids, which are much smaller than planets, are sometimes pulled out of the asteroid belt by the force of Jupiter’s gravity. * Many of these asteroids then travel toward the inner solar system -where they can collide with Earth. ## Comet : Comets are cosmic snowballs of frozen gases, rock, and dust that orbit the Sun. * When frozen, they are the size of a small town. * When a comet’s orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. * The dust and gases form a tail that stretches away from the Sun for millions of miles. ## How to recognize a meteorite? * **Fall:** a meteorite actually observed falling * **Find:** a meteorite not seen to fall but found sometime later (even 2 Ga!) * 1000 falls and 31,000 finds as on 2006. * So, falls are rarer than finds. ## Geological Time Scale | Younger | Eon | Era | Period | Epoch | Today | |---|---|---|---|---|---| | | | | Quaternary | Holocene | 11.8 Ka | | | | | | Pleistocene | | | | | | | Pliocene | | | | | Neogene | | Miocene | | | | | | | Oligocene | | | | Cenozoic | | Paleogene | Eocene | | | | | | | Paleocene | 66 Ma | | | | Mesozoic | Cretaceous | | | | | Phanerozoic | | Jurassic | | | | | | | Triassic | | 252 Ma | | | | | Permian | | | | | | Paleozoic | Carboni- Pennsylvanian | | | | | | | ferous Mississippian | | | | | | | Devonian | | | | | | | Silurian | | | | | | | Ordovician | | | | | | | Cambrian | | 541 Ma | | Older | Proterozoic | | | | 2.5 Ga | | | Archean | | | | 4.0 Ga | | | Hadean | | | | 4.54 Ga | ## Types of Meteorites * There are three main types of meteorites: - Iron meteorites: which are almost completely made of metal - Stony-iron meteorites: which have nearly equal amounts of metal and silicate crystals - Stony meteorites: which mostly have silicate minerals * Each group can be split into many more classes and types depending on the minerals, structure and chemistry. ## Iron meteorites * Composed of iron and nickel and are extremely dense. * They are pieces of the cores of asteroids. * Early in Solar System history, asteroids melted and the dense iron-nickel metal sank to the center to form a core - much like the Earth has a core. * Iron meteorites are the samples of the cores of ancient worlds. * While they are rare among meteorites seen to fall to Earth (only a few percent), they are among the most common type of meteorites in our collections, because they can be recognized long after their fall, are very different from Earth rocks, and are resistant to weathering. * **One of the most distinguishing features of meteorites is the presence of the Widmanstatten pattern - the distinctive series of bands in geometric patterns.** * This pattern is created by the intergrowth of two different iron-nickel minerals formed during very slow cooling (a few degrees every million years) in the core of the asteroid. * The presence of nickel is a universal feature of iron meteorites. ## Stony-iron meteorites * Stony-iron meteorites consist of almost equal parts iron-nickel metal and silicate minerals including precious and semi-precious gemstones. * They are considered some of the most beautiful meteorites. * There are two different types of stony-iron meteorites: 1. Pallasite 2. Mesosiderite * **Pallasites have a continuous base of Ni-Fe and enclose olivine grains of euhedral forms.** * Pallasites are thought to be samples of the boundaries between a metal core and the silicate, olivine-rich mantle around it. * Mesosiderites form when debris from a collision between two asteroids is mixed together. * In the crash, molten metal mixes together with solid fragments of silicate rocks * **Record the history of both meteorites** ## Stony meteorites * Stony meteorites, the most common type of meteorite, are generally composed of approximately 75-90% silicate minerals, 10 - 25% nickel-iron alloy, and trace amounts of iron sulfide. * Stony meteorites account for ~94% of observed meteorite falls * There are two main types of stony meteorite: * (a) chondrites (some of the oldest materials in the solar system) and * (b) achondrites (including meteorites from asteroids, Mars and the Moon) ## Chondrites * Over 4.5 billion years old * Most primitive and pristine rocks in the solar system * Distinctive appearance * **Contain chondrules, millimeter-sized grains of silicate material that are thought to have condensed straight from the solar nebula.** * Appear to have remained essentially unchanged since their formation in the early Solar System * Never been melted! * Chondrites have been subdivided into three main classes - * (A) Ordinary * (B) Carbonaceous, and * (C) Enstatite chondrites * **Carbonaceous chondrites - most basic type** * Contain up to 20% water by weight. * As well as substantial amounts of carbon (present mainly as organic compounds such as amino acids) and oxidized elements. * Highest proportion of volatile elements * **Ordinary Chondrites:** also contain oxidized and volatile elements, but to a lesser degree than the carbonaceous chondrites * Their parent asteroids are thought to have formed in the inner asteroid belt. * **Enstatite Chondrites:** The main elemental constituent of is iron in its metallic or sulfide state. This is in contrast to ordinary and carbonaceous chondrites in which iron is mostly present as oxides and tied up in silicates. * The high metal, low oxygen content for enstatite chondrites suggests that these meteorites may have originated in the inner Solar System. ## Achondrites * Achondrites include meteorites from asteroids, Mars and the Moon. * They are igneous, meaning at some point they were melted into magma. * When magma cools and crystallizes, it creates a concentric layered structure. * This process is known as igneous differentiation. * The rocky planets Mercury, Venus, Earth and Mars were formed in this way, giving them planetary crusts, mantles and cores. * Achondrites can tell us a lot about the internal structure and formation of the planets, including our own. ## Bulk Silicate Earth * Earth differentiated into crust, mantle, outer core and inner core relatively quickly (within 3 million years of formation * **BSE should be similar to carbonaceous chondrites in terms of refractory lithophile elements (Al, Ca, Ti, Sc, V, REE, U, TH....) & Sm-Nd & Lu-Hf should not deviate more 5% from chondritic values** * The present-day "silicate" part of the Earth, that is, the mantle and the crust (both oceanic and continental), makes up 67.5% of the whole Earth's mass, while the metallic core makes up the remaining 32.5%. * **Bulk Silicate Earth refers to the original chemical composition of the silicate part of the Earth, after the separation of the metallic core but prior to the differentiation of the crust. For this reason, the Bulk Silicate Earth composition is synonymous with the composition of the Earth's “primitive mantle.”**