Presolar Grains

MineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More grains that formed before our solar systemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids.. These tiny crystalline grains are typically found in the fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giantGiant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence. These red stars have a relatively cool surface whose core has burned most of its hydrogen. Red giants lose parts of their atmospheres and thus provide new elements into interstellar Click on Term to Read More (AGB) stars before being incorporated in the molecular cloudAn interstellar gas cloud that is dense enough to allow the formation of molecules and comprised of a cold dense complex mixture of interstellar gas and dust roughly 75% hydrogen and 21-24% helium. Clouds contain trace amounts of other molecules, of which well over 100 different types have now been Click on Term to Read More from which the solar systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with formed. More exotic formation mechanisms include mass transfer within a binary starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. system. Presolar grains survived the collapse of the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed., and also the subsequent formation of planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More because they consist of refractory minerals. Presolar grains in meteorites are recognized by their isotopically anomalous and highly unusual composition.

Peter Hoppe’s May, 2000 abstract in the paper “Presolar dust grains from meteorites and their stellar sources” provides an excellent baseline summary¹ that has been updated to include new research²:

Primitive meteorites contain small concentrations (ppbParts per billion (10⁹). Click on Term to Read More to ppmParts per million (10⁶). Click on Term to Read More) of presolar dust grains that have survived largely unaltered the processes that led to the formation of the solar system. Minerals identified to date include diamondOne of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest naturally occurring substance (according to the Mohs scale) in terms of resistance to scratching. There are Click on Term to Read More, silicon carbidePresolar interstellar dust grain found in CM and E chondrites; its formula is SiC. Click on Term to Read More (SIC), graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More, silicon nitride (Si₃N₄), corundumCrystalline form of aluminium oxide, Al2O3, found in Ca-Al-rich inclusions (CAIs). Corundum-bearing  CAI are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk¹. Click on Term to Read More (Al₂O₃), spinelMg-Al oxide, MgAl2O4, found in CAIs. (MgAl₂O₄), hiboniteRefractory mineral, Ca-aluminate (CaAl12O19) that occurs in terrestrial metamorphic rocks and in CAIs of many chondrites. Meteoritic hibonite tends to be blue as seen in the meteorite Isheyevo (Ch/CB). Hibonite is one of the most refractory minerals found in primitive meteorites. Click on Term to Read More (CaAl₁₂O₁₉) and titanium oxide (TiO₂). These grains exhibit large isotopic anomalies indicative of a stellar origin. Variations in the isotopic ratios of the major elements and of many trace elements contained in the grains range over more than 4 orders of magnitude. The presolar dust grains preserve memories of both nucleosynthesis in the parent stars and galactic chemical evolution. Most of the silicon carbide and corundum grains formed in the winds of red giant and asymptotic giant branchPath on the Hertzsprung-Russell diagram corresponding to the changes that a star undergoes after He burning ceases in the core. At this stage, the C core shrinks and drives the expansion of the envelope, and the star becomes a red supergiant. Click on Term to Read More stars (AGB). Most graphite grains, some SiC and corundum grains, and all silicon nitride grains originated most likely in supernovaStellar explosion that expels much or all of the stellar material with great force, driving a blast wave into the surrounding space, and leaving a supernova remnant. Supernovae are classified based on the presence or absence of features in their optical spectra taken near maximum light. They were first categorized Click on Term to Read More ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More. A few SiC and graphite grains appear to have a novaStar that, over a period of a few days, becomes 10^{3 to} 10⁴ times brighter than it was previously. Novae are observed about 10-15 times per year in the Milky Way. Click on Term to Read More origin. The origin of the diamonds is still unknown but at least a small fraction apparently comes from supernovae. Diamonds are only 2 nm in size. The other types of presolar grains are larger and range from ~0.2 to 20 μm. These sizes are larger than those inferred for dust in the interstellar mediumMaterial between the stars, consisting of gas, dust and cosmic rays (high energy charged particles moving at nearly the speed of light). It comprises ~10% of visible matter in the disk of our Galaxy (Milky Way). Until recently it was generally assumed that silicates in the ISM were amorphous, but Click on Term to Read More but are comparable to the sizes of interstellar dustGrains of carbon and silicate ~0.1-1.0 mm in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction and scatters incident starlight, particularly blue light (which has a wavelength comparable to the dust grain's size), causing reddening. Cooling of interstellar gas and Click on Term to Read More in the heliosphereRegion of space containing plasma and magnetic fields of solar origin; a cavity carved in the interstellar medium by the flow of the solar wind. Click on Term to Read More identified by the Galileo and Ulysses spacecraft missions.