Glossary S

SCALAR - Quantity that has only magnitude as opposed to both magnitude and direction. For example, mass is scalar quantity. By convention in physics the word "speed" is a scalar quantity, having only magnitude, while the word "velocity" is used to denote both the speed and the direction of the motion and is thus a vector quantity.

SCHREIBERSITE - Ni-Fe phosphide, (Fe,Ni)₃P, common in iron and stony-iron meteorites.

SCHWARZCHILD RADIUS - Radius of the event horizon surrounding a nonrotating black hole. Its size is given by:

For a one solar mass star this is ~3 kilometers.

SERPENTINE - Fe-Mg phyllosilicate, (Fe,Mg)₃Si₂O₅(OH)₆, produced by aqueous alteration of olivine and pyroxene; abundant in the matrixes of CI and CM chondrites.

SHATTER CONE - Conical fragment of rock with regular thin grooves (striae) that radiate from the apex of the cone. Shatter cones range in size from less than one centimeter to more than one meter across and are formed in fine-grained brittle rocks, such as limestone or quartzite. They result from the high pressure, high velocity shock wave produced by an impacting meteorite.

SHELL BURNING - Nuclear "burning" regions that occur in shells surrounding a star's core (see diagram). For example, helium burning might take place in the core (where the hydrogen has been exhausted) with a shell of hydrogen burning surrounding it. Stars may have more than one region of shell burning during their stellar evolution, each shell with its own nuclear reactions.

SHEPHERD SATELLITE - Satellite whose gravitational effect on a planetary ring helps preserve the ring's shape. Examples are two satellites of Saturn, Prometheus and Pandora, whose orbits lie on either side of the F ring.

SHERGOTTITE (SHE) - Most abundant type of SNC meteorites believed to have come from Mars, with 17 known examples by mid-2002; the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or hypabyssal (shallow plutonic) origin, and resemble terrestrial rocks more closely than do any other achondrite group. They all have exceptionally young crystallization ages of 150-200 Ma, and usually show signs of severe shock metamorphism (typically plagioclase has been converted to maskelynite). Shock metamorphism probably occurred when the shergottites were blasted off the Martian surface.

SHOCK MELTING - Complete melting of shocked material at pressures in excess of 90 GPa at which instantaneous temperature increases exceed 1500 K.

SHOCK METAMORPHISM - Metamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second). Kinetic energy is converted into seismic and heat energy almost instantaneously, yielding pressures and temperatures far in excess those in normal terrestrial metamorphism. On planetary bodies with no atmosphere, smaller impacting bodies (even micrometeorites) can produce shock metamorphic effects as observed in meteorites. Observed effects include planar deformation features (PDFs), diaplectic glass, and melting. Effects are observed in different minerals at different pressures.

SHOCK STAGE - A petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".

SHOCK WAVE - Abrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed of sound.

SHORT-LIVED RADIONUCLIDES (SLRs) - Radioactive isotopes with half lives less than ~20 Ma. SLRs are also called "extinct isotopes" because they have decayed completely into their daughter products, and are detectable only by isotopic excesses in daughter isotopes. SLRs include: ²⁶Al (t_½ = 0.73 Ma), ⁴¹Ca (t_½ = 0.1 Ma), ⁵³Mn (t_½ = 3.7 Ma), ⁶⁰Fe (t_½ = 1.5 Ma), ¹⁸²Hf (t_½ = 9 Ma) and ¹²⁹I (t_½ = 16 Ma). Assuming an initially homogeneous distribution of SLRs, the isotopes potentially provide high-resolution chronometers of early solar system events. The resulting SLR ages are presented relative to a standard (see figure below), the absolute age of which must be determined using long-lived radionuclide dating.

SIDERITE - An obsolete term for an iron meteorite.

SIDEROLITE - An obsolete term for a stony-iron meteorite.

SIDEROPHILE ELEMENT - Literally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on Earth's surface.

SILICA - Silicon dioxide, SiO₂; although quartz itself is rare in meteorites, several polymorphs (coesite, tridymite) are not uncommon.

SILICATE - A large group of minerals, whose structures are dominated by the silica tetrahedron, SiO₄⁴, with metal ions occurring between tetrahedra). Silicate mineral are subdivided based upon the kind of linking that occurs between the tetrahedra.

SILCON CARBIDE - Presolar interstellar dust grain found in CM and E chondrites; its formula is SiC.

SIMPLE CRATER - Smallest impact features, consisting of a bowl-shaped interior with smooth walls, an elevated rim, and a shallow inclination hummocky exterior. The interior wall is usually inclined at 20-40° to horizontal, whereas the exterior wall is inclined at 5-15° to horizontal. At larger diameters, simple craters become more complex with transitional forms developing in the 15-25 km diameter range on the Moon, 4-10 km range on Mars and 2-6 km range on Earth. Transitional craters have landslides on the interior wall and a hummocky floor. The photograph shows Moltke on the Moon, a typical simple crater. The crater diameter (measured across the top of the rim) is 6.5 km and the crater depth (measured from the top of the rim to the centre of the floor) is 1.3 km.

SOLAR ABUNDANCES - Amount of elements in the Sun as determined by spectral line intensities. Approximately 60 elements have been identified; the most abundant are listed in the table.

SOLAR APEX - Imaginary point in the constellation Hercules, near the bright star Vega, towards which the Sun is moving. The Sun's velocity relative to nearby stars is ~19.7 km/s. The point on the opposite side of the sky from which the Sun appears to be moving away is called the antapex.

SOLAR ENERGETIC PARTICLE (SEP) NOBLE GASES - Gases more deeply implanted than solar wind noble gases; probably of solar origin, but particles of interstellar origin may contribute as well.

SOLAR FLARE - Sudden eruptions from the surface of the Sun. Flares typically last a few minutes and can release energies equivalent to millions of hydrogen bombs. Flares become frequent near sunspot maximum, when smaller flares can occur daily and large flares can occur about once a week. During a flare the material in the flare may be heated to temperatures of 10 million K; matter at these temperatures emits copious amounts of UV and X-Ray, as well as visible light. In addition, flares tend to eject matter, primarily in the form or protons and electrons, into space at velocities that can approach 1000 km/second. These latter events are coronal mass ejections, and produce bursts in the solar wind that influence much of the rest of the Solar System, including the Earth. The observation of a large flare on the surface of the Sun is usually a signal for increased auroras and related activity several days hence when the ejected burst reaches Earth. Flares are also observed on other stars.

SOLAR LUMINOSITY (L_sun) - Solar luminosity is 4 x 10³³ ergs per second.

SOLAR MASS (M_sun) - Mass of the Sun, M_sun; equal to 1.99 x 10³⁰ kg.

SOLAR WIND - Supersonic flow of high-speed charged particles continuously blowing off a star (mostly e^- and p⁺). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially from all parts of the Sun: the fast wind originates from the coronal holes and the quiet Sun, whereas the slow wind arises from the coronal streamers. Active regions, which lie under closed magnetic loops, are also a source of the slow wind. The solar wind contains roughly equal number of electrons and protons, along with a few heavier ions, and blows continuously from the surface of the Sun at an average velocity of ~400 km/s. The Sun's solar wind leads to a loss of ~10^-14 M_sun per year. The solar wind is fractionated from the photosphere by the forces that accelerate the ions off of the Sun. This fractionation appears to be ordered by the first ionization potential (FIP) of the elements (low-FIP elements tend to be over-abundant and high-FIP elements tend to be under-abundant).

SOLAR WIND (SW) NOBLE GASES - Noble gases implanted by the solar wind into mineral grain surfaces that have been part of an asteroidal regolith; present in "gas-rich" meteorites, but also in lunar soils from the lunar regolith. Elemental abundances in most minerals are moderately fractionated compared to solar, favoring heavy elements.

SOLID SOLUTION - Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with the entire range of compositions possible) or partial (in which only part of the range of potential compositions occurs). For example, olivine shows a complete range of compositions between the end-members forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄) produced by the substitution Mg²⁺ → Fe²⁺.

SPECIFIC GRAVITY - Ratio of the density of a substance to the density of a standard substance. The standard is usually liquid water for solids and liquids and air for gases. The density of liquid water under typical conditions is ~1000 kg/m³. The density of air at room temperature near the surface of the earth is approximately 1.2 kg/m³. Specific gravity is a unitless quantity.

SPECTRAL CLASS - Classification scheme, also called the "Harvard Spectral Sequence," based on the strength of stellar spectral lines, which indicate a star's temperature. Each category in this classification can be subdivided into 10 subclasses using numbers from 0 to 9. For example, the sequence for the O and B subclasses is O0, O1, O2, O3 ... O8, O9, B0, B1... The data column in the table below gives (in order): temperature (K), mass divided by M_sun (M), radius divided by R_sun (R), luminosity divided by L_sun (L), and main-sequence lifespan (T).

Class	Lines	Data	Example(s)	Spectra
O	Ionized He lines strong; multiply ionized metal lines; weak H lines	K = 28,000-50,000; M = 20-60; R = 9-15; L = 90,000-800,000; T = 1-10 Ma	ζ Orionis
B	Neutral He lines moderate; singly ionized metal lines; H lines moderate	K = 10,000-28,000; M = 3-18; R = 3.0-8.4; L = 95-52,000; T = 11-400 Ma	Rigel (B8), Spica
A	Neutral He lines very faint; Balmer H lines dominant, singly-ionized metal lines	K = 7,500-10,000; M = 2.0-3.0; R = 1.7-2.7; L = 8-55; T = 400-3,000 Ma	Vega (A0), Sirius (A1), Deneb
F	H lines moderate, neutral and singly ionized metal lines	K = 6,000-7,500; M = 1.1-1.6; R = 1.2-1.6; L = 2.0-6.5; T = 3-7 Ga	Procyon, Canopus (F0)
G	Singly ionized and neutral metal lines; H lines faint	K = 5,000-6,000; M = 0.85-1.1; R = 0.85-1.1; L = 0.66-1.5; T = 7-15 Ga	Sun (G2), α Centauri (G2), Capella
K	Singly ionized and neutral metals lines strong; molecular bands begin to appear; H lines faint	K = 3,500-5,000; M = 0.65-0.85; R = 0.65-0.85; L = 0.10-0.42; T = 17 Ga	Aldebaran (K5), Arcturus (K2)
M	Ti oxide molecular lines; neutral metal lines strong; molecular lines moderate; H lines very faint	K = 2,000-3,500; M = 0.08-0.65; R = 0.17-0.63; L = 0.001-0.08; T = 56 Ga	Antares, Betelgeuse
L	Strong metal-hydride molecular bands (CrH, FeH), and neutral metals; TiO and VO bands are nearly absent.	K = ~1300-2500; M < 0.09; L = 10-5-10-6	brown dwarf
T	Strong bands of methane (CH4) - like spectrum of Jupiter	K = 1500-2000; M < 0.09; L = 10-6	cool brown dwarf

SPICULES - Spikes of gas that rise through the chromosphere (right). Spicules are rising jets of gas that move upward at ~30 km/sec and last only ~10 mniutes.

SPINEL - Mg-Al oxide, MgAl₂O₄, found in CAIs.

SPIN-ORBIT RESONANCE - State that a body is said to be in if its rotation period and its orbital period are related in a simple way.

S-PROCESS - Slow neutron capture by nuclei in massive stars (left). In the s-process, one starts with existing iron-group nuclei. Therefore, it would only be expected to take place in second-generation stars that collapsed out of the residue of a previous supernova explosion. The flux of neutrons is small enough that rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-decay. These neutrons come from various reactions in the He-burning region of a red giant star. Hundreds to thousands of years may pass between successive neutron captures. In this situation, a seed nucleus will slowly capture neutrons, for example ⁵⁶Fe → ⁵⁷Fe → ⁵⁸Fe → ⁵⁹Fe, followed by ⁵⁹Fe → ⁵⁹Co (β decay). This process builds nuclei by climbing the line of stability, until ²⁰⁸Pb and ²⁰⁹Bi are reached. Beyond this point, no nuclei are stable enough to allow neutron capture to operate: actinides cannot be synthesized by the s-process.

STAR - Self-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.

STARDUST MISSION - Space mission to study Comet Wild 2 (see http://stardust.jpl.nasa.gov/home/index.html). During the encounter, Stardust performed a variety of tasks including making counts of particles encountered by the spacecraft and real-time analyses of the compositions of these particles and volatiles. It also captured cometary particles using Aerogel and stored them for return to Earth. Results indicate that comets are not composed entirely of volatile rich materials but rather are a mixture of materials formed at all temperature ranges, at places very near the early sun (olivine) and at places very remote from it (ices).

STEFAN-BOLTZMANN LAW - Law of blackbody radiation that states that the amount of energy given off by a blackbody per second per unit area (flux) is proportional to the fourth power of the temperature of the blackbody. For flux (power per area) expressed as J/m²s with σ = 5.6703 x 10^-8 W/m²K⁴:

If R is the radius of a star, its luminosity is:

STELLAR EVOLUTION - Changes in a stars luminosity and temperature over its lifetime; conventionally, plotted on an Hertzsprung-Russell (HR) diagram. All stars, irrespective of their mass spend most of their lifetime on the main sequence. The more massive a star, the more luminous and hotter it is. As all stars age, they enter a giant phase (their brightness remains constant, but the effective surface temperature decreases, 7 → 9). This reflects a change in the fusion processes at work within the star: outer layers expand and are no longer of sites nuclear burning. As these layers cool, the star drifts towards the right side of the HR diagram. Subsequent evolution depends on the mass of the star.

For a ~1 M_sun star, when H burning ends, the core temperature is insufficient for He burning to occur. With no source of energy production in the core there is no longer any outward radiative pressure to resist gravitational collapse, and the outer regions of the star start to collapse (9 → 10). Collapse raises the temperature in the H shell and H fusion occurs. Luminosity increases as the core continues to collapse and the temperature in the H shell keeps increasing. The burning shell also provides pressure on the outer layers of the star and causes them to expand. As the layers expand they cool and the star appears to become redder.

After just a few million years the H shell runs out of fuel. Once again the star contracts under its own weight. The compact core may flash into life for a short period and He be fused into C (10). The energy released in the He flash reaches the outer layers and the star becomes a red supergiant (11). Up to half its mass is thrown out into space and seen as a planetary nebula (12) leaving a white dwarf behind.

Stars with a mass of between 8 and 20 M_sun have a more complex evolution. Initially, they evolve in the same way as low mass stars, turning into red giants and undergoing a core He burning phase. However, He burning is no longer the end phase of stellar evolution. When He in the core is exhausted, the additional mass allows stellar collapse to take place and the outer layers to reignite. A cross section through the star at this point would show an outer shell of H burning, an inner shell of He burning, and the core where C burning is taking place. Once the C supply is exhausted, O begins to fuse into Ne; the He shell becomes a C burning shell, the H shell a He burning shell and a new outer layer of H burning forms. Subsequently, Ne can fuse into Mg, into Si, and so on to Cr and Fe.

Each of these stages produces less energy than the previous one and lasts for a shorter. During these final stages the star expands to thousands of times the diameter of the Sun, becoming a red supergiant like Betelgeuse. Iron is the end of the exothermic fusion road: to fuse iron into heavier elements is an endothermic reaction. Fission of Fe into lighter elements also requires an input of energy. The core cools, drawing heat from its surroundings to power the fusion; the outward radiative pressure, which had supported the star for many millions of years, ceases and the star undergoes free fall gravitational core collapse until it reaches nuclear densities (~10¹⁴ g/cm³). The core, which represents a large percentage of the stellar mass, exceeds the 1.44 Chandrasekhar limit for a white dwarf. Protons and electrons in the core are compressed into neutrons, yielding a sphere the size of a large city and the density of an atomic nucleus, held up by neutron degeneracy pressure: a neutron star. Core collapse produces a shock wave that blasts out through the star releasing an enormous amount of energy in a few seconds, equivalent to ~10²⁸ Mton of TNT. The outer layers of the star become superheated plasmas with temperatures high enough to fuse Fe and heavier elements. These outer layers brighten rapidly and are ejected into the interstellar medium at speeds approaching the speed of light. Such an event is a Type II supernova.

Stars with over 20 M_sun evolve in the same way as their slightly less massive companions dying in a supernova explosion, but the core becomes a black hole. A neutron star can mass up to around 3 M_sun. After this point neutron degeneracy pressure is no longer sufficient to prevent core collapse. With nothing left to resist collapse the core condenses into an infinitely small, infinitely dense point called a black hole (singularity).

STISHOVITE - Dense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters.

STONY-IRON METEORITE - Meteorite composed of roughly equal amounts by weight of silicate minerals and Ni-Fe metal. The stony-irons consist of two groups: mesosiderites and pallasites. However, there is gradual shading into metal-rich stony meteorites such as the lodranites (once considered stony-irons) and silicate-rich iron meteorites. Stony-iron meteorites are less abundant than their stony and iron cousins, comprising a total known mass of ~10 tons, which is ~1.8% of the entire mass of all known meteorites.

STONY METEORITE - Meteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely heterogeneous as a group, ranging from samples of primordial matter that have remained more or less unchanged for the last 4.56 Ga (chondrites) to highly evolved younger rocks from differentiated worlds, such as the Moon or Mars (achondrites). They account for 95% of all known falls, the majority (86% of all falls) being chondrites.

STREWNFIELD - Area on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektite, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's flight direction can be roughly determined from the size pattern in the strewnfield. The Australasian strewnfield, which covers ~10% of Earth's surface, is the largest and the youngest of tektite strewnfields. This ~800,000 year-old field includes most of Southeast Asia, and extends across the ocean to include the Philippines, Indonesia, Malaya and Java and reaches far out into the Indian Ocean and south to the western side of Australia. The impact crater may have been between 32 and 114 km in diameter.

STRONG LENSING - Effect due to gravitational bending of light by mass concentrations which results in (1) the strong distortion of background galaxy images (arcs) behind some massive foreground galaxy clusters, and (2) multiple images of the same background quasar behind a foreground galaxy. Over 50 lensed quasi-stellar objects are known.

STRONG NUCLEAR FORCE - Fundamental force binding nucleons within an atomic nucleus and preventing like-charged protons from flying apart. Particles that are acted on by the strong nuclear force (including protons and neutrons) are known collectively as hadrons. The strong nuclear interaction between individual hadrons is believed to be a remnant of a more powerful "color force" (carried by gluons) that acts between quarks inside hadrons.

STRUCTURE FORMATION - Development of organized structures in the universe, ranging from galaxies to clusters to superclusters, and possibly beyond to huge filaments and voids.

SUEVITE - Terrestrial breccia composed of angular fragments of different rock types and glass inclusions. Glass can make up more than half of the volume of suevite. Mineral grains in the rock fragments commonly display shock-metamorphic effects. Suevite was named after a rock found at the Ries Crater in southern Germany (photograph).

SUN - Our parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region.

The core is the hot, dense central region in which the nuclear reactions that power the Sun take place, temperatures range from 8-15 x 10⁶ K with densities from 10-160 g/cm³. It comprises about 25% of the interior radius. The core is the region where the energy of the Sun is produced. Density and temperature are high enough to cause nuclear fusion reactions. These reactions release energy both in the form of γ-rays and particles (in particular neutrinos).

The radiative zone extends from ~25% to 85% of the solar radius. Here, and in the core, the primary transport of energy is by the movement of photons; temperatures range from 0.5-8 x 10⁶ K with densities from 0.01-10 g/cm³. The radiative zone is not transparent. On average, photons only ~2 cm before being scattered in a random direction by an encounter with an electron. The resulting "drunkard's walk" is very inefficient and it typically takes ~170,000 years for energy generated in the core to escape to the surface.

The convective zone starts at ~85% of the solar radius and extends to just below its surface; its density is <0.01 g/cm³. In this region, the change in temperature with increasing radius is so rapid (0.1-5 x 10⁵ K) that the interior becomes unstable and undergoes convection (rapid up and down motion of large packets of gas). Convective movement is responsible for the granulation pattern seen on the surface.

Basic data for the Sun: R_Sun = 7.0 x 10⁵ km; M_Sun = 2.0 x 10³⁰ kg; ρ_Sun = 1.4 g/cm³; Composition: 74% H, 25% He, 1% other elements; T_surface = 5800 K; L_Sun = 3.9 x 10²⁶ W.

SUNSPOT - Regions on the Sun's surface that appear dark because they are cooler than the surrounding photosphere, typically by ~1500-1800 K. Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the Sun by its rotation.

Sunspots travel in pairs (north and south magnetic poles). The pairs are due to magnetic flux tubes on the surface of the Sun that carry energy away causing the surface to be cooler than the surrounding material. The number of sunspots per year varies with an 11 year cycle and the peaks are associated with times of high solar activity (many flares and solar storms). This 11 year periodicity is called the "sunspot cycle".

SUPERCLUSTER - Cluster of galactic clusters.

SUPERNOVA - Stellar 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. There two possible routes to supernova formation, classified as Type I and II. Supernovae leave behind neutron stars or black holes, depending upon the remaining mass.

SYNCHRONOUS ORBIT - State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is in a synchronous orbit, and so presents the same face toward Earth at all times.