Astrophysics

21 Planetary Science terms

This glossary aims to simplify the scientific terms in the field of Astrophysics and Planetary science

Accretion: The growth of planetary bodies from smaller objects by impact, one impact at a time. After formation, bodies are said to have "accreted" from small objects.; Example: In warmer clouds ice crystals may collide with supercooled droplets, freezing them on contact and sticking together, called accretion.
AGB stars: Cool, luminous, and pulsating red giant stars. Most stars in the Universe that have left the main sequence will reach their final evolutionary stage as stars on the asymptotic giant branch (AGB).; Example: Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB stars.
Agglutinates: Common particle type in lunar regolith; agglutinates consist of small rock, mineral, and glass fragments bonded together with glass—a glass that is formed by flash heating when micrometeorite impacts melt the lunar regolith. The heat can also release solar-wind-implanted hydrogen and helium from the lunar regolith, causing vesicles (bubbles) in the quickly-quenched glass. Agglutinates are typically tens of micrometers to a few millimeters in size.; Example: Agglutinates are typically tens of micrometers to a few millimeters in size.
Albedo: The brightness of an object or surface; it is the percentage of incoming radiation (light) that the object or surface reflects.; Example: The global irradiation on a surface can be expressed as the sum of the beam, diffuse and albedo (surface reflection) components.
Andesite: Dark-colored, fine-grained extrusive igneous rock with about 52 to 63 weight percent silica (SiO2). Andesite consists mainly of plagioclase and one or more mafic minerals. The word andesite is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common.; Example: Regarding the circulation of lithics beyond the original production centres, from the EN there is clear evidence of long-distance circulation of obsidian and flint tools, andesite millstones, polished celts and chisels (Demoule and PerlÃ¨s, 1993: 384).
Celestial body: A solid object found in space.; Example: It is thought that this is the result of a collision with another celestial body, taking place during the early stages of the Solar System formation.
Coma: A large cloud of dust and gas which escapes from the nucleus of an active comet.; Example: Such a cloud, termed a coma, is a distinguishing feature of comets and consists of gases and entrained dust escaping from the cometary nucleus when sunlight causes its ices to sublimate.
Cosmic rays: Extremely high-energy subatomic particles that continuously bombard Earth and other planetary bodies from all directions. Most cosmic rays hit and break up atomic nuclei in Earth's upper atmosphere. Airless bodies are not protected and their surfaces (asteroids and the Moon, for example) are exposed. When cosmic rays collide with the atoms of the elements in soils and rocks on a planetary surface, the nuclei of the atoms emit gamma rays — by measuring these gamma rays with a gamma-ray spectrometer it is possible to calculate the abundance and distribution of the chemical elements on the planet's or moon's surface.; Example: Most cosmic rays hit and break up atomic nuclei in Earth's upper atmosphere.
Crater: A hole or depression. Most are roughly circular or oval in outline. On Earth most natural craters are of volcanic origin. On the Moon most are of impact origin.; Example: The Dinosaur Extinction describes the force due to meteorite impact which resulted in the Barringer Meteor crater in northern Arizona as "roughly equivalent to 3.5 million tons of TNT."
Eratosthenian: The geologic history of Earth's Moon has been divided into five broad time periods, or systems. From oldest to youngest, these are pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican. These systems are defined by the number of impact craters on the ground surface (more craters = older surface) correlated to the absolute ages determined for the lunar rock samples returned to Earth by the U. S. Apollo missions and unpiloted Soviet Luna missions. The Eratosthenian system extends from 3.2 billion to 1.1 billion years ago.; Example: The geologic history of Earth's Moon has been divided into five broad time periods, or systems. From oldest to youngest, these are pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican.
Heliocentric distance: The distance from the Sun.; Example: The Heliocentric Distance Where the Deflections and Rotations of Solar Coronal Mass Ejections Occur.
Impact: (see Crater) The forceful striking of one body, such as a meteorite, against another body such as a moon or planet.; Example: These epochs are defined by the number of impact craters on the ground surface.
Ionosphere: "The part of the Earth's upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves," as defined in 1950 by a committee of the Institute of Radio Engineers. The ionosphere is ~ 1000 kilometers above the Earth.; Example: Because the ionosphere is made up of charged particles, it's uniquely reactive to the changing magnetic and electric conditions in space.
Luna: One of two successful series of Soviet unpiloted lunar missions. There were 24 missions to the Moon in the Luna series launched from 1959 to 1976. (The other series, Zond, had 5 lunar missions launched in 1965 to 1970.) The first image of the farside of the Moon was taken by the Luna 3 spacecraft in October, 1959. Three Luna landers (Luna 16, 20, and 24) collected a total of approximately 300 grams of lunar samples and returned them to Earth.; Example: There were 24 missions to the Moon in the Luna series launched from 1959 to 1976.
Lunar basin: An impact crater on the Moon larger than 300 kilometers in diameter with two or more concentric rims and no central peak.; Example: Stretching 1,550 miles (2,500 kilometers) wide and 8 miles (13 km) deep, the South Pole-Aitken basin, as the tremendous hole is known to Earthlings, is the oldest and deepest crater on the moon, and one of the largest craters in the entire solar system.
Martian meteorites: Of the ~60,000+ meteorites collected on Earth, only 124 have been identified as meteorites from Mars. They are generally divided into three groups (all with relatively young ages slightly over 1 billion years old). The groups are the Shergottites, Nakhlites, and Chassignites. A Martian meteorite that does not fall into one of these groups is ALH 84001, a cumulate rock composed mostly of orthopyroxene, which is much older than all the others with an age of 4.5 billion years. A NASA-sponsored research team reported in 1996 that ALH 84001 may contain evidence of past primitive life on Mars--an idea that remains a topic of great debate and continued investigation. (See Mars Meteorites compiled by Ron Baalke from the Jet Propulsion Laboratory.); Example: A Martian meteorite that does not fall into one of these groups is ALH 84001, a cumulate rock composed mostly of orthopyroxene, which is much older than all the others with an age of 4.5 billion years.
Olympus Mons: Largest shield volcano on Mars. Located in the Tharsis region at 18.5oN, 133.2oW, it is 624 kilometers in diameter and rises 18 kilometers over its surroundings. The volcano is rimmed by a 6-kilometer-high scarp. The summit has nested circular depressions, calderas, formed by collapse after repeated withdrawal of magma and eruptions of lava. The last major summit volcanism on Olympus Mons ceased about 150-400 million years ago, based on crater counting.; Example: Olympus Mons is still a relatively young volcano.
Oort cloud: Vast spherical swarm of comets orbiting our Solar System from roughly 2,000 to 100,000 AU. The Oort cloud, while roughly spherical at the largest radius, is wedge-shaped where it merges with the outer planet region in the vicinity of the Kuiper belt of comets. The cloud is divided into different regions of dynamical stability: the Kuiper belt (35-50 AU; affected by planetary perturbations), a dynamically inert region (50-2000 AU; not affected by gravity of planets or stars), the inner Oort cloud (2,000-15,000 AU; affected by galactic tidal forces), and the outer Oort cloud (15,000-100,000 AU; affected by stellar perturbations). The Oort cloud has never been observed, rather its existence is inferred from the careful analysis of the orbits of comets which come in from the cloud. Recent discoveries of objects in the Kuiper belt have confirmed its existence.; Example: The Oort cloud has never been observed, rather its existence is inferred from the careful analysis of the orbits of comets which come in from the cloud.
Plasma: The fourth state of matter beyond solid, liquid, and gas. Plasma may be the most common state of matter in the universe. Our Sun is plasma. In space science, plasma is hot, ionized gas—a mixture of electrons (negative electric charge) and protons (positive electric charge). Plasma conducts electrical currents and responds to electromagnetic fields.; Example: Plasma may be the most common state of matter in the universe.
Solar flare: A region of exceptional brightness in an atmospheric layer of the Sun, often associated with sunspots of complex magnetic fields.; Example: Solar flares are large eruptions of electromagnetic radiation from the Sun lasting from minutes to hours.
Solar wind: Streams of plasma--charged particles (mainly ionized hydrogen and some helium but actually a mixture of all electrons and protons in the Sun)-- ejected from the outer atmosphere (corona) of the Sun and moving outward all the time with velocities in the range 300-500 kilometers per second. For amazing images and information about the Sun and solar wind see the SOHO website for the Solar and Heliospheric Observatory, a project of international cooperation between ESA and NASA.; Example: A plasma subsystem will be used to measure interactions of the solar wind with the planetary magnetic field and interplanetary plasma resonance.