Glossary

6. Glossary#

A list of some terms used in the PSLU course. If you see important terms being used in the notes that you would like to see featured here, please let us know.

Abiotic#: Deriving from physical/chemical processes without the involvement of life.
Accrete#: To gain mass.
Accretion#: The process whereby an object, such as a planet, gains mass.
Astronomical unit#: The mean Earth - Sun distance, ~150 million kilometers.
Autotroph#: An organism using inorganic energy sources (sunlight, energy from rocks) to convert inorganic carbon compounds (e.g., CO2) to organic compounds and energy.
Biosignature#: Any observable that can be attributed to the action of life, although it may not on its own be definitive evidence of life. In this respect fossils are a biosignature, although only thought of as such as we try and push back the earliest evidence for life on Earth. In the context of exoplanets, trace atmospheric gasses are the most frequently considered type of biosignature. SETI’s survey for radio emission from extraterrestrial technological life also a biosignature search.
Bulk silicate Earth#: The integrated composition of Earth’s non-core reservoirs, i.e., atmosphere + oceans + crust + mantle.
CAI#: Calcium-aluminium rich inclusions are found in certain un-differentiated meteorites (particularly carbonaceous **chondrite**s). These grains contain the first minerals to condense out of a cooling nebular gas, thought to occur after our Sun began to fuse hydrogen. Age dating of these grains shows them to be the oldest solids formed in the solar system at 4.5673 billion years old Connelly+2012.
Chemiosmosis#: Chemiosmosis is the process of ions moving across a semipermeable membrane from a high concentration to a low concentration.
Chondrite#: A class of un-differentiated meteorites, including the ‘carbonaceous’, ‘ordinary’, and ‘enstatite’ chondrites. So named because of they contain chondrules, spheres of silicate mineral presumed to have grown from blobs of melt.
Cofactor#: A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein’s biological activity.
Crust#: The outer, chemically distinct, layer of a planet forms from magmas ultimately derived from the planet’s mantle undergoing incomplete (i.e., ‘partial’) melting and that melt moving towards the surface where it solidifies to crust.
Debris belts#: Regions around a star containing material left over from planet formation, such as the asteroid belt in our solar system, or the Kuiper belt.
Differentiated#: See differentiation.
Differentiation#: The process whereby a rocky planet segregates its internal structure into a metal-rich core and silicate-dominated mantle. Can potentially proceed further to the formation of crusts and atmospheres.
Equilibrium temperature#: The blackbody temperature of an object (planet) at equilibrium between incident and emitted electromagnetic radiation.
Fugacity#: The effective partial pressure of a gas, taking into account non-ideality (i.e., inter molecular interactions). Often fugacities are given in bar (\(\mathrm{10^5}\) Pa), and with a reference state chosen at 1 bar, are numerically equivalent to activity (where the relationship between activity and fugacity is \(a=f/f_0\), where f_0 is the reference state).
Habitable#: The property of being able to host life. Typically thought of in the context of the presence of liquid water, given the needs of life on Earth, but in principle a concept that could be generalised if more chemically physically tolerant life is discovered.
Habitability#: See habitable.
Habitable zone#: That region around a star where climatic conditions could in principle be maintained on a rocky planet such that liquid water is stable at its surface.
Heterotroph#: An organism that runs its metabolic processes using input, e.g., reduced organic carbon, from other organisms. Contrast with autotroph.
Hill radius#: The radial distance from a planet within which its own gravitational force is dominant, e.g., in affecting the orbit of pebble-sized material passing near it. Defined as \(R_H = a(m/(M+m))^{1/3}\), where \(a\) is the semi-major axis of the planet, \(M\) is the mass of the star, and \(m\) is the mass of the planet.
Hill sphere#: The 3D region described by a planet’s Hill radius.
Hot Jupiter#: A planet of roughly Jupiter’s mass (~300 Earth masses) that is highly irradiated by its host star, and consequently has a high equilibrium temperature. These planets typically have orbital periods \(<\)10 days.
Ice#: In the context of planets, ‘ice’ is used to refer to molecular components such as water (\(\sf H_2O\)), ammonia (\(\sf NH_3\)), and methane (\(\sf CH_4\)), i.e., species heavier than \(\sf H_2\), the dominant gas-phase constituent of protoplanetary disks. These species, which might be gas in the inner hot regions of a protoplanetary disk, will condense beyond their respective ice lines, forming part of the solid building blocks of planets.
Ice line#: The radial location in a protoplanetary disk where a gas phase species condenses into a solid phase, an ‘ice’. E.g., the water ice line in the solar system is thought to have been located around the orbit of Jupiter.
Igneous#: A rock that formed by solidification of a magma.
Instellation#: The amount of light received by a planet from its host star. Units of \(\text{Wm}^{-2}\).
Last Universal Common Ancestor#: LUCA refers to the most recent common ancestor of all current life forms on Earth. It is not the first life form but represents the final organism from which all known life evolved. LUCA is thought to have existed around 3.5-4 billion years ago and likely had a relatively complex cellular structure, including genetic material (RNA or DNA), metabolic pathways, and the ability to replicate. It serves as a key reference point in understanding the evolution of life and the shared molecular features of living organisms today.
Magma#: A molten silicate liquid, possibly containing crystals and gas bubbles.
Magma ocean#: When there is an extensive and sustained body of magma at a planet’s surface. Such as is created from a giant impact, from the heat of accretion early in a planet’s life, or may be sustained permanently on irradiated exoplanets.
Main sequence#: That period of a star’s life during which it is fusing H in its core into heavier elements. The main sequence lasts for different lengths of time according to stellar mass, with more massive stars fusing H faster and so having shorter lives. For reference, the main sequence lifetime of our Sun will be approximately 10 billion years.
Mantle#: The rocky interior to a planet beneath its crust, typically comprising most of the non-metal mass of a rocky planet.
Mantle plume#: An buoyant upwelling of rock withing a planet’s mantle, often triggering volcanism at the surface. The buoyancy is typically driven by the upwelling being hotter than the surrounding mantle. Such thermal upwellings originate at thermal boundary layers in the planet, e.g., at the core mantle boundary.
Metal#: This is one of those words that means different things to different communities. For astrophysicists, this can mean ‘elements heavier than He’, i.e., most of the periodic table. For Earth and Planetary Scientists, ‘metal’ refers to material properties of matter, meaning a material like iron or iron alloys (Earth’s core being a notable example), with high thermal and electrical conductivity.
Metallicity#: For astronomers, the metallicity of a star, refers to the metal content of that star. Iron is the element normally used to trace metallicity, with the ratio of iron to hydrogen in the star, relative to solar, used to describe the metallicity, [Fe/H]
Metamorphism#: The transformed of a rock by pressure and/or temperature into a new mineral assemblage, with possible textural modification as well, e.g., the generation of banding or shearing.
Metamorphosed#: See metamorphism.
Nucleosynthesis#: The formation of elements in either primordial ‘big-bang’ nucleosynthesis (producing hydrogen, helium and lithium), or stellar nucleosynthesis. The latter is now recognised to include a range of processes from nuclear fusion in stellar cores, nuetron capture in stellar envelopes, and neutron star merger events.
Mid-ocean ridge#: The topographic expression of two tectonic plates pulling apart
Partial melting#: The process in any multi-component system whereby the solid reacts to form a liquid of one composition and a residual solid of another composition. Added together, liquid and solid have the same composition as the whole system, by mass conservation. The distinct physical chemical properties of the liquid and solid, however, allow differentiation to operate to produce, for example, compositionally distinct planetary crusts.
Partial pressure#: In a mixed gas, the partial pressure is the pressure that a an individual gas in the mixture would have on its own, if it were the only gas filling the volume. For an ideal gas, i.e., one where intermolecular forces can be ignored, then the total pressure \(p\), is equal to the sum of the partial pressures, and the partial pressure of gas \(i\) is given by \(p_i = x_i \cdot p\), by the product of the mole fraction of the molecule in the mixed gas with the total pressure.
Pebble accretion#: A regime of planetary accretion in which aerodynamic drag enhances the rate of solid mass gain by a growing planet. The need for aerodynamic interaction between the accreting material and the gas of the protoplanetary disk means that this accretion mechanism is only efficient for solids with a radius that moderately couples them to the flow of the gas (i.e., ‘pebbles’): larger objects feel insufficient drag, whereas very small material is coupled to the gas and swept past the growing planet.
Pebble isolation mass#: The mass a planet reaches at which point it can no longer accrete pebbles from the environment around it, having already accreted those in its immediate vicinity.
Pentose Sugar#: A pentose sugar is a type of sugar that contains five carbon atoms. These sugars are important building blocks in nucleic acids (DNA and RNA) and other biological molecules. The two most common pentose sugars in biology are ribose and deoxyribose.
Phase#: A chemically uniform and physically distinct region of matter. For example, a rain drop is a distinct phase compared with the air it falls through; an ice cube is a distinct phase from the water it floats on; and in a granite, each type of mineral present, quartz, feldspare etc. are distinct phases.
Planetesimal#: An object held together under its own self-gravity, of order kilometers to hundreds of kilometers in size. Can be differentiated, or not.
Plate tectonics#: The movement of rigid plates across the surface of a planet, with deformation localised to their margins (e.g., subduction zones and mid-ocean ridges).
Protoplanetary disk#: The disk-shaped annulus of gas and dust surrounding a young star out of which planets form. The ratio of gas to dust in protoplanetary disks is roughly 100:1 by mass, and their lifetimes are of order 10 million years.
Radial drift#: The movement towards or away from the star of solid material in a protoplanetary disk. This movement occurs due to the rotation of the gas in the protoplanetary disk being generally slower than the orbital velocity required to avoid falling inwards (due to the gas being supported by pressure as well as its orbital speed). The result is for solids to drift inwards as they experience a headwind trying to move through the slow moving gas.
Reducing#: A substance that has a propensity to donate electrons to another material, thereby ‘reducing’ it. E.g., in placing iron metal, Fe, next to water, H\(_2\)O, the iron metal can reduce the water by the reaction \(\text{Fe} + \text{H}_2\text{O} = \text{H}_2 + \text{FeO}\).
Regioselectivity#: Regioselectivity refers to the preference of a chemical reaction to occur at one specific position or site on a molecule when multiple reactive sites are available. In essence, it describes the tendency of the reaction to produce a particular isomer or product at a preferred location on the molecule.
Retrosynthetic analysis#: Retrosynthetic analysis is a problem-solving technique used in organic chemistry to design a synthetic pathway for complex molecules. It involves working backward from the target molecule to simpler starting materials through a series of logical steps. This method focuses on breaking down the target compound into smaller, more manageable precursor structures, called synthons, which can be derived from commercially available or easily synthesized compounds.
Ribozyme#: Ribozymes (ribonucleic acid enzymes) are RNA molecules that can catalyse specific biochemical reactions, similar to the action of protein enzymes.
Semi-major axis#: The longest radius of an ellipse and mean distance of an object from its primary, e.g., a planet from its star, or satellite from its planet.
Siderophile#: An element with an affinity for moving into the metal phase. E.g., Fe, Ni, Pt.
Silicate#: A mineral whose major structural unit comprises Si-O bonds. E.g., quartz, or olivine, the latter being a major constituent of shallow planetary mantles.
Short lived radionuclide#: A radioisotope that decays sufficiently rapidly that, as of today, no natural occurrence of the isotope exists, i.e., it is ‘extinct’. Notable examples include \(\mathrm{^{26}Al}\), which drove heating and differentiation of planetesimals in the early solar system.
Stereoselectivity#: Stereoselectivity refers to the preference of a chemical reaction to produce one stereoisomer over others when multiple stereoisomers are possible. Stereoisomers have the same molecular formula and atom connectivity but differ in the spatial arrangement of atoms or groups.
Subduction#: The process whereby a tectonic plate is pushed into the interior of a planet. Only known to have happened on Earth.
TCA cycle#: The TCA cycle, also known as the Krebs cycle or Citric Acid cycle, is a key metabolic pathway that plays a central role in cellular respiration, the process by which cells extract energy from organic molecules. The TCA cycle is a series of chemical reactions that generate energy through the oxidation of acetyl-CoA (a product of carbohydrates, fats, and proteins) into carbon dioxide (CO₂) and high-energy electrons that are transferred to carrier molecules (NADH and FADH₂). These electrons are later used in the electron transport chain to produce ATP (adenosine triphosphate), the primary energy currency of cells.
rTCA cycle#: The rTCA cycle (or reverse TCA cycle) is a biochemical pathway that is essentially the reverse of the conventional TCA cycle. The reverse TCA cycle is a series of reactions that essentially “reverses” the steps of the TCA cycle, utilizing CO₂ as a carbon source to synthesize key metabolites like acetyl-CoA. This cycle is energy-consuming and is a part of carbon fixation pathways in certain organisms, meaning it is used to convert inorganic carbon (CO₂) into organic compounds that can be used for growth and metabolism.
Tectonic plate#: A rigid outer layer to a planet bounded by regions where it is being created (mid-ocean ridge), subducting, or slipping past another plate. Only known to operate on Earth.
Unconformity#: A gap in time.