# Glossary

Amazonian
  A geological period in Mars’ history, from 2.9 billion years to the present day.

Axial tilt
  The angle between a planet’s rotational axis and its orbital plane (north pole).

Clathrates
  A chemical substance into which molecules of a different substance can become trapped. They usually have a cage or lattice structure.

Cross bedding
  Layering seen in beds of sedimentary rocks that are at an angle to the horizontal. They can represent ripples or dunes.

Crystallisation
  The ordering of atoms or molecules into a defined crystal structure. In geology, represent the formation of crystals on cooling from an igneous melt.

Deltaic
  An area with a delta, usually at the mouth of a river.

Dessication
  The process of drying, i.e., the loss of water from a substance.

Discontinuity
  In geology, a disruption to sedimentary beds caused by a change in physical or chemical conditions.

Dissociate
  Dissociation is the process in which molecules of a substance separate or split, usually when a solvent.

Ecliptic
  The plane of the Earth's orbit about the Sun. Also the path of the Sun in the sky (on the celestial sphere) during the course of a year.

Electromagnetic radiation
  Energy in the form of waves that have both electrical and magnetic properties.

Erosion
  The process by which soil and rock particles are broken down and moved elsewhere by wind, water or ice.

Gas chromatography-mass spectrometer
  A scientific instrument that can separate and identify molecules within a substance.

Geological record
  Rock strata laid down over geological time.

Geomorphology
  The study of the physical features on a planet’s surface and their relation to its geological history.

Hesperian
  A geological period in Mars’ history spanning from 3.7 to 2.9 billion years ago.

Hydrogen bond
  A weak bond between two molecules that results from the attraction of a hydrogen within a module to an atom in a neighbouring molecule because of their electron charge.

Hydrothermal
  The presence and action of hot water. Could refer to hot water systems on Earth or Mars.

Igneous
  Rocks formed by the cooling and crystallisation of molten magma or lava.

Ions
  Positive or negative forms of atoms in which electrons have been gained or lost from the atomic structure.

Laser altimeter
  An instrument on board an orbiting spacecraft that is used to investigate the topography of a planet’s surface, using laser pulses.

Meteorites
  Rocks ejected from a Solar System object (e.g. Mars or asteroids) because of an impact event, that are delivered to the Earth’s surface.

Noachian
  A geological period in Mars’ history spanning from 4.1 to 3.7 billion years ago.

Organic compounds
  Molecules composed predominantly of carbon and hydrogen.

Pixel
  A small dot or square that makes up an image. The more pixels there are, the better the resolution of the image.

Polar
  A term applied to molecules where there is a charge difference between its constituent atoms.

Polarity
  The relative degree to which molecules are polar.

Rotation period
  The time it takes for a planet or other object to take a single revolution around its axis of rotation. For Earth, this is 23.93 hours.

Scanning electron microscope
  A powerful microscope that uses electrons to image objects at high resolution. Can also be used to determine chemical composition of objects.

Sedimentary
  Rocks formed by the transport and deposition of sediments produced by erosion and weathering of pre-existing rocks, or the precipitation and accumulation of substances from solution.

Sol
  A sol is a martian day, 24 hours 39 minutes in length.

Solvent
  A liquid that is able to dissolve substances.

Spectroscope
  An instrument for measuring the spectrum of electromagnetic radiation.

Sublimation
  The phase change of a substance directly from solid to gas, for example from water ice to water vapour.

Suspension
  A fluid mixture containing fine solid particles.

Wavelength
  The distance between one part of a wave profile and the next identical part of a wave profile.

Weathered
  To have been subjected to weathering, the process of breaking down rocks through the action of rainwater, extremes of temperature, and biological activity. It does not involve the removal of rock material.

OpenLearn - The search for water on Mars
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