Planet Four Talk

What does that mean? – a Glossary

  • wassock by wassock moderator

    Here’s a few of the terms you’ll find being used in the discussions going on here, if you come across a term that you think should be in this list please let one of the moderators know.


    The proportion of the incident light or radiation reflected by a surface, typically that of a planet or moon. Dark surfaces have low albedo; bright surfaces have high albedo. There are a number of recognised 'albedo features' on Mars. These dark areas are plainly seen in many images. Eg

    Aphelion - The point in a planets orbit when it is furthest from the Sun; for Mars this is 249 million km. Planets are at perihelion when nearest to the Sun; for Mars this is 207 million km.

    Araneiform - The technical name for 'spiders'.

    Cryptic Region

    There is a region of the South Pole denoted "Cryptic" which is markedly colder than the rest of the polar region. It's vaguely between 150 and 310 degrees W. Doesn't seem to be a consensus on why, best bet may be that it is a thick layer of CO2 covered by a thick layer of dust which stops it melting in summer.

    This from mschwamb:
    A lot of fan and spot activity has been associated with the cryptic terrain.

    The cryptic terrain in Piqueux et al. 2003 was described as:

    "These regions of the seasonal CO2 ice caps exhibit low albedo, characteristic of the underlying regolith material, despite retention of substantial CO2 ice covering, as evidenced by the observed brightness temperatures. It has been hypothesized that the simplest explanation for this ‘‘cryptic’’ terrain is the development in these regions of large-grained ‘‘slab’’ ice, which is transparent to visible light [Kieffer et al., 2000]. Alternatively, these dark and cold regions may be due to contamination of the seasonal ice with dust."

    Hansen et al. 2010 - describe the cryptic region as:

    Cryptic terrain (a large region near the south pole of Mars termed ‘‘cryptic” because even though its albedo darkens in the spring it retains the 145 K temperature of CO2 ice until late in the season) which contains a host of phenomena that are inferred to arise from local gas jet activity (Kieffer, 2000; Christensen et al., 2005; Kieffer et al., 2006). Inca City is outside the cryptic region however it also shows evidence of gas jet activity in the form of fan-shaped deposits and morphology of the eroded surface.

    Glitch - Term for the image processing or camera artifacts in some of the images, typically a very bright coloured dot or a line.

    HiRise - High Resolution Imaging Science Experiment. The HiRise website hosts the full size images from which the ones you look at here are cropped.

    Ice - In these pages Ice will normally mean solid carbon dioxide (dry ice). If someone is talking about water ice they will normally say so to make the distinction.

    Inca City

    A region of Mars around 82°S, 67°W, originally named by the Mariner 9 team in 1972. It features a series of ridges which bear some similarity to the foundations of a large building.

    enter image description here

    Katabatic Wind - A strong wing which normally blows in a single direction and is usually caused by very cold air effectively falling from a region of high altitude down the slope. On earth this type of winds are common in the Antarctic, blowing from the high level plains inland out towards the coast and the same sort of effect occurs at the south pole on Mars as well

    Perihelion - The point in a planets orbit when it is nearest to the Sun; for Mars this is 207 million km. Planets are at aphelion when furthest from the Sun; for Mars this is 249 million km.

    Regolith – The top layer of particulate material, usually meaning the surface immediately beneath the ice layer.

    Solar Longitude (Ls) - This is used to measure time within a Mars year. Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, 270° at winter solstice. Full details here:

    Spider – A network of interconnected channels on the surface of Mars, typically in polar regions subject to annual ice cover and retreat. There are a number of distinct types of spider around and a system for classifying them may be part of a future project.

    Sublimation – The process of a solid turning directly to gas. This term is normally used to refer to the thawing of CO2 ice which goes directly from ice to gas without passing through a liquid phase. On Earth ice normally melts to become water which then evaporates to water vapour (gas). But if conditions are right Ice can undergo sublimation without turning to water first – think of a patch of snow which gradually disappears without leaving a wet patch or trail of water.

    Yardang – A surface feature which appears as a series of roughly parallel ridges, thought to be formed by the effect of strong prevailing winds. Similar features are seen in some deserts on Earth. Also referred to as ‘tramlines’