Mars glossary

Every term you will meet while exploring Mars, in plain English. They come to life on the interactive Mars globe, where the terminator, the season and the orbiters are all live.

Mars itself

Terminator: The moving line between Martian day and night. A point on the equator travels the planet's 21,300 km circumference in one sol, so the line sweeps past at roughly 865 km per hour. Craters and dune fields look deepest near it, where the Sun grazes the surface sideways. It is the line this globe draws, in real time, from the actual positions of Mars and the Sun.
Areoid: Sea level on a planet with no sea. The areoid is the gravitational equipotential surface derived from Mars Orbiter Laser Altimeter data, and it does the job Earth's geoid does: every Martian elevation is quoted against it. Curiosity landed at -4,501 m on the areoid and has since climbed about 995 m above that point; Perseverance landed at -2,570 m and climbed about 800 m out of Jezero.
Areographic coordinates: Latitude and longitude on Mars, areo- for Ares as geo- is for Earth. The modern IAU standard is planetocentric latitude with east-positive longitude, which is what every pin on this globe uses. Older publications used planetographic latitude and west longitude, which is why one site turns up with two very different numbers: Viking 1 is 22.273 N, 312.054 E in the modern convention and 22.483 N, 47.94 W in the old one.
Airy-0: The crater that fixes zero longitude on Mars: a small one on the floor of the crater Airy, in Sinus Meridiani, picked out of Mariner 9 imagery by Merton Davies in 1972 and formalised as Airy-0 in 2003. Its size is not agreed even inside the IAU gazetteer, which publishes 0.50 km against the point geometry and 0.79 km against the mapped outline. The longitude system as a whole is pinned only to about 250 m on the surface, which is why Martian mean solar time is known to a few seconds and no better.
Regolith: The layer of broken rock and fine dust covering Mars. It is not soil in the biological sense: Phoenix's wet chemistry laboratory found perchlorate salts at 0.4 to 0.6 per cent of the sample by mass, which makes the ground chemically reactive and destroys organic molecules in a hot oven, reopening the argument about what Viking's biology experiments actually measured. The same dust is what the storms lift, and what blacked out Opportunity's solar panels for good in 2018.
Phobos and Deimos: The two moons, both small, dark and lumpy. Phobos is about 22 km across and laps Mars in 7 h 39 m, faster than the planet turns, so from the ground it rises in the west and sets in the east; Deimos is about 12.5 km across and takes 30 h 18 m. Both keep one face towards Mars, and both reflect about 6 per cent of the light that reaches them, roughly as dark as fresh asphalt. Nothing has ever landed on either, and where they came from is still an open question.
Sample depot: Sealed sample tubes left on the ground for a later mission to collect. Mars has exactly one, at Three Forks on the floor of Jezero crater, where Perseverance laid out ten titanium tubes between 21 December 2022 and 28 January 2023, spaced 5 to 15 m apart in a zigzag so a small helicopter could reach each of them. It was the first sample depot built on another world. All ten are still lying there: Mars Sample Return was defunded in January 2026 and no mission is scheduled to fetch them.

Time and seasons

Sol: One Martian solar day, local noon to local noon: 24 hours 39 minutes 35.244 seconds. It has its own word so that nobody writes "day" and leaves the reader guessing which planet is meant. It is not the same as one turn of Mars against the stars, which takes 24 h 37 m 22.66 s, because the planet also moves about half a degree along its orbit each day and has to turn that little bit further to bring the Sun back to the meridian. A team working on Mars time starts 39 minutes later every day and walks right around the Earth clock in about five weeks.
Ls (solar longitude): The Martian calendar is an angle. Ls is the direction of Mars from the Sun, measured from the northern spring equinox: Ls 0 is that equinox, Ls 90 the northern summer solstice, Ls 180 the northern autumn equinox and Ls 270 the northern winter solstice. Papers about Mars date their observations in Ls rather than in months, because Ls says what the season is in any year and a month name would not.
Mars Year: The numbering scheme for Martian years, proposed by Clancy and colleagues in 2000, counting from the northern spring equinox of 11 April 1955. The zero point is arbitrary; its convenience is that the great global dust storm of 1956 falls inside MY 1. Piqueux and colleagues extended the count in 2015, back to MY -184 in 1607 and forward to MY 100 in 2141. Mars Year 39 begins on 30 September 2026 and runs to 17 August 2028.
Airy Mean Time and Coordinated Mars Time: Mean solar time on the Martian prime meridian is called Airy Mean Time, and it is the closest thing Mars has to a world clock. The name Coordinated Mars Time circulates for a civil scale built on it, but no body has defined or adopted one, and there are no Martian time zones. So every place keeps its own local mean solar time, which is Airy Mean Time plus one Mars hour for every 15 degrees of east longitude: Curiosity at Gale crater runs about four Mars hours ahead of Perseverance at Jezero, and neither clock means anything at the other site.
Mars Sol Date: A running count of sols with no mission attached, the Martian equivalent of the Julian Date. Sol zero falls on 29 December 1873, an epoch chosen because Earth and Mars mean solar midnights very nearly coincide there. Mission sol numbers cannot be compared with each other, because Viking, Phoenix, Curiosity, InSight and Perseverance call landing day Sol 0 while Pathfinder, Spirit and Opportunity call it Sol 1. Anything that has to line up across missions and decades uses the Mars Sol Date instead.
Perihelion, aphelion and dust storm season: Mars's orbit is a long way from circular: an eccentricity of 0.0935 against Earth's 0.0167, swinging from 206.65 million km from the Sun at perihelion to 249.26 million at aphelion, so the planet gets about 45 per cent more sunlight at the near end. Perihelion falls at Ls 251, in southern spring, and that is when the dust rises. Regional storms happen every year; roughly once every three or four Mars years one of them wraps the planet. Eight planet-encircling storms have been confirmed since 1956, the most recent in 2018.

Orbits and tracking

Areocentric orbit (periapsis and apoapsis): An orbit around Mars, with its low and high points called periapsis and apoapsis, the Mars equivalents of perigee and apogee. The shape tells you what the spacecraft is for. Mars Reconnaissance Orbiter flies a tight 255 by 320 km loop to photograph the ground at about 30 cm per pixel, while the UAE's Hope sits on a 20,000 by 43,000 km ellipse, more than two sols a lap, far enough out to watch a whole hemisphere of weather at once.
Sun-synchronous orbit: A near-polar orbit whose plane keeps pace with Mars's march around the Sun, so the spacecraft crosses a given latitude at roughly the same local solar time on every pass. The lighting is then comparable from one pass to the next, and from one year to the next, which is what makes long-term change detectable at all. MRO's science orbit is sun-synchronous, 255 by 320 km and 112 minutes a lap, with periapsis over the south pole; Odyssey's local mean solar time was frozen near 5 p.m. by a manoeuvre.
Aerobraking: Using the top of the atmosphere as a brake. A spacecraft captures into a long ellipse, then dips its lowest point into the air and lets drag shave a little energy off every orbit, for months. Mars Odyssey made 332 drag passes in 77 days, cutting an 18.6-hour orbit to about 1.8 hours; NASA Langley records that the propellant saved cut more than 200 kg from the spacecraft, which is what let it fly on a Delta II class rocket at all. ExoMars TGO made 952 atmospheric passes between March 2017 and February 2018, worth about 1.02 km/s.
Entry, descent and landing (EDL): The few minutes between the top of the atmosphere and the ground, about seven of them on the American landers the phrase "seven minutes of terror" was coined for. Mars is the awkward case: the air is thick enough to burn a spacecraft and too thin to stop it, so every lander has needed a heat shield, then a parachute, then rockets or airbags, in that order. Nobody on Earth can intervene. One-way light time was 13.8 minutes for Curiosity and 11 minutes 22 seconds for Perseverance, so a Mars landing is always watched as a recording.
Relay: Rovers hardly ever talk to Earth directly. Curiosity's direct link home runs between about 500 bits and 32 kilobits per second, while its UHF hop to an orbiter a few hundred kilometres overhead reaches up to 2 megabits per second, so 99.9 per cent of Perseverance's surface science goes home through an orbiter. NASA's Mars Relay Network lists four: ExoMars Trace Gas Orbiter, Mars Reconnaissance Orbiter and Mars Odyssey carrying the traffic, with Mars Express held as backup. Together they move roughly 3.2 gigabits a day, which is everything Mars sends home.

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