Planitia

Planitia (plural planitiae, from the Latin word for "flatness" or "low plain") is an official descriptor term in International Astronomical Union (IAU) planetary nomenclature used to designate a broad, low-lying plain or smooth basin floor situated below the mean reference elevation (equipotential datum) of a planet, satellite, or dwarf planet. Contrasted directly with a planum (an elevated plateau or high plain standing above datum) and a vastitas (a circum-polar lowland plain of global extent, used only for Vastitas Borealis on Mars), planitiae encompass some of the largest resurfacing provinces in the Solar System: the giant impact basins and northern lowlands of Mars (Utopia Planitia, Acidalia Planitia, Isidis Planitia, Chryse Planitia, and Elysium Planitia), the regional basaltic lowlands covering ~70% of Venus (Atalanta Planitia, Guinevere Planitia, Lavinia Planitia), the flood-basalt floor of Mercury's Caloris Planitia, and the actively convecting nitrogen-ice glacier basin Sputnik Planitia on Pluto. Because planetary lowlands serve as the terminal gravitational sinks for surface drainage, groundwater migration, volcanic flood lavas (fluctus), and volatile condensation, planitiae represent primary targets for landing spacecraft and astrobiology exploration.

On Earth, "lowlands" and "highlands" are measured relative to mean sea level. Because rocky planets and icy moons lack global liquid surface oceans, planetary geodesists define a reference equipotential surface (geoid, areoid, or aphroditoid) or mean planetary radius to distinguish a low-lying planitia from a high-standing planum:

• Mars (The Areoid): Originally defined during the Mariner 9 and Viking missions as the elevation where atmospheric carbon dioxide (CO₂) pressure equals the triple-point pressure of water (610.5 Pa or 6.105 mbar at 273.16 K), the Martian datum was redefined in 2001 using Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) and gravity-field tracking as the equipotential gravitational + rotational surface whose mean equatorial radius is 3,396.19 km. Martian planitiae sit at elevations of −2.0 to −5.2 km below this datum, whereas Martian plana (such as Lunae Planum and Solis Planum) stand at +1.5 to +5.0 km above datum.
• Venus (Planetary Mean Radius): From Magellan synthetic aperture radar altimetry, the Venusian datum is fixed at the planet's mean spherical radius of 6,051.84 km. Broad rolling and lowland plains lying between −2.0 and +0.5 km relative to this radius are designated planitiae (named after mythological heroines).
• Mercury and Pluto: On Mercury (MESSENGER MLA datum = 2,439.4 km) and Pluto (New Horizons stereo datum = 1,188.3 km), planitiae correspond to deep impact basins (Caloris Planitia, Sputnik Planitia) depressed 2 to 4 km below the surrounding cratered uplands.

Nearly all major planitiae on Mars lie north of or straddle the global Martian dichotomy boundary, where crustal thickness thins to 10 to 38 km (compared to 58–72 km beneath the southern highlands) and where Hesperian Martian outflow channels (valles) discharged floodwaters and sediment (Tanaka et al., 2014):

1. Utopia Planitia (46.7° N, 117.5° E; Diameter ~3,300 km): The largest recognized circular impact basin inside the northern lowlands, excavated during the Pre-Noachian eon (>4.1 Ga) and accompanied by a prominent positive gravity anomaly (mascon). Explored at the surface by NASA's Viking 2 lander (1976) and CNSA's Zhurong rover (2021–2022), and probed from orbit by the Mars Reconnaissance Orbiter SHARAD radar, western Utopia Planitia (between 39° N and 49° N) hosts a subsurface stepped mesa and scalloped-depression permafrost deposit containing 50% to 85% pure water ice (H₂O) mixed with dust, 80 to 170 meters thick—storing a water volume equivalent to Lake Superior (~12,000 km³) beneath just 1 to 10 meters of dry regolith (Stuurman et al., 2016).
2. Acidalia Planitia and Chryse Planitia (30°–50° N, 320°–350° E): Chryse Planitia is a 1,600-km circular lowland embayment into which Mars's largest Hesperian outflow channels—Kasei Valles, Maja Valles, Tiu Valles, Simud Valles, and Ares Vallis—debouched from southern chaos terrains (landed on by Viking 1 in 1976 and Mars Pathfinder in 1997). Chryse drains directly northward into Acidalia Planitia, which hosts fields of kilometer-scale pitted cones interpreted as sedimentary mud volcanoes driven by rapid dewatering of buried flood sediments, alongside fields of remnant knobs (colles).
3. Isidis Planitia (12.9° N, 87.0° E; Diameter ~1,500 km): A ~3.9 Ga impact basin interrupting the dichotomy boundary west of Syrtis Major Planum; its western margin contains Jezero Crater, where NASA's Perseverance rover is sampling a Noachian river-delta deposit for ancient microbial biosignatures.
4. Elysium Planitia and Amazonis Planitia: The youngest lowland volcanic plains on Mars, surfaced by Late Amazonian flood basalts (10 to 500 Ma) erupted from the Cerberus Fossae fissures. Where these lavas flowed over ice-rich substrates, steam explosions constructed swarms of rootless volcanic cones (colles) and rafted platy-ridged lava pack-ice. NASA's InSight geophysical lander operated on western Elysium Planitia from 2018 to 2022, measuring Martian seismicity and crustal thickness.

On Venus, low-lying planitiae (such as Atalanta Planitia, Lavinia Planitia, Helen Planitia, Guinevere Planitia, and Sedna Planitia) cover more than two-thirds of the planetary surface. Magellan radar imagery and Soviet Venera 9, 10, 13, and 14 X-ray fluorescence landers proved that these plains consist of tholeiitic and alkaline flood basalts emplaced over the past 300 to 750 million years (Basilevsky & Head, 1998).

Venusian planitiae are pervaded by anastomosing networks of compressional wrinkle ridges (dorsa) spaced 10 to 30 km apart, broader compressional ridge belts (Laverna Dorsa), pancake domes, coronae, and lobate lava aprons (fluctus). Geodynamic modeling of gravity and topography shows that deep circular basins like Atalanta Planitia (2 km below datum) and Lavinia Planitia sit directly above cold mantle downwellings, where sinking lithospheric drips pull the surface downward into a regional basin and compress the overlying basalt sheets into ridge belts.

In July 2015, NASA's New Horizons spacecraft revealed one of the most dynamic planitiae in the Solar System: Sputnik Planitia, the bright western lobe of Pluto's heart-shaped feature Tombaugh Regio (McKinnon et al., 2016; Trowbridge et al., 2016):

• Impact Basin Geometry and True Polar Wander: Sputnik Planitia occupies an elliptical, 1,050-by-800-km ancient impact basin depressed 3 to 4 km below Pluto's water-ice bedrock highlands. Because this deep topographic cold trap condensed a 3-to-10-km-thick load of dense solid molecular nitrogen ice (N₂) mixed with carbon monoxide (CO) and methane (CH₄), plus an underlying uplifted lens of liquid water–ammonia ocean, the positive mass anomaly reoriented Pluto's rotational axis via true polar wander until Sputnik Planitia locked onto the anti-Charon tidal axis near the equator.
• Polygonal Rayleigh-Bénard Convection Cells: Completely devoid of detectable impact craters (setting a maximum surface age of <10 million years), the surface of Sputnik Planitia is divided into dozens of convex polygonal and ovoid cells 16 to 40 km across, bounded by shallow troughs 50 to 100 meters deep containing dark floating blocks of water-ice crust (Al-Idrisi Montes). At Pluto's surface temperature (~37 K) and modest radiogenic heat flux (~3 mW m⁻²), solid β-N₂ ice has a low viscosity (~10¹⁵ to 10¹⁷ Pa·s) and undergoes active Rayleigh-Bénard thermal convection, overturning the nitrogen glacier at centimeters per year and sublimating pits that release N₂ into Pluto's tenuous atmosphere.

What is the difference between a planitia and a planum?

Both terms designate broad, flat plains in IAU planetary nomenclature, but they are separated by elevation relative to the planet's reference datum (planetary "sea level"). A planitia is a low-lying plain or basin situated below the reference datum (such as Utopia Planitia at −4.5 km on Mars), whereas a planum is a high plateau or elevated tableland standing above the reference datum (such as Syria Planum at +5 km on Mars or Lakshmi Planum at +3.5 km on Venus).

Why are there no impact craters on Pluto's Sputnik Planitia?

Unlike Pluto's ancient water-ice highlands (which are heavily cratered over 4 billion years), Sputnik Planitia is filled with a 4-km-thick glacier of mechanically soft, crystalline nitrogen ice (N₂). Driven by gentle heat escaping from Pluto's rocky core, this solid nitrogen slowly boils in a pattern of 30-km-wide Rayleigh-Bénard convection cells that rise in the center and sink along the edges at several centimeters per year, completely erasing any new impact craters in less than 500,000 to 10 million years.

Why do most Mars landers and rovers land in a planitia?

To slow down safely from hypersonic orbital entry (~20,000 km h⁻¹), Mars spacecraft rely on atmospheric drag against an entry heat shield and a supersonic parachute before firing landing retrorockets. Because the Martian atmosphere is thin (~6 mbar at datum), landing inside a deep planitia (such as Utopia, Chryse, Isidis, or Elysium Planitia, 2 to 5 km below datum) provides several extra kilometers of denser atmospheric column for aerodynamic braking, as well as flat, boulder-sparse sedimentary or volcanic plains.

1. Tanaka, K. L., et al. (2014). Geologic Map of Mars. U.S. Geological Survey Scientific Investigations Map 3292. https://doi.org/10.3133/sim3292
2. Stuurman, C. M., Osinski, G. R., Holt, J. W., Levy, J. S., Brothers, T. C., Kerrigan, M., & Campbell, B. A. (2016). "SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars." Geophysical Research Letters, 43(18), 9484–9491. https://doi.org/10.1002/2016GL070138
3. McKinnon, W. B., Nimmo, F., Wong, T., Schenk, P. M., White, O. L., Roberts, J. H., et al. (2016). "Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour." Nature, 534(7605), 82–85. https://doi.org/10.1038/nature18289
4. Trowbridge, A. J., Melosh, H. J., Steckloff, J. K., & Freed, A. M. (2016). "Vigorous convection as the explanation for Pluto's polygonal terrain." Nature, 534(7605), 79–81. https://doi.org/10.1038/nature18016
5. Basilevsky, A. T., & Head, J. W. (1998). "The geologic history of Venus: A stratigraphic view." Journal of Geophysical Research: Planets, 103(E4), 8531–8544. https://doi.org/10.1029/98JE00487