Planum

Planum (plural plana, from the Latin neuter noun for "plateau" or "high level ground") is an official descriptor term in International Astronomical Union (IAU) planetary nomenclature used to designate an elevated plateau or high-standing flat plain situated above the reference equipotential datum of a planet or satellite. Serving as the high-elevation counterpart to a low-lying planitia and the broad regional equivalent of a smaller flat-topped mesa (mensa), plana span three major planetary geological settings across the Solar System: (1) continental-scale tectonic and volcanic plateaus on Venus, epitomized by Lakshmi Planum in Ishtar Terra; (2) thick Hesperian flood-basalt plateaus and polar layered ice caps on Mars (Lunae Planum, Solis Planum, Syrtis Major Planum, Meridiani Planum, Planum Boreum, and Planum Australe); and (3) smooth volcanic or cryovolcanic high plains on Mercury (Borealis Planum), Io (Danube Planum), and Triton (Cipango Planum). In planetary geophysics and astrobiology, plana preserve multi-kilometer stratigraphies of flood volcanism, lithospheric thrust faulting along wrinkle ridges (dorsa), Noachian–Hesperian aqueous hematite and sulfate evaporites, and orbital Milankovitch climate records locked inside polar water- and carbon-dioxide-ice sheets.

The archetype of a tectonically bounded planetary plateau is Lakshmi Planum (68.6° N, 339.3° E) in western Ishtar Terra on Venus (Roberts & Head, 1990; Kaula et al., 1992):

• Topographic Elevation and Geometry: Covering roughly 1.8 × 10⁶ km² (twice the area of Earth's Tibetan Plateau relative to planetary surface area), Lakshmi Planum is a smooth, polygonal volcanic plateau perched 3.0 to 4.5 km above the mean Venusian datum (6,051.84 km radius).
• Surrounding Fold-and-Thrust Mountain Belts (Montes): Unlike any other plateau in the Solar System outside Earth, Lakshmi Planum is ringed on all sides by linear compressional mountain chains (orogenic belts) where crustal shortening thickened the Venusian crust to 50–80 km: Akna Montes (rising 4 km above the plateau on the west), Freyja Montes (6.5 km high on the north), Danu Montes (on the south), and Maxwell Montes (the highest mountain range on Venus, reaching 10.7 km above datum on the east). Radar-bright ribbon-and-fold tessera blocks outcrop as basement rafts within the plateau and along its outer slopes (Fortuna Tessera and Itzpapalotl Tessera).
• Internal Caldera Volcanism and Geodynamic Origin: The smooth interior of Lakshmi Planum is resurfaced by flood basalts erupted from two large volcanotectonic calderas—Sacajawea Patera (233 × 140 km) and Colette Patera (149 × 100 km). Geophysicists debate two end-member models for Lakshmi Planum: a convergent mantle downwelling model, in which cold sinking mantle currents dragged buoyant crustal blocks together to fold the peripheral mountain belts around a rigid central craton, versus a mantle plume upwelling model, in which a thermal diapir uplifted and underplated the plateau while gravity-sliding and peripheral compression buckled its margins.

On Mars, more than two dozen named plana stand at elevations of +1.0 to +6.0 km above the MOLA areoid datum, falling into three geological provinces:

1. Circum-Tharsis Hesperian Wrinkle-Ridged Plateaus

Surrounding the eastern and southern flanks of the Tharsis volcanic rise is a radial staircase of elevated flood-basalt tablelands—Lunae Planum, Syria Planum, Sinai Planum, Solis Planum, Thaumasia Planum, and Daedalia Planum—bounded on the north and east by the canyons of Valles Marineris and Martian outflow channels such as Kasei Valles. Erupted during the Early Hesperian Period (~3.7–3.5 Ga) as stacked sheets of low-viscosity basaltic lava 1 to 4 km thick, these plana were subsequently compressed by Tharsis lithospheric flexure and global cooling into spectacular, regularly spaced concentric belts of wrinkle ridges (Lunae Dorsa, Solis Dorsa).

2. Syrtis Major Planum and Meridiani Planum

• Syrtis Major Planum (8.4° N, 69.5° E; ~1,300 km wide): First drawn by Christiaan Huygens in 1659 as the earliest recognized albedo feature on another planet, Syrtis Major was long thought to be a lowland sea or plain until MOLA laser altimetry revealed that it is an elevated Hesperian low-relief shield volcano and flood-basalt plateau rising to +1 to +2 km around the central calderas Nili Patera and Meroe Patera, surfaced by wind-swept dark olivine- and pyroxene-rich basaltic sands (undae).
• Meridiani Planum (0.2° S, 357.5° E): Located atop the western margin of Arabia Terra, Meridiani Planum was explored from 2004 to 2018 by NASA's Opportunity Mars Exploration Rover. Beneath a surface lag of crystalline gray hematite (α-Fe₂O₃) concretions ("blueberries"), Opportunity documented the Burns Formation: a Late Noachian / Early Hesperian sequence of cross-bedded aeolian and interdune playa sandstones cemented by 20% to 40% magnesium, calcium, and iron sulfates (including the acidic sulfate mineral jarosite, KFe₃(SO₄)₂(OH)₆), proving that shallow, fluctuating acidic groundwater repeatedly evaporated across the plateau (Squyres et al., 2004; Grotzinger et al., 2005).

At both poles of Mars, the IAU applies the descriptor planum to the kilometer-thick domes of the Polar Layered Deposits (PLD): Planum Boreum at the north pole and Planum Australe at the south pole (Byrne, 2009; Phillips et al., 2011):

• Planum Boreum (North Polar Plateau): A 1,000-km-wide, 3.0-km-thick dome of finely laminated water ice (H₂O > 95%) and silicate dust (~5% by volume), containing ~1.14 × 10⁶ km³ of water ice (equivalent to a global water layer ~8 meters deep) resting atop a basal sand-rich unit (Rupes Tenuis and Olympia Planum) that supplies the surrounding circumpolar dune sea (Olympia Undae). Planum Boreum is carved by a clockwise pinwheel of katabatic-wind and sublimation spiral troughs (Chasma Boreale, 500 km long and 2 km deep) that expose hundreds of alternating ice-and-dust layers driven by Mars's 51,000-year axial precession and 120,000-year obliquity cycles over the past 4 to 5 million years.
• Planum Australe (South Polar Plateau): A 1,200-km-wide, 3.7-km-thick water-ice and dust plateau (~1.6 × 10⁶ km³) capped by a bright seasonal and permanent veneer of solid carbon dioxide (CO₂ dry ice). Mars Reconnaissance Orbiter SHARAD radar sounding discovered that buried beneath the residual cap of Planum Australe lie three distinct beds of pure CO₂ ice totaling ~9,500 to 12,500 km³—enough frozen carbon dioxide to double Mars's present atmospheric surface pressure (from 6 mbar to ~12 mbar) whenever high axial obliquity (>35°) vaporizes the polar deposit into the atmosphere.

Beyond Venus and Mars, high-standing plana record distinct tectonic and cryovolcanic processes across the Solar System:

• Borealis Planum on Mercury: Covers the northern polar lowland/plateau province mapped by MESSENGER, consisting of smooth volcanic flood basalts erupted ~3.7–3.8 Ga that buried ancient impact craters to depths of >1–2 km and were subsequently deformed by high-relief wrinkle ridges and lobate scarps.
• Tectonic Plana on Io (Danube Planum, Dodona Planum, Ethiopia Planum): Flat-topped crustal blocks uplifted 2 to 5.5 km along high-angle reverse faults triggered by global subsiding-shell compression, frequently surrounded by basal debris aprons and volcanic paterae.
• Cipango Planum and Medamothi Planum on Triton: Broad, flat-topped cryovolcanic tablelands resurfaced by viscous sheets of water–ammonia (H₂O–NH₃) ice emerging from central vents such as Leviathan Patera.

How does a planum differ from a mensa and a planitia?

All three terms describe flat-topped or planar terrain in IAU nomenclature, differentiated by scale and elevation. A planitia is a regional low-lying plain below the planetary datum (such as Utopia Planitia). A planum is a broad, regional elevated plateau standing above datum (hundreds to >1,500 km across, such as Lakshmi Planum or Lunae Planum). A mensa is a much smaller, steep-walled, flat-topped mesa or tableland (typically 10 to 100 km across) isolated by erosion along cliff scarps, such as Cydonia Mensae on Mars.

Why is Lakshmi Planum on Venus surrounded by tall mountain ranges?

Lakshmi Planum is a buoyant, continental-scale crustal block standing 3 to 4 km above the mean surface of Venus. Whether driven by a converging mantle downwelling that pulled surrounding crust inward against a rigid plateau core or by peripheral compression around a rising mantle plume head, horizontal tectonic forces crumpled the borders of Lakshmi Planum into folded and thrust-faulted mountain chains (Akna, Freyja, and Maxwell Montes) rising up to 10.7 km high.

What do the ice layers inside Mars's Planum Boreum and Planum Australe tell scientists?

Just like ice cores drilled in Greenland and Antarctica on Earth, the thousands of alternating light (ice-rich) and dark (dust-rich) layers exposed inside the spiral troughs of Planum Boreum and Planum Australe act as a high-resolution tape recorder of Martian climate over the past several million years—tracking how cyclical swings in Mars's axial tilt (obliquity) shifted water ice and CO₂ between the poles and mid-latitude glaciers.

1. Roberts, K. M., & Head, J. W. (1990). "Lakshmi Planum, Venus: Characteristics and models of origin." Earth, Moon, and Planets, 50(1), 193–249. https://doi.org/10.1007/BF00142395
2. Squyres, S. W., et al. (2004). "In Situ Evidence for an Ancient Aqueous Environment at Meridiani Planum, Mars." Science, 306(5702), 1709–1714. https://doi.org/10.1126/science.1104559
3. Grotzinger, J. P., et al. (2005). "Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars." Earth and Planetary Science Letters, 240(1), 11–72. https://doi.org/10.1016/j.epsl.2005.09.039
4. Byrne, S. (2009). "The Polar Deposits of Mars." Annual Review of Earth and Planetary Sciences, 37(1), 535–560. https://doi.org/10.1146/annurev.earth.031208.100101
5. Phillips, R. J., Davis, B. J., Tanaka, K. L., Byrne, S., Mellon, M. T., Putzig, N. E., et al. (2011). "Massive CO₂ Ice Deposits Sequestered in the South Polar Layered Deposits of Mars." Science, 332(6031), 838–841. https://doi.org/10.1126/science.1203091