Corona (Planetary Geology)

Corona (plural coronae, from the Latin word for "crown") is a major class of circular-to-elliptical volcano-tectonic landform in planetary geology defined by a prominent concentric annulus of tectonic ridges, graben, and faults surrounding an interior topographic plateau, dome, or depression. First resolved as ovoid ring structures (ovoidy) on Venus in 1983 by the Soviet Venera 15 and Venera 16 radar orbiters and imaged in 1986 on Uranus's icy satellite Miranda by NASA's Voyager 2 spacecraft, coronae are the surface expression of buoyant mantle or cryospheric diapirs (plumes) impinging against the base of a stagnant planetary lithosphere. Over 515 to 540 coronae have been cataloged on Venus from NASA's Magellan synthetic aperture radar and altimetry dataset, spanning 60 km to 2,600 km in diameter at Artemis Corona—the largest corona in the Solar System. Within astrobiology and comparative planetology, coronae demonstrate how Venus releases internal radiogenic and secular heat through episodic plume-lithosphere interactions and localized plume-induced subduction in the absence of global Earth-style plate tectonics, while Miranda's icy coronae record tidal heating and solid-state ice diapirism on outer Solar System ocean worlds.

Diagnostic Structural Architecture

Unlike impact craters—which are excavated rapidly by hypervelocity bolides and display sharp, overturned ejecta rims with hummocky floors—Venusian coronae evolve over tens of millions of years through multi-stage magmatic and tectonic deformation beneath a 9.2 MPa, 737 K carbon dioxide (CO₂) atmosphere. As standardized by Ellen R. Stofan, Gerald G. Schaber, Daniel M. Janes, and Suzanne E. Smrekar (1992–1997), a mature Venusian corona consists of four concentric morphological elements:

1. The Interior Core (Dome, Plateau, Rim-Only, or Depression): The central region inside the tectonic ring displays diverse topography—ranging from a broad volcanic dome rising 1 to 2.5 km above the surrounding plains (e.g., Aramaiti Corona), a flat-topped plateau, a complex saddle, or an interior basin depressed 0.5 to 2.0 km below the regional datum. The interior typically hosts abundant volcanic edifices, including small shield fields, radar-bright lobate lava flows, and steep-sided pancake domes.
2. The Concentric Tectonic Annulus (10–150 km Wide): The defining crown of the corona is an elevated topographic rim or ring zone densely packed with tens to hundreds of closely spaced concentric tectonic structures—including normal-fault graben, thrust-fault ridges, and transpressional folds—often accompanied by older radial fractures similar to those of smaller arachnoids.
3. The Peripheral Moat (Flexural Trench): Approximately 60% of Venusian coronae are partially or completely encircled outside their elevated rim by an arcuate, crescent-shaped topographic trough or moat 0.5 to 3.5 km deep and 20 to 100 km wide (such as the deep peripheral trenches of Latona Corona, Quetzalpetlatl Corona, and Artemis Corona).
4. The Outer Flexural Rise (Forebulge): Seaward of the peripheral moat, the elastic lithosphere flexes upward under the weight of the corona rim or downward pull of a delaminating slab to form a broad, low-amplitude outer topographic arch (100–300 m high) analogous to the outer trench swell of terrestrial oceanic subduction zones.

Topographic and Tectonic Subclasses

Based on Magellan radar clinometry and radar altimetry profiles across more than 500 structures, Stofan et al. (1992, 2001) classified Venusian coronae into two primary sequence groups:

• Type 1 Coronae (~86% of catalog): Structures surrounded by a continuous (>180° arc) concentric tectonic annulus, further subdivided into Concentric (~52%), Concentric-Double-Ring (~11%), Radial-Concentric (~9%, transitional toward novae and arachnoids), Asymmetric (~10%), and Multiple coalesced coronae (~4%).
• Type 2 ("Stealth") Coronae (~14% of catalog): Topographic rim-and-moat rings identified in Magellan altimetry whose concentric fracture annulus occupies <180° of arc because extensive post-tectonic basaltic lava flooding has buried portions of the faulted ring.

Spatially, Venusian coronae have a mean diameter of ~210–250 km and are non-randomly distributed: roughly 68% occur along linear extensional rift belts (chasmata)—such as the Parga Chasma and Hecate Chasma chains, where scores of coronae are strung like beads along lithospheric extension zones—while ~21% reside on large volcano-tectonic rises (such as Beta, Atla, and Themis Regiones) and ~11% occur as isolated features in lowland plains alongside arachnoids and ancient tessera blocks.

In classic geodynamic models pioneered by Ellen R. Stofan, Daniel M. Janes, Steven W. Squyres, and Anna Gülcher (1991–2020), a Venusian corona evolves through three sequential thermo-mechanical stages as a hot, buoyant mantle diapir (a spherical or mushroom-shaped thermal plume head 50 to 300 km in radius, ΔT ≈ 150–300 K hotter than the ambient mantle) rises toward the base of the stagnant Venusian lithosphere:

Stage 1: Diapir Ascent, Dynamic Uplift, and Radial Fracturing

As the buoyant mantle diapir ascends through the upper mantle, normal dynamic stresses above the plume head push the overlying crust and lithosphere upward into a broad, circular topographic dome (0.5 to 2.0 km high) accompanied by pressure-release partial melting of peridotite that erupts flood basalts across the summit. During this early doming phase, radial tensile hoop stresses fracture the brittle upper crust into a starburst of radial graben and dikes ( structurally resembling a nova or early arachnoid).

Stage 2: Lithospheric Impingement, Flattening, and Trench Delamination

When the rising diapir hits the strong mechanical boundary layer at the base of the elastic lithosphere (or encounters the neutral buoyancy horizon at the crust–mantle Moho), it can no longer rise vertically and instead spreads laterally outward like a pancake. This lateral viscous spreading transforms the central dome into a flat-topped plateau while pushing a ring of compressed, thickened crust outward to build the elevated concentric rim and outer flexural moat.

As demonstrated in three-dimensional thermo-mechanical numerical simulations by Anna Gülcher, Taras Gerya, and colleagues (2020), when a hot mantle plume penetrates into a warm, ductile Venusian lithosphere, the dense, cold lower-lithospheric rim detached around the plume perimeter becomes gravitationally unstable (ρ_lith > ρ_{asthenosphere}). The cold lithospheric ring peels downward (delaminates) or initiates short-lived, radially outward-retreating plume-induced subduction (roll-back), pulling down the arcuate peripheral trench (such as Artemis Chasma) and flexing the outer forebulge.

Stage 3: Thermal Cooling, Gravitational Relaxation, and Rim Collapse

Over 20 to 100 Myr, the flattened diapir loses heat by conduction into the surrounding lithosphere and solidifies, removing both thermal buoyancy and dynamic upward stress. Deprived of deep buoyant support, the central plateau subsides below the surrounding plains to form an interior depression bounded by inward-dipping concentric normal faults (graben), while viscous gravitational relaxation slumps the elevated rim. Using Magellan Doppler gravity tracking and contemporary tectonic strain analysis, Gülcher et al. (2020) identified at least 37 large coronae on Venus (including Artemis, Quetzalpetlatl, and Latona) that exhibit positive free-air gravity anomalies, high apparent depths of compensation (40–150 km), and unrelaxed trench-and-outer-rise topography indicative of presently active mantle plume upwelling. Furthermore, in 2023 Robert R. Herrick and Scott Hensley discovered direct morphological changes in a 2.2 km volcanic vent on Maat Mons (adjacent to corona-chasma rift systems) between Magellan SAR orbits separated by eight months in 1991, confirming ongoing volcanism on Venus.

Outside of Venus, the only other planetary body with officially named coronae is Miranda, the innermost and smallest of the five major spherical moons of Uranus (mean radius R = 235.8 km, bulk density ρ = 1.20 g cm⁻³, composed of ~60% water ice H₂O and ~40% silicate rock). During its January 1986 flyby, Voyager 2 imaged Miranda's southern hemisphere at sub-kilometer resolution, revealing three giant trapezoidal-to-ovoid coronae—Inverness Corona (~234 km across, containing the bright chevron), Arden Corona (~318 km across), and Elsinore Corona (~323 km across)—cutting across ancient cratered terrain.

Rather than silicate magma plumes, Miranda's coronae were formed by solid-state cryospheric diapirism when Miranda was temporarily captured into a 3:1 mean-motion orbital eccentricity resonance with Umbriel (or a 5:3 resonance with Ariel). Tidal dissipation heated Miranda's deep interior, causing warm, lower-density Ice I_h diapirs (potentially doped with ammonia NH₃ or methanol antifreeze above a transient subsurface ocean explored in outer Solar System ocean worlds by future cryobot concepts) to rise and spread beneath the brittle cryogenic ice crust, generating concentric extensional tilt-blocks and normal-fault scarps up to 10 km high.

1. Stofan, E. R., Sharpton, V. L., Schubert, G., Bindschadler, D. L., Janes, D. M., Squyres, S. W., & Stefanick, M. (1992). "Global distribution and characteristics of coronae and related features on Venus: Implications for origin and relation to mantle processes." Journal of Geophysical Research: Planets, 97(E8), 13347–13378. https://doi.org/10.1029/92JE01314
2. Smrekar, S. E., & Stofan, E. R. (1997). "Corona Formation and Heat Loss on Venus by Coupled Upwelling and Delamination." Science, 277(5330), 1289–1294. https://doi.org/10.1126/science.277.5330.1289
3. Pappalardo, R. T., Reynolds, S. J., & Greeley, R. (1997). "Extensional tilt blocks on Miranda: Evidence for upwelling and resulting diapiric stress." Journal of Geophysical Research: Planets, 102(E6), 13369–13379. https://doi.org/10.1029/97JE00802
4. Gülcher, A. J. P., Gerya, T. V., Montési, L. G. J., & Munch, J. (2020). "Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus." Nature Geoscience, 13(8), 547–554. https://doi.org/10.1038/s41561-020-0606-1