Appearance
Venus's Giant Eye: How Low-Angle Faults Reveal a Planet's Volcanic Past
Coronae are among the strangest structures in the solar system: roughly circular features, hundreds to thousands of kilometres across, each ringed by a fractured annulus of ridges around a sunken middle. They are unique to Venus, and what makes them has been argued about for thirty years.
This study looked at one of the largest, Atahensik Corona, an eye-shaped structure 700 by 900 kilometres across in southeastern Aphrodite Terra. The result is a sequence in which the same faults move one way and then reverse, which turns out to settle an argument rather than add to it.
Working with what Venus allows
Venus is difficult to study for an unglamorous reason: the surface is at 450 °C under crushing pressure, so a long-lived lander is not currently realistic. Almost everything structural we know comes from orbital radar, mostly NASA's Magellan mission in the early 1990s. The resolution is far below what we have for Mars, and it is enough to establish geometry and cross-cutting relationships if you are careful about what you claim.
Atahensik is about the size of Texas and has four things worth naming: deep arcuate troughs curving around the north and south, reaching 2,400 metres below the surrounding terrain; elevated fractured ridges forming the rim, over 6,600 metres high; broad outer rises swelling beyond the troughs across more than 2,000 kilometres; and an interior that has subsided. It sits in a chain of coronae and connects to Dali Chasma, a major linear trough system running for hundreds of kilometres.
Faults that reversed
The central observation is a set of large fault zones along the steep inner slopes of the arcuate troughs. They run for hundreds of kilometres, they dip gently toward the corona centre at 22° to 42° rather than steeply, and in places the fault surface itself is partly exposed as terrace-like features in the radar.
That exposure is the evidence, and it is worth being precise about why. A thrust fault only shows its intersection with the surface, and even that is usually buried under its own debris. You do not get to see the plane. But if a thrust is later reactivated as a normal fault and the hanging wall slides back down, the smooth underside of the old fault plane is uncovered. That is what appears in the images.
So these faults began as thrusts, under compression, which fits the rest of the evidence: asymmetric troughs, outer rises and gravity signatures all describe a lithosphere being pushed together and bent downward, much like a subduction zone in its earliest stage. They then reversed, becoming low-angle normal faults as the hanging wall slid back down and peeled the plane open.
The fracture sequence
Mapping the fractures across the corona gives a chronology. Radial fractures came first, radiating from the centre as a plume rose and domed the crust, with magma-filled dikes driving outward. Oblique fractures followed, spiralling slightly, which may record the plume shifting position. Concentric fractures came last, parallel to the crest of the outer rise, opening as the lithosphere flexed downward. Some of those have widened into small grabens, which is the standard signature of extension on the upper surface of a bending plate and is exactly what you find at ocean trenches on Earth.
Two smaller coronae give the sequence
Atahensik on its own is one snapshot. Set beside two smaller coronae in eastern Eistla Regio, Didilia at 400 by 450 kilometres and Pavlova at 550 by 650, it becomes a stage in something.
Neither of the smaller two has Atahensik's deep encircling troughs, and both have the same elevated, heavily fractured central volcanic edifice inside a fractured ridge annulus. The difference that matters is vertical: in Didilia and Pavlova the central volcano stands higher than the rim. At Atahensik the centre has dropped below it. Reading the three together gives an evolutionary sequence rather than three unrelated structures.
The four stages
Uplift and radial fracturing. A mantle plume pushes into the lithosphere and domes the crust. Radial fractures open across the swell as dikes drive toward the surface.
Volcanic construction and lateral spreading. An edifice builds, fed by a spreading magmatic reservoir, and its weight begins to load thermally weakened lithosphere. The plume spreads sideways at depth and the outer slopes steepen. This is the stage Didilia and Pavlova are in.
Overthrusting and trough formation. The spreading plume reaches cooler, intact lithosphere at its margins. Hot weak material cannot push cold strong material aside, so the fractured ridge annulus thrusts outward over it along low-angle thrusts instead. That loading bends the cooler lithosphere down into the deep arcuate troughs, and the plate flexes back up beyond them to form the outer rises, cracking concentrically at the crest. The rim stands high because the crust there is doubled where it has been thrust over intact lithosphere.
Subsidence and reactivation. Plume activity wanes, the interior cools and loses its thermal support, and the horizontal compression that drove the overthrusting fades. The elevated annulus is now perched with nothing holding it up, and the old thrusts reactivate as low-angle normal faults, letting the hanging wall slide back down and inward and exposing the fault planes as terraces.
Dali Chasma does the same thing
The same low-angle faults, with the same evidence of reactivation, turn up along Dali Chasma, a relatively straight trough northwest of Atahensik. Dali has usually been read as a simple extensional rift. If the faults there have the same history, it more likely began as a trench with flexural bending and overthrusting and was pulled apart afterwards, with the extension reusing the existing faults.
The friction problem
There is a mechanical objection that has to be taken seriously. A thrust dipping around 30° should not reactivate as a normal fault. With a typical rock friction coefficient near 0.75 it locks up long before it can slip the other way. Something has to weaken it, and the candidates are a severely weakened fault zone from grain-size reduction and thermally activated crystal plasticity during prolonged faulting, frictional melting producing a thin lubricating film along the plane, or elevated fluid pressure from volatiles at depth, which Venus lacks at the surface but may retain below it.
On Earth the same reactivation happens where clays or mylonite zones weaken a fault, or where fluid pressure effectively floats the overburden. Which of these operates on Venus is not resolved by this work.
Why the reversal settles an argument
Venusian chasmata and circumferential troughs have been interpreted three ways: as extensional rifts, as trenches formed by compression and incipient subduction, or as vertical adjustments over mantle upwelling and downwelling with little horizontal motion.
A multi-stage history means these are not competing answers. The structures formed under compression, through overthrusting and flexural bending, and were then reworked by extension as the interior collapsed. Both readings are correct about different chapters. The same thing happened in the Scandinavian Caledonides, where a mountain belt collapsed under extension and drove its thrusts to reactivate as low-angle normal faults.
What it implies about Venus
Radial fracturing and the central edifices put focused mantle upwelling at the origin of coronae, which was already the leading view and is strengthened here.
The thrusts, asymmetric troughs, outer rises and gravity pattern together support plume-induced incipient subduction, proposed by Sandwell and Schubert in the 1990s and demonstrated experimentally by Davaille and colleagues. Wherever hot weak plume material meets cold strong lithosphere, that boundary becomes a compressional zone: the corona material thrusts outward, the lithosphere bends down, and you get a trench.
It also means coronae are not static. They pass through stages over tens of millions of years, and the topographic classes that have been catalogued are probably snapshots at different points in one life cycle rather than different kinds of object.
More broadly, this is a planet shedding heat without plate tectonics. Venus appears to run in something between Earth's mobile plates and a genuinely stagnant lid, and building and collapsing coronae may be one of its main ways of moving heat from the interior to the surface.
What would test it
This work is at the limit of what Magellan can support. Two missions should change that: ESA's EnVision with the VenSAR radar, and NASA's VERITAS with VISAR, both planned for the 2030s and both capable of resolutions comparable to what Mars already has.
The specific things worth looking for: striations or grooves on the exposed fault planes, which would confirm the kinematics directly rather than by inference; other large coronae with the same reactivation, which would show whether this is general or particular to Atahensik; the depth at which these faults root; and smaller coronae caught at earlier stages, which would test the evolutionary sequence rather than assume it.
The paper. Kenkmann, T., Karagoz, O., and Veitengruber, A. (2024). Structural analysis and evolution of large Venusian coronae: Insights from low-angle faults at coronae rims. Planetary and Space Science. doi.org/10.1016/j.pss.2024.105955
The related work on what weakens these faults enough to move is Venus's Molten Fault Lines.