Hotspot Venus
The surface of the earth-like planet Venus features structures that may have been formed by mechanisms that are also found inside the Earth, as simulations conducted by ETH-Zurich professor Taras Gerya reveal.
The surface of Venus is an inhospitable place. It is hot (over 400 degrees Celsius) and the air pressure in its hostile atmosphere is almost 100 times higher than on Earth. Due to its bowl-like structure primarily made of solid components, however, it is one of the four planets in our solar system that are most like Earth and thus of interest to science. Although Venus seems geologically “dead” nowadays, it is littered with scar-like, ring-shaped structures that bear testimony to a turbulent history and cannot be found on Earth. These unique structures are referred to as novae and coronae.
Two structures of the same origin?
Both structures, as astro and geophysicists suspect, stem from a period when Venus was geologically active and are supposed to have formed through volcanic and related tectonic processes. There is speculation that the entities were formed through an interplay between the rigid crust and convection currents in the mantle that transport hot rock material to the surface and carry cooled rock back down. However, it is unclear whether their formation is interconnected and one structure emerges from the other or they develop completely independently.
Based on computer simulations, Taras Gerya, a professor of geophysics at ETH Zurich, has now managed to recreate the striking surface structures for the first time and thus provide a plausible explanation for their formation. The results of his simulations have just been published in the journal Earth and Planetary Science Letters.
In his thermomechanical model, he simulated defined processes inside Venus for the first time in 3D. The simulations suggest that novae form first and coronae can develop from them over millions of years. Consequently, the geophysicist concludes that both structures may well stem from the same origin.
Evolution of a ring structure
So far, sixty-four novae measuring 100 to 300 kilometres in diameter have been identified on Venus. The ring-shaped structures in Venus’ crust are interspersed with star-shaped fracture zones that are formed through as yet unknown tectonic and magmatic processes.
Structural analyses have already revealed that novae can also form structures that are similar to those of coronae. In contrast to novae, these 513 structures are sometimes extremely complex: their outermost ring is raised. A trench separates them from another ring-shaped ridge, which leads into a depression before the central area of the structure stands out. Like the Artemis Corona, coronae can be up to 2,600 kilometres in diameter and contain a large number of small volcanoes.
According to Gerya, previous, exclusively two-dimensional models for the reconstruction of the entities originated from a lithosphere (crust and areas of the upper mantle) that was too cold, thick and rigid. However, the latest studies suggest that Venus’ lithosphere is relatively warm, thin and ductile, as the researcher writes. Moreover, such a lithosphere was recently discovered on Venus among seemingly active hotspots – volcanoes that are fed by hot mantle material. The hot rock material wells up in form of a large “mushroom” to the surface of the planet from the depths of the mantle. As long as it does not break the surface, it is referred as “mantle plume”. If plume-generated magma penetrates the crust, a hotspot forms where lava erupts.
Simulating“mantle plumes”
Gerya now factored the thermal changes and viscosity in the mantle and crust into his model based on the assumption that Venus’ mantle plumes are 30 to 100 kilometres in diameter and penetrate a thin, very warm lithosphere. The simulations reveal that the partly molten plume initially causes the crust to bulge. The plume-generated hot magma then flows upwards and a large, rising magma region forms in the crust with an internal convection current. A giant nova mountain with the typical star-shaped cracks forms above this hot region.
The simulation photos also reveal that a nova structure can develop into a corona over millions of years if molten rock material is able to rise to the surface of Venus from the magma reservoir. One of the simulations even resembles the Aramaiti Corona. However, Gerya also notes that the simulated structure is around three times smaller than the original.
If a nova transforms into a corona in the model, the rim of the nova mountain breaks inwards concentrically while a wedge of partly molten crustal rock from the nova’s interior is continuously obducted outward over the Venus surface through the convection current under the nova’s centre. An outer ring of concentric normal faults within down bending crust and an inner ring with concentric thrust faults within the obducting wedge form, along with a trench between these two rings.
“The small and medium-sized novae and coronae simulated particularly bear striking similarities to those we can observe on Venus,” says Gerya. However, the model is not without its limitations: it cannot generate all the structures observed in nature. Consequently, although it is highly plausible that novae and coronae are caused by mantle plumes, the ETH-Zurich professor cannot rule out other formation mechanisms.
Lack of water goes against plate tectonics
Whether novae or coronae are still formed on Venus today remains a mystery. According to Gerya, however, the models reveal that large nova mountains are relatively short-lived structures that can only exist above active crustal magma regions. There is currently some evidence of active hotspot volcanic activity, such as on Idunn Mons, which is similar to a nova mountain. Nor can the possibility that certain plate-tectonic processes once took place on Venus in a similar way to on Earth be ruled out. Whether this is still the case today seems unlikely as water plays a key role in the formation and recycling of crust through subduction processes and nowadays there is not a single drop at all on the entire 400-degree-celsius surface of Venus.
Further reading
Gerya T: Plume-induced crustal convection: 3D thermomechanical model and implications for the origin of novae and coronae on Venus, Earth and Planetary Science Letters, published online 20th Febr 2014, DOI: external page 10.1016/j.epsl.2014.02.005external page