Coronae on Venus.
THE CORONAE OF VENUS
 
Coronae are unique to Venus. Coronae (corona is Latin for crown) are large geological structures that appear to have resulted from a combination of volcanic and tectonic processes. The exact way in which they have formed is still the cause of much debate among planetary scientists, but understanding how coronae work may be the key to understanding how Venus has evolved.
 
Most coronae are circular in plan but some are elliptical or pear-shaped. They usually consist of a topographic rim which encircles a low interior, commonly below the level of the surrounding plains. They range in size from 70 kilometres across to 2000 kilometres, though most are in the 200 kilometres to 400 kilometres size range. The huge corona Artemis, to the south of Aphrodite Terra, is exceptional.
 

Atete Corona


Artemis Corona


Corona chain in Parga Chasma
 
Artemis Corona
 
They generally have fractures radiating outwards and those with intense radial fractures were nicknamed "arachnoids" because the fracture pattern looked like spiders and cobwebs. Most large coronae have a ring of intense concentric fractures on their rim.
 

Close-up of fractures in Artemis Corona
 
Coronae usually have a great deal of volcanic activity taking place on or around them. Clusters of domes or cones are often seen on the rim or basins of coronae, sometimes with larger volcanoes. The most common volcanic features are the lava flows which frequently occur on the rim where the surface has ruptured.
 

Shield volcanoes in Boann Corona
Aine Corona
Mantle plume
 
Many ideas have been put forward to explain how coronae form. The most widely accepted explanation is that they are formed by a blob or plume of hot material rising through the mantle. The hot material doesn't break through the crust but pushes it up. When it eventually cools, the surface subsides, leaving behind a basin encircled by a ridge. Plumes allow Venus to lose heat by transporting heat from the hot mantle to the surface.