Arachnoid

Arachnoids are the members of the volcano-tectonic family whose concentric rings lie inside and whose radial troughs or ridges lie outside. In corona-novae the arrangement is exactly the reverse.

The term was coined by the scientists of the Soviet Venera probes (Barsukov et al. 1986). The radial fractures around the ring look like a spider, hence the name arachnoid, “spider-like”. The fractures also join up and in places link arachnoids into chains. This network may even be compared with a spider’s web, with the arachnoids sitting at its nodes (see the figure).

Arachnoids are smaller than 200 km; their average diameter is 129 km. The fractures are narrower than 10 km but may reach 100 km in length. The radial lineaments may be extensional troughs or compressional ridges (wrinkle ridges). Some arachnoids have a raised centre, others a sunken one. Lava flows and even small shield volcanoes may occur inside them. Volcanism is not typical of their surroundings, however. So they are formed mainly by magmatic processes below the surface.

The radial fractures are directly connected to the concentric rim. In coronae, by contrast, there is no structural link between the nova and the rim (Kostama and Törmänen 2007).

Arachnoids form above local magma upwellings. They may differ from coronae in the depth that the mantle flow reaches. In places the lineaments are several ring diameters long. So they certainly do not form from the magma upwelling itself. More likely, they form as dykes advance.

According to Crumpler and Aubele (1999), Venus has 265 arachnoids. There are four large clusters of them, on lower-lying plains.

They have no descriptor term of their own. Individual arachnoids are named as a Corona or a Patera.