Nova, Radial fracture center

A raised volcanic structure with troughs spreading out like the rays of a star.

The story of this structure’s name is as exciting as its geology. Everyone in astronomy knows novae: these are exploding stars. The word means “new” in Latin. A faint star suddenly brightens and appears in the sky as a new star. The rayed star shape that we often draw has nothing to do with the Latin word. So “new” has no direct link to the shape of a star.

Soviet researchers first proposed the name astrum, meaning “star”, for these features. The International Astronomical Union accepted Astrum as a descriptor term for naming them. Yet in the past twenty years not one such name has been approved. Instead, these structures are named as a mountain (Mons), a corona (Corona) or a dome (Tholus). Some take their name from a linear fracture system (Linea). A name may then read, for example, Bécuma Mons astrum (see the plate). To avoid Latin names with mixed metaphors, some researchers call these structures “radial fracture centres”.

In relief, novae can rise like cones or be flat. Some have subsided, and others rise like plateaus. Almost half of them, however, lie inside a round rim: they are corona-novae (Krassilnikov and Head 2003).

The fractures may radiate from an “unmarked” centre or from a fissure (vent). In some novae the diverging fractures have no geometric centre at all. A nova may consist of one fracture system or of several visibly different ones.

They are 50–300 km across. Raised novae represent the first stage of nova evolution (for example, Bécuma Mons). Later, circular features form around the nova: a raised ring or a trough. The relief of flat novae has slowly levelled out. Subsided novae already resemble collapsed calderas.

They typically lie near large deformation zones and trough systems. They are also found on plains, near tesserae and in volcanic areas.

Similar structures on Earth (and Mars) are giant dyke swarms below the surface. These never develop into coronae. On Earth they are instead early signs that a continent is breaking apart.