Necrosignature

A type of technosignature indicating the former existence of a civilization which has since collapsed. Necrosignatures typically appear as signs of a civilization growing and expanding, such as industrial gases building up in the atmosphere of its homeworld, followed by an event (either abrupt, such as a surge of radioisotopes, or gradual, like planetary warming) and no apparent further activity. This usually signals the civilization in question encountered an existential threat which it could not overcome. Further remote observation can be undertaken in an attempt to discover the reason for its demise, as well as whether a new civilization is likely to have risen by the time a relativistic mission arrives.

Necrosignatures are most commonly seen on the homeworlds of species which failed to develop spacefight and colonization, since once this is done it is unlikely any single event will be able to wipe the entire civilization out.