Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 232

These dykes are considered to be impact melts that are driven into the expand-
ing cavity as high-velocity turbulent flows of superheated low-viscosity silicate
liquid that penetrate downwards into fissures and cracks at the bottom of the
crater.
Despite the variety of forms of evidence that can be used to identify impact
structures it can still be difficult to identify these features when they are of
great age and havebeensubjecttoextensive erosion and post-impact deforma-
tions. The three main diagnostic criteria for identification (historical record of an
impact event; meteorite fragments or contaminated meteorite material in rocks
such as enriched siderophile element abundances; and shock metamorphic fea-
tures such as shatter cones or planar deformation features or solid state or
fusion glasses) may not always be immediately obvious. In such cases, other cri-
teria such as circular gravity and magnetic anomalies, impact-produced igneous
melts or breccia dykes, circular geochemical anomalies, and linkage between
distal ejecta layers to specific structures may need to be found.
Barnouin-Jha and Schultz (1998)discovered that ejecta lobateness resulting
from instabilities created in an atmospheric vortex ring are a useful diagnostic
criterion. The atmospheric instabilities or waves are generated by the advancing
ejecta curtain and the number of ejecta lobes on the ground increases with the
size of the advancing vortex size scaled to its core radius and the square root of
the Reynoldsnumber(description of the degree of turbulence) of the flow in the
vortex ring. The vortex ring forms behind the impermeable portion of the ejecta
curtain, as it passes through the atmosphere, by flow separation and rapidly
begins to entrain ejecta out of the curtain. As the distance between the vortex
ring and the curtain increases, the vortex ring becomes unstable, creating waves.
These waves force the vortex to evolve from a laminar to a turbulent regime (as
described by the Reynolds number). The vortex ring then drops towards the
target surface before it continues to scour and entrain ejecta as it advances
along the ground surface. Finally, the vortex ring disintegrates into multiple
lobes of ground-hugging flows of ejecta debris resembling a turbidity flow. There
is good correlation between observations of ejecta lobateness and theoretical
calculations, and observations of lobes on Mars and Venus.
Twoofthe main factors that play a part in the preservation of prehistoric
impact craters and events are the magnitude of the event and the degree of
consequent deformation, recrystallisation and melting. Impact models for aster-
oids with diameters larger than 10 km, especially where they impacted on thin
and hot Archaean (the most ancient of Earth's rocks) crust, suggest that the
thermal and magmatic effects of a large magnitude impact can be expected
to obliterate the proximal impact deformation signals through recrystallisa-
tion, shearing and melting. This means that shock deformations and melting