CRATERING MECHANICS ON VENUSPRESSURE ENHANCEMENT BY THE ATMOSPHERIC OCEAN
Geophysical Research Letters, 19 (21):
2115--2118 (1992)Аннотация
The mass per unit area of the Venusian atmosphere is approximately
10(6) kg/m2, equivalent to a 1-km-deep water ocean. An important
effect of this öcean" on cratering is pressure enhancement due to
multiple reverberations during the compression stage. We consider
impactors with diameters, D, less than a scale height (approximately
15 km). Such impactors drive before them bow shocks with standoff
distances of DELTA approximately 0.05 D into the lowermost atmosphere.
Reflection of this shock at normal incidence from the basalt surface
raises the pressure in the atmosphere and surface, but initially
not as much as a direct impact would. Several shock reverberations
should occur, however, as the shock crossing time in the trapped,
compressed atmospheric layer is much less than either the shock crossing
time in the impactor or the time for the impactor to penetrate the
atmospheric layer. With each reverberation we estimate new Hugoniots
for shocked atmosphere, impactor, and target. The first is treated
as an ideal gas of gamma = 1.2-1.3, and the latter are modeled with
Murnaghan equations of state and Gruneisen GAMMAs is-proportional-to
rho-1. The asymptotic pressure enhancement for a 20 km/s carbonaceous
chondrite impactor (modeled as serpentine) striking basalt at 20
km/s, relative to the atmosphereless 1-D impedance match pressure
(approximately 550 GPa), is a factor of approximately 1.4. This maximum
enhancement factor increases with velocity, reaching 1.6 for serpentine
moving at typical Venus-impacting cometary velocities (approximately
65 km/s), and is greater for more compressible impactors (comets).
Each reverberation sends a shock pulse into the surface or impactor
that overtakes the previous ones, hence at the end of the compression
stage both impactor and target are shocked to a higher pressure than
would occur in the absence of the atmosphere. The impactor is effectively
denser or stiffer than in the atmosphereless case, but as the energy
and momentum delivered are essentially the same, cratering efficiency
should be unaffected to first order. However, fusion and vaporization
efficiency should be enhanced for impactors whose impact velocities
are below (whether initially or due to atmospheric drag) the threshold
for complete melting or vaporization, respectively, in the absence
of an atmosphere.