Hydrocode modeling is a fundamental tool for the study of melt
production in planetary impact events. Until recently, however, numerical
modeling of impacts for melt production studies have been limited to
vertical impacts. We present the first results of the investigation of
melt production in oblique impacts. Simulations were carried out using
Sandia's three-dimensional hydrocode CTH, coupled to the SESAME equation
of state. While keeping other impact parameters constant, the calculations
span impact angles (measured from the surface) from 90° (vertical )
impacts to 15°.
The results show that impact angle affects the strength and distribution
of the shock wave generated in the impact. As a result, both the isobaric
core and the regions of melting in the target appear asymmetric and
concentrated in the downrange, shallower portion of the target. The use
of a pressure-decay power law (which describes pressure as function of
linear distance from the impact point) to reconstruct the region of
melting and vaporization is therefore complicated by the asymmetry of the
shock wave. As an analog to the pressure decay versus distance from the
impact point, we used a "volumetric pressure decay", where the pressure
decay is modeled as a function of volume of target material shocked at
or above the given shock pressure. We find that the volumetric pressure
decay exponent is almost constant for impact angles from 90° to
30°, dropping by about a factor of two for a 15° impact.
In the range of shock pressures at which most materials of geologic
interest melt or begin to vaporize, we find that the volume of impact
melt decreases by at most 20% for impacts from 90° down to 45°.
Below 45°, however, the amount of melt in the target decreases
rapidly with impact angle. Compared to the vertical case, the reduction
in volume of melt is about 50% for impacts at 30° and more than
90% for a 15° imapct. These estimates do not include possible
melting due to shear heating, which can contribute to the amount of
melt production especially in very oblique impacts.
Studies of melt production in vertical impacts suggest an energy scaling
law in agreement with the point source limit. An energy scaling law,
however, does not seem to hold for oblique impacts, even when the impact
velocity is substituted by its vertical component. However, we find
that for impact angles between about 30° and 90° (a range that
includes 75% of impact events on planetary surfaces) the volume of
melt is directly proportional to the volume of the transient crater
generated by the impact.
COLOR FIGURES (To download GIF files click on the figures)
Peak shock pressure contours in the plane of impact (i.e., the plane
perpendicular to the target surface that includes the projectile's line
of flight) for the various simulations. A projectile 10 km in diameter is
drawn for scale. The vectors from the center of the projectile show the
direction of impact for the various oblique impact simulations.
