Article,
Hydrocode modeling of oblique impacts: The fate of the projectile
METEORITICS & PLANETARY Science, 35 (1): 117--130 (2000)
Abstract
All impacts are oblique to some degree. Only rarely do projectiles
strike a planetary surface (near) vertically. The effects of an oblique
impact event on the target are well known, producing craters that
appear circular even for low impact angles (> 15 degrees with respect
to the surface). However, we still have much to learn about the fate
of the projectile, especially in oblique impact events. This work
investigates the effect of angle of impact on the projectile. Sandia
National Laboratories' three-dimensional hydrocode CTH was used for
a series of high-resolution simulations (50 cells per projectile
radius) with varying angle of impact. Simulations were carried out
for impacts at 90, 60, 45, 30, and 15 degrees from the horizontal,
while keeping projectile size (5 km in radius), type (dunite), and
impact velocity (20 km/s) constant. The three-dimensional hydrocode
simulations presented here show that in oblique impacts the distribution
of shock pressure inside the projectile land in the target as well)
is highly complex, possessing only bilateral symmetry, even for a
spherical projectile. Available experimental data suggest that only
the vertical component of the impact velocity plays a role in an
impact. If this were correct, simple theoretical considerations indicate
that shock pressure, temperature, and energy would depend on sin(2)
Theta, where Theta is the angle of impact (measured from the horizontal).
However, our numerical simulations show that the mean shock pressure
in the projectile is better fit by a sin Theta dependence, whereas
shock temperature and energy depend on sin(3/2) Theta. This demonstrates
that in impact events the shock wave is the result of complex processes
that cannot be described by simple empirical rules. The mass of shock
melt or vapor in the projectile decreases drastically for low impact
angles as a result of the weakening of the shock for decreasing impact
angles. In particular, for asteroidal impacts the amount of projectile
vaporized is always limited to a small fraction of the projectile
mass. In cometary impacts, however, most of the projectile is vaporized
even at low impact angles. In the oblique impact simulations a large
fraction of the projectile material retains a net downrange motion.
In agreement with experimental work, the simulations show that for
low impact angles (30 and 15 degrees), a downrange focusing of projectile
material occurs, and a significant amount of it travels at velocities
larger than the escape velocity of Earth.
