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Melt production in oblique impacts

Melt production in oblique impacts
E. Pierazzo, H.J. Melosh

Icarus 145, 252-261, 2000

ABSTRACT

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.

(See also Ann. Rev. Earth Planet. Sci 28, 141-167, 2000 )

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.

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