VERIFICATION OF IMPACT MELT AND VAPOR DETERMINATION METHODS IN CTH

CTH is an Eulerian shock physics analysis code with a variety of applications, including planetary impacts, e.g. [1-2], that require accurate melt and vapor representation. Verification studies allow a comparison of melt and vapor determination methods in order to better understand the strengths and limitations of using such methods. Although the ANEOS equation of state (EOS) is able to provide phase and entropy, CTH until recently was unable to pass them through to the end user. Instead, analyses used maximum pressure, P_max, as a surrogate for entropy, S, or final release-state temperature, T_f, to determine melt and vapor within CTH and other hydrocodes [3]. This work compares the use of methods based on P_max, S, and T_f to determine melt and vapor mass, and verifies the results of [3] in preparation for studying multi-dimensional, strength and porosity effects.

A series of 1-D and 2-D impact calculations using CTH with ANEOS provided a comparison of melt and vapor determination methods. In the 1-D calculations, pressure, temperature, and entropy of melting for several materials and impact velocities were examined. Comparison between the P-S Hugoniots from ANEOS and the melt/vapor entropy used in Pierazzo et al.’s work verified that ANEOS and CTH are performing as expected. In the 2-D calculations, we recorded the impact melt and vapor mass determined from P_max, S, and T_f from several impact velocities and materials. For verification with Pierazzo et al. results, initial calculations used no strength, gravity or porosity. Melt and vapor masses recorded from the P_max and T_f methods agreed for most materials modeled. These results demonstrate that with a good EOS, both maximum pressure and final release temperature can be used to determine melt and vapor mass in an impact scenario. Because final release temperature can be used, multi-dimensional, strength, and porosity effects can be explored in more detail than with the Pierazzo et al. method based on P_max alone. Future work will extend these properties to 3-D for better understanding of multi-dimensional effects, especially associated with impact obliquity.

[1] Crawford, D. A. (2011), LPSC 42, Abs. #2112.

[2] Schultz, P. H. & Wrobel (2012), K.E., JGR-Planets 117.

[3] Pierazzo et al. (1997), Icarus 127.