Post‐Shock Temperature and Free Surface Velocity Measurements of Basalt

Basalt is the most common rock type on planetary surfaces. Post‐shock temperature and particle velocity measurements constrain the equation of state of basalt and provide fundamental information about the outcome of planetary impact events. A high‐speed, infrared, four‐wavelength pyrometer, developed at Los Alamos National Laboratory (LANL), is used with customized front end optics at the Harvard Shock Compression Laboratory for concurrent observations of particle velocity and free surface thermal emission. In an experiment on Columbia River basalt released from a peak shock pressure of 28.9±0.2 GPa, the apparent post‐shock temperature is wavelength dependent. The 3.5 and 4.8‐μm channels record apparent temperatures between 605 and 630 K, using an emissivity range of 0.7–1.0. The 1.8 and 2.3‐μm channels record apparent temperatures of ∼700 K and ∼800 K, respectively. The pyrometry data are well fit by a two component temperature distribution: (1) a predominantly 565–610 K free surface, in good agreement with the 570 K predicted by the basalt EOS in the shock physics code CTH, and (2) a small area fraction of 1700–2000 K hot spots. The model is in good agreement with inferred basaltic meteorite hot spot temperatures; however, the hot spot model is not unique. Free surface velocity measurements are slower than predicted by CTH, indicating a steeper release path than in the model equation of state.