LIQUID VISCOSITY MEASUREMENTS OF LUNAR HIGHLAND AND MARE IMPACT MELT SIMULANTS: JSC-1A, STILLWATER ANORTHOSITE, AND STILLWATER NORITE

Energy released during impact cratering events can rapidly heat silicate materials to superliquidus temperatures and generate gravity-driven melt flows that behave, ostensibly, like lava flows. However, the rheology of impact melt flows on the Moon and other planetary bodies is poorly constrained. To address this, we characterized three lunar simulant materials (JSC-1a, Stillwater anorthosite, and Stillwater norite) using differential scanning calorimetry (DSC) and viscometry. Liquidus and glass transition temperatures (T_liq and T_g) determined by DSC were 1325°C and 665˚C for JSC-1a; 1550°C and 840˚C for the anorthosite; and 1452°C and 780˚C for the norite. Liquid viscosity measurements were made for each liquid, by concentric cylinder viscometry at superliquidus conditions and by parallel plate viscometry just above T_g. These measurements span viscosity (η, Pa s) ranges of 0.6–1.9 log units and 9.0–12.3 log units, respectively. The data were then fit by Vogel-Fulcher-Tammann (VFT) equations, allowing viscosity to be extrapolated to higher temperatures. For JSC-1a, log η = -2.18 + (3532.8/[T(K)-676.7]), where η is viscosity in Pa s. For Stillwater anorthosite, log η = -3.12 + (5154.7/[T(K)-739.8]). For Stillwater norite, log η = -3.07 + (5142.3/[T(K)-686.7]). RMS values for each fit are 0.08 log units for JSC-1a, 0.01 for the anorthosite, and 0.02 for the norite. At 2000°C the liquid viscosity of JSC-1a is 1.1 Pa s, anorthosite is 1.7 Pa s, and norite is 1.5 Pa s, suggesting that most impact melts will have similar viscosities at their initial very high temperatures. Further data will be collected during subliquidus experiments for each composition to determine how the rheology of each material evolves upon cooling and crystallization, where their behavior is expected to diverge strongly.