Rheological investigation of lunar highland and mare impact melt simulants

Energy released during impact cratering events can rapidly heat silicate materials to superliquidus temperatures. This can generate gravity-driven melt flows that appear to behave like lava flows. The rheology of impact melts 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. We determined liquidus and glass transition temperatures (Tliq and Tg respectively) and made bulk viscosity measurements for each material by concentric cylinder viscometry at superliquidus conditions (between 1320 and 1600°C) and by parallel plate viscometry just above Tg. These measurements span viscosity (η) ranges of 4-80 Pa s and 1 × 109 to 2 × 1012 Pa s, respectively, and allow viscosity to be extrapolated to higher temperatures. At 2000°C, the liquid viscosity of JSC-1a, anorthosite, and norite are predicted to be 1.1 Pa s, 1.7 Pa s, and 1.5 Pa s, respectively, suggesting that most impact melts will have similarly low viscosities at their initially superliquidus temperatures. Thermodynamic modeling using MELTS supports our experimental observations that the anorthosite and norite samples crystallize over a narrow temperature interval upon crossing the liquidus, suggesting that melt flows of similar composition would quickly cease flowing as the melt cools. Modeling of the JSC-1a sample predicts modest crystallization for ∼125°C of undercooling before a rapid increase, again in accordance with our experimental observations. This suggests that cooling-limited melt flows of basaltic (mare-like) compositions would likely be thinner and flow farther than anorthositic (highlands-like) compositions.