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- Title
Temperature and Density on the Forsterite Liquid‐Vapor Phase Boundary.
- Authors
Davies, E. J.; Duncan, M. S.; Root, S.; Kraus, R. G.; Spaulding, D. K.; Jacobsen, S. B.; Stewart, S. T.
- Abstract
The physical processes during planet formation span a large range of pressures and temperatures. Giant impacts, such as the one that formed the Moon, achieve peak pressures of 100s of GPa. The peak shock states generate sufficient entropy such that subsequent decompression to low pressures intersects the liquid‐vapor phase boundary. The entire shock‐and‐release thermodynamic path must be calculated accurately in order to predict the post‐impact structures of planetary bodies. Forsterite (Mg2SiO4) is a commonly used mineral to represent the mantles of differentiated bodies in hydrocode models of planetary collisions. Here, we performed shock experiments on the Sandia Z Machine to obtain the density and temperature of the liquid branch of the liquid‐vapor phase boundary of forsterite. This work is combined with previous work constraining pressure, density, temperature, and entropy of the forsterite principal Hugoniot. We find that the vapor curves in previous forsterite equation of state models used in giant impacts vary substantially from our experimental results, and we compare our results to a recently updated equation of state. We have also found that due to under‐predicted entropy production on the principal Hugoniot and elevated temperatures of the liquid vapor phase boundary of these past models, past impact studies may have underestimated vapor production. Furthermore, our results provide experimental support to the idea that giant impacts can transform much of the mantles of rocky planets into supercritical fluids. Plain Language Summary: Collisions onto planets during planet formation can reach extreme pressures and temperatures. Giant impacts, like one that may have made the moon, reach pressures of millions of atmospheres. Decompression from such high pressures causes the material that makes up planets to melt and vaporize. To help predict the post collision structure of planets, we performed laboratory experiments on the mineral forsterite (Mg2SiO4) that recreates the extreme conditions of these collisions. We used the Z Machine, a facility at Sandia National Laboratories that can launch projectiles up to 40 km s−1 (almost 90,000 miles per hour), to measure the properties of forsterite once it has decompressed. We find that previous models of forsterite melting and vaporization do not match the experimental results found here. The results provide experimental support that giant impacts yield planets that may have mantles that are partially supercritical fluids, that behave like liquids and gases. Key Points: We performed reverse impact experiments and shock‐and‐release experiments to probe the density and temperature of the liquid‐vapor phase boundaryThe experimentally determined liquid‐vapor dome does not agree with commonly used equation of state modelsSupercritical post‐impact states are easier to achieve than previously modeled
- Subjects
ORIGIN of planets; FORSTERITE; SUPERCRITICAL fluids; THERMODYNAMICS; ENTROPY
- Publication
Journal of Geophysical Research. Planets, 2021, Vol 126, Issue 4, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2020JE006745