Zhongyi Xie

Meteorite Studies Illuminate Phase Transition Behavior of Minerals under Shock Compression

Some shock wave researchers have long contended that phase transitions of minerals under shock compression occur more rapidly than under comparable static compression conditions. Other researchers argue that phase transition behavior under shock compression does not differ from observations of static high pressure behavior. Many meteorites contain high‐pressure phases that are ascribed to impact. These high‐pressure phases are found within or adjacent to so‐called melt veins, sheets of material that was once molten and was quenched via conduction to surrounding material. Possible mechanisms for melt vein formation on impact include adiabatic shear and jetting. Thermal analysis of melt vein solidification and cooling, together with knowledge of phase stability fields and conditions for metastable survival of high‐pressure phases, constrains the shock conditions and provides evidence that the observed reconstructive phase transitions occurred via the same nucleation and growth mechanisms observed in static ...

Ejection mechanisms for Martian meteorites

At least 35 meteorites have been identified as Martian. The Martian origin of these meteorites is not in dispute. It is generally accepted that the meteorites were ejected from Mars as a result of asteroid or comet impacts. However, there is no agreement on the detailed mechanism by which these meteorites were accelerated to the Martian escape velocity of 5 km/s. The simplest mechanism, that the meteorites were accelerated by a strong shock, implies a minimum shock pressure of about 65 GPa. Evidence from the meteorites themselves implies that none of them has been subjected to shock pressures in excess of about 45 GPa. Measurements of the magnetic properties of Martian meteorite ALH 84001 indicate that the ejection event did not heat it above its curie temperature of about 40 C, implying a maximum shock pressure during ejection of less than 15 GPa. We have not been able to reproduce recent calculations that predict low‐pressure shock ejection at high velocity. We explore the possibility that Martian meteo...