THE EFFECT OF WATER CONTENT ON ROCK FRAGMENTATION DURING RAPID DECOMPRESSION: PRELIMINARY RESULTS AND POSSIBLE IMPLICATIONS FOR THE FORMATION OF FLUIDIZED EJECTA

Martian rampart craters, characterized by fluidized ejecta blankets, are unique in the solar system. Currently, there is a debate as whether pyroclastic flow or ground surge style mechanisms are responsible for the emplacement of fluidized ejecta. The purpose of this study is to test the hypothesis that the decompression of water across the vaporization curve during the excavation stage of impact cratering results in an overall shift toward smaller ejecta particles. This may result in an increase in finer particles that may become suspended in the atmosphere and deposited in a manner similar to a ground surge or collapsing column. To test this hypothesis, fragmentation experiments on sandstone (30% open pore space) from the northern Eldorado Mountains, Nevada, were conducted using a shock-tube apparatus at the Ludwig-Maximilians-Universität in Munich, Germany. Rock samples containing about 0%, 15%, 30%, 45%, 60%, 75%, and 90% of their open pore space filled with water were subjected to 15 MPa at 177ºC and 300ºC and then rapidly decompressed to about 1 atm and 20ºC. Experimental results show that, when the vaporization curve for water is crossed, water in open pore space has a significant effect on the grain size distribution of the decompressed sandstone samples. When compared to the fragmentation of a dry sample, samples with water in about 15 – 50% open pore space had much smaller grain sizes. Samples with 100% water in open pore space showed an increase in fines and larger particles and a decreased number of intermediate particles. Large particles from the 100% water experiments had a more blocky grain shape indicating that decompression of water played a more significant role in fragmentation than the control experiments. When the vaporization curve for water is not crossed, water added to open pore space has the effect of reducing the overall porosity of the rock and increasing the fragmentation threshold.