The effects of water vaporization on rock fragmentation during rapid decompression: Implications for the formation of fluidized ejecta on Mars

Crater and ejecta morphology provide insight into the composition and structure of the target material. Fluidized ejecta surrounding Martian rampart craters are thought to result from the addition of water to the ejecta during impact into a water-rich (ice or liquid) regolith. Here we test experimentally an alternate hypothesis. We propose that the decompression of a rock-water mixture across the water vaporization curve during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta. This enables the ejecta to flow with little or no liquid water present. To test this hypothesis, fragmentation experiments were conducted on sandstone (28 vol% open porosity) from the northern Eldorado Mountains, Nevada, using a shock-tube apparatus at the LMU Munich, Germany. Rock samples with 0-92% of their open pore space filled with water were pressurized to 15 MPa at 177 °C or 300 °C and rapidly decompressed. As the water vaporization curve is crossed, the water in the pore space rapidly flashes to steam causing, together with the expanding gas in the water-free pore space, the sample to fragment. The presence of water has a significant effect on the grain size distribution and grain shape of the fragmented rock samples. In comparison with (dry) control samples, samples with water with 15-50% open pore space exhibit much smaller grain sizes. The predominant grain shape of dry as well as partially water-saturated samples is bladed, reflecting fracturing parallel to the decompression front. Samples with >80% water in open pore space had an increase in fines and larger particles but less intermediate sized particles. Fragments from experiments with >80% water in open pore space also displayed a more equant grain shape, indicating that the decompression of water caused fracturing independent of the orientation of the decompression front. These results may provide insight into the morphology of Martian rampart craters. We propose here that even relatively low water contents in the target (~16%) may be sufficient to produce a significant increased proportion of fines allowing the ejecta to flow with little or no water present.