CONSTRAINING ALTERATION CONDITIONS ON MARS FROM EXPERIMENTALLY ALTERED BASALT

Major occurrences of hydrous alteration minerals on Mars have been found in Noachian impact craters formed in basaltic targets and detected using visible/near infrared (VNIR) spectroscopy. It has been reported recently that at least nine craters in the northern plains apparently excavated thick layers of lava and sediment to expose phyllosilicates as well. This discovery indicates that the planet was globally altered by water in the past but does not constrain formation conditions for phyllosilicate occurrences, which have important implications for the evolution of the surface and the biological potential on Mars. No experimentally controlled and well characterized analog materials that simulate martian shock metamorphism and alteration conditions currently exist for calibrating either remote sensing or in-situ measurements of Mars. Therefore, a series of experiments have been initiated that are designed to document the alteration phases produced under controlled Mars-like conditions on both unshocked basaltic material and the same material shock metamorphosed in the laboratory to produce a reference suite of samples for calibrating remote sensing and in-situ measurements on Mars.

A well characterized natural sample of basaltic tephra (HWMK725) from Mauna Kea, Hawaii that has a composition similar to the average Mars basaltic soil and Meridiani compositions inferred on a S-, Cl-, and Br-free basis has been chosen as the starting material. The relative abundance of phases in HWMK725 is glass>plagioclase feldspar>olivine>titanomagnetite. The variables affecting the alteration assemblages created in the experiments have been limited to temperature and pH. Temperatures chosen for the experiments (80^oC and 160^oC) represent hydrothermal conditions typical of those that could be initiated by meteorite impact into a hydrous target. Water chemistry for the experiments ranges from acidic to neutral pH representing the evolving environmental conditions throughout early Mars (Noachian) history. Results of alteration experiments on these Mars analog materials as characterized by VNIR and Mössbauer spectroscopy prior to laboratory shock will be presented and compared to the results of alteration experiments by Golden et al., 2005 on the same starting material but under different alteration conditions.