Experimental Investigation of the Effect of Meteoritic Impacts on Clays on Mars

Introduction: Phyllosilicates have been detected on the surface of Mars by both OMEGA/MEx and by CRISM/MRO [1-3]. Phyllosilicates have been found in some of the oldest terrains on Mars and are thought to date back to the Noachian era. These clays have especially been detected in the ejecta and central peaks of small impact craters [4, 5]. The exact processes that occurred to form these clays are still unclear. It has been suggested that clays were formed in the earliest history of Mars through the activity of liquid water on the surface [6]. Other possible mechanisms include formation by the hydrothermal processes caused by meteoritic impacts [5, 7]. This investigation deals with the effects of shock pressures and temperatures on the spectral properties of clays and how these pressures and temperatures may be modeled. Investigating shock effects on phyllosilicates will help us determine if clays formed prior to an impact event, and are thus affected by it, or after by the impact-induced hydrothermal processes. The latter case implies that some clays in craters may not be as old as previously thought [5]. Methods: The shock experiments were carried out using a two-stage light gas gun at the Institute of Astronautical Science, Japan Aerospace Exploration Agency (JAXA) [8, 9]. The light gas gun has a target chamber of 500 mm in diameter by 1000 mm long. The projectile collides with a sample holder in the target chamber under 40 Pa at room temperature. This gas sun can accelerate a projectile to about 4 km s -1 . The projectiles used were polycarbonate cylinder, 7 mm in diameter and 4.5 mm in height, with a stainless steel head 4 mm in diameter and 1 mm in height. Stainless steel (SUS304) or brass sample holders (100 mm in diameter and 20 mm in height) were placed in the target chamber. Five shock experiments were conducted – two with loose powder with projectile speeds of 2.25 km s -1