The Significant Contribution of Impact Glass to the Martian Surface Record

Introduction and Background: The surface of Mars displays a well-preserved record of impact bombardment extending back billions of years. Unlike on Earth, remnant ejecta products from these impacts are collected and preserved on or near the present-day Martian surface. Depending on factors such as target properties and depositional environment, the diverse nature and fate of such materials will produce a widerange of products draped over a variety of geologic surface units. Previous estimates, based on observations of impacts into soft sediments (loess) in Argentina (prime terrestrial analog for Martian sediments), suggest that the accumulation of global distal melts on the Martian surface since the Hesperian should be considerable [1]. This study quantifies these predictions. It integrates hydrocode models of impact melt generation with calculations of ballistic ejecta delivery across a rotating Mars in order to estimate post-Noachian melts on the present-day surface accumulated over the last 3 to 4 Ga [2]. The mafic crust of Mars should yield a wide assortment of impact products, e.g., distal glasses, rapidly quenched melts, and crystalline melts, that have experienced varied degrees of crystallization along with influences of chemical and physical weathering (thus resulting in a diversity of spectral signatures) [1, 3]. Identification of these products distinctly as mafic impact glasses, as opposed to volcanic remnants, is therefore challenging. Regardless, the present results indicate that melts generated from the mafic crust will contribute widespread breccias and tektitic glasses to the near-surface record. Procedure: This study uses a combination of analytical and computational approaches to model impactglass generation and distribution on Mars. The CTH shock physics hydrocode package [4] is used to determine melt generation mass estimates, ejection angles, and ejection speeds for impacts modeled under ambient Martian conditions (12 km/s impacts of a dunite projectile into a basaltic target surrounded by a Martian atmosphere, see Fig. 1). A series of detailed equations of ballistic motion, adapted to accurately account for planetary rotation effects [see 5], are then applied to yield global cumulative melt estimates as distributed by all mapped large impact craters (>100 km in diameter) produced since the Hesperian (~3.5-0 Ga), 22 craters in all. The results presented here incorporate only fully melted, distal ejecta products (e.g., tektite-like glasses with ejection velocities of ~2.7 km/s and greater). However, impacts into unconsolidated sedimentary targets have been found to generate more complex assemblages of melted ejecta, such as breccias containing small percentages (as little as 10-20%) of actual melt volume due to inclusion of mineral clasts [6]. In addition, recent studies have indicated that porous targets may generate five times more melt mass, relative to non-porous targets [7]. Taken these factors into account, the results from our code calculations (impacts into non-porous crustal materials, see Fig. 1) should be viewed as highly conservative estimates.