The generation and distribution of martian impact melt/glass: A computational study with implications for the nature of dark surface materials

Introduction: Recent studies [1-2] propose that widespread dark deposits across Mars could be the result of well-preserved impact products accumulated since the late Hesperian (distal tektite-like glasses and lower velocity melt-matrix breccias). The relatively low impact velocity on Mars (compared to Earth) along with the sedimentary nature of the martian crust, however, has led to the assumption that significant impact melts may not be produced on Mars [3-4]. In contrast, Schultz and Mustard [1] found, based on spectral reflectance and the TES instrument, that accumulated distal deposits would be substantial enough to suggest an alternative interpretation for glassy andesitic materials. The present study re-examines the possible contribution of melt across Mars by incorporating the CTH hydrocode [5] into a global ejecta dispersal model that includes Coriolis effects. The results confirm that substantial melt will be generated and widely distributed. Such materials may have a significant contribution to large concentrated regions of dark mobile deposits found on the surface. Background and Procedure: Effect of Rotation: Previous studies [2, 6-7] showed that accurate account of the Coriolis force is essential in detailed modeling of ejecta distributions; the Coriolis terms must be incorporated directly into the angular momentum equation of the spherical ballistics. In particular, the deposition locations of distal ejecta from high angle (closer to vertical) impacts are significantly affected, resulting in substantial concentrations of deposits at rather unexpected locations. The cumulative effects of such deposits over time could have global implications and thus accurate incorporation of the Coriolis force is crucial for the present study. Computational vs. Analytical Models: Analytical ejecta-scaling models [e.g., 8-11], while indispensable and highly utilized, cannot provide a completely realistic, fully integrated model of ejecta mechanics (generation and distribution). Hydrocodes such as the CTH shock physics analysis package [5] allow more comprehensive tracking of specific impact cratering phenomena, including melt generation [e.g., 12]. Procedure: This study maps the distribution of impact melt across the surface from all large (>100 km in diameter) Hesperian-aged (or younger) martian craters using a series of detailed ballistic equations that have been altered to include rotational effects [see 6]. Actual melt fractions (along with melt generation velocity distributions) were calculated using a CTH computational model of a 10 km/s impact under martian conditions (melt fractions obtained were scaled accordingly for each crater based on diameter). Ejecta mass estimates were obtained from an impact simulation that modeled the formation of a Lyot-sized crater – 220 km in diameter at ~30N, ~330W. Comparisons with previous analytical approximations show reasonable consistency (see Table 1). Differences stem primarily from the uncertainty in the empirical, material-dependent constants found in the analytical equations.