Effects of Rocks on Neutron and Gamma-ray Production in Martian Surface Soil

We studied the effects of a dry rock sitting on a 3% water-containing martian-surface soil on neutron and gamma-ray fluxes. Rocks with radii of ~25 cm and bigger significantly affect these fluxes and the flux ratios of certain gamma rays. Introduction: The rock surface abundances on Mars are inferred from orbital data to vary from almost zero to ~30% [1]. These rock abundances are consistent with images from landers and rovers [J. Keller, priv. comm., 2005]. Several regions on Mars appear to be very rocky or very dusty with some compositional differences [e.g., 2], although more detailed analyses of regions with and without thick dust covers show that even the rockiest regions are still mainly dust [3]. The effects of rocks and soil on gamma-ray fluxes were investigated for several cases [4], although only rocks both much smaller and much larger than neutron-interaction lengths were studied. However, many rocks are of intermediate sizes. For this study, we did numerical simulations using a 3D code for rocks of various sizes on a martian soil. Calculations: We did the numerical simulations with the Monte Carlo N-Particle eXtended (MCNPX) code [5]. Our typical models for MCNPX calculations of gamma-ray production rates [6] were spherical with a scale of Mars to model of 1 to 1. However, it is not feasible to hit enough cosmic radiation on a small size (i.e., R=20 cm) rock on such a martian surface. We adapted a box model with reflecting walls that has the same atmospheric features and depth profiles as the spherical MCNPX calculation. This model shows the effect of rock soil mixture without the geometric problems of placing small rocks on the surface of a full sized Mars. The surface neutron fluxes for cases using reflecting walls and a large sphere were found to be almost identical. Therefore, for the purpose of this calculation, the reflecting-wall model is used. This model has an isotropic radiation source surface at the top of the atmosphere. We used a box with 80 cm dimensions. We investigated five rock sizes ranging in radii from 5 cm to 30 cm on the surface. We assumed that the rock had almost no water. These rocks cover from 1.2% to 44% of the surface area in our box. The rock is located with its center in the center of the surface, with half of the rock above the ground and half below the ground level. A soil composition of 3% water and a composition similar to the rock for all elements except water and Cl were used. Most elemental concentration ratios of rock to soil are 1.04 except H (0.03), O (0.98), Cl (0.02), and Fe (1.02). The densities for rock and soil were 2.5 g/cm and 1.5 g/cm, respectively. We used a standard martian atmosphere with a thickness of 16 g/cm. To obtain neutron fluxes around the rock, a cylindrical mesh tally option was chosen with radii from 0.5 cm to 40 cm at 0.5 cm increments. Layers with depths of 0.5 cm were used near the surface.