The Near Side Megabasin of the Moon

Introduction: A very old, very large basin has been found to underlie the region of large maria on the near side of the Moon. The strong differences between the near and far sides of the Moon have led to searches for such a basin in the past [1], [2], [3]. The Topogrd1 elevation database [4] from the Clementine mission was used to locate the basin. Its central depression covers more than half the Moon, including nearly all of the near side. The ejecta of this basin forms the highlands of the far side. The center of this feature is at 7 degrees North latitude and 21 degrees East longitude, with a radius of 3050 km (101 degrees of arc). A model of this Near Side Megabasin and its ejecta field, together with a model of the South Pole – Aitken Basin, account for over half of the standard deviation of the topography of the entire. Moon. Ejecta of large basins: The ejecta of basins of moderate size such as Orientale can be approximated as if they impacted a flat Moon: curvature of the spherical Moon can be neglected for the rounded rim and for the ejecta blanket within about twice the basin radius. A correction can be made for the far field ejecta by assuming it was all launched from a single radius within the basin. However, for basins of the size of the South Pole – Aitken Basin or more it is important to consider the trajectories of ejecta launched from each radius within the basin, considering the profile of velocity as a function of the internal radius. The model should account for ejected material that escapes the Moon, for focussing of the ejecta at the antipode of the basin center, and for ejecta that passes the antipode and forms a second deposited layer. Such a model was constructed, starting with an empirical “flat Moon” model derived from examination of selected basins, using Clementine elevation data [5]. This model is based on Topogrd2, the quarter-degree model [4] from Clementine. Velocity profile: A velocity profile was estimated from the “flat Moon” model. The ejecta profile depends on the radial ejection profiles of volume per angle of azimuth, the launch angle, and the launch velocity [7]. The volume rate and launch angle were assumed to be constant and the velocity profile was adjusted to produce the ejecta profile of the “flat Mooon” model (see Figure 1). Orbital and spherical effects: The equations of motion of the ejecta thrown into orbit were used to determine the external radius as a function of the internal radius of launch. The depth of the deposit was determined by dividing the incremental volume of the launched material by the derivative of the external radius as a function of the radius of ejection (to account for the spreading out of ejecta). The spherical shape of the Moon was further taken into account by multiplying the depth by the ratio of the ejection radius to the radius of the circle of deposition (a function of the arc distance from the center of the basin). Figure 1: Velocity profile that produces the typical depth profile of basin ejecta [5. To find the velocity for a particular basin, multiply the normalized velocity by the square root of the radius of the basin.