The topography and gravity of Mare Serenitatis: implications for subsidence of the mare surface

Abstract

The long wavelength topography of Mare Serenitatis was analyzed using topographic data obtained by the Clementine laser ranging instrument (LIDAR). The topography shows that the lowest elevations in the mare surface occur near the margins of the basin. The present mare surface reflects a long period of volcanism, subsidence, and deformation. Subsidence is generally attributed to a Gaussian-shaped, superisostatic load from the mare basalts that results in flexure of the lunar lithosphere. Gravity data from Lunar Prospector suggest that the basalt sequence in the interior of Serenitatis is generally uniform in thickness and thins rapidly at the margins. This suggests that the topographic lows in the basin do not coincide with areas where the mare basalts are thick but rather occur where the basalts thin. The topographic lows also do not appear to coincide with accumulations of the youngest mare basalt units. The long wavelength topography of Mare Serenitatis may reflect subsidence influenced by pre-mare basalt basin topography and preexisting zones of lithospheric weakness.

Introduction

Mare Serenitatis is one of the largest and most circular maria on the Moon (Fig. 1). It is approximately 600 km in diameter and has a ring structure that may extend out to 880 km (see Solomon and Head, 1979, Fig. 2). Based on its degraded appearance, the Serenitatis basin is thought by some to be one of the oldest on the Moon (Wilhelms, 1987). However, radiometric ages of Apollo 17 samples suggest that it is a young basin (Staudacher et al., 1978). The mare basalts that subsequently flooded the Serenitatis basin are dominated by two tectonic landforms, mare ridges (or wrinkle ridges) and arcuate rilles. Mare ridges are found in nearly all lunar maria and typically occur both radial to and concentric with the centers of circular mare filled basins (Strom, 1972; Bryan, 1973; Maxwell et al., 1975). They are generally thought to be compressional features (Bryan, 1973; Howard and Muehlberger, 1973; Muehlberger, 1974; Maxwell et al., 1975; Lucchitta 1976, Lucchitta 1977; Maxwell and Phillips, 1978; Sharpton and Head 1982, Sharpton and Head 1988) resulting from a combination of folding and thrust faulting (Plescia and Golombek, 1986; Watters, 1988; Golombek et al., 1991; Watters, 1991; Watters and Robinson, 1997; Schultz, 2000). Arcuate rilles are linear to arcuate troughs with flat floors and steep walls (see Wilhelms, 1987). These troughs are interpreted to be graben formed by extensional stresses (Baldwin, 1963; McGill, 1971; Golombek, 1979). Lunar graben have a simple geometry and often occur in parallel or echelon sets, concentric to circular maria (Golombek, 1979; Wilhelms, 1987). Most lunar graben are located along basin rims and cut both mare and basin material (Wilhelms, 1987).

The origin of the stresses that formed the mare ridges and arcuate graben associated with mascon maria is generally thought to be due to subsidence of the mare basalts (Bryan, 1973; Maxwell et al., 1975; Lucchitta 1976, Lucchitta 1977; Maxwell, 1978), although global contraction or a combination of subsidence and global contraction have also been suggested (Muehlberger, 1974; Solomon and Head, 1978). In an effort to evaluate the pattern and nature of subsidence of the mare surface, the long wavelength topography of Mare Serenitatis is analyzed using topographic data obtained by the laser ranging instrument (LIDAR) flown on the Clementine spacecraft. The characteristics of the mare fill and the subsurface structure of Mare Serenitatis is also inferred from Lunar Prospector gravity data.