Mascons as structural relief on a lunar ‘moho’

Abstract

A mechanism for the creation of lunar mascons is proposed that requires no abnormal density materials or major density inversions. The mascons are produced by mantle plugs upwelling into giant impact basins punched through the lunar crust followed by volcanic filling of the remainder of the crater above the plug. It is explicitly shown that continued volcanic filling is not inhibited by the attainment of isostatic equilibrium. If the density contrast between the lunar crust and mantle is 0.5 g/cm3, the minimum depth of the crust is 45 km in the Imbrium region. The strength of the lunar mantle must be somewhat greater than the strength of the earth's mantle; however, this is not inconsistent with a composition essentially similar to the composition of the earth. The approximately linear relationship between the peak gravity anomaly and the mare basin diameter suggests that for craters less than 200 km in diameter, the strength of the moon coupled with its crustal thickness are adequate to preclude significant mantle plug upwelling and mascon formation. With better density data the mascons may well be seen to result from combination of several effects requiring some modification of present geometry based on plug and topographic effects alone. Nevertheless, given a lunar (or Martian) crust and. mantle capable of healing giant impact basins by normal isostatic and volcanic processes, then mantle plug mechanics should make a significant contribution to the geologic evolution of the basin and to the associated gravity anomaly.

Lunar Multiring Basins and the Cratering Process

“…Using this approach, the subsurface structure of the basin is modeled without invoking scaling laws that may or may not be applicable to the largest craters. For instance, it has been concluded from these studies that the lunar Moho (i.e., the crust-mantle boundary) is substantially uplifted beneath the multirings basins (e.g., Wise and Yates 1970, Bills and Ferrari 1977, Thurber and Solomon 1978, Phillips and Dvorak 1981, Bratt et al 1985. This uplift has commonly been interpreted as resulting from the excavation of large amounts of crustal material and the subsequent rebound of the crust and mantle beneath the basin floor.…”

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

“…la). The positive anomaly is attributed to the uncompensated mare basalts (Phillips et al, 1972) and the presence of a dense mantle "plug" formed by post-impact mantle rebound (Wise and Yates, 1970;Solomon and Head, 1979) (for a summary discussion of the relative contribution of mantle material and the basalt fill to the gravity see (Konopliv et al, 2001). The grid was generated by truncating LP 165P at degree and order 1 10.…”

Improved Gravity Field of the Moon from Lunar Prospector

“…However, in the context of this model, the question of the respective contribution of mantle plug and subsequent mare fill to the observed positive mascon gravity anomaly is a matter of debate. Some believe that the mare fill is of relatively low density and that most of the positive mascon anomaly is due to the high-density mantle plug (17). Others argue that the mass anomalies responsible for the five principal mascons (in particular, Mare Serenitatis) are thin and near the surface with the dominant contribution coming from a high-density mare fill (relative to the crust) rather than a mantle plug (18).…”

“…After a giant impact, according to one scenario, the extensive excavation of lunar material resulted in crater relaxation, a strong thermal anomaly, and high amounts of stress within the crust (16 ). The heating and weakening of the crust allowed an upwelling of denser mantle material, resulting in excess mass near the center of the basin (17)(18)(19). This mantle rebound resulted in uplift of the crust-mantle boundary (or Moho) and the formation of a dense mantle "plug."…”