Gravity Constraints on the Structure of the Marius Hills and Aristarchus Plateau Volcanic Fields, the Moon

The Marius Hills in central Oceanus Procellarum is the largest lunar volcanic dome complex. It is 200 by 250 kilometers across and contains more than 300 volcanic domes and cones and 20 sinuous rilles. Individual domes are up to 25 kilometers across. On the rest of the lunar mare, only about 200 volcanic domes are known, emphasizing the unusual nature of the Marius Hills. The Aristarchus Plateau in northern Oceanus Procellarum is 170 by 220 km across. It contains the Moon's largest pyroclastic deposit (49,000 km²) and 36 sinuous rilles.

Gravity observations provide additional constraints on the structure of these volcanic fields. Existing gravity data resolves the regional structure in the volcanic fields but not individual domes. In the Marius Hills, the maximum free-air anomaly of 183 mGal (at spherical harmonic degree 70) occurs near the densest concentrations of domes. Less than 30% of the gravity anomaly is due to the surface topography. Most of the observed gravity anomaly in the Marius Hills is due to buried, high density material. In this regard, the Marius Hills are similar to some highland volcanos on Mars such as Syrtis Major and Tyrrhena Patera (Kiefer, Earth Planet. Sci. Lett., 2004). The density contrast that produces the Marius Hills anomaly is most likely due to basalt that fills pore space in the brecciated highland crust beneath the volcanic field. Thus, the gravity observations constrain the overall thickness and volume of volcanic activity in the Marius Hills. For plausible density values, the volcanic unit is at least 3 to 4 kilometers thick. In Aristarchus, the gravity high (108 mGal) is offset to the south-east from the topographic peak. It appears likely that buried, high density material occurs near the gravity maximum. On-going work will quantify the thickness and lateral distribution of this material.