GRAVITY CONSTRAINTS ON THE SIZE AND THICKNESS OF THE SCHILLER-SCHICKARD CRYPTOMARE DEPOSITS, THE MOON

Cryptomare deposits on the Moon are ancient mare basalts that were subsequently buried by younger deposits of impact basin ejecta. Understanding the spatial distribution and total volume of the cryptomare is crucial to understanding the Moon’s earliest volcanic history, particularly prior to the end of the heavy bombardment at 3.8 Ga. At present, cryptomare are mapped primarily on the basis of exposure in the ejecta blankets of young craters (termed dark halo craters) and by spectral detection of mafic geochemical anomalies due to mixing of the basalt into the overlying basin ejecta deposits. Because lunar basalts are typically denser than highland breccias, it should also be possible to map cryptomare deposits using gravity observations.

As a proof of concept, I am using Lunar Prospector gravity observations to study cryptomare deposits in the Schiller-Schickard region on the southwest portion of the Moon’s nearside. These deposits are covered by ejecta from the Orientale basin and thus are > 3.8 Ga in age. After removing the contributions of the topography from the gravity field, there are three prominent gravity highs in this region: in the center of Schiller-Zucchius (S-Z) impact basin (260 mGal), Wargentin crater (70 mGal), and east of Schickard crater (67 mGal). All three regions have been mapped as having cryptomare in Clementine multispectral imagery, with estimated thicknesses of 100-200 meters. The gravity observations require that the Wargentin and east of Schickard cryptomare units are 2-3 km thick for plausible densities, reflecting the ability of gravity data to detect totally buried material which is undetectable in remote sensing data. The S-Z basin deposits could be much thicker, but may also have a mascon uplift of the crust-mantle interface contributing to the gravity anomaly. On the other hand, extensive cryptomare deposits have also been mapped in and north of Schickard crater. These deposits are not evident in the gravity data, perhaps because they are too thin to produce a significant gravity signal.