Lunar Sub - Surface Objects Using Surface Gravimetric Surveys

Introduction: Surface gravity surveying is a well developed terrestrial geophysical technique, used to detect subsurface density variations, which in turn can reveal subsurface geologic features. Here we perform a feasibility study to determine what major science questions can be answered by a rover-mounted, highly compact gravimeter and gravity gradiometer being designed by Gedex Inc.; a Toronto-based geophysics company. Several satellite and manned missions to the Moon have provided gravitational data. Most recently, the GRAIL (Gravity Recovery and Interior Laboratory) survey produced the highest resolution global gravity field from a satellite mission to date, of ~13km [1]. However, because the wavelength resolvable by these satellites is about equal to their altitude, even the highest resolution satellite gravity surveys are not sensitive enough to detect fine-scale sub-surface structures on the Moon. Therefore, many of the details about the lunar sub-surface geology have been derived primarily from the manned Apollo missions [2]. In particular, the Apollo 17 mission carried a gravimeter, which was used to measure the local gravity field near the landing site [3]. This was the first and only successful gravity survey on the surface of the moon. Now, with privatized space travel to the moon becoming more accessible than ever, and with advances in space gravimetry instrumentation, performing a high-resolution gravity survey on the surface of the moon is entirely viable. Methods: Gedex Inc., is developing two types of gravity-measuring instruments suitable for use on Lunar rovers. The first of these is a compact (10x10x15 cm, 1.5 kg) gravimeter, able to measure the complete gravity vector with a repeatability target of 1 milliGal on the Moon. A breadboard of this instrument has been built and tested, and a flight-test version is under development. The second is a gravity gradiometer, designed to measure the full gravity gradient tensor, with a target repeatability of 1 Eotvos; that instrument is at a preliminary design stage. Gradiometers are much more sensitive to near surface features, whereas gra-vimeters are more effective at detecting deeper subsur-face features. With these estimates of repeatability for these instruments , we have conducted preliminary modeling to predict the magnitude of the gravity and gravity gradient signals produced in response to targets of interest in the lunar subsurface.