Subsurface density structure of Taurus‐Littrow Valley using Apollo 17 gravity data

Abstract

The Traverse Gravimeter Experiment (TGE) from the Apollo 17 mission was the first and only successful gravity survey on the surface of the Moon, revealing the local gravity field at Taurus‐Littrow Valley (TLV). TLV is hypothesized to be a basalt‐filled graben, oriented radial to Serenitatis basin. We implemented modern 3‐D modeling techniques using recent high‐resolution Lunar Reconnaisance Orbiter topography and image data sets to reinvestigate the subsurface structure of TLV and constrain the volcanic and tectonic history of the region. Updated topography led to significant improvements in the accuracy of free‐air, Bouguer, and terrain corrections. To determine the underlying geometry for TLV, we tested a range of possible thicknesses, dips, and wall positions for the graben fill. We found that the thickness and position previously determined by Talwani et al. (1973) represent our preferred model for the data, but with walls with dips of 30°, rather than 90°. We found large model misfits due to unmodeled 3‐D structure and density anomalies, as well as parameter trade‐offs. We performed a sensitivity analysis to quantify the parameter trade‐offs in an ideal future survey, assuming dominantly 2‐D geological structure. At the TGE survey noise level (2.5 mGal), the fill thickness was constrained to ±150 m, the wall angle to ±5∘20∘ and the wall positions to ±1 km of the preferred model. This information can be used to inform the design of future lunar gravimetry experiments in regions similar to TLV.

A surface gravity traverse on Mars indicates low bedrock density at Gale crater

“…Gravimetry, the precise measurement of gravitational fields, is used in terrestrial geophysics to detect and map density variations in Earth's subsurface at a variety of scales. A ground-based gravity survey on another Solar System body was made by the Traverse Gravimeter Experiment (TGE) carried to the Moon by Apollo 17, which was used to infer the thickness of a basaltic lava flow filling the Taurus-Littrow valley (3,4). In principle, gravimetric measurements from surface rovers could probe the subsurface to depths not possible with existing in situ analytical techniques, while also revealing finer-scale structure than is possible from orbital gravity surveys.…”

Slope Deformation Associated With Recent Tectonism and the Lasting Effect of Local Subsurface Geometry in the Taurus‐Littrow Valley, Apollo 17 Landing Site

“…In fact, Urbancic et al (2017), by interpreting subsurface density structures of TLV using Apollo 17 gravity data (Talwani et al, 1973), suggest that valley walls dip 30°. TLV was infilled with basaltic material (Wolfe et al, 1981), whose thickness is estimated to be about 1 km (Talwani et al, 1973;Wolfe et al, 1975;Urbancic et al, 2017). However, subsurface geometry reconstruction is based on simplified physical models of TLV, which leads to discrepancies between the models and the data.…”

“…However, mass wasting processes must have shallowed such initial steep angle of valley slopes (Kreslavsky & Head, 2016;Wolfe et al, 1981). In fact, Urbancic et al (2017), by interpreting subsurface density structures of TLV using Apollo 17 gravity data (Talwani et al, 1973), suggest that valley walls dip 30°. TLV was infilled with basaltic material (Wolfe et al, 1981), whose thickness is estimated to be about 1 km (Talwani et al, 1973;Wolfe et al, 1975;Urbancic et al, 2017).…”

The wedge-pentahedra method (WPM): Topographic reduction of local terrain in the context of solar system surface gravimetry and robotic exploration

“…Heck and Seitz (2007). However, this assumes a flat-topped rectangular top surface, which again is only an approximation of the real surface (Urbancic et al, 2017). The prism approximation therefore leads to a manifold of plateaux, giving a discontinuous surface and neglecting the surface slopes.…”