Structure of the South Pole-Aitken Basin from KAGUYA Selenodesy Data

Abstract

The South Pole-Aitken basin (SPA) is the largest (2500km in diameter), deepest and presumably oldest impact basin in the solar system. SPA was characterized by an ellipse with axes 2400 by 2050 km, but more precise topography and gravity data are necessary to discuss the structure of large basin like SPA. We use localized representation of gravity potential where Slepian functions were used to estimate the gravity field over certain areas of the Moon and a spherical cap area with a radius of 40 degree from the SPA center. The direction of an ellipse denoting the depression is found to be similar to the previous result. The region with the thinnest crust is offset southward from the center of SPA. We analyzed interior structure of small basins in and around SPA. We interpret that a positive gravity anomaly corresponds to a Moho uplift. Just around the rim of SPA, a distinct Moho uplift beneath Schrödinger corresponds to the presence of olivine and obscure circular structure Amundsen-Ganswindt has a significant Moho uplift, suggesting a buried impact structure.

Geologic structure generated by large‐impact basin formation observed at the South Pole‐Aitken basin on the Moon

“…This depression could correspond to the “possible fourth ring” described by Hiesinger and Head [] for the SPA basin; however, in their study it was tracked only partially. The central depression is smaller than the previously suggested area (1250 × 1150 km) [ Sasaki et al , ], based on its lower elevation and thinner crust, and it is shifted slightly northward compared to the lowest elevation (Figures a and ). However, the current lowest elevation may not need to correspond completely to a geologic structure, because the topography changes with relaxation and melt cooling after basin formation.…”

Constraining the size of the South Pole-Aitken basin impact

“…Because the melt sheet has no strength, the basin center should have a relatively flat floor with little topographic relief. This is seen in Kaguya data where the central region of the basin is at a depth of about 5 km relative to the Moon's mean radius (Sasaki et al, 2010). Beyond about half of the basin radius, the effect of impact heating is minor and would not affect the strength of the crust near the basin rim.…”

“…The gravity-derived profiles suggest the crust beneath the center of SPA has a thickness of $30-55 km, gradually thickening towards $60 km nearer the basin's edge (radial distance of $1200 km). Unlike the majority of gravity-derived crustal profiles for smaller lunar basins, SPA does not appear to possess a thickened annulus of crust around its basin rim (see Wieczorek and Phillips, 1999;Hikida and Wieczorek, 2007;Sasaki et al, 2010Sasaki et al, , 2011. By comparison to the gravity-derived profiles, our steepest LTG (50 K/km) therefore appears to best fit the gravity data, as the other, shallower, LTGs all produced a prominent thickened annulus.…”

“…The shallower LTGs exhibit a more pronounced thickened crustal annulus. Inferred crustal thicknesses taken from Neuman et al (1996), Wieczorek and Phillips (1999), and Sasaki et al (2010), are included for comparison. The rapid change in elevation at the top of crust is a result of the cell size and the spherical nature of the modeled target.…”

“…These data confirm a (thin) layer, less dense than the underlying mantle, must be present under the basin center; the presence, therefore, of mantle at the surface is inconsistent with both gravity and topography data. Assuming an average crustal composition and density over the basin floor (i.e., neglecting the observed SPA compositional anomaly), data from Kaguya indicates a crustal thickness of 30 km at SPA's center, gradually thickening to 60-80 km outside of the basin (Sasaki et al, 2010;Ishihara et al, 2009); inferred crustal thickness in the inner region is therefore thinner than its surroundings and the lunar average ($50 km: Wieczorek et al, 2006). This result has been used as evidence for upper crustal material being absent in the Inner SPAT.…”