Geol28103/30/15

Elliptical structure of the lunar South Pole-Aitken basin. Garrick-Bethell and Zuber, 2009

PurposeThe authors attempt to quantify the shape of SPA using topography, iron and

thorium content of the region. They demonstrate that SPA is an elliptical structure formed by an oblique impact; an outer ellipse fit to topographic contours follows ~√2 scaling. The shape of the SPA basin has implications for understanding spatial geological, mineralogical, and geophysical consequences of the impact.

Methods and ResultsTopography data from the Clementine laser altimeter and iron and thorium

from the Lunar Prospector gamma-ray spectrometer are used to study the region. Iron is chosen to represent deep crustal or upper mantle mineralogy and thorium is a proxy from KREEP-rich material. The authors also use high angle photos from the Apollo 8 Hasselblad camera and Clementine UVVIS data to compare their results to other studies of SPA.

A best-fit ellipse is found using data points for the perimeters of each dataset (-2000m contour, 7.5-8 wt.% FeO contour, 1.25 ppm Th). Multiple subsets of data points are used to determine errors on the fits. Neither thorium nor iron ellipses show obvious structure outside of their boundaries; only topography is used to estimate an outer ring (which is most obvious in the north and northeast of the basin). This outer ring has a lower a/b ratio than the inner ellipses and passes through more isolated structures.

Elevations are higher in the northern half of the ellipse and topographic contours are sharper in the north; thorium and iron likewise show higher abundances in the north. This work agrees with previous topographic studies (Wilhelms et al., 1987; Stuart-Alexander, 1978) and the ellipses the authors define match with the mafic mineralogy as characterized by spectral data.

DiscussionAn oblique impact explains the north-south symmetry in the data. This

impact deeply excavated the inner basin, which shows a geochemical signature (Fe, Th), and left a less deeply excavated outer terrace, the border of which follows √2 scaling along the semi-minor axis. Many lunar craters show scaling of ring diameters by multiples of √2 according to statistical analysis by Pike and Spudis (1987). Pike and Spudis (1987) invoke a standing wave bounded by the resonant cavity to create this specific scaling. It is unclear how ring scaling works in elliptical basins, though the ellipses in this study suggest scaling changes with azimuth.

Sharper topography, increased iron and thorium, and larger volumes of mare basalt in the north of the basin suggest asymmetry either in the excavation depth (different up-range and down-range excavation flow fields) or pre-existing difference in crustal thickness and element distribution (see Figure). It is not possible to constrain the impactor direction without gravity data. The contribution of SPA to lunar orientation is also discussed.

Geol28103/30/15

Figure 1: Best-fit inner and outer ellipses for (a) Clementine topography, (b) ULCN2005 topography (c) Lunar Prospector thorium abundance and (d) Lunar Prospector iron (FeO) abundance

Discussion Questions1) The data is “plane-projected from the often-assumed SP-A basin center”. Why

do we assume this center? How do the best-fit ellipses change if you choose a different center or a different projection?

2) The authors define the outer edge of the thorium and iron ellipses based on abundance contours (1.25 ppm and 7.5-8 wt. % respectively). Do these abundances have physical meaning (i.e. we expect these values for background levels) or are they chosen only because they fit with the expected elliptical shape?

3) The authors make a big deal of the semi-major axis scaling by √2, but the Pike and Spudis (1987) paper was criticized earlier in the semester; does this new finding add validity to that scaling? What does that mean for the formation of SP-A given the proposed formation mechanisms leading to this particular scaling? Why does scaling change azimuthally?

4) Do we expect iron and thorium abundances to be constant over spatial scales the size of SPA? Would the impact lead to homogenization or further segregation of these elements?

5) How has GRAIL data added to our understanding of the excavation depth and impact direction of SP-A?