Abstract
AbstractMascon basins on the Moon are large craters that display significant positive free‐air and Bouguer gravity anomalies. An important question is why is not every large crater a mascon, as less than half have been previously determined to be. We detrend the free‐air, topographic, and Bouguer gravity anomalies and find that most large basins (28 of 41) display mascon characteristics (e.g., strong positive Bouguer anomalies narrower than the surface rims). Negative free‐air gravity annuli surrounding the central highs generally are absent in the Bouguer gravity, implicating surface topography. We propose that beneath a forming large basin, the relatively narrow transient crater drives mantle uplift, while upward and inward collapse forms the surface topography. Furthermore, the nonmascon basins are all ancient and heavily degraded, indicating a postimpact evolutionary process. Our results suggest that mascon formation is the standard for large impacts on the Moon and by extension on other terrestrial planets.
Highlights
[3] In light of the imminent release of the global, highresolution gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission [Zuber et al, 2012], we reexamine all large craters on the Moon, finding that most display the characteristics of a mascon basin
Since the days of Apollo [Müller and Sjogren, 1968], it has been known that many large impact basins possess a centrally located, positive free-air gravity anomaly, in perplexing contrast to the negative anomaly that should arise from what is, in essence, a big hole in the ground
One might expect that all large craters on the Moon with characteristics of a mascon basin should be common and perhaps pervasive
Summary
[2] Large impact craters provide a window into the crustal structure of terrestrial planets. While this exercise does not represent an exhaustive study of the structure beneath Hertzsprung, this example does demonstrate that this simple crustal structure produces the bulls-eye gravity signature of the Category C basins It appears that the negative and surrounding positive annuli surrounding mascon basins in the free-air gravity are largely products of the surface topography; no subsurface mass anomaly is usually required to explain the gravity signal exterior to the central gravity high. It all depends on whether the subsurface mass excess (as reflected by the Bouguer high) is strong enough to balance or overcome of the negative gravity associated with the low surface topography of the basin’s center Even for those cases where this central free-air peak achieves strongly positive values, it is improper to classify such a basin as superisostatic in its entirety; it is only superisostatic in its central region, surrounded by an annulus that is subisostatic (suggesting competing and thereby partially cancelling loads on the lithosphere). Category B basins, on the other hand, tend to be fully isostatic, as demonstrated by the small free-air gravity anomalies at all radii, while the Category A basins are subisostatic, the negligible Bouguer anomalies indicating the free-air gravity largely reflects the remaining surface topography
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