The Structure of Lunar Basins: Implications for the Basin-Forming Process

Details The Structure of Lunar Basins: Implications for the Basin-Forming Process The Structure of Lunar Basins: Implications for the Basin-Forming Process

Jan 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997lpico.922...63w&link_type=abstract

Large Meteorite Impacts and Planetary Evolution, p. 63

Mathematics

Gravitational Fields, Lunar Gravitation, Lunar Maria, Topography, Lunar Craters, Meteorite Craters, Selenology, Lunar Crust, Mathematical Models, Clementine Spacecraft, Thickness, Structural Basins, Lunar Mantle

Scientific paper

Using lunar gravity and topography data obtained from the Clementine missions, a global crustal thickness map was recently generated. This model had a two-layered crust; gravity anomalies were assumed to be primarily to upper crustal thickness variations, and a spherical analog of Parker's algorithm was used in computing the potential anomalies due to finite amplitude relief along the surface, intracrustal interface and Moho. This dual-layered model of lunar crustal structure has several important properties: (1) the upper crust (about 30 km thick) was entirely removed during the formation of the major nearside basins. This is consistent with the fact that the ejecta for these basins shows a considerable noritic component, which presumably was derived from the lower crust. (2) The large South-Pole Aitken basin also had its entire upper crust removed, but has a 30-km-thick lower crust present. This basin apparently did not excavate into the lunar mantle, which is consistent with spectral reflectance studies fo the regolith. (3) Surrounding most basins is an annulus of thickened crust. This is a result of a low "gravity moat" that surrounds most multiring basins. (4) The crust below the Crisium Basin is extremely thin (about 5 km). By adjusting the parameters of the above gravity model, it is possible to get a zero total crustal thickness beneath the basin. Thus based on gravity studies, Crisium is the only basin that could possibly have excavated mantle material. In this study we use the above crustal thickness model, as well as the lunar gravity field, to study several aspects of the basin-forming process (1) what was the size of the excavation cavity? (2) How much lower crustal material was excavated? (3) Is the gravity moat surrounding the basins due to crustal thickness variations, lower density ejecta deposits, or brecciated bedrock (i.e., the "strength crater"? (4) Were the basins in isostatic equilibrium before or after the emplacement of mare basalts? Radially averaged crustal thickness profiles for the major basins all show that the upper crust is significantly thinned and the the moho is substantially uplifted beneath these basins. We use a relatively simple technique to reconstruct the size of the excavation cavity. Within the annulus of thickened crust, the Moho is "restored" to its average value outside of the basin. It is assumed that the resulting cavity is a good approximation of the excavation cavity (that portion of the preimpact crust that is ballistically ejected from the transient crater). Although this method does not take into account any basin modification effects such as stumping, nor does it restore the uplifted crater rim material to its original location, given the uncertainty in the lunar gravity field and the assumptions that went into the gravity modeling, we will assume that the excavation cavity is adequately described to the first order by this method. A table shows the diameter of the excavation cavity, Dex (which is equivalent to the diameter of the transient cavity), the depth of excavation, hex and the volume of material that was excavated, Vex. A best fit between hex and Dex yields hex = 0.07 Dex. Although this correlation is not great (R = 0.54), the excavation depth always lies between 0.12 and 0.02 of the excavation cavity diameter, which is consistent with most previous estimates. A good correlation (R = 0.82) is found between the transient cavity diameter and the most prominent ring diameter, D, for the multiring basins, given by Dt = 0.41D + 134 (most prominent ring diameter taken). This correlation is very similar to that given by Spudis. We are currently in the process of determining the pre- and postmare isostatic state of the nearside mascon basins. These results should give an indication of the variability in strength of the lunar lithosphere with time. Additionally, the origin of the gravity moats that surround most basins is being investigated. Particularly, we are attempting to discern whether these gravity lows are caused by (1) thickened crust due to the excavation flow during crater formation, (2) low-density ejecta, or (3) brecciated in situ bedrock. If it can be shown that either (2) or (3) is the most likely cause, estimates of the ejecta thickness or "strength crater" diameter may be able to be obtained. (Additional information is contained in the original.)

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