Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p42a..05w&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P42A-05
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 5460 Physical Properties Of Materials, 6022 Impact Phenomena (5420, 8136), 6235 Mercury, 6250 Moon (1221)
Scientific paper
Impacts have been important in the evolution of the Moon's crust and regolith, and impact melt products (breccias, glasses, and "highland basalts") account for a huge fraction of all sampled nonmare materials. As we endeavor to unravel the history of the Moon using these materials, it is important to understand that three fundamentally distinct types of impact melt occur. This discussion will focus on the Moon, but the same tripartite classification of impact melts should be applicable to Mercury. However, on Earth, Venus and to some extent Mars, atmosphere and even hydrosphere have major complicating influences. The fate of impact melt is closely tied to a parameter that Melosh has termed melting/displacement ratio, i.e., volume of impact melt over volume of transient crater; hereafter m/d. For any given energy of impact in large-scale cratering events, d is sensitive to the local gravity, g. As a result, m/d is systematically lower on a planet with modest g, such as the Moon, than it is on Earth. But also, for any given g, m/d increases with the energy (crater size) of the impact. As one consequence, it is easy to show that most of the total volume of impact melt throughout lunar history, M, was generated by a very small number (probably much less than 10) of the largest impacts. In "small" lunar events, with final crater D < ~ 100 km and m/d of order 1-4%, calculations based on Maxwell's z model indicate that roughly half of the impact melt is ejected from the transient crater; and the fraction not ejected will be dispersed within the thoroughly disintegrated but relatively cool shallow-subcrater crust. In the largest events (most notably South Pole-Aitken) that contribute most to M, m/d can be as high as 50%. The z models show that only about 1/4 of m is ejected from the transient crater, yet these largest events so dominate M that they still dominate the total inventory of ejected impact melt, M*. The ejected impact melt winds up cooling rapidly, dispersed in thin puddles that are choked with cool solids entrained during the process of landing and outflow. The unejected component of impact melt, in these largest events, meet a very different fate. The zone of unejected melt initially extends completely through the crust into the mantle. Although there must still be a tendency to assimilate with disintegrated cooler solids, the sheer size of the melt zone guarantees that it will not quench, but rather undergo slow cooling, more or less in the style of a conventional igneous intrusion. Thus, unejected impact melt of large events is fundamentally different from its ejected counterpart, which in terms of cooling and mixing with solid debris is more akin to impact melt (whether ejected or not) from small events. Of course, surface samples will under-represent this deep-origin but volumetrically dominant type of impact melt. A third category that must be recognized is impact melt that is ejected as such a fine spray that it quenches to glass during ballistic transport. Such glasses have the important virtue of never undergoing mixing with originally non-adjacent solids. They are significant constituents of the surface regolith, but their total contribution, even to M*, must be tiny. Our z-model results indicate that in high m/d impacts, average depth provenance of ejected melt is substantially deeper than the average for unmelted ejecta. For example, in a model with m/d = 0.35, as for a roughly Imbrium-sized lunar basin, average ejected&melted depth provenance is 1.40x average ejected&unmelted depth provenance. The impact-melt rocks of the sampled part of the Moon are materials that were, on average, ejected from deeper levels than the non-impact-melted materials (fragmental breccias and unbrecciated pristine rocks).
Kallemeyn Gregory W.
Warren Harry P.
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