Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..confr..34k&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 34
Mathematics
Logic
Composition (Property), Craters, Ejecta, Landforms, Lunar Maria, Lunar Surface, Lunar Topography, Selenology, Geochronology, Mathematical Models, Stratigraphy, Lunar Crust
Scientific paper
Varying surface ages of lunar light plains in northern-nearside latitudes indicate an origin of these smooth terrae units not exclusively related to Imbrium and/or Orientale impact ejecta. and subsequent processes. Multispectral data seem to support a more diversified history for many of these plains. The nature, ages, stratigraphic position, composition, and mode of emplacement of lunar light plains have been discussed with controversy for more than three decades. Covering about 5% of the lunar terra surface, the relatively low albedo plains are the most distinctive terra landforms after the more crater-like ejecta of the fresh basins. Morphological properties, like their smoothness, lower crater densities, their superposition on the "Imbrian Sculpture," and frequent occurence as crater fill, are mare-like. Other features, such as relative (compared to mare basalts) high-albedo and geological/stratigraphical setting, are more highland-like. Not surprisingly, light plains were seen as both volcanic and impact related deposits. The Cayley Plains, a type locality in the central-nearside highlands, has been chosen as the landing site of the Apollo XVI mission partly to help resolve these interpretations. The astronauts collected samples - highly brecciated rocks - and concluded that the Cayley was indeed of impact origin. These findings have been extrapolated to stratigraphically similar plains units on the nearside, focusing on the Imbrium and Orientale impacts as responsible events for resurfacing terrae environment to form light plains. In addition, theoretical modeling mechanisms have been provided that could explain how basin and crater ejecta were able to make up for the smoothness of light plains by stirring up local material through secondary-impact related processes, or mega impact induced seismic shaking. However, subsequent age determinations showed that some light plains cannot be correlated to the Imbrium or Orientale event, the last two basin-forming impacts to occur on the Moon that had the ability to resurface areas thousands of kilometers away from their target site. An unknown form of highland volcanism was proposed as a contributing process in light-plains formation. The question remained unanswered whether processes other than impact-related processes were responsible for the formation of these enigmatic geological units, and how these processes might have worked. Focusing on light plains in the northern-nearside highlands, a chronological approach has been chosen to address these questions, and compositional information from multispectral data has been included to support our investigations. Mapping the northern-nearside light plains, earlier workers have recognized the stratigraphically and morphologically obvious bimodal distribution of smooth terrae units north and northeast of Mare Frigoris. The older of these plains (IP-1; based on stratigraphic relations and surface-crater densities) show gradual transition into (Imbrium-impact) Fra Mauro Formation units, whereas the younger unit (IP-2) cannot be related to this relatively nearby impact event. Instead, it was proposed that the impact of Orientale, despite the fact it is several thousand kilometers away, could have smoothed these terrae units with its ejecta stirring up local highland material, an interpretation that seems to be not very compelling, as these younger plains show quite homogeneous surfaces over extended areas. Determining precise surface ages of smooth surface units should help with getting a chronology of plains emplacement in these latitudes. Based on the principle of crater-frequency distribution measurements and adjusting the cumulative crater-frequency distributions to a lunar standard distribution, and "fixing" the Orientale and Imbrium event with the absolute ages obtained by radiometric measurements of Apollo samples to this distribution, one is able to determine reliable absolute-age data for surfaces after measuring the crater-frequency distribution on it. As the decline in impact frequency is rather steep during the first billion years in lunar history, the discrimination between surfaces of different age can be determined to a relatively high degree of reliability. Forty-five smooth plains areas north and northeast of Mare Frigoris have been mapped for crater frequency. Some light plains have been disturbed by secondary cratering impact to a degree that made deduction of reliable age data impossible, so only 27 crater counts yielded useful absolute ages. A histogram of how these plains ages are distributed is shown. For reference, the Imbrium and Orientale events are marked at their respective time of impact at 3.90 Ga, and 3.84 Ga respectively. There are several observations that can be made in this distribution: (1) the ages do not cluster around one or two peaks; instead, the majority of light plains formed in a rather broad time span between 4.0 Ga and 3.65 Ga; (2) there are some light plains that seemed to have formed before the Imbrium impact event; ( 3) light plains formation peaked at the time of, or shortly after, the Orientale impact event; (4) light plains formation continued a significant time span after the Orientale basin impact; (5) there are light plains ages that are remarkably younger than the age of the Orientale basin, exposing surface ages that are only slightly older than the mare-basalt ages of their neighboring units at the eastern margin of Mare Frigoris; and (6) the time span of light-plains surface formation seems to extend from around 3.95-4.0 Ga to 3.60-3.65 Ga. (Additional information contained in original)
Gerhard Neukum
Head James W. III
Koehler U.
Wolf Ursula
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