Lunar and Planetary Science XXXI 1822.pdf

LUNAR LIGHT PLAINS IN THE NORTHERN NEARSIDE LATITUDES: LATEST RESULTS ON AGE DISTRIBUTIONS, SURFACE COMPOSITION, NATURE, AND POSSIBLE ORIGIN. U. Koehler1,2, J.W. Head III 1, G. Neukum2, U. Wolf2; 1Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912, U.S.A. (ulrich_kohler@brown.edu), 2DLR, Rutherfordstrasse 2, 12489 Berlin, Germany.

Abstract: Surfaces of light plains in the north- local material through processes related to secondary- ern latitudes of the lunar nearside do not show a uni- impact [7], or even mega-impact induced seismic form age distribution. Instead, these ages vary over a shaking [8]. Consequently, the initially preferred time range of more than 350 My, with units being interpretation of a volcanic origin has been com- significantly younger than the Orientale impact event. pletely outruled. But despite these findings, the ori- Multispectral observations show compositional in- gin and nature of lunar light plains seem not conclu- homogeneities. It is concluded that the origin of light sively resolved. This work’s focus is on ages and plains cannot exclusively be attributed to ejecta depo- composition of light plains in the northern nearside sition of the last basin-forming events on the Moon. hemisphere as a key area for addressing specific Introduction: Light plains cover about five questions related to these enigmatic units. In par- percent of the lunar terrae [1,2]. Their widespread ticular, with measured crater-size frequency distribu- distribution, the smooth surfaces—embedded in tions it can be shown that ejecta redistribution from much rougher highland—and albedo values distinc- only the two last basin-forming events is not suffi- tively lower than the surroundings (but higher than cient to explain resurfaced plains units of broadly mare-basalt), give light plains some importance in varying ages. In addition, results from multispectral understanding the geologic history of the Moon dur- data show that northern light plains are not of homo- ing and past the period of the late heavy bombard- geneous surface composition. As a consequence, ment. From their stratigraphic position relative to additional processes to ejecta redistribution from ba- horizons like the “Imbrian Sculpture”, it seems quite sin impacts are considered in this study. obvious that many light plains are related to the pro- Ages of Light Plains: Age determinations by cesses of impact-ejecta deposition following the for- the method of crater-size frequency distribution mation of the youngest basins on the lunar surface. measurements (calibrated to the radiometric ages of The events that led to the formation of the Imbrium Apollo samples) of light plains showed that the time or Orientale basin are the most likely candidates re- of resurfacing is not uniformly distributed in the cen- sponsible for this process. This conclusions is mainly tral highlands [9]. In many cases these ages do not based on investigations of the plains in the equatorial match the estimated time of the Imbrium respectively nearside highland. But similar interpretations can be Orientale impact, the two last events of global conse- made for the plains in the polar latitudes [3]. quences on the Moon. Instead, volcanism occurring But there has been controversy over nature and in this highland environment has again been proposed origin of lunar light plains since three decades. Mor- [10] to contribute to the formation of some light phological properties like smoothness, low crater plains. Neither do surface ages in the northern lati- densities, and occurrence in topographic lows are tudes show a uniform distribution. Earlier workers more mare-like. Other features such as relative [3] have recognized the bimodal age distribution of (compared to mare basalts) high albedo and geologi- smooth terra units north and northeast of Mare Frig- cal setting within terra environment are more typical oris. The older of these plains (unit Ip1) show a grad- of highland. The prevailing view, though, was that ual transition into the Fra Mauro Formation of Im- the light plains’ origin was of volcanic nature. Partly brium-impact origin, whereas the younger unit (Ip2) in order to confirm this a typical plains surface has is more difficult to be related with the Imbrium event. been chosen as target for the Apollo 16 mission: The It was therefore suggested that Ip2 plains might have Cayley Plains in the central-nearside highlands. The their origin in the Orientale impact event [6]. astronauts, surprisingly, returned brecciated rocks Fourty five plains north of Mare Frigoris have which implied an origin related to (basin) impact [4], been mapped for crater-frequency counts on Lunar consequently then favoured as theory over that of Orbiter IV frames. Areas with unreliable data (poor lunar highland volcanism (summarized in [5]). image quality, units too small, high number of secon- The findings have been extrapolated to strati- dary-impact craters) have been excluded, resulting in graphically similar plains units on the nearside, with 27 new crater statistics. The following observations the Imbrium and Orientale impact events as likely are considered most interesting: 1.) The ages do not sources for the resurfacing of topographic lows [6]. cluster around one or two peaks, as should be ex- In addition, theoretical modelling and observations pected if the light plains formation is directly related from nuclear explosions on Earth provided mecha- to the impacts of the Imbrium and/or Orientale basin. nisms that could explain how basin and crater ejecta 2.) Instead, the time span ranges from about 4.0 were able to make up for the smoothness and vast By—slightly older than the Imbrium impact—to 3.65 extent of light plains by stirring up large quantities of By, which is clearly younger than the impact age of Lunar and Planetary Science XXXI 1822.pdf

NORTH-POLAR LUNAR LIGHT PLAINS: U. Koehler et al.

Orientale, assumed to have occurred at ~3.84 By. 3.) inside the Imbrium basin, have to be considered. Areally, a significant proportion of light plains Conclusions: The fact that we are dealing with formed post-Orientale. 4.) A correlation between light plains varying in age over more than 350 mil- ages and geographic position can be seen: plains lion years is a strong argument for assuming that the >3.90 By are focused near the pole, in and around the ejecta–distribution from the Imbrium respectively craters Byrd, Main, and Scoresby; plains £3.90 By Orientale impact event cannot exclusively be respon- and ³ 3.80 By can be found in the center of the north- sible for resurfacing the highland terrain to form the ern highlands, filling craters like Meton, Barrow, light plains we see today. The deposition, and subse- Arnold, Baillaud, and Neison; and plains £3.80 By quent processes of secondary impact of ejecta from can be identified in the extended smooth plains area these impacts, though, very likely played a key role W and S of crater Gärtner, NE of Epigenes, E of De- in forming this unique morphology. Additional pro- Sitter, and the areas bordering Mare Frigoris. 5.) The cesses that may have contributed to the plains’ for- above age variations are similar to what can be ob- mation include ballistic erosion and sedimentation served at the central-nearside highland plains [11]. from regional late Imbrian and early Eratosthenian Multispectral Data: High-resolution multis- impacts, and the incorporation of products from non- pectral data from the Galileo and Clementine mis- mare volcanism yet not identified. sions provided compositional information to our References: [1] Wilhelms D.E. and McCauley J.F. know-ledge. Based on previous work [12], and con- (1971), USGS Misc. Geol. Inv. Map, I-703. [2] Howard et firmed by these studies, it can be seen that the north al. (1974), Rev. Geophys. Space Phys., 12, 309 [3] Luc- polar plains show a distinctive albedo behaviour. chitta B.K. (1978), USGS Misc. Geol. Inv. Map, I-1062. Albedo variations between different plains are mini- [4] Young J.W. et al. (1972), Apollo 16 Prelim. Sci. Rep., 5-1–5-6. [5] Wilhelms D.E. (1987), USGS Prof. Paper mal, significantly less than between highland units in 1348, 20-22, 215-220. [6] Boyce J. et al. (1974), LPS V the area. Selected spectra from small young impact Proc., Vol. 1, 11-23. [7] Oberbeck V.R. (1975), Rev. Geo- craters on light plains show that at least two distinct phys. Space Phys., 13, 337-362. [8] Schultz P.H. and Gault classes of light-plains surfaces exist in the area of D.E. (1975), The Moon, 12, 159-177. [9] Neukum G. et al. interest, one with typical highland characteristics, and (1975), The Moon, 12, 201-229. [10] Neukum G. (1977), the other with affinities to mare-basalt surfaces. The The Moon, 17, 383-393. [11] Koehler U. et al. (1993), LPS latter can be found in suspiciously close vicinity to XXIV abstr., 2, 813-814. [12] Mustard J.F. and Head J.W. the edge of Mare Frigoris, leading to the assumption (1995), LPS XXVI abstr., 2, 1023-1024. [13] Spudis P.D that cryptomare bodies are hidden under the light (1978), LPS IX Proc., 3, 3379-3394. plains’ brighter surface material. Though, the exis- Fig.1: Color-ratio composite of northern nearside lati- tence of one contiguous cryptomare unit, or several tudes from high-res Clementine VIS-NIR data. North is on smaller, non-contiguous units could not be verified top. Mare Frigoris is bright orange, light plains are dark based on the existence of dark-haloed craters so far orange, influenced by blue rays from impacts like Ana- (but remains a viable explanation for the mafic sig- xagoras (NW corner) or Thales (E margin). Highland is natures in the Galileo and Clementine spectra). In purple-blue. Area imaged is 0˚-50˚E/50˚-70˚N. Crater in this context forms of non-mare volcanisms, like the foreground is Aristoteles. possible occurrences on the Apennine Bench [13]