Age and origin of the lunar northern light plains

AGE AND ORIGIN OF THE LUNAR NORTHERN LIGHT PLAINS. C. M. Poehler1, H. Hiesinger1, and C. H. van der Bogert1.1Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany ([email protected]). Introduction: Lunar light plains are morphologically similar to dark mare plains, but differ in albedo, namely the light plains are brighter and have an albedo closer to the adjacent highlands [1]. Due to the morphological similarity a volcanic origin for light plains was proposed [2]. However, the Apollo 16 mission revealed that light plains consist mostly of impact breccias [3]. This led to several new hypotheses proposing an origin by one or two basin-forming events or several local impact events, and the deposition of fluidized ejecta [e.g., 1,4,5,6]. Meanwhile, other lines of evidence still seem to support a volcanic origin [7,8]. To investigate whether light plains have common ages and/or origins, we determined the absolute model ages (AMAs) of light plains in the northern hemisphere (see also [9]) to expand on existing data for light plains close to the equator [7], in the southern hemisphere [10], on the northern nearside [8], and around Orientale basin [11]. Method: On the basis of Lunar Reconnaissance Orbiter Wide Angle Camera image data (100 m/pixel) [11], we identified and characterized the light plains in our study area extending from the north pole to 45°N (Fig. 1). Shadowing effects prohibited light plains identification above 80°N latitude. On the nearside, mare basalts in Mare Frigoris and Oceanus Procellarum cover the area up to about 60°N. We performed crater size-frequency distribution (CSFD) measurements and determined absolute model ages (AMAs) using the production and chronology functions of [12,13,14]. The CSFD measurements were made using CraterTool [15] in ArcGIS, and we used Craterstats [16] to determine the corresponding AMAs. The light plains units chosen for CSFD measurements were carefully examined for homogenous surfaces and special care was given to exclude secondary craters. Results: We determined AMAs for 24 light plains occurrences. The light plains show AMAs ranging from 3.43+0.096/-0.28 to 3.98+0.019/-0.022 Ga (Figs. 1, 2). The light plains were divided into three different units Ip, Ip1, and Ip2 by [17]. Unit Ip1 was described as the older stratigraphic light plains unit with more craters than the younger Ip2. Light plains that cannot be distinguished into an older or younger unit were grouped as Ip [17]. We measured eight occurrences for each Ip type. Plains type Ip1 shows both the youngest (3.43 Ga) and oldest (3.98 Ga) AMAs. Some areas show signs of resurfacing in their CSFDs. For example, in Area 12 (61.1°N, 121.2°W), the light plains in a 40-km diameter crater gives two ages: 3.98+0.06/-0.071 Ga, representing an underlying older surface, and 3.43+0.096/- 0.28 Ga, interpreted as the emplacement age of the light plains. Craters with a diameter smaller than 400 m appear to be in equilibrium. Two other areas also give two ages for the underlying surfaces: 3.93/3.60 Ga and 3.72/3.64 Ga. The older ages could indicate the presence of underlying cryptomare. From the areas mapped as Ip1, four occur as crater fills and four are on intercrater plains. Some of the areas mapped as Ip1 show CSFDs that can be easily fit to derive an AMA. The other areas mapped as Ip1 seem to be either (1) affected by secondary cratering, which appears as small bulges in the cumulative CSFDs, or (2) exhibit equilibrium at small crater diameters. The Ip2 plains type occurs mostly on the nearside and to the east of the Ip1 units. Areas mapped as Ip2 show AMAs between 3.51 and 3.84 Ga. Five of these light plains fill large craters; the other three are intercrater plains. While Ip2 was proposed to be younger than Ip1 [17], the ages obtained in this study do not confirm this stratigraphic relationship, because both unit types span about the same time interval. However, in small-scale areas some differences can be observed. Fig. 1. Locations and absolute model ages (AMAs) for light plains units in our study region from 45°N-80°N shown on LRO WAC mosaics [NASA/GSFC/ASU]. Two areas directly east of Meton crater were selected to look at the small-scale age differences between the different types of light plains units. These areas border each other and were mapped as Ip1 and Ip2 [18]. Ages obtained for these plains show a younger AMA of 3.71+0.025/-0.03 Ga for the Ip2, whereas the AMA for the Ip1 is 3.84+0.32/-0.42 Ga. Thus, in this case we can confirm a stratigraphic relationship where Ip2 is younger than Ip1. On a larger scale, as shown in Fig. 2, this stratigraphy could not be supported since both Ip1 and Ip2 include light plains of both older and younger ages. Discussion: The two youngest large basins the Moon are Orientale (3.68-3.8 Ga [18,19]) and Imbrium basin (3.85 Ga [18]).The long time span over which the light plains of the study area formed excludes an origin by one or even two major events. With several light plains postdating the formation of the youngest basin (Orientale), an origin solely via basin forming events can also be excluded. Nevertheless, our data shows a peak in the number of plains formed around 3.8 Ga. This peak becomes even more significant when combining our data with previous AMA determinations in other areas of the Moon (Fig. 3) [7,8,9,10]. The peak in ages around 3.8 Ga correlate to Orientale and possibly Imbrium basin formation, which might indicate a contribution to the formation of light plains by either one of these events either directly as ejecta or by triggering volcanism in those areas. To investigate a potential volcanic origin of the light plains, we looked for volcanic features in the light plains in our study. While we were able to identify some structures that could be wrinkle ridges and might be linked to volcanic activity, most light plains in the study area did not show any evidence of typical volcanic features. On the basis of all available CSFD measurements for light plains [7-10, this work], it appears that light plains in the northern hemisphere might be younger than those in the southern hemisphere. In addition, light plains in the western hemisphere seem to be younger than those in the eastern hemisphere. This supports an origin related to the Orientale basin for light plains units close to the basin. Conclusions: The absence of volcanic features in the areas we studied supports an impact, rather than a volcanic origin, for light plains materials. The broad range of ages for light plains across the Moon precludes an origin via a small number of basin-forming events. Nevertheless, groupings of ages are likely associated with particular basin events, such as Orientale. Acknowledgements: This project receives funding from the European Union Horizon 2020 research and innovation programme under grant agreement No 776276. References: [1] Eggleton and Schaber (1972) Apollo 16 PSR, 29-7–2916. [2] Wilhelms and McCauley (1971) USGS I-703.. [3] Muehlenberger et al. (1972) Apollo 16 PSR, 6-1–6-8. [4] Oberbeck et al. (1974) PLPSC 5, 111-136. [5] Head (1974) Moon 11, 327-356. [6] Oberbeck et al. (1974) PLPSC 5, 111-136. [7] Neukum (1977) Moon 17, 383-393. [8] Köhler et al. (2000) LPSC 31, 1822. [9] Pöhler et al. (2019) LPSC 50, 2310. [10] Hiesinger et al. (2013) LPSC 44, 2827. [11] Meyer et al. (2016) Icarus 273, 135-145. [11] Robinson et al. (2010) Space Sci. Rev. 150, 81-124. [12] Neukum et al. (2001) Space Sci. Rev. 96, 5586. [13] Hiesinger et al. (2000) JGR 105, 29239-29276. [14] Crater Analysis Working Group (1979) Icarus 37, 467-474 [15] Kneissl et al. (2011) PSS 59, 1243-1254. [16] Michael and Neukum (2010) EPSL 294, 223-229.. [17] Lucchitta (1978) USGS I-1062. [18] Whitten et al. (2011) JGR 116, E6. [19]Wilhelms (1987) USGS Prof. P 1348. Fig. 2. Age distribution of light plains AMAs in the northern lunar hemisphere sorted into units according to [17]. Fig. 3. Distribution of light plains AMAs in this study combined with [7,8,9,10].