China's Chang'e-5 Landing Site: An Overview

<p><strong>Introduction</strong></p><p>The Chang’e-5 (CE-5) mission is China’s first lunar sample return mission. CE-5 landed at Northern Oceanus Procellarum (43.1°N, 51.8°W) on December 1, 2020, collected 1731 g of lunar samples, and returned to the Earth on December 17, 2020. The CE-5 landing site is ~170 km ENE of Mons Rümker [1], characterized by some of the youngest mare basalts (Em4/P58) on the Moon [2,3], which are never sampled by the Apollo or Luna missions [4]. This study describes the geologic background of the CE-5 landing site in order to provide context for the ongoing sample analysis.</p><p><strong>Northern Oceanus Procellarum</strong></p><p>Northern Oceanus Procellarum is in the northwest lunar nearside, and the center of the Procellarum-KREEP-Terrane [5], characterized by elevated heat-producing elements and prolonged volcanism. This region exhibits a huge volcanic complex, i.e., Mons Rümker [1], and two episodes of mare eruptions, i.e., Imbrian-aged low-Ti mare basalts in the west and Eratosthenian-aged high-Ti mare basalts (Em3 and Em4/P58) in the east [2]. The longest sinuous rille on the Moon [6], Rima Sharp, extends across Em4/P58. Both the Imbrian-aged (NW-SE) and Eratosthenian-aged (NE-SW) basalts display wrinkle ridges, indicating underlying structures, with different dominant orientations [2].</p><p><strong>Young Mare Basalts</strong></p><p>The Em4/P58 mare basaltic unit, on which CE-5 landed, is one of the youngest mare basalts on the Moon. Various researchers found different CSFD results; however, all of them point to an Eratosthenian age for Em4/P85 (1.21 Ga [2], 1.33 Ga [7,8], 1.53 Ga [3], 1.91 Ga [9]), and there are minor age variations across Em4/P58 [3]. Em4/P58 mare basalts have high-Ti, relatively high-olivine and high-Th abundances, while clinopyroxene is the most abundant mineral type [2,3]. Em4/P58 mare basalts cover an area of ~37,000 km<sup>2</sup>, with a mean thickness of ~51 m and volume of ~1450-2350 km<sup>3</sup> [3]. No specific source vents were found within the unit, and Rima Sharp is the most likely source region for the Em4/P58 mare basalts [3].</p><p><strong>Scientific Significance of the Returned Samples</strong></p><p>The scientific significance of the young mare basalts is summarized in our previous studies [2,3]. In [3], we first summarized the 27 fundamental questions that may be answered by the returned CE-5 samples, including questions about chronology, petrogenesis, regional setting, geodynamic & thermal evolution, and regolith formation (<strong>Tab. 1</strong> in [3]), especially calibrating the lunar chronology function, constraining the lunar dynamo status, unraveling the deep mantle properties, and assessing the Procellarum-KREEP-Terrain structures.</p><p><strong>References</strong></p><p>[1] Zhao J. et al. (2017) JGR, 122, 1419–1442. [2] Qian Y. et al (2018) JGR, 123, 1407–1430. [3] Qian Y. et al. (2021) EPSL, 555, 116702. [4] Tartèse R. et al. (2019) Space Sci. Rev., 215, 54. [5] Jolliff B. L. et al. (2000) JGR, 105, 4197–4216. [6] Hurwitz D. M. et al. (2013) Planet. Space Sci., 79–80, 1–38. [7] Hiesinger H. et al. (2003) JGR, 108, 1–1 (2003). [8] Hiesinger H. et al. (2011) Geol. Soc. Am., 477, 1–51. [9] Morota T. et al. (2011) EPSL, 302, 255–266.</p>

Lunar chronology as determined from the radiometric ages of returned lunar sample

We have heard earlier in this Discussion Meeting that from the systematics of Sr and Pb isotopes in lunar samples, it is possible to ascertain that the Moon had a solid crust about 4.6 Ga ago, that is, very soon after the formation of the solar system. In addition, it would seem that the major ring-basins on the Earth side of the Moon were all formed before 3.8 Ga ago. After the formation of the basins by impact, there was extensive magmatic activity in the form of basalt flows, expecially in the major ring-basins. About 3.1 Ga ago, all major lava-flow activity on the Moon had ceased. This outline of lunar chronology is accepted in practically all proposed interpretations of the radiometric ages. The first 600 Ma of lunar chronology is not as clear. During this time the multi-ring basins were formed. It has been proposed by G. Turner and by G. J. Wasserburg at this meeting, that the multi-ring basins formed in a rather short interval of time, perhaps as short as 100 Ma. This would imply an intense cratering rate some 3.9 Ga ago and a rapid decline in cratering rate thereafter. Such an event would have probably greatly altered the other bodies in the solar system, especially the Earth, and as such is of no small significance. It has been pointed out previously by Tera et al. (1974) that an alternative interpretation of the radiometric data is that widespread, simultaneous metamorphism (which the systematics of the lead isotopes in the highland rocks seems to imply) could as well result from a single widespread event such as Imbrium as from a multiple basin-forming sequence in a short period of time. In that case, the chronology of the multi-ring basins is an open question.

Outline of a lunar chronology

We present here an outline of lunar chronology and evolution based on analyses of the isotopic parent-daughter systems 87Rb-87Sr, U-Th-206Pb-207Pb-208Pb and 40K-40Ar. An overview of the chronology will first be given, followed by an outline of the observational basis. A more complete discussion of 40K-40Ar results and their interpretation is presented in the paper by G. Turner in this volume. While the body of data on lunar materials is limited, the chronology for lunar evolution appears to be rather well defined. The samples which have been investigated represent mare basalts [returned by the Apollo missions (11, 12, 15 and 17) and by the Soviet Luna 16 mission] and terra rocks, which include non-mare basalts, anorthosites, troctolites and norites but are predominantly comprised of complex breccias [returned by Apollo 12, 14, 16 and 17 and Luna 20]. The mare basalts are associated with the late stage lava flows which covered the mare basins. These flood basalts have been broken up by impact processes but for the most part are associated with the local areas and have not been subject to major transport or metamorphism by impact. The highland rocks predate the mare lava flows but are not clearly associated with a particular magmatic or impact process. They may have been excavated from considerable depths and transported over wide distances. Impact metamorphism is certainly one of the critical stages in their development.