Yuqi Qian, and 5 co-authors, reported in early 2021 thatChang’e-5, China’s first lunar sample return mission, is targeted to land in northern Oceanus Procellarum, within a region selected on the basis of:
1) its location away from the Apollo-Luna sampling region,
2) the presence of the Procellarum KREEP Terrane (PKT),
3) the occurrence of one of the youngest lunar mare basalts (Em4), and
4) its association with Rima Sharp.
In order to provide context for returned sample analyses, the authors conducted a comprehensive study of the regional and global settings, geomorphology, composition, mineralogy, and chronology of the Em4 mare basalts. Superposed on Imbrian-aged low-Ti basalts, Em4 covers 37,000 km2 and is composed of Eratosthenian-aged (∼1.53 Ga), high-Ti basalts with a mean thickness of ∼51 m and a volume between ∼1450 and 2350 km3. Minor variations in TiO2 and FeO abundance occur within the unit and the thorium content averages ∼6.7 ppm, typical of PKT mare basaltic regolith. No specific source vents (e.g., fissures, cones, domes) were found within the unit.
The authors showed that Rima Sharp is actually composed of three major rilles, whose source vents are located outside of, and which flow into, and merge in Em4, suggesting that they may be among the sources for Em4. Regolith thickness averages ∼7 m and there is abundant evidence for vertical and lateral mixing; the most likely sources of distal ejecta are Aristarchus, Harpalus, and Sharp B craters. Returned samples from local and distant materials delivered by impact will thus provide significant new insights into lunar geochronology, inner Solar System impact fluxes, the age of very young mare basalts, the role of the PKT in the generation of mare basalts, the role of sinuous rilles in lava flow emplacement, and the thermal evolution of the Moon.
The authors concluded that:
1) The CE-5 landing region is within the distinctive northern Oceanus Procellarum region, which:
a) lies within the unique PKT,
b) has the highest concentration of red spots,
c) contains the greatest mare basalt age range on the Moon,
d) is the site of the youngest mare basalts,
e) shows a diversity of smaller gravity anomalies, and
f) is characterized by a distinct polygonal gravity gradient anomaly.
2) The Em4 unit is in the east of the CE-5 landing region and covers an area of 37,000 km2 with a mean thickness of ∼51 m and has a volume between ∼1,450 and 2,350 km3. ∼7 m of Thrich regolith develops on the top of the Em4 mare basalts. Distal materials are mixed with the regolith by impacts, mostly from Aristarchus, Harpalus, and Sharp B craters.
3) Em4 mare basalts have a mean age of ∼ 1.53 Ga, with some variations, and the youngest mare basalts are located to the south, northwest, and northeast of Em4. The TiO2 and FeO (ilmenite) contents have a slight trend toward decreasing abundance with time. Em4 is the youngest mare unit in the landing region (∼1.53 Ga), and is thus a priority target for the CE-5 mission.
4) Em4 mare basalts are a type of high-Ti lunar mare basalt located in the PKT, but not typical of that terrain. CPX is the dominant mafic mineral, followed by OPX and OLV. CPX, OPX, OLV, and PLG values are homogeneous within Em4; the highest ilmenite (TiO2) abundances occur in the eastern part of Em4 near the Mairan domes and in southwest Em4.
5) No obvious volcanic source vents within the unit are found that could have contributed to the Em4 mare basalts.
The authors found that Rima Sharp is actually composed of three major rilles (Rima Sharp, Rima Mairan, and Rima Louville); source vents of each rille are
located outside Em4, but the rilles enter Em4, and are likely to be among the most important sources for the unit.
6) Because of the distinctive characteristics of the Em4 mare basalts, any samples returned from Em4 for terrestrial laboratory analysis will address a very wide range of fundamental problems regarding the geological and thermal evolution of the Moon.
This study provides a fundamental guide for future researchers.
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