Chang��e-5 lunar samples shed new light on the Moon

Abstract

<p>The Chang��e-5 (CE-5) mission, the first return of lunar samples to Earth since the Apollo and Luna missions more than 44 years ago, landed on one of the youngest mare basalt units (1.0-3.0 Ga, based on superposed crater counts), located at middle latitude (~43��N) far from previous landing sites. On December 17, 2020, the sample capsule returned to Earth with 1731 grams of lunar soil collected from the upper few centimeters of the surface and from an ~1 meter-long core drilled into the lunar regolith. This paper summarizes the main discoveries of the CE-5 samples allocated since July 12, 2021, and measured with state-of-the-art analytical techniques. Physical property studies indicate that the CE-5 soil is mature, with a peak particle size of ~50 ?m (in volume), and a particle size distribution similar to the sub-mature and mature Apollo lunar soils (<1 cm). The soil sample contains basalt and mineral fragments, impact melt breccia, agglutinates, and glasses. The basalt fragments can be divided into several petrographic types, likely crystallized from the same lava flow at different depths and cooling rates. The CE-5 basalt Pb/Pb SIMS analyses yielded a crystallization age of 2.030 �� 0.004 Ga, extending the duration of lunar volcanic activity by ~1.0~0.8 Ga. This age, in turn, has helped to calibrate the widely applied lunar crater chronology model. The isotopic ratios of Pb, Nd and Sr indicate that the contribution of a KREEP component in forming CE-5 basalt is limited (<0.5%), excluding high concentrations of heat-producing radioactive elements in their mantle source. The isotope analyses of H, Cl, and S reveal that the mantle source is dry, which cannot account for the prolonged volcanism observed in the CE-5 landing region. A possible explanation is that the CE-5 mantle source contains enhanced clinopyroxene-ilmenite cumulate (~20%), which reduces the melting temperature by ~80��C. The REE-, FeO-enrichment of the CE-5 basalt can be attributed to a low degree of partial melting followed by extensive fractional crystallization. The CE-5 soil has also recorded a two-billion-year history of meteorite impact and solar wind irradiation. A few exotic fragments have been recognized (some with high-pressure silica phases) and are likely ejected from distant lunar highlands. The U-Pb dating of impact glass beads reveals at least 17 main impact events. New space weathering effects, especially the formation of Fe³⁺, have been found. <i>In situ</i> reflectance spectra and laboratory analyses of CE-5 soil show the presence of water (in the form of H, OH, and/or H₂O). The solar wind hydrogen was implanted and concentrated in the outermost rims (<100 nm) of soil grains, with a temperature (hence latitude)-dependent maximum water concentration of up to ~2 wt%.</p>