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Chang’E-5 reveals the Moon’s secrets to a longer life

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Ross N. Mitchell
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Chang’E-5 reveals the Moon’s
secrets to a longer life
Ross N. Mitchell
1,2,
*
1
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*Correspondence: ross.mitchell@mail.iggcas.ac.cn
Received: October 17, 2021; Accepted: October 18, 2021; Published Online: October 22, 2021; https://doi.org/10.1016/j.xinn.2021.100177
ª2021 The Author. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Citation: Mitchell R.N. (2021). Chang’E-5 reveals the Moon’s secrets to a longer life. The Innovation 2(4), 100177.
The Moon is not only important because it enchants us in the night sky.
The Earth-Moon system is critical for Earth’s surface environment and
even life, with lunar gravity controlling Earth’s tides and rate of rotation. The
origin of the Moon, from either a giant impact or a shrinking hot debris
disk, remains debated and carries implications for either early Earth or
even Earth’s origin, respectively. The Apollo and Luna missions provided
abundant samples informing the earliest lunar history, namely the evolution
of the “lunar magma ocean,”where less dense plagioclase crystallized and
floated up to the surface, lending most of the lunar surface its white appear-
ance from this anorthositic crust, but the dark patches on the Moon—its
“mare”basalts, named for the large “sea”of volcanic plains—record the rest
of lunar history. The youngest rocks from the Apollo and Luna missions
and lunar meteorites are only dated at 2.8–2.9 billion years ago (Ga)
(Figure 1A).
China’sChang’E-5 mission thus sought a new frontier: discover the
younger rocks on the Moon to see what they could reveal about the longevity
of lunar geologic activity. Based on crater counting chronology (see below),
the selected landing site targeted potentially some of the youngest rocks
on the Moon (Figure 1B). It was first critical to establish, as published in Na-
tional Science Review,
1
that the samples collected by Chang’E-5 from the
highly impacted lunar regolith indeed had compositional affinity to mare ba-
salts. How long did mare volcanism last, and, if volcanic activity persisted
quite late in lunar history, what was the Moon’s secret to a longer life? These
are the questions that the Chang’E-5 samples, the first lunar samples re-
turned to Earth in 44 years, can begin to answer. However, as with any
new data, as some questions are answered, new ones always arise.
Youngest Moon rocks
Two geochronology studies have been conducted on the Chang’E-5 ba-
salts, both employing the Pb-Pb isochron method (
204
Pb/
206
Pb versus
207
Pb/
206
Pb), which has proven reliable in the study of Apollo basalts. The
first geochronology study of Chang’E-5 was published in Science
2
on October
7
th
and reported two types of ages: one age (1,963 ±57 million years ago
[Ma]) is based on 18 analyses of two rock fragments; the other age
(2,011 ±50 Ma) relies exclusively on Zr-bearing minerals ideal for dating
due to U enrichment with negligible non-radiogenic Pb (lunar initial Pb).
The second study reporting Chang’E-5 geochronology, published in Nature
3
on October 19
th
, yielded a strikingly precise age of 2,030 ±4Mabasedon
51 grains of Zr-rich minerals from 47 rock fragments. The age on Zr-rich min-
erals from the Science study
2
agrees well with the robust 2,030 ±4Maage.
3
Zirconium-bearing minerals were only <5mm wide, thus the 3-mm spot size
of one study,
3
compared with 7mmintheother,
2
also provided additional
accuracy by avoiding terrestrial, laboratory-derived Pb contamination along
grain boundaries. Irrespectively, the two independent laboratories in Beijing
arrived at indistinguishable ages within uncertainty. The Chang’E-5 basalts
are thus the youngest dated Moon rocks and they extend the duration of
mare volcanism 800–900 million years longer than previously known
(Figure 1A).
Cratering chronology confirmation
The Moon is a unique resource for our being able to date the surfaces of
other moons and planets in the solar system, some of which can only be
dated using “crater counting”chronology. Generally, older surfaces have
ABC
Figure 1. Age and location of Chang’E-5 (A) Timeline of lunar history. LMO, lunar magma ocean. Any volcanism after 2 Ga is highly localized. Cratering ages (B) and
thorium contents (C) of mare basalts. In (A)–(C), red dots indicate the age and location of Chang’E-5. In (B)–(C), landing sites are indicated with dots for Apollo (light blue)
and Luna (dark blue).
ll The Innovation 2, 100177, November 28, 2021 1
The Innovation
Commentary
accumulated more impact craters and younger surfaces have less. However,
translating the crater count of a rocky region into an approximate age relies
heavily on the impact cratering flux in the solar system. Luckily, not only does
the Moon preserve a long impacting history but it is close enough that we are
able to sample rocks and precisely date them with radioisotopic dating. This
allows the cratering chronology curve to be calibrated. If the impacting rate
were constant, then the curve would be straight, but, based on Apollo sam-
ples, it is not, making calibration critical. Specifically, between the old and
young ages of these existing samples, the cratering curve exhibits a signifi-
cant change in shape, but there was a 2-billion-year-long gap in dated sam-
ples, leaving this critical portion of the curve unanchored. The 2 Ga
Chang’E-5 age squarely fills this gap and anchors the cratering chronology
curve.
2,3
A mechanism for young volcanism
Dating the youngest mare basalts thus begs the question of how volcanic
activity on the Moon was sustained for so long. The small Moon is less than
30% the radius of Earth. With such a large surface-to-volume ratio (3/r,where
ris the sphere’s radius), the Moon should have cooled rapidly and become
geologically inactive relatively early. Nonetheless, three isotopic systems
(U-Pb, Sm-Nd, and Rb-Sr) all consistently indicate that the young Chang’E-5
basalts formed by the melting of the lunar mantle.
3,4
How did the interior
of the Moon keep warm for so long? On the same day as the second geochro-
nology study,
3
two other studies were published in Nature
4,5
that investigated
two possible mechanisms for how volcanic activity on the Moon lasted as
late as 2 Ga.
Is the lunar mantle wet or dry?
One possible explanationis elevated water contents in the Chang’E-5 lunar
mantle source because water lowers the melting temperature, thereby
requiring less heat to generate melt. Wat er abundances of the Moon’sinterior
are a highly controversial topic, with estimates spanning three orders of
magnitude that range between an essentially anhydrous lunar mantle to
very elevated water abundances on par with Earth’s mantle. Attaining the wa-
ter content from the lunar mantle as calculated from the measured contents
of the basaltic rocks requires backtracking the effects of magmatic evolution.
The measurements of both volatile (namely, water) contents and hydrogen
isotopes in both late-forming apatite minerals and ilmenite-hosted melt inclu-
sions are together able to track the zigzagging of water contents back to the
source. Following mantle melting, three magmatic stages cause water con-
tents to increase (fractional crystallization), decrease (H
2
degassing during
melt extraction), and then increase again (crystallization of apatite). Back-
stripping these effects, a maximum water abundance of 1–5mgg
1
is esti-
matedforthemantlesourceoftheChang’E-5 basalts.
5
Compared with
Apollo basalts from ca. 4.0–2.8 Ga, the very modest water abundances of
themantlesourceoftheChang’E-5 basalts are among the lowest measured.
Such dry mantle thus rules out any role for water in generating the Moon’s
youngest basalts.
Non-KREEP origin
Another possible explanation for young lunar volcanism is an old and pop-
ular idea inspired by several lines of evidence. Within the mare basalts lies the
Procellarum KREEP Terrane, where the landing site of Chang’E-5 was specif-
ically chosen. KREEP is an acronym for a distinctive geochemical component
of some lunar rocksrich in the elements K, rare-earth elements (REEs), and P.
During the evolution of the lunar magma ocean, as plagioclase floatedupinto
the crust and mafic cumulates sunk down into the mantle, incompatible
elements like K and P got enriched in the residual melt, thereby potentially
leaving a KREEP-rich layer in between the mantle and crust. If abundant in
the upper mantle beneath Chang’E-5, radiogenic elements (U, K, and Th)
may have provided sufficient heat to melt the lunar mantle at such a young
age. Orbital observations indicate that Th is abundant at the surface in the
region (Figure 1C), but whether the underlying mantle is also radiogenic is un-
known. Furthermore, the KREEP component has never been found as a bona
fide rock type, nor hasit been tested from samples of young mare basalt. The
Chang’E-5 basalts thus offered a promising test of the idea of radiogenic
heating, but the results are quite unexpected. At face value, the Chang’E-5
basalts are indeed richer in incompatible elements than the Apollo and
Luna basalts. However, as extensive fractional crystallization can also
achieve this concentration, such an observation can arguably be a false-pos-
itive identification of the contribution of KREEP in the mantle source. Thus,
isotopes resilient to magmatic evolution must be used. Both Rb-Sr and
Sm-Nd isotopes indicate a depleted mantle source,
4
in stark contrast with
a KREEP affinity, which is also consistent with reported Pb isotopes.
3
The
young Chang’E-5 basalts from the KREEP Terrane have a non-KREEP origin.
4
Thus, the prevailing notion of heat-producing elements melting the lunar
mantle cannot account for young volcanic activity.
A convection mechanism?
Chang’E-5 thus rules out two leading mechanisms for the Moon’syoun-
gest volcanism. Rejecting hypotheses is science at its most definitive, but
of course the enigma of the Moon’s geologic longevity now only deepens.
Both the water and KREEP hypotheses are related to shallow mantle compo-
sition. Another proposed mechanism, the lunar megaregolith as a poor
conductor, is an even shallower explanation. Alternatively, one might look,
literally, for a deeper explanation. Mare basalts are notably only found on
the nearside of the Moon. This hemispheric asymmetry may be attributed
to degree 1 mantle convection (one upwelling, one downwelling), where the
upwelling of core-heated mantle on the nearside caused the melting for
mare volcanism. Whether the Sm-Nd isotopes or the mantle water content
of Chang’E-5 at a much younger age support the continuation of a mantle
convection cycle can be further investigated. Pending the discovery of sup-
porting evidence, if lunar mantle convection was sluggish, it could have
delayed the dissipation of internal heat.
REFERENCES
1. Li, C., et al. (2021). Characteristics of the lunar samples returned by Chang’E-5 mission.
Natl. Sci. Rev. nwab188.
2. Che, X., et al. (2021). Age and composition of young basalts on the Moon, measured
from samples returned by Chang’e-5. Science, eabl7957. https://doi.org/10.1126/sci-
ence.abl7957.
3. Li, Q.L., et al. (2021). Two billion-year-old volcanism on the Moon from Chang'E-5
basalts. Nature. https://doi.org/10.1038/s41586-021-04100-2.
4. Tian, H.C., Wang, H., Chen, Y., et al. (2021). Non-KREEP origin for Chang’E-5 basalts in
the Procellarum KREEP Terrane. Nature. https://doi.org/10.1038/s41586-021-04119-5.
5. Hu, S., He, H., Ji, J., et al. (2021). A dry lunar mantle reservoir for young mare basalts of
Chang’E-5. Nature. https://doi.org/10.1038/s41586-021-04107-9.
ACKNOWLEDGMENTS
Wei Yang provided assistance with data collection. Wei Yang and Zichen Xu provided
assistance with figure preparation. Funding was provided by the Natural National Science
Foundation of China (41888101, 41890833) and the Key Research Program of the Institute
of Geology & Geophysics, Chinese Academy of Sciences (IGGCAS-201905).
Commentary
2The Innovation 2, 100177, November 28, 2021 www.cell.com/the-innovation
The Innovation
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Two billion-year-old volcanism on the Moon from Chang’E-5 basalts
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A dry lunar mantle reservoir for young mare basalts of Chang’E-5
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  • NATURE
The distribution of water in the Moon’s interior carries implications for the origin of the Moon1, the crystallisation of the lunar magma ocean2, and the duration of lunar volcanism2. The Chang’E-5 (CE5) mission returned the youngest mare basalt samples, dated at 2.0 billion years ago (Ga)3, from the northwestern Procellarum KREEP Terrane (PKT), providing a probe into the spatiotemporal evolution of lunar water. Here we report the water abundances and hydrogen isotope compositions of apatite and ilmenite-hosted melt inclusions from CE5 basalts. We derive a maximum water abundance of 283 ± 22 μg.g-1 and a δD value of -330 ± 190‰ for the parent magma. Accounting for a low degree partial melting of the depleted mantle followed by extensive magma fractional crystallisation4, we estimate a maximum mantle water abundance of 1-5 μg.g-1, suggesting that the Moon’s youngest volcanism was not driven by abundant water in its mantle source. Such modest water contents for the CE5 basalt mantle source region is at the low end of the range estimated from mare basalts that erupted from ca. 4.0-2.8 Ga5,6, suggesting that the mantle source of CE5 basalts had become dehydrated by 2.0 Ga through previous melt extraction from the PKT mantle during prolonged volcanic activity.
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Age and composition of young basalts on the Moon, measured from samples returned by Chang'e-5
Article
  • Oct 2021
  • SCIENCE
Sample return shows late lunar volcanism Measuring physical samples of Solar System bodies in the laboratory provides more information than is possible from remote sensing alone. In December 2020, the Chang’e-5 mission landed on the Moon, collected samples, and returned them to Earth. Che et al . analyze two fragments of volcanic lunar basalt collected by Chang’e-5. Radiometric dating using lead isotopes indicated that the rocks formed from magma that erupted about 2 billion years ago, later than other volcanic lunar samples. The abundance of extinct radioactive elements in the rock is too low for radioactive heating to have produced the magma. Another, thus far unknown, source must be responsible for the late lunar volcanism. —KTS
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