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Chang’e 5 Lunar Sample Return Mission: A Brief Background and Summary:

James W. Head,

Department of Earth, Environmental and Planetary Science, Brown University,

Providence, RI 02912 USA (james_head@brown.edu)

What is Chang’e 5?: Chang’e 5 is a fundamental part of the Chinese Lunar Exploration Program (CLEP), a long term robotic program that will pave the way for further human and robotic exploration by the People’s Republic of China (PRC). Our colleague, geochemist and cosmochemist Academician OUYANG Ziyuan (Institute of Geochemistry, Chinese Academy of Sciences), is one of the parents of the CLEP program, advocating early on in the Chinese Academy of Sciences (CAS) and government circles for scientific and resource exploration of the Moon. The program is administered by the Chinese National Space Administration (CNSA) and has several major evolutionary elements: Phase 1-Orbital missions (Chang’e 1 and 2); Phase 2-Soft landers and rovers (Chang’e 3 and 4, Yutu 1 and 2 rovers, and the Magpie Bridge orbital communications satellite, enabling the first robotic landing on the lunar farside); Phase 3-Sample Return (Chang’e 5 and Chang’e 6); and Phase 4 (Polar research and exploration). The CLEP missions are named for the Chinese Moon goddess, Chang’e. Evidence that the Chang’e missions and CLEP are preparing for human exploration can be seen in the two human footprints that make up the central part of the CLEP logo (Figure 1); in graphic combination with a crescent Moon, this symbolizes the Chinese character for “Moon”. In a recent major space conference in China (2020 China Space Conference), the basic design of a heavy rocket, Long March-9, was announced and this is likely to be the rocket used to send taikonauts (Chinese astronauts) to the Moon. Why is Chang’e 5 important?: Chang’e 5 is the first robotic lunar sample return mission since the Soviet Union Luna 24 mission returned samples from Mare Crisium in 1976, 44 years ago. As amply demonstrated by the treasure trove of samples returned to Earth by the Apollo Lunar Exploration Program (Apollo 11-12, 14-17) and the Soviet Union Luna missions (Luna 16, 20 and 24), analysis of returned samples in Earth laboratories with very sophisticated equipment (chemical, mineralogic, petrologic, isotopic, geochronologic measurements, etc.) is absolutely critical for an in-depth understanding of the origin and evolution of the Moon. Furthermore, the Apollo-Luna sample zone of the Moon, while critical to our understanding, was undertaken in an area that comprises far less than half the lunar surface, and may be dominated by the influence of a small number of large impact basins (Imbrium, Serenitatis). Subsequent data from orbital remote sensing missions have shown a wider diversity of rock types, mineralogies and mare basalt unit ages than represented in the Apollo-Luna sample collections. Lunar scientists have been advocating for robotic sample return missions to these many different

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critical areas in order to address a host of fundamental questions remaining from earlier exploration (1). The CLEP Chang’e 5 mission is just such a sample return mission. Among the many outstanding and unaddressed questions related to the origin and evolution of the Moon (and thus all planetary bodies) (1) are:

1) How long did the Moon remain “active” in its interior, as manifested by the ascent and eruption of lunar mare basalt lavas? Some recent workers have hypothesized (on the basis of orbital data) that volcanism could have been active in the recent geologic past. The number of superposed impact craters on stratigraphically young mare basalts in Oceanus Procellarum (outside the Apollo-Luna zone) suggests that these lava flow units may be as young as a billion years old (2), an age about 2 billion years younger than the vast majority of lavas sampled from the Apollo-Luna zone.

2) What does the detailed nature of these youngest basalts tell us about the location and nature of activity in the lunar interior? Was the melting shallow or deep, did the source region contain volatiles (e.g., water), what are the implications of the high titanium content?

3) What is the true character of the impact flux (the number of projectiles impacting the Moon as a function of time) in the last several billion years? The flux is known to decrease with time from early Solar System history but the exact magnitude and rate of this decrease is poorly constrained. Returning samples from a unit with many fewer craters will permit the calibration of the lunar flux curve for these young ages. This is critically important because the lunar flux curve is the basis for extrapolation to the chronology of all of the terrestrial planets, including Earth.

4) What is the origin and influence of the Procellarum-KREEP Terrane (PKT), a region in the northwest nearside of the Moon that is anomalously rich in radioactive elements, such as thorium. How and when did this unique and distinctive terrane form, and how did its presence influence the generation of mantle melting and formation of young mare basalts in the region? Is the focus of young lunar activity in northwest Oceanus Procellarum due to its location in the PKT?

5) What is the history of the magnetic field of the Moon? Originally thought to have been active only in the early part of lunar history, more recent analyses have shown that it was active in middle lunar history, observations carrying important implications for constraining models of the lunar interior. Was the lunar magnetic field active in later lunar history? We currently have no samples to address this question, but returning samples from the young mare basalts would enable this question to be addressed. Where will Chang’e 5 land?: To address these types of fundamental questions Chinese scientists and engineers have been working together (3-6) to target the Chang’e 5 lunar sample return mission to Northwest Oceanus Procellarum, located in the middle of the PKT, and the site of some of the youngest extensive mare basalt volcanic units on the Moon (2). The broad candidate landing region (Figure 2) covers the wide array of mare basalt history (2), and includes the northern part of the Rümker Hills, one of three major volcanic complexes in Oceanus Procellarum (3). Within this broad target region, there is an extensive unit in the eastern part that represents the youngest mare basalt lava flows in the region (4-5) (Figure 3). Once Chang’e 5 is in orbit, should orbital observations and spacecraft telemetry further indicate that this unit is a safe place to land, scientists have suggested that this is one of the most promising landing region for maximizing scientific return (4-5).

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How does Chang’e 5 get there and when will it all happen?: Chang’e 5 will launch on a Long March-5 rocket (Figure 4) from the Wenchang launch center on Hainan Island, off the coast of China in the South China Sea. This time of year is the typhoon season in this region and recent typhoons have passed over the island, but the Long March-5 rocket carrying the Chang’e 5 mission spacecraft was recently rolled out to the launch pad and is currently ready to go. The launch window is early on November 24th 2020 (Beijing Time). After it reaches lunar orbit, it will undergo checkout and site validation in preparation for separation of the lander, and its subsequent descent and landing on the lunar surface (7). What will Chang’e 5 do?: Following the landing of the Chang’e 5 lander on the lunar surface (Figure 4), surface sampling operations will commence after local site inspection and sample acquisition planning. Two sampling methods will be employed, representing a major update and improvement from Luna 24 in 1976 (8): 1) The Chang’e 5 lander drilling operations will drill and collect a regolith core up to ~ 2 m in length (9). 2) A robotic arm will also be used for scoop sampling of shallow lunar regolith soils. The plan is to collect and return to Earth at least a total of ~ 2 kg of lunar regolith (soil and small rocks), and perhaps more, up to the maximum Chang’e-5 sample return capacity of ~ 4 kg. Due to the sample acquisition and storage hardware, and the sample ascent stage, the remaining scientific payload is limited to two important experiments: 1) a visible near-infrared spectrometer to characterize the surroundings and assist in sampling strategy, and 2) lunar ground penetrating radar to assess the subsurface regolith structure and layering below the lander (10-11). Both of these experiments were also onboard former CLEP lunar landing missions, Chang’e-3 and Chang’4. Lunar ground penetrating radar is critical in correlating the structure and layers of the lunar soil and understanding its origin, and is a very important addition to the exploration capabilities in the post-Apollo-Luna era (12). Upon completion of surface operations, and storage of the samples in the Chang’e-5 lander ascent stage, the ascent stage will lift off from the lunar surface (Figure 4), rendezvous with the orbiter, and transfer the samples for return to Earth, and recovery from the landing region in the Dongfeng Landing Area in Inner Mongolia. This complex series of operations (robotic lunar sample ascent, rendezvous, sample transfer in lunar orbit, departure from lunar orbit and return to Earth; Figure 4) has never before been attempted (13) and can be viewed as something of a “dress rehearsal” for the future Mars sample return mission envisioned by NASA and ESA. The lunar orbital rendezvous maneuvers will also obviously serve as operational experience in China’s preparation for their human lunar exploration missions, which are believed to commence around ~2030. What will be done when the Chang’e 5 samples are returned?: Once the lunar samples are returned to Earth, they will be transported to Beijing and stored in the National Astronomical Observatories of China (NAOC), Chinese Academy of Science (CAS), where primary curation and initial descriptions and measurement will be undertaken. As part of the safekeeping strategy, some samples will be stored permanently at Hunan University to avoid any potential loss due to natural disasters. Some samples will also be used for outreach purposes. In an approach similar to that employed by NASA with the Apollo samples, Chang’e 5 samples

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will be initially studied to provide an assessment of the main characteristics of the diversity and abundance of the returned samples. The detailed sample allocation and distribution policy of the Chinese National Space Agency (CNSA) is expected to be formally announced soon, but is thought to initially involve teams of Chinese scientists and their national and international collaborators. Formal applications for sample access will of course be required, to ensure sample security and to optimize the scientific return. What major scientific results do we expect from the Chang’e 5 sample analysis?: As outlined above, analysis of the Chang’e 5 returned lunar samples will address many fundamental problems remaining from the Apollo-Luna era (1), including: 1) the age and nature of the youngest phase of lunar volcanism; 2) calibration of the impact flux curve, 3) the rate of relatively recent impact bombardment in the inner Solar System, 4) the nature of the lunar interior in the last third of lunar history; 4) the duration and strength of the lunar magnetic field; 5) the origin and nature of the radioactive-element-rich PKT terrain and its influence on melting of the interior; and 6) the nature of other widespread units in this unsampled region from the selectin of fragments brought in by ejecta from distant craters in the mare and highlands of the region (14). Selected References:

(1) National Research Council (2007). The scientific context for exploration of the Moon. Washington DC: National Academies Press.

(2) Hiesinger, H., J. W. Head, U. Wolf, R. Jaumann, and G. Neukum (2011), Ages and stratigraphy of lunar mare basalts: A synthesis, in Recent Advances and Current Research Issues in Lunar Stratigraphy, edited by W. A. Ambrose and D. A. Williams, pp. 1-51, Geological Society of America Special Paper 477.

(3) Zhao, J., Xiao, L., Qiao, L., Glotch, T. D., & Huang, Q. (2017). The Mons Rümker volcanic complex of the Moon: A candidate landing site for the Chang’E-5 mission. Journal of Geophysical Research: Planets, 122, 1419–1442. https://doi.org/10.1002/2016JE005247

(4) Qian, Y. Q., L. Xiao, S. Y. Zhao, J. N. Zhao, J. Huang, J. Flahaut, M. Martinot, J. W. Head III, H. Hiesinger, and G. X. Wang (2018), Geology and scientific significane of the Rümker region in northern Oceanus Procellarum: China's Chang'E-5 landing region, J. Geophys. Res., 123, 1407-1430, doi: 10.1029/2018JE005595.

(5a) Qian, Y. and Xiao, L. and Head, J. W. (2020) The Young Mare Basalts in Chang’E 5 Mission Landing Region, Northern Oceanus Procellarum, 51st Lunar and Planetary Science Conference, Abstract #1459, Lunar and Planetary Institute, Houston.

(5b) Qian,Yuqi, Long Xiao, James W. Head, Carolyn H. van der Bogert, Harald Hiesinger, Lionel Wilson (2020) Young Lunar Mare Basalts in the Chang’e-5 Sample Return Region, Northern Oceanus Procellarum, in press, Earth and Planetary Science Letters.

(6) Mallapaty, S. (2020) China set to retrieve first Moon rocks in 40 years, Nature 587, 185-186 (2020) https://doi.org/10.1038/d41586-020-03064-z.

(7) Weiming Xu, Yu Hongxuan, Hao Jiang, Peng Tong, Yaowu Kuang, Ming Li, and Rong Shu, (2020) Navigation Doppler lidar sensor for precision landing of China’s Chang’E-5 lunar lander, Appl. Opt. 59, 8167-8174.

(8) Basilevsky, A. T., B. A. Ivanov, A. V. Ivanov, and J. W. Head III (2013), Clarification of sources of material returned by Lunar 24 spacecraft based on analysis of new images of the landing site taken by Lunar Reconnaissance Orbiter, Geochemistry International, 51, 456-472,

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doi: 10.1134/S0016702913060025. In Russian: Geochimia, 6, 510-528, doi: 10.7868/S00167525130 60022.

(9) Qian, Y., L. Xiao, S. Yin, M. Zhang, M. Zhao, S. Zhao, Y. Pang, J. Wang, G. Wang, and J. W. Head III (2020), The regolith properties of the Chang’e-5 landing region and the ground drilling experiments using lunar regolith simulants, Icarus, 337, 113508, doi: 10.1016/j.icarus.2019.113508.

(10) Jianqing Feng, Yan Su, Chunlai Li, Shun Dai, Shuguo Xing, Yuan Xiao (2019) An imaging method for Chang'e−5 Lunar Regolith Penetrating Radar, Planetary and Space Science 167, 9-16, https://doi.org/10.1016/j.pss.2019.01.008.

(11) Yuan Xiao, Yan Su, Shun Dai, Jianqing Feng, Shuguo Xing, Chunyu Ding, Chunlai Li (2019) Ground experiments of Chang’e-5 lunar regolith penetrating radar, Advances in Space Research 63, 3404-3419.

(12) Basilevsky, A. T., A. M. Abdrakhimov, J. W. Head III, C. M. Pieters, Y. Z. Wu, and L. Xiao (2015), Geologic characteristics of the Luna 17/Lunokhod 1 and Chang'E-3/Yutu landing sites, Planetary and Space Science, 117, 385-400, doi: 10.1016/j.pss.2015.08.006.

(13) Fei Li, Mao Ye, Jianguo Yan, Weifeng Hao, Jean-Pierre Barriot (2016) A simulation of the Four-way lunar Lander–Orbiter tracking mode for the Chang’E-5 mission, Advances in Space Research 57, 2376-2384, https://doi.org/10.1016/j.asr.2016.03.007.

(14) Xie, M., Xiao, Z., Zhang, X., & Xu, A. (2020). The provenance of regolith at the Chang'e-5 candidate landing region, Journal of Geophysical Research: Planets 125, e2019JE006112, https://doi.org/10.1029/2019JE006112.

Figure 1: Logo of the Chinese Lunar Exploration Program (CLEP).

Figure 2: Chang’e-5 Landing Region in Northern Oceanus Procellarum (White Box; Qian et al., 2018; (4)).

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Figure 3: Young mare units (Em3, Em4; blue) in the easternmost part of the CE-5 landing region (Figure 2). The basemap is a Kaguya MI color composite map. From (5).

Figure 4. Overview of the Chang’e 5 Launch Vehicle and the Chang’e 5 mission profile (https://www.planetary.org/space-images/change-5-mission-profile, https://www.planetary.org/articles/china-new-lunar-missions.)