Dark side of the Moon: the first samples arrive on Earth

China’s Chang’E 6 probe, launched on 3^rd May  2024¹, brought back to Earth the first sam­ples from the far side of the Moon. The mis­sion car­ried sev­er­al inter­na­tion­al pay­loads, includ­ing the French instru­ment DORN² (Detec­tion of Out­gassing RadoN). This was the first active French instru­ment deployed on the Moon’s sur­face, and its mis­sion was to mea­sure radon, a radioac­tive gas con­tin­u­ous­ly pro­duced in the regolith, in order to study the ori­gin and dynam­ics of the thin atmos­phere (known as the exos­phere) sur­round­ing our satel­lite. It also aimed to bet­ter under­stand the spa­tial and tem­po­ral vari­a­tions in this gas, which had been detect­ed from orbit by the Apol­lo 15–16, Lunar Prospec­tor and Kaguya-SELENE mis­sions, and which sug­gest­ed the pres­ence of high­ly local­ized out­gassing activ­i­ty, pos­si­bly cor­re­lat­ed with cer­tain seis­mic events.

DORN was designed and built at IRAP³, the Insti­tute for Research in Astro­physics and Plan­e­tol­ogy, and is a project man­aged by the Cen­tre nation­al d’é­tudes spa­tiales (CNES) in col­lab­o­ra­tion with CNRS, Toulouse III – Paul Sabati­er Uni­ver­si­ty, and the Insti­tute of Geol­o­gy and Geo­physics of the Chi­nese Acad­e­my of Sci­ences (IGG-CAS). This instru­ment is the first col­lab­o­ra­tion between France and Chi­na in the field of plan­e­tary explo­ration. Its dis­cov­er­ies will help us to bet­ter under­stand the migra­tion of gas­es on the sur­face of the Moon (the most acces­si­ble body with­out an atmos­phere) and, more gen­er­al­ly, the inter­ac­tions and exchanges between plan­e­tary sur­faces and their space environment.

The sam­ples from the new 54-day mis­sion are very dif­fer­ent from those col­lect­ed by pre­vi­ous mis­sions, which all came from the vis­i­ble side of the Moon. Chang’E land­ed on 2^nd June 2 2024 at a pre-select­ed site (the Apol­lo crater) inside a very large impact basin 2,400 km in diam­e­ter, known as the South Pole-Aitken (SPA), which may have exposed lunar man­tle rocks on the sur­face. The cho­sen site is cov­ered with dark, cooled lava flows (basalts), which resem­ble the lunar seas found main­ly on the vis­i­ble side of the Moon (also of vol­canic ori­gin), but which are scarce­ly present on its hid­den side.

One of the aims of recov­er­ing sam­ples from this region is to under­stand the asym­me­try between the vis­i­ble and hid­den sides of the Moon, and to under­stand why vol­canic activ­i­ty there has been dif­fer­ent. The sam­ples – over 2 kg of regolith and lunar rocks – were obtained using a drill and robot­ic arm⁴. The load then lift­ed off from the lunar sur­face, docked and trans­ferred to the re-entry cap­sule, which remained in orbit, before return­ing to Earth.

“DORN record­ed almost 110 hours of data in orbit and 20 hours of data dur­ing sur­face oper­a­tions,” explains Pierre-Yves Mes­lin, sci­en­tif­ic man­ag­er of the DORN mis­sion at IRAP.  He and his team are cur­rent­ly analysing the data.

For the Chi­nese sci­en­tists, the first analy­ses of the sam­ples car­ried out in the lab­o­ra­to­ry using radio­met­ric dat­ing, based on mea­sure­ments of lead, rubid­i­um and stron­tium iso­topes, indi­cate the pres­ence of basaltic rock frag­ments dat­ing back 4.2 and 2.8 bil­lion years, thus reveal­ing the exis­tence of at least two vol­canic events in this region. The vol­canic activ­i­ty that took place on the far side appears to have occurred at dif­fer­ent ages from the vol­can­ism that is char­ac­ter­is­tic of the vis­i­ble side, which main­ly dates back more than 3 bil­lion years, accord­ing to esti­mates based on the analy­sis of lunar sam­ples brought back by the Amer­i­can ‘Apol­lo’ and Sovi­et ‘Luna’ mis­sions. These new dis­cov­er­ies were pub­lished by Zex­i­an Cui and his col­leagues from the Guangzhou Insti­tute of Geo­chem­istry of the Chi­nese Acad­e­my of Sci­ences in the jour­nal Sci­ence⁵, and by Qian Zhang’s team from the Bei­jing Insti­tute of Geol­o­gy and Geo­physics in the jour­nal Nature⁶. The lat­ter study dates cer­tain sam­ples back 4.2 bil­lion years.

The researchers believe that the thin crust beneath the SPA basin may have allowed vol­can­ism to per­sist in this region. They have been able to show that it was prob­a­bly not the con­tent of radioac­tive ele­ments (ura­ni­um, tho­ri­um, potas­si­um-40), high­ly enriched in cer­tain vol­canic plains on the vis­i­ble side, that was respon­si­ble for this vol­can­ism. But the rel­a­tive rar­i­ty of traces of vol­can­ism on the hid­den face remains to be explained.

The non-basaltic sam­ples also col­lect­ed will pro­vide a bet­ter under­stand­ing of the for­ma­tion of the pri­ma­ry lunar crust (light-coloured high plateaux char­ac­ter­is­tic of the far side of the Moon), the effect of impact process­es on this crust, and per­haps access to the com­po­si­tion of the lunar man­tle, pos­si­bly exposed in the SPA basin. 

“These analy­ses will make it pos­si­ble to refine pre­vi­ous obser­va­tions made by remote sens­ing, which have shown that the far side of the Moon is dif­fer­ent from the vis­i­ble side in terms of geo­physics (dif­fer­ences in the thick­ness of the crust, for exam­ple) and the chem­i­cal and min­er­alog­i­cal com­po­si­tion of the rocks,” explains Pierre-Yves Mes­lin. For exam­ple, the quan­ti­ties of potas­si­um, rare earths and phos­pho­rus (known togeth­er as KREEP) dif­fer from one side of the Moon to the other.

The giant impact that cre­at­ed the SPA basin could be the cause, since it was pow­er­ful enough to redis­trib­ute lunar mate­ri­als. For exam­ple, it could have trans­port­ed mate­ri­als rich in KREEP towards the vis­i­ble side of the Moon, leav­ing the man­tle on the hid­den side deplet­ed in KREEP.

Isabelle Dumé