# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 6 Conclusions  
Daily use of REEs by modern day society is placing an increasing demand for extraction of these elements from limited terrestrial resources. With an increasing population, a projected 8.5 billion by 2020, it is natural to consider exploration and characterization of extraterrestrial REE sources in order to sustain our reliance on raw materials and energy reserves. Earth's nearest neighbor in space, the Moon, is a logical first target both in terms of evaluating resource potential. Following formation of the Moon as the result of a giant impact between proto-Earth and an impacting Mars-sized body \(c\). 70-100 Myrs after the onset of Solar System formation, molten material (a primordial magma ocean) began to solidify and differentiate. After -99% crystallization a KREEP (potassium, rare-earth element, phosphorus)-rich layer concentrated incompatible elements, a geochemical signature detected in many lunar samples today. Today, KREEP-rich lithologies are associated with enrichments in thorium on the lunar near-side (in the PKT), as observed by the gamma ray spectrometer on the Lunar Prospector. The volume of KREEP-rich lithologies associated with this region of the Moon is estimated at 2.2 \(\times\) 10\({}^{8}\) km\({}^{3}\), with potential reserves of REEs at -2.25 \(\times\) 10\({}^{14}\)-4.5 \(\times\) 10\({}^{14}\) kg. The majority of lunar samples contain REE-bearing phases as minor or trace components, typically apatite and/or merrillite with KREEP-like lithologies almost always containing merrillite. The presence of merrillite has the potential to account for up to 3% of a lithologies volume in the PKT however, this does not permit the classification of REEs on the Moon as ores. While REE abundances of trace phases in lunar rocks are high, their abundances are low compared to terrestrial ones meaning there is to date, no geological evidence to support mining the Moon for REEs yet. With the development of future technologies capable of mapping at higher resolutions, and in situ exploration by future missions, economically viable lunar resources may yet be discovered. However, other potential resources do currently exist on the Moon--including helium-3, water, oxygen, and metals (Fe, Al, Mg)--which will likely be fundamental to future space exploration. Beyond the Moon, samples from Mars and chondritic meteorites are also host to REEs, predominantly hosted in merrillite, in addition to apatite, whitlockite, xenotime, and recently-identified chevkinite-perrierite. These minor phases dominate the REE budget with abundances 1-2 orders of magnitude higher than their bulk host rock. However, as with the Moon, REEs have not yet been found in high enough concentrations in these extraterrestrial samples to warrant their classification as an ore resource. The concentrations of REEs in martian merrillites for example are several orders of magnitude lower than lunar merrillites, a difference attributed to the planetary bodies' crystallization history during the early evolution of the Solar System. While the relative concentrations and abundances of the REEs in the phases that have been characterized to date provide crucial insights into the differentiation history of planetary objects, current and future missions to the Moon, Mars, and other nearby objects in our Solar System may yet reveal one or more extraterrestrial REE resources that one day will be utilized. We thank Guest Editor [PERSON] for the opportunity to contribute to this special issue on the criticality of rare-earth elements. We are also very grateful to two anonymous reviewers who provided comments and thoroughly constructive feedback on an earlier draft of this manuscript. Our contribution was greatly improved by their input and suggestions. [PERSON] designed the structure, collected the materials to be included in the review of REEs in planetary materials, and wrote 90% of this article.