# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 1 Introduction  
\"Space may be vast, but many of the most valuable resources--especially those convenient to Earth--are limited. Our Moon may be one of the most promising sites for mining, energy capture, and spaceship refueling, but a limited amount of useable land exists, with an even more limited quantity of useable water. The problem is not limited to the Moon. Every resource is limited. The question thenis who, if anyone, should have the right to the riches of space? Space is an international zone, and so is, in a sense, the heritage of all humanity.\" [1]  
The rare-earth elements (REEs) are a group of transition metals which include the lanthanide series of the periodic table (lanthanum to lutetium), in addition to scandium and yttrium. Despite the nomenclature, they are not rare with cerium (Ce) being the 25 th most abundant element on Earth [2] at an average value of 60 ppm. This is compared to <1 ppm for the least abundant REEs (thulium (Tm) and lutetium (Lu)) [3]. In addition, the occurrence of REEs is often associated with U-Th mineralization, where uranium ore deposits often contain a significant concentration of REEs [4]. The REEs are as abundant as tin, lead, and cobalt in Earth's lithosphere, and are more abundant that gold and silver [5]. However, they are dispersed throughout the rock record and only exist in concentrated, economically viable, deposits in certain areas of the world with the largest accumulation of REEs being found in the Bayan Obo deposit in China [6]. The REEs are widely used on Earth by humankind on a daily basis. They are integral components of smart phones, computer monitors, flash drives, lightbulbs, camera lenses, catalysts, and magnets, and are fundamental to many military-based technologies. However, as of 2012, REEs had been produced from less than 20 minerals with bastnasite (REECO3(F,OH)), monazite ((REE,Th,Ca,Sr)(P,Si,S)O4), xenotime ((REE,Zr)(P,Si)O4), loparite ((Na,REE,Ca)(Ti,Nb)O3), parisite (CaRE2(CO3)3(F,OH)2), and Al-clays, accounting for the majority of REE production [2]. From the USGS Mineral Commodity Summary in 2014 [7], Earth's REE reserves totaled 140 million tonnes, with almost half of those resources in China (55 million tonnes) and the United States (13 million tonnes). Figure 1 summarizes the years of REE reserves left on Earth based on recent mining practices and technologies, alongside their concentration in Earth's crust. Most critical are the reserves of Europium (Eu), the only rare earth metal to have <1000 years remaining as a resource. It is used daily, in conjunction with phosphors, to generate red in televisions which use cathode ray tubes, and is a common component in compact fluorescent bulbs [8]. Also from Figure 1, lanthanum (La, which is three times more abundant than lead (Pb) in Earth's crust; [8]), Ce, praseodymium (Pr), neodymium (Nd), holmium (Ho), and yttrium (Y) all have reserves <2500 years. Respectively, these elements are used in mischmetal for spark effects in movies (La), in the oxide form as an abrasive for polishing glass (Ce); generating the green color in fake cubic-zirconia peridot (Pr), in high power magnets (Nd), in magnetic resonance imaging (MRI) machines to concentrate  
Figure 1: Estimated reserves of selected rare-earth elements (REEs: \(x\)-axis) in years (shown on left hand \(y\)-axis) and their abundance in the Earth’s crust (right hand \(y\)-axis). The colored bar represents the number of years that element has left as a reserve. The corresponding star symbol illustrates the concentration of that element in the Earth’s crust (ppm). Data from [4]. the magnetic field (Ho), and in barium copper oxide powders for use in superconductors (Y; [8]). Figure 2 summarizes the short term (within 5 years), and medium term (5-15 years), criticality of REEs (in addition to cobalt (Co), gallium (Ga) indium (In), lithium (Li), and tellurium (Te)). The following elements are both highly valuable to clean energy, and are experiencing a high supply risk, both in the short term, and medium term: Dy, Eu, Nd, Tb, and Y.