# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 5 Lunar Resources
### REEs beyond the Moon  
The only other planet from which samples are known to have been derived from is Mars. Yet compared to Earth and the Moon, relatively little is known about the differentiation and geological evolution of the red planet (e.g., [155]). To date, there have been no sample return missions hence our understanding of Martian geology is based on the 107 meteorites (as of 1 August 2017, [156], and their associated pairs) that have survived passage through Earth's atmosphere. This number is considerably less than the 344 lunar meteorites identified so far, and their pairings (as of 1 August 2017, [157]) and the \(>\)1000 samples brought back by the Apollo missions (including rock samples and lunar regolith, [158]). From the REE characteristics of martian meteorites, it has been shown that Mars differentiated very early in the history of the Solar System (\(\sim\)4.5 Ga) to form a metallic core, a silicate mantle, and a crust, and broadly differentiated to form a LREE-depleted reservoir (low La/Yb, high Sm/Nd) and an inferred, complementary LREE-enriched reservoir (high La/Yb, low Sm/Nd; [159]), the latter of which remains to be directly sampled (see discussion later; e.g., [160; 161; 162; 163; 164; 165; 166; 167]). Unlike the lunar sample collection and the lunar magma ocean model (Figures 3 and 4), Mars lacks an Al-rich anorthositic crust. This observation is attributed to crystallization of garnet during Mars magma ocean crystallization (e.g., [159; 165]). As Mars is larger than the Moon (\(\sim\)twice the diameter), higher pressure in the planet's interior would have promoted the crystallization of high pressure phases, such as garnet. This would have had a significant impact on the crystallization sequence of a magma ocean as garnet contains Aluminum. From experiments at pressures of 3-5 GPa, the following sequence for martian magma ocean crystallization has been proposed: olivine \(\rightarrow\) olivine + orthopyroxene \(\rightarrow\) orthopyroxene + clinopyroxene \(\rightarrow\) clinopyroxene + garnet + ilmenite \(\rightarrow\) clinopyroxene + garnet [168]. At higher pressures, the crystallization of garnet occurs earlier, but between the crystallization models is the consistent absence of plagioclase (see [168]). Following \(\sim\)99.5% martian magma ocean crystallization, a late-stage liquid enriched in incompatible elements (including KREEP) has been modelled. The overall geochemical characteristics of this evolved reservoir are very similar to lunar KREEP [168]. As presented earlier within the context of the Moon, REE-bearing minerals are volumetrically rare in extraterrestrial samples. This can be in part attributed to the scarcity of felsic igneous rocks (as sampled to date) and the lack of high-temperature fluids which would concentrate these elements. With no sample return missions having yet been completed to Mars, mineralogical and chemical information is derived from (1) elemental surface maps obtained by Mars-orbiting satellites (e.g., the Gamma-Ray Spectrometer (GRS) on board Mars Odyssey); (2) landers and rovers on the martian surface (e.g., the Mariner missions, Mars 3, Viking 1, Viking 2, Pathfinder, Opportunity, Phoenix, and most recently Curiosity); and (3) meteorites. Maps from the GRS instrument have summarized the distribution of chlorine, calcium, potassium, iron, silicon, and hydrogen across the martian surface, where like the Moon (Figure 5), thorium exhibits a high degree of correlation with potassium [169; 170]. From these results, the martian surface has been shown to be broadly composed of basaltic material (iron, silicon, calcium), sulfur, chlorine, hydrogen, potassium, and thorium in addition to elements that were not mapped by the GRS (e.g., magnesium and sodium; [170]). More recent results from analyses of martian soil in Gale Crater by Curiosity's onboard instruments (ChemCam and APXS (Alpha Particle X-ray Spectrometer)) reveal a dominance of silicon, titanium, aluminum, iron, magnesium, calcium, and sodium.