Extra-terrestrial settlements require local sources of raw materials, and key to these, are the ability to extract metals from local sources. The efficient utilisation of local materials and resource recovery from space could reduce the mass, cost and environmental constraints imposed on space missions1–3. The lunar regolith has large amounts of oxygen, silicon and metals4,5, while near-Earth asteroids and comets can contain substantial amounts of metals, oxygen, hydrogen, carbon and, in some cases, also nitrogen. In fact, some of the largest known metal-rich asteroids are located in the middle and outer part of the asteroid belt, between the orbits of Jupiter and Mars, at 2.7–3.0 au from the sun (4.0 × 108 – 4.5 × 108 km). These solar bodies have been considered as the parental bodies of iron meteorites, enstatite chondrites, stony-iron, and metal rich carbonaceous chondrites6–8. Near Earth asteroids (NEAs) can contain valuable platinum group metals (PGMs), as well as iron, nickel and cobalt in concentrations greater than on Earth9,10. For instance, the spectral signature of asteroids 1986 DA and 2016 ED85 are quite similar to asteroid 16 Psyche11, the largest metal-rich body in the solar system (radius of ca. 113 km). Both NEAs have surfaces with 85% metal (mostly iron, cobalt and nickel) and 15% of silicate material12. These metals, if recovered, could provide a ‘local’ source of materials essential for establishing a human settlement in space or on other terrestrial bodies. PGMs, for example, can be used as catalysts to enhance oxygen and hydrogen production, whereas iron, cobalt and nickel can be used as raw materials for infrastructure. Asteroids can also have a significant concentration of selenium and tellurium13,14, which can be used as raw materials for the construction of solar panels, fuel cells, thermoelectric generators, energy storage and/or organic synthesis15–18.
Over the last 60 years, different process technologies have been conceptualised and demonstrated for oxygen and hydrogen production from lunar regolith and Martian soil1,4,19–25. Even though, the extraction of rare earth elements from basal rocks using microorganisms in different gravity regimes has recently been demonstrated on the International Space Station26, the recovery of metals has been treated solely as a by-product rather than a target resource. The Fray-Farthing-Chen (FFC) Cambridge process considers the solid-state electrochemical reduction of metal oxides to metals in molten salt (e.g., CaCl2) at temperatures above 900 ˚C27–29. This method can enable both the extraction of oxygen (up to 45%) from lunar regolith and the simultaneous production of metal alloys, such as Al/Fe, Fe/Si, Ca/Si/Al, as by-products30. Similarly, carbothermic reduction can enable the extraction of iron alloys from Martian soil at 1,120 ˚C by cooling the carbon monoxide produced from carbon dioxide electrolysis31. Although, the proposed methodologies have shown great potential for oxygen production as the main goal, they also have significant drawbacks. Mineral processing operations are needed firstly to concentrate the mineral phases of interest and reject the unwanted material. The energy consumption of these processes is expected to be around 50 GJ/tonne of metal, which accounts about 45% of the total energy spent on modern mining on Earth32. It must be noticed that contrary to the vast utilisation of fossil fuels as energy source for mining on Earth, solar energy will be the primary power source in space33. Furthermore, whilst the electrochemical reduction processes of solid compounds in high temperature molten salts has only been demonstrated at a laboratory scale34, it is clear that the technology will need the continues re-supply of suitable electrodes, most likely from Earth, as they are normally made of carbon or graphite which will not be available for their direct use in space. In addition, similarly to traditional pyrometallurgical processes, e.g., calcination, roasting and smelting, a robust infrastructure in terms of shell and lining structure will also be necessary for working in high temperature operating conditions. The high-temperatures proposed in these technologies also constitute a significant safety concern and require extensive, expensive and thermal management systems to prevent overheating in environments with little or no convective dissipation. Consequently, the development of cost-efficient, low-temperature, simple and safe extraction/recovery processes that can be implemented in space is still desirable. Until now, comminution, beneficiation and metal production have all been developed and refined over time with base assumptions that come inherently from the physical and chemical conditions found on Earth, and they cannot be directly implemented in space35.
In order to meet the energy challenges for in-situ resource utilisation (ISRU) deployment2, electrochemical methodologies driven by solar photovoltaic energy conversion represents the most promising methodology for minerals and metals processing. Electrochemical methods have already been demonstrated as an alternative methodology for solubilising metal oxides in non-aqueous ionic solvents such as deep eutectic solvents (DESs)36. DESs have been widely studied in different applications, but mostly in metal processing and electrodeposition37. They have the great advantage of being easy to prepare by combining a Lewis base and acid. They also have low toxicity, are not flammable, and can have low vapour pressures38,39, capable of enduring the hard vacuum of space while remaining in the liquid phase. The most commonly investigated systems have been quaternary ammonium salts with hydrogen bond donors (HBD) such as urea, ethylene glycol and glycerol40. The utilisation of inorganic chloride salts-based DESs have also been investigated41–44, which opens the opportunity to synthesise novel DESs by utilising in space materials. The presence of calcium, aluminium, iron, and zinc, for instance, have already been reported in regolith4,25,45, whereas the presence of halogen gas (chlorine, bromide) have been reported in the atmosphere of some planets like Mars and Venus46. Urea and glycerol could also be produced in space from waste and therefore, be considered as potential HBDs47–51.
This work aims to investigate the proof-of-concept of a methodology for extracting metals from meteorite proxies of asteroids using non-aqueous deep eutectic solvents (DESs). Two type of chondrites meteorites (NWA 13876 and NWA 7160) and one iron-rich meteorite (Campo del Cielo) were investigated. The occurrence of minerals and metal phases was investigated by automated SEM-EDX analysis. The samples were subjected to chemical micro-etching experiments with iodine and iron(III) chloride as oxidising agents in a DES formed from the mixture of choline chloride (ChCl) and ethylene glycol (EG). Etching depth and dissolution rates were determined by analysing the 3D topography of the samples before and after etching.