Prospecting the solar system

The most Earth-like planet in the solar system in terms of size, composition, layering and gravity, is Venus. It is possible that Venus hosts ancient geologic terrains similar to those that host many ores on Earth and it may once have had plentiful water to assist in the concentration of ore bodies, says Vicki Hansen, planetary geologist at the University of Minnesota, US.

‘Using Venus as a guide, we might better understand Earth’s early years and how mineral resource concentrated, for example, in greenstone [ancient basaltic rocks]. It would allow us think about Earth [and its resources] in a new way,’ says Hansen.

However, surface temperatures on Venus average 462°C and pressure exceeds 90 Earth atmospheres, equivalent to diving 1km below Earth’s oceans. The atmosphere is mostly CO₂.

‘The CO₂ is present as supercritical fluid and is incredibly corrosive. Mining as we do on Earth just wouldn’t work,’ says Hansen.

However, the pressure and temperature at around 50 to 60km altitude on Venus are very close to Earth’s and this region has been proposed as a site for data collection or even future colonisation via floating cities. And it has plenty of CO2 for generation of oxygen and carbon, for building plastics; at this altitude, above the sulfurous clouds, solar power could also be deployed.

‘The atmosphere at 50km is a fantastic environment for our technology and materials. We could fly a glider-type craft and collect data about the planet, perhaps using solar cells to recharge the glider above the clouds,’ says Hansen.

The ‘Venus atmospheric maneuverable platform’(VAMP) proposed by Roland Polidan of California-headquartered Northrup Grumman Corporation is just such a craft – an inflatable, floating glider with a wingspan of about 150 feet; in 2015 the company started a science advisory board to look at possible missions. Also, a recent NASA exploratory program will focus on the mission architecture and vehicle concept for a 30 day crewed mission into Venus’s atmosphere. The trip from Earth would be 30-50% shorter than to Mars.

The US Apollo spaceflight missions from 1969 to 1972 revealed that the moon has a crust, mantle and core, with a crust rich in calcium and aluminium due to the silicate mineral ‘plagioclase’ [(Na, Ca)Al1_-2Si_2-3O₈]. Other lunar minerals contain iron and titanium.

The Moon’s powdery soil could be microwaved or sintered to produce rigid material for possible use in roads, radiation shields, landing pads and buildings. Lunar soils are rich in silicon, iron and aluminium. Iron would be the easiest of the metals to extract. ‘You’d simply take iron oxide from rock and reduce it to native metal using hydrogen or a halogen gas,’ says Paul Spudis at the Lunar and Planetary Institute in Texas, US. ‘Interestingly, iron on the Moon is very strong, stronger than stainless steel and a very good electrical conductor because it is produced in a completely anhydrous environment.’ It’s unlikely aluminium or titanium would be extracted and processed because of the high energy requirement.

However, the real pay dirt on the Moon lies beneath the permanently dark areas at its poles – water. ‘There are several hundreds of millions to billions of metric tonnes of water ice at both poles,’ says Spudis. ‘You can “crack” it into hydrogen and oxygen. With solar electric power, you can combine those in a fuel cell to make electricity. So this is a medium of energy storage and also the most powerful chemical rocket propellant we know of.’

Ice on the Moon also harbours methane, carbon dioxide, and ammonium compounds. Water and soil could be heated up to extract these volatiles, then separated through fractional distillation; however, the form or distribution of water is unknown. ‘It may turn out to be a real bonanza, but it could turn out to be an economic bust. The ice may be too dispersed and at low concentrations,’ Lewis explains. Nonetheless, the potential excites many; cold traps at the poles contain nitrogen, carbon and phosphorus compounds, notes Spudis; all useful elements.

The carbon molecules could be used to make plastics for spare parts at a lunar outpost. ‘You form methane by combining the hydrogen and carbon. Once you’ve got methane, you use the Fischer-Tropsch process to polymerise it and make long-chain hydrocarbons,’ says Metzger. The chemistry is well known, but hardware in a form suitable for use on the Moon has not been developed – yet.
One final possibility is to harvest helium-3, a light isotope of helium found on lunar dust grains that could be used in future nuclear fusion reactors on Earth.

On Earth, iron, nickel and chemically similar elements became concentrated in the core. Asteroids do not have this sort of differentiated geology, so the terrestrial concept of ore veins buried in hard rock is irrelevant, for example.

Rocky asteroids called chondrites that have not been modified by melting or differentiation are attractive for mining. They have a chemical composition comparable to what you might get if you ran Earth through a giant blender, mixing its core, mantle and crust. 

Consequently, even the average asteroid will have several times the concentrations of iron and platinum group metals, for example. ‘Asteroids already have pure metal. You just crush it up and use a mechanical separator to separate metal from rocky pieces,’ explains Metzger.

Many near earth asteroids are especially rich in iron-nickel-cobalt alloy. The iron and nickel can be volatilised as carbonyl compounds at temperatures near 200°C; these can be separated by fractional distillation. Among the byproducts are cobalt, platinum group metals and semiconductor elements for possible use in photovoltaics.

The platinum group metals - platinum, osmium, iridium, rhodium, ruthenium and palladium - are sufficiently valuable to be worth returning to Earth.  ‘However this requires that you process large quantities of iron, nickel and cobalt and remove them, presumably for local use in space, to concentrate the valuable ones,’ says Lewis. Ferrous metals are an obvious choice for structures in space.

Asteroid miners would avoid strategies that rely on crushing hard rock. Initial mining would likely target chondritic asteroids with free regolith. ‘A metal monolith would be very difficult to process,’ confirms Lewis, who recently authored Asteroid mining 101: wealth for the new space economy. ‘However, even among near Earth asteroids, we know some of them experience night time temperatures below the brittle-ductile transition temperature of metal. This would make the material extremely brittle and any collision would pulverise the metal and leave a sandbank of iron-nickel alloy particles.’

Carbonaceous chondrite asteroids can be heated to extract the volatile elements and useful products could include water, oxygen, hydrogen, carbon dioxide, hydrocarbons, nitrogen, sulfuric acid and hydrochloric acid. These offer the ingredients to make rocket propellant, which could be methane, ethane, propane, butane or methanol.

Mars is a complex, differentiated planet with a wide variety of resources. There may be rich veins of metal or other resources of high economic value, but we haven’t done enough exploration. ‘We don’t have the infrastructure or experience or heritage to know where to look. Therefore we need a different strategy,’ says Metzger. ‘To use whatever we find at our feet.’

This chimes with Nasa’s focus on ‘In Situ Resource Utilisation’ (ISRU) for Mars. Two raw resources are being targeted. First up: the atmosphere. The Martian atmosphere is 95% CO₂, with nitrogen and argon making up most of the remainder. Senior research engineer Diane Linne at Nasa has been working on a chemical processing plant to extract CO₂ from the air. ‘One problem is that the atmospheric pressure is about 1/100th that of Earth’s, which makes it hard to run a chemistry plant.’

Mechanical compression, cryo-freezing and rapid absorption/desorption pumps are all being considered. Once CO₂ is captured, solid oxide ceramics run at temperatures around 800°C would be used to reduce CO₂ to CO and oxygen ions via electrolysis. But there is another option: to employ a Sabatier reactor to generate methane and water. First, electrolyse the water from soil to get hydrogen and oxygen. Save the oxygen. Then combine hydrogen with the CO₂ in a Sabatier reactor [300-400°C and a nickel catalyst] to give oxygen and methane at a ratio of 4:1, Linne explains. A rocket engine burns at a ratio of 3.5-3.2:1, so there would be enough oxygen left over for life support.

The water could come from surface regolith or targeted material. ‘The more ubiquitous surface material you’d need to heat to 300°C and would get about 2% water. However, there may be water rich deposits of materials such as gypsum,’ says Nasa engineer Julie Kleinhenz.

Gypsum is nice because it releases at lower temperatures of around 150°C and can result in up to 8% water.’ Orbital data could be used to target specific landing sites where there are deposits rich in gypsum.

The processing unit for generating propellant ‘would be no taller than a person, like a smallish refrigerator,’ says Kleinhenz. That would generate enough propellant to fill a waiting ascent vehicle for the manned mission in less than two years. All this will likely be powered by fission reactors, rather than solar power.

The in situ approach applies to more futuristic Martian resources too. ‘We’ve been developing processes that can break down ordinary mineral sand to get the metal out,’ says Metzger. One example is molten regolith electrolysis, where you run electric current through molten material to generate oxygen and a metal mixture. ‘But it is not our baseline plan to make metals during a Mars mission.’

Regolith offers other possibilities, as Bernard Foing at ESA notes: ‘The Moon and Mars have volcanic regolith soil that can be used for construction, but also as substrate for plant growth.’

The massive gas giants Saturn and Jupiter are unappealing for resource extraction because of the energy toll needed to exit their huge gravity wells. But their moons hold attractions.

‘Titan is a huge playground for organic chemistry,’ says Geoff Collins, planetary geologist at Wheaton College, US. UV rays break down methane in Titan’s upper atmosphere, which is rich in nitrogen, and all kinds of reactions ensue. A wide spectrum of organic molecules are created and their residues pile up in huge fields of sand dunes, or are washed down river canyons whenever there’s a methane rainstorm, he adds. The geological definition of sand here refers to the size of the grains rather than composition; on Titan, the dunes are probably a mixture of large, sticky organic molecules.

‘Titan’s surface organics exceed Earth’s oil reserves. It is a giant organic factory that could support some surface activities, but at a low temperature of -180°C,’ says Foing at ESA. Sunlight here is limited, so power sources would be needed. ‘The escape velocity is 2.6 km/s [versus 11.2 km/s for Earth], so one could consider launching [hydrocarbons] and using it for a Saturnian orbital village.’ Foing adds. 

Water ice is the main bedrock of the outer solar system. ‘CO₂ is sometimes mixed into the ices around Jupiter and Saturn, but as you go farther out you also see more exotic ices [due to lower temperatures] like CH₄, NH₃ and N₂,’ says Collins. The small moon Enceladus studied during the Cassini mission contained an ocean of water. Water is a raw material for rocket propellant and source of oxygen.