The Conditions on the Moon

When considering pressure on the lunar surface it is important to distinguish between absolute measured pressure and the expected pressure range of operation, as this will be increased by a prolonged human and robotic presence on the lunar surface. The absolute pressure measured at night on the lunar surface by equipment placed on the Apollo 14 and 15 missions  was almost 15 orders of magnitude lower than that on Earth. That's a lot.

So the natural pressure on the Moon is extremely low, but that will actually increase with human and robotic pressence in the future. Did you know that the ascent stage of the Apollo missions actually doubled the mass of the atmosphere of the Moon for a while! That's a wild statistic. But even with that relative increase in pressure, we will still be operating in a hard vacuum.

The vacuum conditions on the Moon has wide-ranging physical and chemical effects. Take, for example, the standard passive cooling techniques used for most industrial pumps and motors which rely on convective heat transport. Under vacuum conditions convective heat transport is  not a viable method of cooling. Instead, equipment that includes conductive and radiative heat management will need to be used.

There are also significant physiochemical effects. One of these is the effect on phase transitions between condensed and dispersed phases, namely that material will start to evaporate and sublimate at lower temperatures than required on Earth. Liquid water can't exist at these pressures, and even rocks can start to sublimate (that is evaporate from a solid form) if heated up enough in the vacuum. There are certainl ways we can use these effects to our advantage in astrometallurgy, but we also need to be careful not to overlook them.

The synodic period on the Moon, a lunar day, is 708.72 hours. This results in two Earth weeks of constant access to sunlight and an equal time with no access to sunlight. When the sun is up, the illumination on the lunar surface averages 1361 W/m^2. This is the same flux that hits the upper atmosphere of Earth, however, on the Moon this solar flux is not attenuated by the atmosphere and weather.

The temperature on the lunar surface sits around 120 ° C during the day, and during the long night can get down to temperatures below -180 ° C. Designing equipment that can even survive these extreme temperature fluctuations is difficult. One of the reasons we never know how long a lunar rover will survive is because they tend to shut down during the night and we don't know if they'll ever wake up.

One of the other big issues is power generation. We'd love to use solar power to power everything in space, but that means that in equitorial regions on the Moon we have to survive with no solar power for 2 weeks at a time. That means either really large batteries, or some other sort of energy solution.  There are some areas on the poles that see sunlight for longer than that, look up the 'peaks of eternal light', so you can see how that would be a useful place to set up shop. In fact that's one of the many reasons that the Artemis missions are aiming for the south pole!

The Lunar dust is the fine (<50 µm) portion of the lunar soil or 'regolith' material. The regolith on the Moon is formed by meteoric impact and doesn't undergo the weathering that dirt on Earth does. As a result the lunar dust is very abrasive and, due to its glassy composition, fine size, and large surface area to volume ratio, is extremely prone to building up an electrostatic charge. In layman's terms:

"It's abrasive, it's sticky, and it's everywhere!" M.S.

Following the Apollo program a NASA report on the effect of the dust identified nine main areas in which the dust was an issue in terms of its effect on Extra-Vehicular Activity (EVA) on the Moon. These areas were:

• Vision obscuration, 
• False instrument readings, 
• Dust coating and contamination, 
• Loss of traction, 
• Clogging of mechanisms, 
• Abrasion, 
• Thermal control problems, 
• Seal failures, and 
• Inhalation and irritation. 

Outside of being annoying to deal with, the dust probably won't hinder metallurgical extraction processes much itself. However, when considering mineral processing, an area that traditionally is notirious for creating dust, significant modifications will need to be made to equipment to ensure that dust generation is minimised

Most mineral processing technologies rely heavily on water for operation. While water in the form of ice is predicted to be available on the Moon in permanently shadowed regions (PSRs), this water will be an expensive commodity in its own right.

But beyond that, even assuming a source of water can be found to enable the use of water in metallurgical processing, a second significant issue arises when considering the practicalities of using water at all. The pressures present on the lunar surface are well below the triple point of water meaning that in order to stop the spontaneous evaporation of the water the pressure of the processing system would need to be artificially increased. Similarly, in order to stop solidification or vaporisation of the water at average day and night temperatures on the Moon, the system would need to be heavily temperature regulated. Given the difficulty of creating large, pressurized, temperature regulated, dust proof environments to enable the use of water, it is important to consider alternatives to water-based mineral processing technologies. 

There are however water free options. Some mineral sands processing techniques for example like electrostatic and magnetic separation could be operated. Similarly there are dry gravity and pnumatic processes that can be used. Recent work out of Japan has even demonstrated vibratory separation of material based on the 'brazil nut effect' (we didn't make that up...). So thre are options. But it's safe to say that due to the lack of water, mineral processing on the Moon is going to look very different to what we normally do here on Earth.