The Space Resources Compendium
Why Are Mineral Deposits Common on Earth, But Rare on the Moon?
Hint: Water makes all the difference.
Nearly 4.5 billion years ago, a small planet (known as Theia) collided with early Earth, ejecting a massive amount of material into space. The ejected material eventually recombined to form our Moon. This event, explained in the Giant Impact Theory, suggests that the Moon is partially composed of the outer layers of both the small planet and early Earth. While the elemental composition of the Moon is strikingly similar to that of Earth’s outer layers, particularly the mantle, the minerals available on these two planetary bodies are strikingly different.
The shared origin and the mixture of materials from both planetary bodies account for the remarkable resemblance in their elemental compositions. However, the impact and subsequent reformation processes led to variations in the concentration of volatile elements and compounds resulting in the Moon that is much drier than Earth. This difference has played a key role in the evolutionary paths of each body’s geology.
Earth boasts a diverse mix of elements that forms the backbone of its varied landscapes and deep interiors. Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium all play their parts in creating the planet’s crust, mantle, and core. Predominantly, Earth’s crust is a haven for silicate minerals with feldspar and quartz being some of the most common constituents. These silicate minerals are a direct result of a rich elemental interplay, combined with the varied geological processes that the Earth undergoes. Similarly, the Moon possesses a mantle and crust rich with silicate minerals with an elemental palette containing oxygen, silicon, magnesium, iron, calcium, and aluminum. Olivine, pyroxene, and plagioclase feldspar (anorthite) minerals are reported to make up a significant portion of its crust, while other oxide minerals such as ilmenite are also constituents.
However, it is the presence of abundant liquid water on Earth that truly sets these two planetary bodies apart. Water has affected our planet’s mineralogy through processes that the arid landscape of the Moon has never experienced.
Water facilitates the formation, alteration, and distribution of minerals through the Earth’s crust. One of the most significant contributions of water to Earth’s mineral wealth is through abundant hydrothermal processes. As water circulates through the Earth’s crust, it allows for the movement and concentration of various minerals. The dynamic nature of water, combined with Earth’s internal heat, creates a conduit for minerals to be transported, altered, and deposited in concentrated pockets. Such processes produce a host of concentrated ore and mineral deposits, rich in valuable metals like copper, gold, and zinc. These concentrated mineral pockets make the recovery of such resources profitable and therefore worthwhile.
In addition to internal hydrothermal processes, weathering processes on Earth also owe their efficacy to liquid water. Water, especially when slightly acidic, can break down rocks and minerals near the Earth’s surface. This breakdown results in the dissolution and redepositing of these minerals. It also leads to the formation of entirely new mineral types which cannot be found on the Moon. Clays and a myriad of secondary minerals owe their existence to such water-led weathering processes.
The vast oceans that cover the Earth are not just bodies of water but also crucibles where mineralogical processes occur. Carbonate minerals, such as calcite and dolomite, are birthed from the interactions between water and atmospheric or volcanic CO₂. These carbonate formations not only add to Earth’s mineral diversity but also play a crucial role in the global carbon cycle, acting as sinks that regulate atmospheric carbon dioxide levels.
In hot regions where bodies of water often undergo evaporation, a rich film of precipitated minerals is often left behind. Here, we find another class of mineral deposits known as “evaporite deposits” which manifest as vast salt flats abundant in minerals like halite (salt) and gypsum. These regions, often shimmering white under the sun, are a testament to the mineralogical power of water.
In stark contrast is our Moon. It remains untouched by the influence of flowing water and dynamic hydrothermal processes. The Moon lacks any atmospheric water vapor, and the absence of liquid water means it hasn’t been privy to the same mineral-forming processes that Earth has. The concept of hydrothermal activity, so prevalent and influential on Earth, is non-existent on the Moon. As a result, the Moon simply doesn’t have the diverse mineral deposits that we readily find here on Earth, making it much more challenging and costly to recover lunar resources profitably.
Additionally, the weathering processes that the Moon undergoes are vastly different. Instead of water-based weathering, lunar surfaces experience what is termed as ‘space weathering.’ This includes micrometeorite impacts and interactions with the solar wind. The lack of water-based weathering means the Moon does not benefit from the creation of secondary minerals that diversify Earth’s mineralogy. The absence of vast oceans or any significant water bodies on the Moon also means that there are no carbonate formations or evaporite deposits. Such classes of minerals, which add to the Earth’s mineralogical richness, are conspicuously absent from the Moon’s portfolio.
While the Earth and the Moon might share an origin story and have some elemental similarities, their mineralogical tales are vastly different. The presence of water on Earth has been a game-changer, acting as a catalyst and medium for geological processes that have enriched our planet’s mineralogy.
As humanity continues to look beyond its home, understanding the profound influence of water in shaping the mineralogical narratives of celestial bodies will be paramount. The lack of hydrothermal process on the moon means that lunar minerals and desirable elements are dispersed across the surface and subsurface uniformly in low concentrations. Extracting these minerals would require processing vast amounts of lunar regolith to recover even a small quantity of valuable material. This makes the recovery of such minerals much more complex and costly.
Given this reality, both government and private companies seeking to recover resources from the lunar surface must be well-informed about what/where minerals are available, their abundance, and the mining processes required to convert them into usable commodities. Mining on the Moon is not an impossible endeavor, but it will require significant forethought, prospecting, and data analysis to be worthwhile.
Jarrett Dillenburger is a materials chemist by training and a space scientist in training. He currently studies Interdisciplinary Space at the University of Luxembourg. Follow him on LinkedIn and Twitter.
