The same chemistry of planet-formation can, in reverse, generate water from lunar regolith with a dash of hydrogen and a few rays of sunlight.
In situ resource utilisation (ISRU) of water on the Moon and Mars remains one of the greatest limiting factors for future colonisation of our closest Solar System neighbours. While permanent ice is likely present on the lunar south pole, this promises to be an exceptionally limited resource with low odds of renewability, not to mention multinational competition. Production of water from other sources would greatly enhance our ability to create permanent bases and use the Moon as a stepping stone to Mars and the outer Solar System.
While water is necessary for human consumption and for growing our crops, it can also be electrolysed into H2 and O2. This molecular oxygen is what we, as humans, breathe. It is also one of the more common principal components of rocket fuel and could allow the Moon to be used as a refuelling stop for future colonisation beyond. Transporting water from the Earth to the Moon is prohibitively expensive, and lunar water is almost certainly underabundant for sustained human habitation. Another vein for generating water on the Moon is that it would greatly increase the odds of successful, long-term human expansion into the Solar System. The key to finding a means of generating water lies in the composition of materials that will already be present when humans return to the Moon.
The principal element of water is oxygen. Lunar regolith, as with most minerals, is around 50% oxygen by composition, and oxygen is oxygen regardless of what molecules it happens to reside in at the moment. Harvesting of mineral oxygen for the production of water or O2 has remained an ongoing challenge. The chemical structure of the mineral network locks the oxygen in and is dependent upon the constituent oxygen atoms for its stability. Consequently, few alternatives for other molecular products containing oxygen are energetically favourable, utilising rocks as a starting material. However, recent work has shown that molecular hydrogen may be the key to unlocking this element in situ.
First rocks (and planets) from water
Every element on the periodic table, excluding the 90% or so of atoms in the universe that are hydrogen, was formed in the core of a star. Yes, a notable amount of the present-day helium is primordial, but since most stars spend most of their lifetimes turning isotopes of hydrogen into helium, every element that is not hydrogen has some, if not all, of its population originating from stellar nucleosynthesis or other processes thereafter.
Of course, complex astronomical structures comprised of elements from across the periodic table are now present in the present epoch of time. Planets are the most notable of such for their chemical complexity, but even these are believed to form from ever smaller bodies going back in their formation history. The smallest original components of rocky bodies like the Earth, Moon, and Mars are dust grains, often micrometre-sized particles comprised of a chemical network populated largely by iron, magnesium, silicon, and, yes, oxygen. However, the connection in the formation of these grains from their constituent atoms is uncertain, and the difference in size between the atomic components and the final bulk material, even in grains, represents a gulf of more than a factor of one million. Our group has been among the first to attempt to bridge this gap, and we have found special chemistry along the way.
A roller coaster ride or a run down an alpine ski hill will never go higher than where the original lift takes you. Friction, wind resistance, and heat loss rob the coastie or the skier of bits of their momentum, keeping them from ever being able to return to the top of the hill without an additional input of energy. Similarly, chemical reactions in space behave in much the same manner. Ideally, the molecular products will be at the bottom of the hill, and even if there are some ups and downs along the way, none of the ups are as high as the top of that first hill where the reactants initially join together.
The reaction of water with metal hydrides appears to perform in this way, just like skiers. With hydrogen so ubiquitous in the universe, we have assumed that diatomic metal hydrides can form in the atmospheres of most stars. Many such hydride molecules have been observed in our own Sun as well as in stars from our stellar neighbourhood. Furthermore, water is one of the ten most abundant molecules in space, and reactions of metal hydrides with water perform the roller coaster ride ideally. They go up and down just like a ski run, but, again, the ups do not surpass the first lift.
Additionally, these reactions generate H2, or molecular hydrogen, the most common molecule in the universe. Such H2 production would be hidden in the vast abundance of this molecule (like N2 production would be here on Earth), but it also stabilises the final metal oxide molecular product by translating away the excess energy. These reactions also produce a slightly larger molecule containing a metal-oxygen bond, which can react again in the same manner to produce larger metal-oxide clusters and additional H2 molecules. Eventually, this continued roller coaster reaction generates larger and larger clusters, eventually up to very small grains, each with the bonds necessary to produce minerals and dust grains.
These dust grains eventually stick together (or collide violently) and form rocky bodies, ultimately leading to planets. Hence, our work is showing that the rocky minerals such as enstatite, quartz, forsterite (the major endmember of olivine), and related materials likely gained their oxygen originally from water. If the oxygen could be transmuted back into water, however, colonisation of the near Solar System would suddenly become much more practical.
Water from rocks
Theoretically, all that must be done to produce water from rocks is the reverse of what nature appears to do to form dust grains and planets. Simply, oxygen-based minerals in the presence of H2 and some form of energy produce water. However, the practicality of such a reaction is much more than the relatively straightforward theoretical result would suggest.
The chemistry of this process requires counter-intuitive and seemingly circuitous pathways to arrive at the ultimate solution. Initially, the H2 will react with the metal oxide surface. To do so, the hydrogen molecule must parallel an exposed metal-oxygen bond. One hydrogen atom is then linked to the metal atom (likely iron, magnesium, or silicon) while the other is linked to the oxygen atom. This process ‘cracks’ the H-H bond and actually weakens the underlying metal-oxygen bond. The hydrogen atom on the metal, next, migrates onto the oxygen, where the other hydrogen is still bonded. Water forms as a result and can be easily desorbed from the surface.
While the departure of the oxygen as a water molecule leaves a gap in the mineral, the resulting destabilisation is, in fact, a good thing for ISRU applications. The weakened mineral structure is subsequently much more likely to break down in the presence of additional molecular hydrogen and energy. Granted, metal hydrides (MgH2, SiH4, FeH2/3) will also be produced in addition to water, but the densities of these materials differ significantly from water, implying that they can be readily separated. These byproducts are also highly reactive, but if they are isolated in small amounts, they can be utilised, as well. Further energy reactions on the metal hydride byproducts would remove their hydrogen atoms and reform them into H2. The hydrogen can now be recycled for additional water production. The resulting metal atoms would form into macroscopic metal grains and could be readily turned into silicon wafers or iron filings for other applications, and lunar colonies would greatly benefit from a ready source of these raw materials for both building materials and electronics, in addition to the water produced.
Can it work?
The first consideration for developing such technology to produce water from rocks is the material itself. The vast majority of the oxygen in any mineral body will be locked inside the rock. In order to access anything beyond the surface oxygen, the material would have to be processed and ground up into a powder, ideally, where the surface area is notably increased. Additionally, this material would need to be more than just smooth pebbles, but have microscopic protrusions and multiple edges. Coincidentally, lunar regolith is notoriously sharp, with a large surface area for the typical grain size. While this property wreaks havoc on EVA suits and air locks, the jagged, fine, high-surface-area lunar dust grains covering Earth’s nearest neighbour appear to make it ideal for surface chemistry like H2 cracking.
The second consideration for this technology is the source of energy. Some experiments working in this area require ovens operating in the realm of thousands of degrees or even lasing the materials with infrared light. However, our theoretical results imply that simple sunlight may be enough to push the skier of this chemical reaction back up the hill to the top of the lift. This requires a variety of wavelengths from the ultraviolet to the infrared, but all are within the standard energy realm of sunlight. Of course, concentrating sunlight would be ideal, but this can be passive through the use of lenses or other physical processes and would not require extensive hardware or additional electricity.
The most energetically costly step for the chemistry of producing water from rocks is the migration of the hydrogen atom from the metal atom onto the oxygen to create the actual water molecule. Such a process can take place via one of two pathways. One is through several intermediate chemical steps, but none of which require ultraviolet light. The other pathway involves a direct approach that demands a higher energetic cost. The question then becomes: Are several lower-energy steps more efficient than a single higher-energy process? In the realm of photon collisions, the fewer steps, the better. While slightly fewer high-energy, UV photons are produced, the difference is negligible. Catching one fly ball is much easier than having to catch three in a row, even if those three never went as high as the other one. Consequently, even though the single step is higher in energy, this process is the more efficient route.
The third consideration is where to find the needed hydrogen. While lunar regolith is peppered with hydrogen atoms and molecules within, the volume of water that could be produced from this source of hydrogen is inefficient for generating water from a tractable amount of regolith. Large volumes of material would have to be processed for a relatively small amount of water. The amount of available hydrogen is the limiting step. However, our approach promises to use much more of the oxygen if hydrogen is on hand.
As such, an initial shipment of hydrogen would certainly be required for shipment to the location of this chemical reactor, and subsequent deliveries would likely also be needed to top off the hydrogen levels. However, these shipments would be far less frequent than standard water shipments, and transporting the hydrogen solely would be a significant reduction in payload mass (and cost) over the same volume of water. Additionally, the produced water itself is recyclable for both human and agricultural use, even though loss is inevitable. More useful, though, is that the production of O2 from water for life support would create hydrogen as a byproduct. This hydrogen, as well as that harvested from the production of metal raw materials from metal hydrides, would allow for a ‘rebreathing’ of the hydrogen for multiple uses. Hence, recycling the hydrogen is possible and would increase the efficiency of its usage as a relatively limited resource. While in situ hydrogen would be ideal and sources for it on the Moon are possible, once the hydrogen is present, its usage would theoretically trend towards its maximal utilisation in the production of water.
Finally, theory is good, but demonstration is better. Recent work published by the School of Earth and Space Exploration at Arizona State University has demonstrated exactly this. The iron endmember of olivine, fayalite, was placed in an anvil. An infrared laser was used to heat the iron oxide material while hydrogen gas was passed over it. The result was the creation of a dense iron alloy within the anvil as well as an escaping steam of . . . water. This isn’t just theory any longer.
Future outlook
Our theoretical computations will continue to explore this chemistry. Our quantum chemical techniques can explore the granular, step-wise chemistry that allows the production of water from rocks to take place. We can change individual properties, such as chemical composition, and can determine the energy costs and most efficient pathways that result from such changes. These insights will allow for subsequent creativity and new experiments that can develop the technology in an informed manner.
We have already shown the production of water from the forsterite, enstatite, and corundum monomers as well as periclase and quartz monomers, dimers, and trimers. We have also received funding from NASA (Grant 80NSSC26K7044) to explore this chemistry for both planet formation and ISRU water production from fayalite, spinel, and hematite, among others. Hence, many of the most common minerals of terrestrial planets will be explored for their ability to donate oxygen to the cause of generating water for ISRU applications beyond the Earth.
Please Note: This is a Commercial Profile
This article will feature in our upcoming space Special Focus Publication.
Source link
