A team of European researchers have for the first time, quantified oxygen production via solar pyrolysis, a process that uses concentrated sunlight to heat lunar soil to release oxygen as a gas. Jack Robinot and collaborators report their results in Advances in Space Research. The findings of this study mark a significant milestone in space manufacturing technology, providing the first-ever direct experimental quantification of oxygen yield from the thermal decomposition of lunar regolith using focused solar energy.

As space settlement advocates are aware, the primary bottleneck for long-term human habitation on the Moon and elsewhere in the solar system is the logistical challenge of resupply of resources from Earth. They know that In-Situ Resource Utilization (ISRU) is the key to long term sustainability by harvesting and processing local materials into valuable resources like oxygen, water, and metals.

Oxygen is obviously of particular importance, not only because it’s needed for human life support, but also for rocket fuel. Liquid oxygen typically constitutes approximately 80% of the mass of rocket propellant. Producing oxygen directly on the Moon could dramatically reduce the cost of lunar and deep-space missions by eliminating the need to transport heavy fuel up from Earth’s gravity well. And of course it is a component of breathable air for settlers.

The lunar surface is covered in regolith composed mainly of silicate minerals and metal oxides. Despite the absence of an atmosphere, oxygen is the most abundant element on the Moon, making up about 45% of the regolith by weight.

When comparing the over twenty techniques that have been proposed for oxygen extraction—including carbothermal reduction and molten salt electrolysis—most require imported consumables like hydrogen or methane. Solar vacuum pyrolysis is advantageous because it:

• Requires no consumables as it utilizes only solar energy and natural vacuum, both in abundance on the Moon.
• Reduces logistics as there are no reagents to recycle or transport from Earth.
• Lowers reaction temperatures because the low pressure of a vacuum environment favors the reduction of metal oxides, allowing the process to occur at more manageable temperatures.
• Provides high efficiency via solar concentrators which can deliver high thermal power densities (up to 350 W/kg) without the energy conversion steps required by electrical systems.

By way of historical context, the concept of vaporizing lunar regolith was first explored in 1971 using samples of lunar soil returned by Apollo 12. Early researchers like Steurer (1982) proposed the theory of solar pyrolysis, calculating potential yields of 17%, but no solar experiments were conducted at that time.

Subsequent studies by Senior (1992) and Šeško (2024) successfully heated simulants and observed qualitative evidence of oxygen release, such as pressure increases or changes in material composition. However, technical challenges—including glass window breakage and the limitations of mass spectrometry—prevented these researchers from precisely quantifying the amount of oxygen produced. The current study bridges this gap by introducing a new analytical method using a trace analyzer to quantify oxygen within a carrier gas.

Before conducting experiments, the researchers modeled the behavior of heated lunar regolith through thermodynamic analysis of EAC-1 simulant (European Astronaut Centre-1, a high-fidelity lunar regolith simulant developed by the European Space Agency). They used the Gibbs energy minimization method via HSC Chemistry 10 (a process modeling platform widely used in the metallurgical and chemical industry). The model simulated the behavior of EAC-1 regolith simulant—which closely matches the oxygen content of real lunar soil (approx. 44%)—under temperatures up to 3000°C and pressures ranging from 10 mbar to 3 X 10^-15 bar.

The key findings of the analysis included:

• Temperature Dependence: At a pressure of 10^-2 bar, the optimal reaction temperature was roughly 2600°C.
• Pressure Impact: Lowering the pressure significantly reduces the required reaction temperature. At lunar surface pressures, the reaction could occur at temperatures as low as 950°C.
• Oxygen Species: High temperatures and low pressures promote the formation of monoatomic oxygen (O), though this study focused on quantifying diatomic oxygen (O₂).

The researchers noted that while the model provides a “target” for yield, it assumes a closed system at equilibrium. In practice, the experimental reactor will be an open system where species are constantly removed, shifting the equilibrium to promote further reduction via Le Chatelier’s principle.

Digging in to the setup and methodology, the experiments were conducted using a specialized solar reactor at the French national laboratory PROMES (Procédés, Matériaux et Énergie Solaire, or in English, Processes, Materials and Solar Energy), which is managed by the French National Centre for Scientific Research (CNRS).

The components of the apparatus included:

• Solar Concentrator: A 2-meter diameter parabolic dish focusing sunlight into a 2 cm focal spot.
• Tracking and Control: A heliostat provided real-time solar tracking, and a system of shutters precisely modulated the solar flux delivered to the sample.
• Reactor Chamber: A glass vacuum sphere where the regolith pellet sat on a water-cooled steel holder.
• Gas Analysis: An argon carrier gas was injected to sweep released oxygen into a high-precision oxygen trace analyzer (0.1 ppm to 1% range).
• Condensation System: A refrigerated copper condenser and a porous stainless steel filter captured volatilized metal species to protect the vacuum pump.

The procedure was initiated by placing a 3.38-gram pellet of EAC-1 simulant in the reactor. The chamber was evacuated and then pressurized with 10 mbar of argon. Solar power was increased in stages with the following observations:

1. 380 W: Initial melting was observed, but no oxygen was released.
2. 650 W: A brief oxygen peak occurred, likely due to the reduction of volatile oxides like Na₂O and K₂O.
3. 1200 W – 1460 W: Sustained oxygen production began as the sample reached approximately 1800°C.

The key results of the experiment achieved the first direct determination of oxygen reaction yield for this process:

• Total Oxygen Extracted: 35 mg.
• Mass Yield: 1.05% of the total processed sample weight.
• Extraction Efficiency: This represents 2.47% of the total oxygen available within the regolith simulant.
• Energy Yield: Approximately 31 mg of O₂ per kWh.

The study observed that oxygen production happens in distinct stages: an early release during initial melting followed by a more sustained extraction period at peak temperatures. During the process, the vaporized material caused visible “opacification” (clouding) of the reactor’s glass window, which likely reduced the amount of solar energy reaching the sample toward the end of the run.

For characterization of the by-products, after the experiment the researchers analyzed the remaining residue and the deposits found throughout the reactor using SEM/EDS, XRD, and Raman spectroscopy.

With respect to mass balance, of the original 3.38 g sample, 1.82 g remained as a “glassy residue” on the holder, while approximately 1.1 g was vaporized and deposited elsewhere. The final mass recovery was 92%.

An elemental and phase analysis revealed:

• Residue: Contained non-volatile elements like Aluminum (Al), Calcium (Ca), and Titanium (Ti), along with some Magnesium (Mg) and Silicon (Si).
• Deposits:
  • On the holder: High concentrations of Sodium (Na) and Iron (Fe).
  • On the window: Primarily Silicon (Si) and Iron (Fe).
  • On the condenser: Mostly Silicon, with some Iron and Magnesium.
• Crystalline Phases: XRD analysis of the original EAC-1 sample showed it was highly crystalline (containing minerals like augite, forsterite, and anorthite), whereas the residue present in the experiment was largely amorphous glass.

In the Discussion section of the paper the researchers found that the experimental yield of 1.05% was slightly lower than the 1.37% predicted by the thermodynamic model for conditions of 10 mbar at 1800°C. The discrepancy was attributed to several factors:

1. Open vs. Closed System: The model assumes everything stays in the reactor, while the experiment continuously pumps gases out.
2. Kinetics: Real-world reaction rates and temperature distributions within the pellet are not accounted for in basic equilibrium models.
3. Argon Dilution: The argon carrier gas is a “double-edged sword.” While it allows for accurate quantification and prevents oxygen from recombining with metals, its presence increases the total pressure, which works against the pyrolysis reaction.

Although these numbers do not sound significant, the study demonstrates that solar vacuum pyrolysis is a viable, reagent-free method for extracting oxygen from lunar regolith. Beyond oxygen, the results suggest that fractional separation of metals is possible, as different elements vaporize and condense at different locations and temperatures. These findings are promising to inform follow-on studies to develop in-situ metal refining processes to provide feedstock for building lunar infrastructure.

Future work will focus on lowering operating pressures to further increase oxygen yield, customizing the reactor to allow for high carrier gas flow without increasing total chamber pressure, and development of advanced kinetic models that account for temperature gradients within the regolith.

By successfully quantifying the oxygen yield, this research achieves an important benchmark for designing the next generation of hardware intended to support a sustainable human presence on the Moon. Once scaled up, solar pyrolysis factories could supply breathable air in situ for hotels like those planned by GRU Space and other dwellings in communities envisioned by Lunar Cities, a StellarWorld company.

Cutting-edge technology enabling settlement of the solar system and beyond – Art by Rick Guidice / NASA