# Quantification of oxygen production from solar pyrolysis of lunar regolith

Jack Robinot, Sylvain Rodat, Stéphane Abanades, Eric Bêche, Alexis Paillet,

Aidan Cowley

► To cite this version:

Jack Robinot, Sylvain Rodat, Stéphane Abanades, Eric Bêche, Alexis Paillet, et al. Quantification of oxygen production from solar pyrolysis of lunar regolith. Advances in Space Research, 2026, 10.1016/j.asr.2026.02.003. hal-05512237

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# Quantification of oxygen production from solar pyrolysis of lunar regolith

Jack Robinot<sup>a</sup>, Sylvain Rodat<sup>a</sup>, Stéphane Abanades<sup>a,*</sup>, Eric Bêche<sup>a</sup>, Alexis Paillet<sup>b</sup>, Aidan Cowley<sup>c</sup>

Received 27 November 2025; received in revised form 18 January 2026; accepted 1 February 2026

## Abstract

The extraction of oxygen from lunar regolith is a critical step toward enabling sustainable human presence on the Moon, reducing reliance on Earth-based resupply. Among various proposed in-situ resource utilization (ISRU) methods, solar vacuum pyrolysis offers a promising, reagent-free approach by leveraging the Moon's natural vacuum, abundant regolith and solar energy resources. Conventional oxygen extraction techniques, such as carbothermal reduction or molten salt electrolysis, require imported reactants, posing challenges for long-term lunar operations. Here, we demonstrate that concentrated solar pyrolysis under low vacuum can efficiently extract oxygen from EAC-1 lunar regolith simulant while simultaneously producing metallic byproducts. Using concentrated solar energy with a maximum flux of 4.8 MW/m² under low vacuum conditions at 10 mbar, we extracted 35 mg of oxygen, corresponding to a mass yield of 1.05 %. This represents 2.47 % of the total oxygen available in the regolith simulant recovered as gaseous oxygen. These values provide the first ever quantification of oxygen production from regolith solar pyrolysis. Moreover, process intensification routes are further identified. These findings indicate that oxygen production occurs in distinct stages, with early release during regolith melting, followed by sustained extraction at higher temperatures. This study also suggests that fractional separation of metals might be possible, proposing a potential pathway for in-situ metal refining on the Moon. Solar vacuum pyrolysis requires no consumable or reductants and operates under conditions naturally available on the lunar surface, enhancing its feasibility for large-scale ISRU applications. This work provides new insights into solar vacuum pyrolysis as a process for lunar oxygen extraction and metal separation. These results reinforce the strategic importance of ISRU for enabling long-duration lunar missions, reducing Earth-based supply dependency, and laying the groundwork for autonomous life support, construction, and fuel production using local resources.

© 2026 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

**Keywords:** Concentrated solar energy; Lunar regolith pyrolysis; Space exploration; ISRU; Solar thermal reduction of oxides; Oxygen extraction

## 1. Introduction

Ensuring a long-term human presence on the Moon hinged on minimizing reliance on Earth-based supplies and harnessing local resources starting with oxygen and

water to support human activities, and later metals to develop and expand lunar infrastructure such as habitats, factories or power generation systems. In-Situ Resource Utilization (ISRU) offers a strategic pathway for meeting this challenge by harvesting and transforming lunar materials in valuable resources. Oxygen is one of the most critical components given its dual role in life support and rocket propulsion. Extracting oxygen from lunar regolith

<footnote><sup>a</sup> CNRS, Processes, Materials and Solar Energy Laboratory (PROMES-CNRS), 7 Rue du Four Solaire, 66120 Font Romeu, France</footnote>

<footnote><sup>b</sup> National Centre for Space Studies (CNES), 18 Avenue Edouard Belin, 31401 Toulouse, France</footnote>

<footnote><sup>c</sup> European Space Agency, European Astronaut Centre (ESA-EAC), Linder Höhe, Cologne 51147, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: stephane.abanades@promes.cnrs.fr (S. Abanades).</footnote>
will offer sustainability for human activities. Upcoming mission architectures require in Earth-orbit refueling. Approximately 80 % of rocket fuel mass is liquid oxygen. Instead of shipping propellant from Earth, liquid oxygen could be produced on the moon and shipped to Earth orbit. Lunar oxygen production would permit to substantially reduce refueling costs, thanks to absence of atmosphere and low lunar gravity that facilitates launch.

On the lunar surface, the most abundant oxygen-bearing material is the lunar regolith, the layer of soil covering the Moon's surface and composed primarily of silicate minerals and metal oxides. It results from billions of years of continuous meteoritic bombardment, space weathering, and solar radiation exposure, which have pulverized the lunar crust into a mixture of crystalline and amorphous particles (McKay et al., 1991). The major components of lunar regolith include silicates such as plagioclase feldspar ((Ca,Na)(Al,Si)₄O₈), pyroxenes ((Ca,Fe,Mg)₂Si₂O₆), and olivine ((Mg,Fe)₂SiO₄), along with other minerals such as ilmenite (Fe,Mg)TiO₃, and various forms of spinel (MgAl₂O₄) (Papike et al., 1991). Despite the lack of a Moon atmosphere, oxygen remains the most abundant element on its surface, constituting up to 45 % of the regolith's weight. Over twenty techniques have been proposed for extracting oxygen from lunar regolith, each with distinct advantages and challenges (Taylor and Carrier, 1993). Pyrometallurgical methods such as carbothermal or hydrogen reduction, or electrometallurgical techniques like molten regolith electrolysis, require consumables including methane, hydrogen and electrodes. The approach studied here, is the concentrated solar vacuum pyrolysis which represents a particularly attractive alternative. This method harnesses the Moon's abundant solar irradiance under vacuum conditions to thermally decompose metal oxides inside the regolith, releasing oxygen gas and reduced metallic species (Robinot et al., 2025; Schlüter and Cowley, 2020). It requires no consumables that must be recycled or transported from Earth, significantly reducing logistical constraints. Additionally, the lunar vacuum minimizes the reaction temperature as reduction of oxides is favored by low pressures. Heat generated by concentrated solar energy is of a particular interest when considering lunar applications. The absence of atmosphere ensures higher solar constant than on earth, with approximately 1361 W/m² on the moon (Kaczmarzyk et al., 2018) and the energy can be directly used without needing a conversion step. The specific thermal power achievable with solar cells typically lies between 20 and 50 W/kg, while flexible solar concentrator can deliver up to approximately 350 W/kg (Colozza, 2009). Solar vacuum pyrolysis could operate solely with heat generated by solar concentration, making it a uniquely self-sustaining process.

The vaporization of lunar regolith was first mentioned in 1971, when investigations were conducted using a Knudsen cell to study the vaporization processes of Apollo 12 lunar samples. This study determined that alkali oxides were the first to vaporize at temperatures below 1100 °C,

followed by Fe oxides at temperatures below 1300 °C. Mg and Si oxides were observed to vaporize around 1500 °C, while Al, Ca, and Ti oxides required significantly higher temperatures, in the range of 1600 °C to 1700 °C (De Maria et al., 1971). The concept of solar vacuum pyrolysis for lunar regolith oxygen extraction was first proposed in the early 1980s by Steurer (Steurer, 1982; Steurer and Nerad, 1983). However, no solar-based experiments were conducted at that time. These initial studies primarily focused on establishing the basic principles of the process and estimating potential mass oxygen yields based on theoretical thermodynamic studies, which were estimated to be around 17 % at temperatures between 2200 °C and 2700 °C. The first experimental attempt to apply solar heating to lunar materials was carried out by Senior (Senior, 1992, 1991), who used a Fresnel lens to heat individual lunar minerals, specifically ilmenite and anorthite. Experiments were conducted under vacuum at a pressure of around 10⁻¹ mbar and temperatures from 1566 °C to 2126 °C were reached. The experiment recorded a pressure increase, suggesting gas release, and further Gibbs free energy minimization calculations estimated a theoretical oxygen mass yield ranging from 2 % to 20 %. Later, Sauerborn (Sauerborn, 2005) conducted solar pyrolysis experiments on MS1 lunar regolith simulant at a temperature of 1550 °C and a pressure varying from 8.5 × 10⁻² to 9.1 × 10⁻³ mbar, where a decrease in oxygen content was observed in the pyrolyzed material. Additionally, mass spectrometry analysis confirmed oxygen release, though no precise yield was determined. Matchett (2006) further investigated the process using Gibbs energy minimization modeling, estimating an oxygen yield of approximately 14 %. However, despite observing sample mass loss, no direct oxygen production was detected in the experiments. Both Sauerborn and Matchett encountered technical challenges, such as glass window breakage and overpressure issues during mass spectrometry measurements. Building on Matchett's work, Cardiff (Cardiff et al., 2007) was able to confirm Sauerborn findings, detecting an increase in oxygen concentration using mass spectrometry. Additionally, Shaw et al. (Shaw et al., 2021) modelled staged separation of regolith vapors via Gibbs energy minimization, demonstrating that Na, K, and FeO could be separated using solar vacuum pyrolysis. Experimental work utilizing a solar simulator further supported the feasibility of metal separation under controlled conditions (Shaw, 2023). More recently, Šeško et al. (Šeško et al., 2024) conducted computational modeling by Gibbs energy minimization, which is a classical approach in the literature, to determine expected theoretical oxygen mass yields under various pressures from 10³ mbar to 10⁻⁸ mbar and temperatures from 0 °C to 3500 °C, estimating values between 6.6 % and 14.1 %. Experimental verification was performed using a Fresnel lens and temperatures of 1476 °C to 1626 °C were achieved at a vacuum of 1 × 10⁻⁷ mbar to 1 × 10⁻² mbar. Partial melting of EAC-1 lunar regolith simulant was achieved, and vaporization of Fe, Na, and K was directly observed.
Oxygen production quantification was however not possi-
ble on this configuration. These prior studies laid the foun-
dation for understanding solar vacuum pyrolysis as a
viable ISRU technique. They focused primarily on qualita-
tive demonstrations or partial characterizations of volatile
matter release and material transformations. In this work,
we bridge this knowledge gap by providing, for the first
time, a direct determination of the reaction yield, thanks
to a new analytical method consisting in quantifying oxy-
gen inside a carrier gas with a trace analyzer. The key nov-
elty of this work thus lies in the experimental determination
of the oxygen yield, which is quantified and reported for
the first time. In contrast to previous studies, the experi-
ments presented here were conducted under low-vacuum
conditions (10 mbar). The oxygen yield understood as the
mass of oxygen which can be produced with a certain
quantity of regolith is the key metric for designing future
hardware capable of sustaining long-duration exploration
missions. In summary, the original results of the present
study were the experimental determination of oxygen yields
from concentrated solar pyrolysis of regolith simulant,
which has never been done to date. A temperature of about
1800 °C was reached thanks to an efficient solar concen-
trating system. Additionally, a quantitative mass balance
and characterization of the vaporized materials were
performed.

## 2. Thermodynamic equilibrium modelling of the EAC-1 simulant pyrolysis

Thermodynamic analysis was conducted based on the
Gibbs energy minimization method using HSC Chem-
istry version 10. This method permitted to determine
the equilibrium of the system under specified tempera-
tures and pressures by minimizing the internal energy.
Modelling was conducted on the EAC-1 regolith simu-
lant which has been used in various ISRU applications
(Christen et al., 2025; Figueira et al., 2025; Ginés-
Palomares et al., 2023; Lomax et al., 2020) and is consid-
ered to be similar to Apollo 17 samples, the main differ-
ences being the presence of alkali components,
feldspathoids, hydrated amphibole and chlorite groups
which are absent in real lunar regolith (Engelschön
et al., 2020). Real lunar soil contains on average
44.59 wt% of oxygen, while lower and upper bounds
were reported to be 40.60 wt% and 48.10 wt%. In com-
parison, EAC-1 contains 44.41 wt% of oxygen, therefore
the oxygen content matches that of real lunar regolith.
Real lunar soil composition is well known thanks to
extensive characterizations of Apollo and Luna mission
samples. Neutron activation analysis was used to deter-
mine composition and oxygen content (Papike et al.,
1982). DTA (differential thermal analysis) was carried
out on real Apollo 11 soil, but experiment was per-
formed at ambient pressure and up to 1400 °C (no anal-
ysis was carried out in similar conditions as the solar
pyrolysis experiments), therefore only volatile elements

such as S, C, N, He, were observed to be outgazed from
the sample (Wachi et al., 1971). EAC-1 has the following
oxide composition which is expressed in weight percent:
SiO₂ 44.41 %, Al₂O₃ 12.80 %, Fe₂O₃ 12.20 %, MgO
12.09 %, CaO 10.98 %, Na₂O 2.95 %, TiO₂ 2.44 %,
K₂O 1.32 %, P₂O₅ 0.61 %, MnO 0.20 % (Seško et al.,
2024). To account for the mineral composition of the
regolith, not only were the oxides and their reduction
products considered, but also all possible species that
could form based on the atomic composition of EAC-1
available in the HSC 10 database. In total, more than
500 species were included in the model. Simulations were
performed from experimental pressure of 10⁻² bar (dark
tone) to lunar pressure of 3 × 10⁻¹⁵ bar (light tone).
Both extreme values of the pressure range and an inter-
mediate pressure of 10⁻⁷ bar (medium tone) were com-
puted, and oxygen mass fraction was plotted against
the temperature in Fig. 1.

Furthermore, minerals and metals species were
removed to avoid overcrowding and simplify reading.
Formation of monoatomic oxygen was promoted due to
the high temperature and low pressure. Both O₂ and O
species were represented on the plot as monoatomic oxy-
gen behavior remained uncertain due to its high reactivity;
it can either react with himself to produce diatomic oxy-
gen or oxidize metal species. Thus, it was not taken into
account for theoretical oxygen yield estimation. The oxy-
gen diatomic mass fraction slightly decreased from 8 % to
7 % across the entire pressure range, while the trends
became more stepped. At 10⁻² bar, the optimal tempera-
ture appeared to be at 2600 °C whereas at the lunar pres-
sure, the temperature was much lower (950 °C).
Furthermore, it appeared that decreasing the pressure
by one order of magnitude permits lo lower the reaction
temperature by about 100 °C. As operating a laboratory
reactor at lunar pressure is almost impossible, increasing
the temperature of the reaction seemed to be an attractive
strategy to balance the effect of higher pressure. However,
the results must be put into perspective with the assump-
tions required in the modelling. The system was assumed
to be at equilibrium without considering the kinetics of
the reactions, however, some reactions may not reach
thermodynamic equilibrium. The system is considered to
exhibit homogeneous pressure and temperature. The sys-
tem composition does not evolve during reaction (closed
system assumption), whereas considering the vacuum
pyrolysis, species are vaporized and removed from the
reaction medium. The oxygen is pumped out of the reac-
tor and reduced species are condensed which, in accor-
dance with Le Chatelier's principle, shifts the
equilibrium to promote further reduction. The thermody-
namic model assumes an equilibrium state and a closed
system, whereas experiments involve a non-equilibrium
state and an open system. However, despite those assump-
tions, Gibbs energy minimization permits to estimate opti-
mal temperatures for the reaction, which provides a first
target for reaction yield.
<figure><img src="image_2.png" /><figcaption>Fig. 1. Thermodynamic modelling of the oxygen mass fraction versus temperature at different pressures.</figcaption></figure>

**3. Experimental**

**3.1. Solar reactor and ancillary equipment**

Solar pyrolysis of regolith was conducted inside a dedicated reactor, designed at PROMES-CNRS laboratory for concentrated solar energy experiments under controlled atmospheres. It was previously used for oxides reduction applied to thermochemical fuel production cycles (Abanades et al., 2008; Charvin et al., 2007) and upgraded for regolith pyrolysis. The reactor features a vacuum chamber composed of a glass sphere with two ports. The sample was placed on a water-cooled steel holder, and glass-to-metal sealing was achieved via a screw mechanism fitted with Viton gaskets. A refrigerated copper condenser was positioned above the reactor to collect volatilized species, while a sintered porous filter (PORAL grade 03 316 L stainless steel) located between the reactor and the pump stage captured any material not condensed on the condenser, thereby preventing contamination of the pump. Vacuuming was performed using a scroll pump (Edwards NDX15) with a vacuum limit of $7 \times 10^{-3}$ mbar. The pump outlet was equipped with a T-connector, directing flow either to the atmosphere (via a valve) or to an oxygen trace analyzer (Systech, range: 0.1 ppm to 1 %, precision ± 2 % of reading). The analyzer determined an oxygen concentration inside a gas (here argon), injected coaxially to the sample by a flowmeter (BROOKS SLA5850S). The use of a carrier gas was essential for determining the reaction yield, enabling the online monitoring of oxygen concentration and quantification of oxygen production. This approach contrasts with previous studies, which relied solely on mass spectrometry with a residual gas analyzer only permitting qualitative studies. Pressure was monitored using two capacitance manometers (Brooks CMC Series), positioned at the bottom and top of the reactor. An omicron IN 140/5 pyrometer was used for punctual temperature measurements (at 4.8–5.2 µm assuming emissivity of 1).

Concentrated solar energy was supplied by a 2 m diameter parabolic dish, forming a focal spot roughly 2 cm in diameter. This setup delivered up to 1.5 kW of thermal power and provided a concentration factor of approximately 10,000 therefore reaching a power density of 4.8 MW/m². A heliostat positioned below the parabola enables precise real-time solar tracking, ensuring optimal alignment with the solar radiation throughout the experiment. Solar flux from heliostat was modulated by a system of shutters, enabling precise control of power supplied to the sample. Data acquisition and experimental parameters were operated via a controller (Beckhoff computer) at one-second intervals. Fig. 2 provides an overview of the apparatus used for the solar pyrolysis of regolith.

**3.2. Solar pyrolysis procedure**

A pellet of EAC-1 regolith simulant, with dimensions of 13 mm in diameter and 10 mm in height, was prepared using a hydraulic press at a pressure of 2.5 tons. The pellet was weighed prior to the experiment and then positioned at the center of the water-cooled steel holder inside the reactor. The different parts of the reactor were assembled, and the reactor was then evacuated to a pressure below $10^{-3}$ mbar, which corresponded to the lower pressure measurement limit. Once the pressure dropped this low, the reactor was considered air tight, and argon was then injected inside the reactor to reach a pressure of about 10 mbar. The valve at the pump outlet, which remained opened during the vacuuming, was closed to divert the gas to the $O_2$ analyzer. The analyzer was allowed to stabilize at a minimal value to obtain a baseline, which corresponded to the residual leakage and the impurity inside argon (99.999 % purity). Meanwhile, water-cooling was turned on to cool down the holder and condenser. To ensure a proper heating, the sample had to be precisely positioned at the dish focal point. Thanks to the residual light passing between the shutter blades (roughly 40 W),
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Experimental apparatus used for the solar pyrolysis of the regolith simulant, a) Flowchart of the setup with the reactor, solar concentrating system and the various peripheral equipment, b) Picture of the pyrolysis reactor during sample exposure to concentrated solar energy, one can see the glass reactor getting opacified due to the deposition of the vaporized materials.</figcaption></figure>

a focal point was observed and used to adjust the reactor along the three axes. The sample was then progressively heated by gradually increasing the shutters opening to deliver approximately 40 W to 1500 W. A picture of the partially molten regolith pellet can be seen in Fig. 3.

Regolith was heated to first form a droplet of molten regolith (too high heating rate would cause projection of molten materials on the glass reactor wall). Reactor position was readjusted especially after the melting as the droplet had a lower height than the pellet. Furthermore,
<figure><img src="image_5.png" /><figcaption>Fig. 3. Picture of the regolith pellet during experiment in a partially molten state, with the lower half remaining solid, while the upper surface exhibits a highly reflective droplet of molten regolith. A light blueish plume of gaseous material is visibly expelled from the surface.</figcaption></figure>

during the melting, the droplet tended to slightly slide on the holder. To ensure proper results, experiment was conducted on brightest and cloudless days. The reaction was considered completed when the sample was exposed to the highest flux level and no more oxygen production was observed by the analyzer. Once the reaction completed, the shutters were half and then fully closed to avoid potential thermal shocks due to a too fast cooling. The reactor was then filled with argon up to atmospheric pressure. The apparatus was let to cooldown, water-cooling was stopped and the reactor dismounted. Multiple deposited species were gathered at various places inside the reactor, respectively on the holder, the window, the condenser and the filter. These materials were weighted and collected for further characterization. Data from the instrumentation was then used to determine oxygen production and energy balance. Argon was used as a carrier gas for the purpose of continuous oxygen analysis in the outlet gas to determine oxygen concentration and quantify the amount of oxygen produced. On the experimental point of view, there are two competing processes when argon dilution is used. On the one hand, it increases the total pressure, which is not favorable for the pyrolysis reaction. On the other hand, it avoids oxygen accumulation (sweeping gas), thus shifting the reaction equilibrium toward the products side, and it also ensures products quenching, which favors oxygen recovery by limiting recombination reactions. Oxygen production was calculated by first determining the total volumetric outlet flow rate according to the following formula.

$$Q_{tot} = \frac{Q_{Ar}}{(1 - X_{O_2})} \qquad (1)$$

where $Q_{tot}$ (m³/s) is the total outlet volumetric flow rate, $Q_{Ar}$ (m³/s) is the argon volumetric flow rate, $X_{O_2}$ is the molar fraction of oxygen inside argon in the outlet gas. Oxygen production was then calculated as follows.

$$Q_{O_2} = X_{O_2} * Q_{tot} \qquad (2)$$

where $Q_{O_2}$ (m³/s) is the volumetric flow rate of oxygen, $X_{O_2}$ (ppm) is the oxygen mole fraction, and $Q_{tot}$ (m³/s) was calculated above. Then, the volumetric flow rate, $Q_{O_2}$, was converted in mass flow rate using the ideal gas law.

$$\dot{m}_{O_2} = \frac{P * Q_{O_2} * M_{O_2}}{R * T} \qquad (3)$$

where $\dot{m}_{O_2}$ (g/s) is the mass flow rate of oxygen. Here, the gas mix of argon and oxygen is considered to be at the normal state, so P equals 101325 Pa and T equals 273.15 K. Oxygen volumetric flow rate was calculated above and $M_{O_2}$ is the molar mass of diatomic oxygen, which equals 32 g/mol, and R is the ideal gas constant. Finally, the total oxygen production was calculated by integration of the oxygen production rate.

$$m_{O_2} = \int_{t_1}^{t_N} \dot{m}_{O_2(t)} dt \qquad (4)$$

Oxygen yield was simply calculated by dividing produced oxygen by the mass of the sample.

$$\eta_{O_2} = \frac{m_{O_2}}{m_{EAC-1}} \qquad (5)$$

The following polynomial expression (6) was obtained by dedicated solar flux measurements which correlate shutters opening (SO (%)), and the power concentrated at the focal
point for a normalized direct normal irradiance (DNI) of 1000 W/m².

$$P(SO) = (4.377 * 10^1 + 1.538 * 10^0SO + 1.825 * 10^{-1}SO^2 - 4.990 * 10^{-4}SO^3) * \left( \frac{DNI}{1000} \right) \quad (6)$$

The solar power was determined by polynomial in Eq. (6) based on the real recorded shutters opening and then adjusted to the instantaneous DNI. Energy was then calculated by integration of the power, in a similar manner to the mass of oxygen produced.

# 4. Results and discussion

## 4.1. Oxygen production and mass balance

Solar pyrolysis was performed over a one-hour heating period on a regolith pellet weighing approximately 3.38 g. Argon flowed into the reactor at 0.3 NL/min (Normal conditions: 1 atm and 0 °C), maintaining an internal pressure of about 13 mbar. Initial regolith melting was observed at a power level of 380 W, but no detectable oxygen was released at this stage. Increasing the power to 650 W produced a brief oxygen peak, as shown in Fig. 4a, lasting roughly five minutes from t = 10 min to t = 15 min, possibly attributable to the reduction of volatile oxides Na₂O and K₂O at low temperature. Subsequent power increase up to 1220 W did not yield additional oxygen. However, upon elevating the power to 1200 W at time t = 17 min, oxygen generation resumed with a peak value of 3750 ppm. At time t = 25 min, the shutters were fully opened to reach a power of 1460 W, initiating a 33 min period of steadily declining oxygen production from 2000 ppm to 1500 ppm. At peaking power, temperature was punctually measured to be around 1800 °C, nevertheless the temperature may be underestimated on account of the regolith melt not being a blackbody. During the heating process, vapor deposition caused visible opacification of the reactor glass, as observed in Fig. 2b, but with no mechanical damages. Once oxygen production ceased, the shutters were closed. Mass balance of the species recovered at the end of the experiment is shown in Fig. 4c. A total of 35 mg of O₂ (Fig. 4b) was successfully extracted and quantified, yielding 1.05 % of the total mass of the processed sample. A comparable yield of 1.17 % was obtained through experiment conducted under similar conditions, except for the heating rate (figure provided in supplementary data). The specific energy yield was determined to be 31 mg O₂/kWh. A glassy lump weighing 1.82 g remained on the sample holder, while 1.1 g of material was vaporized and subsequently captured on cold surfaces, holder, window, condenser, and filter. An additional amount of 0.27 g was unaccounted for, likely owing to challenges in completely cleaning reactor components, which hindered the complete recovery of the products mass. Indeed, some deposits remain on the window, filter, condenser or sample

holder. The final mass recovery reached 92 %, which is an acceptable value regarding the small amount of raw feedstock.

## 4.2. By-products characterization

With the yield established, detailed characterization of the solid products was conducted, by electron microscopy, X-ray diffraction (XRD), and Raman spectroscopy to determine chemical and structural properties of the various deposits. Due to material quantities limitations, XRD and Raman analysis were carried out on materials produced during a similar experiment as the one presented earlier. The materials collected were imaged using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) to determine their elemental composition and to assess any compositional changes (Fig. 5). Overall, no significant variation in oxygen concentration was observed among the samples. Although a lower oxygen content was anticipated in the vaporized species compared to EAC-1, the measured values revealed a higher-than-expected oxygen content, most likely due to metal reoxidation after recovery. The glassy residue predominantly contained nonvolatile elements (Al, Ca, Ti) along with some Mg and Si. In contrast, the major elements in oxides found on the sample holder were Na and Fe, with lower amounts of Si and trace levels of Mg, P, and K. Despite their higher volatility, Na and Fe were found in relatively high concentrations, presumably due to rapid redeposition following vaporization. Farther from the focal point, the window predominantly showed Si and Fe, both considered moderately volatile. The condenser contained roughly one-third Si alongside measurable Fe and Mg. Finally, the filter captured species that remained in the vapor phase until reaching the outlet of the system, with primarily Si, moderate proportions of Na and Fe, and trace amounts of Mg, P, and K. Notably, Si was present in every sample, likely reflecting its high initial concentration in the regolith feedstock.

XRD analysis was performed at room temperature using a PANalytical X'Pert Pro diffractometer with CuKα radiation (λ = 0.15418 nm). Symmetric scans were collected over a 2θ range of 10° to 80°, with a step size of 0.017° and a counting time of 100 s per step. Phase identification was carried out using PANalytical's DataCollector and HighScore Plus software packages database was PDF-2 (ICDD-Release 2020) (Gates-Rector and Blanton, 2019). To improve spectral accuracy, the contribution of the AlKα₂ component was removed using the Rachinger correction method (Rachinger, 1948). Reference material lanthanum hexaboride was used to determine instrumental function and expressed by a polynomial function (Beche et al., 2012). Diffraction pattern confirmed that EAC-1 (Fig. 6a), as anticipated, is highly crystalline, with multiple distinct phases clearly identified including augite (Ca₀.95-Mg₀.71Fe₀.25Si₂O₆) (ICDD-JCPDS card N°01-083-2016) or (Ca₀.968Mg₀.578Fe₀.23Ti₁.728Al₀.433Si₂O₆) (ICDD-JCPDS
<figure><img src="image_6.png" /><figcaption>Fig. 4. Experimental results from solar pyrolysis of the EAC-1 regolith simulant. (a) Oxygen concentration (ppm) and solar power (W) received by the sample throughout the experiment, showing the correlation between heating and gas release phases, (b) evolution of total oxygen production (mg) and energy (Wh) during experiment, and (c) mass balance of the materials recovered at various locations within the reactor after the pyrolysis experiment (initially the sample had a mass of 3.38 g).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Atomic composition in weight percent of the EAC-1 reference sample (Šesko et al., 2024) and the collected materials inside the reactor.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. XRD diffractograms of the samples, (a) EAC-1 regolith simulant, and (b) Holder sample. (c) Assignment table of the crystalline phases.</figcaption></figure>

card N°01-083-2075), forsterite (Mg₀.672Fe₀.323)₂SiO₄) (ICDD-JCPDS card N°01-71-0795), anorthite (Ca(Al₂Si₂O₈)) (ICDD-JCPDS card N°01-073-0264) or (Ca₀.86Na₀.14-Al₁.94Si₂.03O₈) (ICDD-JCPDS card N°01-076-0833), magnetite (Fe₃O₄) (ICDD-JCPDS card N°01-080-6407), nepheline (Na₇.85Al₇.85Si₈.₁₅O₃₂) (ICDD-JCPDS card N°01-081-8066) or (Na₃K₀.₅₅(AlSiO₄)₄) (ICDD-JCPDS card N°01-083-4893), and quartz (SiO₂) (ICDD-JCPDS card N°01-074-3486). In contrast, the molten regolith glass was predominantly amorphous, containing localized crystalline domains of Al (ICDD-JCPDS card N°01-089-2837), Si (ICDD-JCPDS card N°01-077-2109), and α-Fe (ICDD-JCPDS card N°01-089-7194). The materials collected on the holder exhibited five prominent crystalline phases (Fig. 6b): magnetite Fe₃O₄(Fe₂.₉₄₅O₄) (ICDD-JCPDS card N°01-080-7683) or Fe₃O₄ (ICDD-JCPDS card N°01-086-1360), metallic α-Fe (ICDD-JCPDS card N°01-80-3816), quartz alpha (SiO₂) (ICDD-JCPDS card N°01-089-8935), cristobalite alpha high (α - SiO₂) (ICDD-JCPDS card N°01-071-6245), and fayalite (Fe₂SiO₄) (ICDD-JCPDS card N°01-071-1667) alongside minor phases such as akermanite (Ca₂Mg(Si₂O₇)) (ICDD-JCPDS card N°01-076-7550), rutile (TiO₂) (ICDD-JCPDS card N°01-088-1174) or anor-

thite (Ca(Al₂Si₂O₈)) (ICDD-JCPDS card N°01-073-0264), and sodium oxide (NaO₂) (ICDD-JCPDS card N°01-074-0900). Meanwhile, the diffractograms of materials recovered on the reactor glass and the filter revealed largely amorphous species, showing only α-Fe (ICDD-JCPDS card N°01-089-7194) as a discernible crystalline phase. Finally, no identifiable phases emerged in the reduced species collected from the condenser, which appeared entirely amorphous by XRD.

The regolith simulant and the produced materials were then characterized by micro-Raman, performed using a Horiba LabRAM HR Evolution Raman equipped with an optical microscope (Olympus K.K. BX 41, x10 Working Distance (WD), x50 (long working distance, LWD), x100 WD objective lens), a charge coupled device detector (1024 × 256 pixels, 26 × 26 µm² by pixel) and two laser beams (HeNe laser with a wavelength of 633 nm, laser power of 17 mW and laser diode with a wavelength of 532 nm, laser power of 34 mW). The excitation source is a 532 nm laser beam and the laser power was reduced to about 8 mW (25 % filter strength) in order to avoid laser-induced effects (e.g. surface heating). The EAC-1 reference sample was first characterized to establish its mineralogical
profile (Fig. 7a). Multiple phases were identified, including olivine (Chopelas, 1991; Ishibashi et al., 2012), with several crystalline orientations, a pyroxene phase likely corresponding to be clinopyroxene (Buzatu and Buzgar, 2010; Wang et al., 2001) and a plagioclase compound (Freeman et al., 2008; Sharma et al., 1983). In addition, the presence of nepheline (NaAlSiO) was also identified (Matson et al., 1986). The glass sample was subsequently analyzed on both its top and bottom surfaces to assess potential compositional variation caused by the sample cooling. Analysis of the top surface (Fig. 7b) exposed to the concentrated solar flux, revealed the presence of an aluminosilicate glass matrix (Neuville et al., 2021, 2008; Sharma et al., 1985), along with Mg-rich olivine ((Mg > Fe)₂SiO₄) as a residual crystalline phase. The broadness of bands in the spectra of glasses results from structural disorder. The silicate glass microstructure is close to an amorphous based-olivine with a Qo (orthosilicate) and Q1 structure with [AlO₄]⁻ anions (strong Raman peak centered at about 880 cm⁻¹). On the bottom surface, phases similar to those in the reference sample were identified, including olivine, clinopyroxene, and plagioclase; however, nepheline was not clearly detected. Raman analysis of the holder sample revealed a complex composition comprising a broad range of mineral compounds, necessitating multiple measurement points to

achieve comprehensive characterization. At the first point, Raman spectra indicated the presence of near-fayalite compound (Chopelas, 1991; Kolesov and Geiger, 2004) along with hematite (Fe₂O₃) (Chernyshova et al., 2007; Zhu et al., 2014). A second spectrum corresponded to an unidentified clinopyroxene compound. A third data point showed features consistent with magnetite (Fe₃O₄) (Hosterman, 2011; Kumar et al., 2014), or possibly a near-stoichiometric (FeₓFe₂₋ₓO₄) or disordered magnetite structure (Lenaz and Lughi, 2017; Rodriguez and Chidambaram, 2015). Another spectrum revealed a glassy phase associated with an orthosilicate and an amorphous based-olivine composition (Le Losq et al., 2017; Neuville et al., 2021). In a subsequent analysis, a near-rutile (TiO₂) (Porto et al., 1967; Samara and Peercy, 1973) compound was detected, alongside a similar orthosilicate glass and an impure Fe rich spinel-type phase (FeM₂O₄). The final spectrum contained signatures of a spinel respectively magnetite or near compound, similar to the previously identified compounds (Hosterman, 2011; Kumar et al., 2014; Lenaz and Lughi, 2017; Rodriguez and Chidambaram, 2015), and near-fayalite (Fe₂SiO₄) (Chopelas, 1991; Kolesov and Geiger, 2004). No Raman-active compounds were detected in the window, condenser, or filter samples. The absence of a Raman signal in the window and condenser deposits was attributed to the presence of metallic Fe (Raman inactive element), as confirmed by XRD analysis. Although Fe was detected by SEM/EDS in the filter sample, metallic Fe was not observed by XRD, possibly due to peak masking by the predominantly amorphous matrix. This probable presence of metallic Fe likely accounts for the absence of a detectable Raman response in the filter sample. Overall, the analysis of the processed materials proved challenging due to their intrinsic characteristics. As oxide-based materials, they exhibited poor electrical conductivity, complicating SEM/EDS measurements and necessitating sample metallization. Additionally, the majority of the samples were amorphous, which hindered phase identification by XRD. In some cases, the presence of metallic phases further limited Raman spectroscopy, as these phases often lack Raman-active vibrational modes.

However, despite those limitations, analysis demonstrated minerals reduction, separation of the oxide species by volatility and production of reduced metals such as Fe. The most volatile oxides Na₂O, K₂O, Fe₂O₃, were predominantly recovered farther from the focal point, on the window, condenser, and filter samples. In contrast, the less volatile oxides Al₂O₃, TiO₂, CaO remained in the glass sample. The holder residue likely formed from the slight dispersion of regolith particles during melting and subsequent condensation of vaporized species onto the holder. These findings are consistent with previous studies (Christen et al., 2025) and highlight the potential of vacuum pyrolysis as a means to separate metals contained in lunar regolith.

<figure><img src="image_9.png" /><figcaption>Fig. 7. Raman spectra collected for EAC-1 regolith simulant and glass sample. (a) EAC-1 Raman spectra in the wavenumber range of 150 cm⁻¹ to 1200 cm⁻¹. Four compounds were detected: olivine (Ol), plagioclase-feldspar (Pl), clinopyroxen (Cpx) and nepheline (Nph). (b) Raman spectrum collected for the glass sample in the wavenumber range of 200 cm⁻¹ to 1200 cm⁻¹.</figcaption></figure>
4.3. Discussion

This study demonstrated the feasibility of solar pyrolysis as a method for extracting oxygen from lunar regolith while producing metallic byproducts. First ever quantification of oxygen yield is now available as a starting benchmark. At experimental conditions of 1800 °C and 10 mbar, an oxygen yield of around 1.37 % was expected by thermodynamic simulation, whereas a yield of 1.05 % was obtained experimentally. Both results appeared to be close, however this comparison would be invalid as the model does not accurately reflect the experiment. Gibbs energy minimization considers the system at equilibrium and with fixed composition, which was not the case in experiments. Kinetic parameters of regolith pyrolysis remain unknown, furthermore the reactor was continuously pumped and species were vaporized and therefore left the system. Kinetics along with temperature distribution should be included for accurate modelling. In contrast, thermodynamics assumes that all species remain in the domain (closed system) at a given homogeneous temperature, which is not the case during experiments as oxygen progressively escapes the reactor while volatiles condense on cooled surfaces. Hence, Gibbs energy minimization cannot fully represent the process conditions and experimental results deviate from the basic thermodynamic approach. Indeed, thermodynamic predictions do not provide the oxygen yields directly achievable in experimental systems operating under open and kinetically limited conditions with continuous removal of product species. This explains the observed discrepancy between thermodynamic simulations and experimentally measured oxygen yield. However, it can be inferred from the model and the literature, that Na₂O, K₂O, Fe₂O₃ were vaporized at lower temperatures than the other oxides, which agrees with the experimental results as they have been observed inside the recovered samples and not in the regolith glass sample. The following oxides to be vaporized were SiO₂ and MgO (both were found inside the regolith glass and the condensate, which indicates partial vaporization). Lastly Al₂O₃, CaO, and TiO₂ were only found in the regolith glass, indicating that the temperatures required for vaporization were not achieved. Additionally, for an identical pressure, higher temperature should be favored as the equilibrium predicted a higher O₂ yield, although a too high temperature would cause O₂ dissociation. With the present experiment, it was impossible to reach higher temperatures as the shutters were fully opened. Reducing the pressure inside the chamber could promote oxygen production, as observed from thermodynamic modelling. The utilization of argon as a carrier gas is a double-edged sword: on the one hand it offers oxygen quantification and yield calculation, on the other hand it causes the raise of total pressure which is detrimental, as pyrolysis reactions are favored by lower pressure. Additionally, argon dilution induces competing effects. Argon acts as a sweeping gas that prevents oxygen accumulation, shifts the reaction equilibrium toward the

products, and promotes product quenching, thereby enhancing oxygen recovery by limiting recombination reactions. However, the associated increase in total pressure is unfavorable for the pyrolysis reaction. Therefore, competing processes will be investigated experimentally in future work as it requires specific reactor customization, which should enable modification of the argon flow rate (without changing the total pressure) or even the removal of the argon injection inside the reactor. Parametric studies on argon flow rate (that dilutes and continuously removes gas species) require an upgrade of the pumping capacities. Quantifying the direct influence of argon on the oxide pyrolysis yield is non-trivial, because argon does not chemically participate in the reactions but instead affects the system through gas-phase dilution. The addition of argon reduces the partial pressures of gaseous species, thereby modifying the equilibrium conditions. Additionally, argon is continuously swept through the reactor, which leads to the removal of oxygen and thus to a continuous reduction in the oxygen partial pressure. The conventional approach to determine production rate for ISRU process involves the calculation of the product of interest within a known mass flow rate of gas (Gott et al., 2024; Haggerty et al., 2025; Lomax et al., 2020). Consequently, it is imperative to identify an alternative solution enabling oxygen quantification without the use of carrier gas. In lunar process operations, argon would have to be separated from the oxygen, which could be achieved either by using an adsorbing material such as zeolite or by cryo-distillation, thus complexifying the process. Therefore, argon utilization should be phased out, as it limits the benefits associated with the pristine lunar vacuum, which enables lower operating temperatures, thus reducing power requirements and increasing yield. Several further development paths have been identified and should be pursued. For instance, future work will consider the integration of a secondary vacuum pump (turbo molecular pump) in the experimental set-up in order to investigate lower pressures (with or without argon flow). The present work provides the first experimental quantification of oxygen yield from regolith pyrolysis under specific conditions. Upcoming research should concentrate on raising oxygen yield, improving reactor design to minimize heat loss, enhancing heat and mass transfer, and scaling the process for continuous operation. Influence of regolith composition should be evaluated in order to find the most suitable feedstock and increase yield. In the present setup, the sample was in contact with a cold surface, which limited the maximum temperature achievable during pyrolysis. To minimize thermal losses and improve heating efficiency, a suitable crucible or cavity material should be employed to thermally isolate the sample. Zirconia could be a good candidate due to its high thermal resistance (2680 °C at ambient pressure, furthermore the material is already oxidized, which should avoid the reaction with the molten regolith).

Characterization of the materials produced highlighted the potential use of the solar pyrolysis as a mean of separating the metal oxides inside the regolith. Fractional distil-
<figure><img src="image_10.png" /><figcaption>Fig. 8. Required regolith mass as a function of oxygen extraction yield.</figcaption></figure>

lation of regolith could be considered; however, it will require a better understanding of the vaporization and condensation mechanism of the oxides inside the regolith, and a precise control of the temperature of the molten regolith and condensing surface. Accurate temperature measurement is also critical for advancing process understanding; however, this remains challenging due to the high-temperature and chemically reactive environment. Thermoelectric methods are unsuitable under these conditions, as they risk degradation or chemical interaction with the reactive species. Pyrometric techniques offer a non-contact alternative, but their implementation is technically demanding. The pyrometer must be able to perform measurements within solar spectrum and requires accurate knowledge of the sample's emissivity, which may vary during pyrolysis, complicating real-time temperature assessment. Additionally, understanding how the lunar environment (fluctuating solar intensity, thermal cycling, day and night cycles) affects the process would be critical for real-world implementation. To conclude, the mass of lunar regolith (kg) required to meet the daily oxygen consumption of an astronaut (0.84 kg O₂/day) as a function of the oxygen extraction mass yield (%) is shown in Fig. 8. At the experimentally-obtained yield, a mass of 80 kg of regolith is required. An exponential decay between regolith mass and yield is observed highlighting the importance of yield improvement, as even small increases can significantly reduce the amount of regolith to be processed.

## 5. Conclusion

This study demonstrated, through both thermodynamic analysis and experimental validation, the feasibility of solar pyrolysis as a reagent-free method for oxygen extraction from lunar regolith while simultaneously producing metallic byproducts.

Firstly, a thermodynamic modelling was carried out at various pressures to estimate temperature requirement

and possible oxygen yields. Approximately, decreasing the pressure by one order of magnitude enabled to reduce the oxygen peak temperature by 100 °C. Caution should be exercised when interpreting these results, given the fixed composition of the system and the thermodynamic equilibrium assumed, which excludes the consideration of kinetic limitations.

Secondly, regolith pyrolysis was investigated experimentally. Oxygen was continuously monitored while solid byproducts were characterized using various methods including EDS, XRD, and Raman spectroscopy. The regolith simulant was heated with concentrated solar energy, under low vacuum at 10 mbar (additionally a flow of 0.3 NL/min of argon was injected inside the reactor as a carrier gas), to quantify the oxygen yield. A yield of 35 mg of oxygen from a 3.38 g regolith pellet was achieved, corresponding to 1.05 % of the processed sample, or 2.47 % of the simulant's total oxygen mass content. Importantly, the two-phase release of oxygen, initiated during sample melting at around 1200 °C and sustained at higher solar power levels, suggests a strong dependence on temperature. Metallic byproducts were recovered inside the solar reactor at various locations. A total mass of 1.3 g was found to be vaporized, and materials characterization appeared to be challenging. The most volatile (Na₂O, K₂O, Fe₂O₃) oxides were vaporized and condensed inside the reactor, whereas the less volatile oxides (Al₂O₃, TiO₂, CaO) remained inside the regolith glass sample at the focal point of the parabola. The experimental quantification of oxygen yield, achieved for the first time under these conditions, provides a benchmark for further investigations.

Finally, this proof-of-concept confirms the viability of harnessing concentrated solar energy to extract oxygen and recover metallic byproducts, even though the oxygen yield remained modest and needs to be increased. Special attention should be paid on the influence of argon dilution and lower pressures are targeted for next steps. The experimental study requires parametric investigations, which will
be addressed with new pumping equipment in order to enable modification of the argon flow rate without changing the total pressure. Future work will thus focus on parametric studies to enable a better understanding of the process and to maximize oxygen yield. Modelling should be further developed considering kinetics and temperature distribution. The solar pyrolysis proved to be a promising technology for oxygen and metal extraction approach, enabling in-situ resource utilization on the Moon. This work represents the first quantification of oxygen yield in a dedicated solar reactor, with identified pathways for further optimization. Integration of an oxygen producing plant on the Moon could drastically reduce reliance on Earth resupply, enabling long-term exploration, habitation, and even industrial activities in lunar environments.

### CRediT authorship contribution statement

**Jack Robinot:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Sylvain Rodat:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Stéphane Abanades:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Eric Bêche:** Writing – review & editing, Visualization, Validation, Investigation, Formal analysis, Data curation, Conceptualization.
**Alexis Paillet:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Aidan Cowley:** Supervision, Resources, Project administration, Formal analysis, Data curation, Conceptualization.

### Data availability

Data will be made available on request.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This study was co-funded by the Occitanie region through the Emergence program (ROAST project - Regolith Only and Sunlight) and by CNES (Centre National d'Etudes Spatiales) in the frame of Spaceship FR project. The authors thank Nicolas Boulet for providing the picture in Fig. 3.

## Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.asr.2026.02.003.

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