Future perspectivesLunar regolith contains important resources that are rare on Earth. For example, helium-3, which is much more abundant on the moon than on the earth, is a vital element for nuclear fusion and future developments in energy sectors. Herein, we report abundant helium bubbles in the glassy surface layer of lunar regolith particles. The disordered atomic packing structure of glass plays a critical role in capturing and retaining the noble helium gas. Compared to the helium dissolved in the lunar crystal lattice that requires high temperatures to release, the helium gas contained in bubbles is much easier to extract by using a mechanical milling strategy at ambient temperatures. This demonstrates promising futures for a lunar gold rush.

1.   Introduction

The moon has a variety of unique resources, which are important supplements and reserves for the terrestrial resources and will have a far-reaching impact on the sustainable development of human society. ³He, an isotope of helium, is a clean, safe and controllable fusion fuel. Unfortunately, ³He is rare on Earth (about 0.5 tons). It has been supposed that ³He is very abundant in lunar rocks and soil grains [1, 2] due to the intense solar wind irradiation on the moon. Exploration of lunar resources, in particular ³He, has become an inevitable trend in world space activities, because the resources not only provide energy for future lunar exploitation but also help to meet the growing energy needs on Earth [3, 4]. Determining the form of existence and the associated abundance of ³He is crucial for effective usage of noble resources.

In previous works, the reservation of helium on the moon in different lunar rocks has been studied [5-8] either by in situ multispectral optical reflectance method [8] or ex situ analytical devices in laboratories [5, 6, 9]. It has been suggested that ilmenite is remarkably retentive of helium [5, 7, 8, 10-13]. Its helium content is closely correlated with the concentration of TiO₂ in ilmenite [6, 8]. The helium atoms were thought to be mostly retained in defects or as solid solutions in lunar regolith particles. Accordingly, the lattice of ilmenite particles is considered to be suitable for capturing ³He [5, 10-12]. To extract the ³He, high temperatures, e.g. 400 C-1500 C, are required [14, 15], because the diffusion rate that determines the release rate of ³He is higher at high temperatures [16-18]. It was found that the concentration of helium is very rich in the region near the surface [19], and it is much higher than that predicted by simulations based on a diffusion model [20]. Therefore, it is intriguing to research why the surface layer can capture more helium.

In this work, we investigated the microstructures of lunar ilmenite particles brought back by Chang’E-5 [21-23] using an advanced spherical aberration-corrected transmission electron microscope (TEM) equipped with electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS). It is found that there are huge amounts of helium bubbles in the glassy surface layer on the ilmenite particles. The glassy surface that is likely formed by irradiation acts as a reservoir to capture and retain helium gas owing to the superb stability of the lunar glass. We estimate the total reservation of helium gas on the moon according to our results.

1.1   Ilmenite particle identification

The Chang’E-5 samples studied in this work were scooped from the lunar regolith surface (samples CE5C0400), which comprise approximately 6.0% ilmenite, 44.5% pyroxene, 30.4% plagioclase, 15.5% glass phases, and 3.6% olivine [24]. To identify the ilmenite particles, the lunar regolith particles were loaded in a scanning electron microscope (SEM) and screened by using back-scattering mode, because the heavy elements, e.g. Fe and Ti are more sensitive in the back-scattering mode. Then, several ilmenite (FeTiO₃) particles containing Fe, Ti and O elements were identified and used for structural characterization. A representative ilmenite particle is shown in figure 1. Some agglutinates that mainly contain Ca, Al, Si, and O are attached to the ilmenite particle. A thin slice was cut from the ilmenite particle by using focused ion beam (FIB) for further TEM and EELS studies.

Figure  1.  An SEM image and EDS elemental mapping for a representative ilmenite (FeTiO₃) particle with attachment of agglutinates. A thin layer is cut for TEM test from the white rectangle site in the first image.

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1.2   Helium bubbles

High-resolution transmission microscopy was applied to study the microstructure of ilmenite particles (see figure 2). As shown in figures 2(a)-(c), the top layer from the surface of the ilmenite particle is a glassy layer with dense disordered atomic packing structures (see the inset high-resolution TEM image). Its thickness is about 40-50 nm. Beneath the glassy layer is a partial glass region with a thickness of about 30 nm before reaching the inner crystalline phase. It is known that the irradiation of high-energy ions can induce the amorphization of crystalline materials, and the degree of the amorphization decreases with the increase of the depth [25-27]. Therefore, the top glass layer and the partially glassy layer beneath on the surface of the ilmenite should be attributed to the irradiation of the solar wind or other high-energy cosmic-ray irradiations.

Figure  2.  TEM images and EELS curves. (a) The bright field TEM image of the ilmenite (FeTiO₃) sample. It has a glassy surface layer of about 50 nm in thickness where abundant helium bubbles are observed. Below the glass layer is first a partial glass region and then the crystalline phase. On the right-hand side are high-resolution TEM images. (b) The zoomed-in image of the glass layer containing many helium gas bubbles. (c) A spherical helium bubble in the glass with clear interface. (d) The EELS curves acquired at different locations in (a); site 1 is a big bubble; site 2 is a small bubble; site 3 is in the glass layer; site 4 is in the crystalline phase. The helium signal at around 22 eV is detected in the glassy layer.

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Remarkably, we find plenty of spherical bubbles with diameters of about 5-25 nm in the glassy layer. Figure 2(d) shows the EELS curves acquired in different locations in figure 2(a). A strong helium peak at around 22 eV [10, 28] is detected in the big bubble (site 1 in figure 2(a)). The helium signal in the small bubble (site 2 in figure 2(a)) is weaker than that in the big bubble. The helium signal in the top glass layer (site 3 in figure 2(a)) is very weak and the signal in the crystalline phase (site 4 in figure 2(a)) is too weak to be detectable.

It can be seen that helium bubbles are mainly retained in the glassy layer (zoomed-in in figure 2(b)). It is noticeable that the helium bubbles are in spherical shapes with clear interfaces (figure 2(c)), which is attributed to the viscous and flowable characteristics of glasses [29-31]. These results suggest that the glassy layer plays a critical role in capturing and retaining helium gas.

The helium density is calculated following [32]: $n_{\text{He}}=\frac{I_{\text{He}}}{I_{\text{ZIP}}{\sigma }_{\text{He}}d}$, where n_He is the helium volume density (in He nm^-3); I_He is the integrated signal of helium; I_ZIP is the integrated intensity of the zero-loss peak; _He = 5.9 10^-6 N m² is the cross section for the He 1s-2p transition [28], and the d is the local thickness at the pixel position of the analyzed He nano-volume. The calculated helium volume density in different helium bubbles ranges from 50 to 192 He nm^-3. The pressure inside the bubbles is estimated according to [33, 34] to be about 1-39 GPa (see details in supplementary information, available online at stacks.iop.org/MF/1/035101/mmedia). The total mass of the helium gas in bubbles is estimated to be about M_He = (8.4-64.5) 10¹⁰ kg with a total volume of helium bubbles of about (2.5-5) 10⁸ m³ (details can be found in supplementary information). Upon the ratio of ³He to ⁴He is about 4 10^-4 [35], the mass of ³He in the bubbles should be up to 0.26 10⁹ kg, which may be 1/10-1/4 of the total reserves of ³He on the moon. According to the estimation by Wittenberg et al [3], the fusion energy produced by about 100 tons of ³He could meet the global needs. Thus, the helium gas retained in lunar glass bubbles will be enough for the needs of about 2600 years on Earth.

Compared to the ³He atoms captured in defects and in solid solutions that are difficult to extract, the helium gas in bubbles is of high quality, which is easier to extract. Mechanical methods at ambient temperatures, e.g. ball milling, can break the bubbles and efficiently release the gas. This is highly appreciated for in situ exploring the ³He gas on the moon.

1.3   Formation and capture mechanisms of helium bubbles

It is intriguing to investigate how and why the helium bubbles were captured and retained in the glass layer. The elemental mapping of the region surrounding a helium bubble was studied by using EDS (figure 3). The Ti and O atoms distribute homogeneously across the glass layer while there are some Fe-rich nano-clusters surrounding the helium bubble. The precipitation of Fe clusters may be attributed to the reduction reaction by the hydrogen atoms in the solar wind. The glassy layer is mainly composed of TiO₂. Even though the solar wind irradiation spans billions of years [24], the damage is limited to a thin layer of only 50 nm. This denotes a high irradiation tolerance of ilmenite, especially with the protection of a glassy layer whose disordered atomic packing structure can greatly decrease the speed of ions in the solar wind.

Figure  3.  EDS mapping surrounding a helium bubble. (a) HAADF TEM image, and corresponding EDS mapping for (b) Fe, (c) O, and (d) Ti. The dashed circles are guiding eyes to the bubble outline. (e) The linear scan of elemental distribution across a helium bubble, see the dashed arrow in (a).

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A schematic illustration for the process to capture and retain the helium gas in ilmenite is shown in figure 4. The ilmenite lattice is more suitable for helium atoms to implant compared to other lunar regolith particles [36, 37]. The high-speed ions in the solar wind implant into the lattice and are trapped in the vacancies and interstitials (figure 4(a)). After a long-time irradiation, the lattice close to the surface is damaged and forms a glassy layer (figures 2 and 4(b)). Even though the crystalline ilmenite can capture many helium atoms, it could not retain them because of the channeling effect of crystalline lattice [38], that is, some orientation in a lattice structure is easier for ions to implant and release. In addition, the hot-cool cycles on the moon promote the release of these trapped atoms in the lattice. When the helium atoms diffuse into the surface glassy TiO₂ layer with disordered and densely packed structure, there is no crystalline tunneling effect, and helium atoms are difficult to escape from the disordered structure and are then retained and form bubbles in the glass. This is the reason why the EELS results show that the helium concentration in the inner crystalline structure is much lower than that in the glassy surface, especially that the helium bubbles all locates in the glassy layer. The ultrahigh stability [39, 40] and dense atomic packing structure [41] of TiO₂ glass can retain the helium gas for ultra-long time. The high pressure in the helium bubbles of about 1-39 GPa also confirms that the glassy TiO₂ is an ideal reservoir to retain helium gas.

Figure  4.  Schematic illustration of the capturing and retaining process of helium (He) on the surface of ilmenite. (a) Helium atoms in the solar wind implant into the lattice of crystalline ilmenite. (b) Surface amorphization caused by long-time irradiation of the solar wind. (c) Helium atoms diffuse into the glassy surface layer and form bubbles.

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In summary, even though the lunar regolith brought back by Chang’E-5 mission is much younger than those from the Apollo mission and Luna mission, it contains large amounts of ³He resource. The ³He gas that is stored in bubbles in the glassy surface layer of the ilmenite particles is more convenient to extract in situ compared to the ³He atoms retained in lattice defects. This work provides a first evidence for the advantage of glass in capturing and retaining ³He on the moon owing to its stable disordered atomic packing structure, which acts as a reservoir of the noble ³He gas. Our findings suggest the possibility for future exploration of ³He resource on the moon.

2.   Methods

2.1   Sample preparation

The Chang’E-5 samples studied in this research were scooped from the lunar regolith surface (samples CE5C0400), which comprises approximately 44.5% pyroxene, 30.4% plagioclase, 15% glass phase, 6.0% ilmenite, and 3.6% olivine. A small number of samples were examined under a binocular optical microscope, Ningbo Yongxin NM910, in an ultraclean environment provided by a glovebox (Mikrouna Inc.). The particles with a size ranging from a few microns to dozens of microns were hand-picked and then transferred to a sticky carbon substrate. The selected particles were characterized by morphologic observations and the elemental mapping in a field emission SEM, Thermo Scientific Verios G4 UC, with the following parameter: acceleration voltage of 15.0 kV and probe current of 3.2 nA. Under the back-scattering mode, the particles containing heavy elements were confirmed and then EDS mappings were acquired on each particle to screen out particles rich in Fe and Ti elements. The FeTi-rich particles were selected to prepare the sections using FIB microscopy with a FEI Helios G4 CX. Regions of interest on the chosen particles were coated with a thick ion beam deposited Pt film (1 m) before ion milling to prevent damage to the particle surface by the ion beam.

2.2   Scanning transmission electron microscope (STEM)-EELS measurements

STEM imaging was performed on a spherical aberration-corrected STEM instrument, Thermo Fisher Spectra 300, equipped with a 5th order aberration corrector. The instrument provided an incident electron probe of 1.3 in full-width-at-half-maximum with a convergent semiangle of 24.6 rad. The beam current of the electron probe is set to be 50 pA and a typical dose level of 0.4-1 10⁸ N m² as used for high-resolution imaging. STEM measurements were conducted at 300 kV accelerating voltage under an ultra-high vacuum level of 1 10^-7 Pa. STEM images were acquired in bright field mode and high-angle annular dark field (HAADF) mode, respectively. As will be discussed later, a layer containing many bubbles, which is located at 10 nm below the sample surface, was observed via STEM. In order to detect the helium, EELS acquisitions were conducted and recorded through a Gatan 1066 with a collection semiangle of 50 rad. The energy resolution determined by the full width at half maximum of the zero-loss peak was approximately 0.5 eV. The EELS mappings were carried out on the regions containing the bubbles. We carefully acquired the EELS signal from the central portion of each bubble for the following accurate estimation of helium density in the bubbles. EDS maps of the He bubbles regions were collected with a Thermo Fisher EDS detector with four probes. Data for EELS and EDS were acquired as spectrum images. The EELS and EDS data analysis were performed using the Digital Micrograph software (Gatan Inc.) and Velox software (Thermo Fisher Inc.), respectively.