INTRODUCTION

During various cosmic processes, a multiphase plasma–dust environment is formed (Kuznetsov et al., 2023; Popel et al., 2018; Zakharov et al., 2020; Johnstone, 2013; Goetz et al., 2022). Such plasma–dust clouds can contain not only plasma, but also dust particles, melt drops, and neutral gas. Impacts of meteoroids and micrometeoroids on the surface of an astronomical body (Gerasimov et al., 1998; Popel et al., 2020), the movement of meteoroids in the atmosphere of planets (Bronshten, 1981; Silber et al., 2018) are processes in which such a multiphase environment is formed.

Existing ways of laboratory modeling suggest direct methods (Mocker et al., 2011; Friichtenicht, 1962; Shu et al., 2012; Semkin et al., 2007, 2009; Hudepohl et al., 1989; Best and Rose, 1999; Igenbergs et al., 1987; Vricella et al., 2017; Chhabildas et al., 1995; Thornhill et al., 2006; Lexow et al., 2013) modeling and “indirect” (Helber et al., 2019; Loehle et al., 2017; Blanchard and Cunningham, 1974; Bones et al., 2016; Gómez Martín et al., 2017; Sorokin et al., 2020a; Stöffler and Langenhorst, 1994; Fulvio et al., 2021; Moroz et al., 1996; Grokhovsky et al., 2020; Bezaeva et al., 2010). Direct modeling methods are based on accelerating the impactor to velocities characteristic of meteoroids of the Solar System, and in indirect methods, certain stages of processes or conditions are reproduced.

The creation of plasma–dust clouds from the substance of meteorites, regoliths, or equivalent artificial mixtures in laboratory conditions makes it possible to study certain issues of space weathering (Prince et al., 2020; Sasaki et al., 2003; Sorokin et al., 2020b), prebiological synthesis (Managadze, 2003; Zaitsev et al., 2016; Ferus et al., 2020; Martins et al., 2013). The synthesis of organic substances from the simplest inorganic substances under conditions of a plasma–dust cloud differs from other possible ways of prebiological synthesis by the presence of a large number of possible plasma-chemical reactions (with atoms and molecules in an excited state, with charged particles, photolytic reactions under the influence of ultraviolet radiation), as well as the presence of particles of dust, which can play the role of both crystallization centers and reaction catalysts (Parmon, 1999; Gerasimov and Safonova, 2008; Peters et al., 2023; Saladino et al., 2013, 2018; Snytnikov, 2002, 2010).

Charged dust is present everywhere in space and the Earth’s atmosphere: in interstellar matter, nebulae and clouds, supernova remnants, protoplanetary disks, in the vicinity of active galactic nuclei, in the Earth’s ionosphere and meteoroid tails, in the tails of comets, in the atmospheres of other planets and in the exospheres of atmosphereless bodies (satellites and asteroids). Organic compounds can be found on individual bodies and dust particles.

Complex chemical reactions could occur on dust particles in the molecular layers of protoplanetary disks and organic compounds could be formed, including simple amino acids (for example, glycine). One can expect the development of processes of synthesis of organic compounds in the transition regions of protoplanetary disks, where the concentrations of charged particles (including charged dust) are still high, but at the same time dense molecular regions are present. Also, the prerequisites for the development of these processes may be stellar flares, which produce high radiation passing into denser layers and precipitation of charges. As a result, there are areas nearby with oppositely charged particles, ions and radical ions, which are attracted and enter into chemical reactions more intensely.

In small meteorites (or in the upper parts of the cometary core), in which internal rocks have not been heated, compounds can exist in the form in which they were deposited on the surface of dust grains during the formation of planets in the protoplanetary disk or formed on the surface of dust as a result of chemical reactions. This is confirmed by observations. During the 2001 Leonid meteor shower, data collected by the Leonid MAC showed that meteoroids ranging in size from 50 µm to 10 cm were rich in CN compounds (Jenniskens et al., 2004) as well as H2, O, H, H2O, CO2, CO, CH4, CH, C2HO (Jenniskens, 2001). O and N2 bands were observed in the spectra of meteor showers in different excited states.

Comets are a source of meteoroids that can enter the Earth’s atmosphere and serve as triggers for the prebiological synthesis of organic substances and the origin of life. Comets and the small meteoroid bodies they generated hardly experienced any thermal metamorphism, and their composition could have preserved more complex substances synthesized on dust particles during the formation of planets (Vasyunin et al., 2008). When a meteoroid passes through the Earth’s atmosphere, high temperatures and concentrations of particles are created in the plasma–dust tail of the meteoroid, which turns out to be favorable conditions for the rapid occurrence of a number of chemical reactions. At the same time, rapid thermal “hardening” of synthesis products can favor the preservation of synthesized organic substances.

It is known that nitrogen is an essential element of many organic compounds necessary for the life of terrestrial organisms. Such compounds are nitrogenous bases (building blocks of nucleic acids and ATP, etc.) and amino acids (structural elements of peptides, proteins, etc.). At the same time, nitrogen gas (the predominant component of the Earth’s atmosphere) is a rather chemically inert substance. But the converted kinetic energy of the meteoroid is enough so that during the interaction of atmospheric gases and vapors (N2, CO2, H2O) with simple and complex substances, as well as individual functional groups of substances present in meteorites (C, N, H, CN, H2O, etc.), such compounds as HCN, NO were obtained, under conditions of high temperatures—nitrogenous bases, sulfur- and fluorine-containing compounds, etc.

In our studies on the synthesis of micro- and nanoparticles in a microwave discharge in metal/dielectric powder mixtures (Gayanova et al., 2023; Skvortsova et al., 2021a, 2023a; Batanov et al., 2013; Akhmadullina et al., 2019; Sokolov et al., 2023) we obtain plasma–dust clouds even under atmospheric pressure of the working gas. We also used this experimental setup to create a cloud of particles from powder samples that replicate the chemical composition of lunar regolith (Skvortsova et al., 2017, 2019, 2021b, 2023b; Batanov et al., 2022; Kachmar et al., 2021). In a cloud containing plasma, evaporated matter, melt drops, dust particles, the gas temperature is about 5000 K, and the plasma density is ~1014 cm–3. During plasma exposure, new substances and phases are formed, and morphological changes in dust particles occur. In this case, the heating process is fast (<50 μs), since it is associated with the development of a low-threshold discharge at the metal-dielectric or semiconductor–dielectric contacts (Batanov et al., 2004; Batanov and Kossyi, 2015), and not with the direct heating of substances in a microwave field (Bykov et al., 2001; Vodop’yanov et al., 2017; Rybakov et al., 2006, 2013; Rybakov and Buyanova, 2018; Egorov et al., 2010). Direct heating of substances in a microwave field is a relatively slow process, even taking into account the possible development of thermal instability. The probability of initiating microwave breakdown in mixtures of powder samples depends on the proximity to the optimum size of conductive particles (corresponding to the thickness of the skin layer), the proportion of conductive particles in the mixture, and the intensity of microwave radiation.

When used in experiments on microwave discharge in mixtures of powders as initial mixtures of mineralogical samples characteristic of cosmic bodies, it becomes possible to simulate plasma–dust environments formed, for example, when a micrometeoroid hits the surface of an atmosphereless body or when a meteoroid moves in the atmosphere of a planet. Moreover, such modeling will make it possible to recreate a multiphase environment with its characteristic processes of transformation and synthesis of substances.

In the present work, we used three types of samples in experiments as starting substances: (1) ilmenite concentrate, since ilmenite is often a phase contained in lunar regolith; (2) lunar dust simulant LMS-1D (Lunar mare simulant—dust) (Long-Fox and Britt, 2023; Madison et al., 2022; Stockstill-Cahill et al., 2021, 2022), as a sample that replicates the mineralogical composition and the distribution of particles by size in the regolith of the lunar seas (Florensky et al., 1975; Slyuta, 2014; Heiken et al., 1991; NASA Lunar sample and photocatalog; Lunnyi grunt..., 1974, 1980; Barsukov and Surkov, 1979) or more precisely, its finest fraction is lunar dust (Kuznetsov et al., 2016; Zakharov et al., 2020; Park et al., 2006); (3) the substance of the Tsarev meteorite (ordinary chondrite L5) (Semenova et al., 1984; Migdisova et al., 1982; Barsukova et al., 1982; Slyuta, 2017), as an example of an original substance of cosmic nature with a complex mineralogical composition. Separately, we have already presented the main results of experiments with the substance of the Tsarev meteorite in (Akhmadullina et al., 2023).

EXPERIMENTAL CONDITIONS

As a source of linearly polarized microwave radiation with a wavelength λ = 4 mm (frequency f = 75 GHz) a gyrotron is used (Litvak et al., 2021). The experiments use pulses with a duration of up to 10 ms and a power of up to 450 kW. Microwave radiation at the gyrotron output is a Gaussian beam. This beam from the gyrotron to the sample is transmitted through a quasi-optical system of focusing mirrors (Fig. 1). At the sample location, the diameter of the microwave beam is 5 cm, which provides average electromagnetic wave intensities of up to 20 kW/cm2.

Fig. 1.
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Experiment scheme: (1) gyrotron, (2) focusing mirror of the quasi-optical path, (3) flat mirror, (4) high-speed camera Phantom VEO 710L, (5) quasi-optical microwave coupler, (6–8) detectors of incident, reflected and transmitted microwave radiation, (9) plasma-chemical reactor, (10) end lenses of transmitting light guides of Avantes AvaSpec spectrometers.

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The powder sample under study is placed horizontally in a thin layer (1–2 mm) in the form of a spot with a diameter of 4–5 cm on the surface of a quartz disk with a diameter of 8 cm. To limit the scattering and collection of the substance during the experiment, it is placed vertically on a quartz disk (6–8 mm thick) and a thin-walled quartz tube with a diameter of 7 cm and a height of up to 10 cm is installed coaxially with it. The test sample, assembled with a quartz disk and a quartz tube, is placed in a vacuum chamber (up to 10–2 Torr) plasma chemical reactor (Fig. 1) so that the center of the powder sample coincides with the axis of the microwave beam. The plasma-chemical reactor has quartz windows transparent to microwave radiation at the bottom and top. Microwave radiation is supplied to the plasma–chemical reactor from the bottom up, i.e., passes through the powder layer, and the discharge will occur on the upper open surface of the powder sample. This is possible because the initial (under normal conditions) absorption coefficient of microwave radiation does not exceed 30%.

To estimate the magnitude and dynamics of the absorption coefficient of microwave radiation in a powder sample and a plasma–dust cloud, a system of balanced microwave measurements is used (Fig. 1). With a time resolution of 200 ns, the level of supplied microwave power, the level of reflected radiation (the sum of reflections from stationary objects and the plasma–dust cloud) and the level of radiation passed through the powder sample and the plasma–dust cloud are recorded.

Registration of emission spectra of plasma and surfaces of solid bodies makes it possible to determine: electron temperature Te (by relative intensity of metal lines); rotational gas temperature Tr (according to the bands of the TiO molecule), which is as close as possible to the translational temperature under microwave discharge conditions; solid surface temperature Ts (along the radiation continuum in the Wien region of the spectrum), which corresponds to the surface temperature of the powder sample. The lines of sight of the spectrometers have three characteristic directions (Fig. 1): from above, the beam passes through the plasma–dust cloud and rests on the upper surface of the powder sample; from the side, the beam passes through the plasma–dust cloud parallel to the surface of the powder sample; from below, the beam passes through the entire layer of the powder sample. In total, several spectrometers with different swaths and spectral resolutions are used in the studies (AvaSpec-3648-U80: range 371–920 nm, resolution 0.36 nm; AvaSpec-ULS4096-CL-2-EVO 2109527U2: range 219–381 nm, resolution ~0.1 nm: 2109528U2: range 379–521 nm, resolution ~0.1 nm; AvaSpec-ULS2048CL-EVO 210952U1: range 520–739 nm, resolution ~0.25 nm) (Sokolov et al., 2023).

The dynamics of the processes of development of a microwave discharge and the evolution of a plasma–dust cloud are recorded by a high-speed video camera Phantom VEO 710L (monochrome, 1280 × 800 pixels, 7400 fps, 680 000 fps with a resolution of 64 × 8 pixels, minimum exposure time 1 μs, ISO sensor sensitivity Mono 6400D, dynamic range 12 bits), which is located above the plasma–chemical reactor (Fig. 1).

DESCRIPTION OF SAMPLES

Ilmenite Concentrate

A commercially available ilmenite concentrate was purchased in the form of a powder with a particle size of no more than 100 μm. The documentation provided by the supplier indicates the mineral composition (ilmenite 95%, rutile 3%, zircon 0.4%) and elemental composition (Table 1), obtained using X-ray fluorescence analysis. The ilmenite concentrate was ground by grinding in an agate mortar for 10 min, and 1 g of the material was used as a sample in the experiment without adding additional metal powder. As mentioned above, low-threshold microwave breakdown occurs in a mixture of metal and dielectric powders (usually a metal mass fraction of 5% is required), or a semiconductor and dielectric. Ilmenite has semiconductor properties, so we initially assumed that a microwave discharge could develop in it without additives. For the prepared ilmenite sample, micrographs were obtained in Figs. 2 and 3. The measurements were carried out using a Zeiss Merlin scanning electron microscope (SEM) equipped with an Oxford Instruments INCAx-act X-ray microanalysis attachment. An accelerating voltage of 10 kV was used to obtain images, which made it possible to simultaneously obtain sufficiently high-quality images of materials and carry out measurements using X-ray energy-dispersive spectroscopy (EDS). The elemental composition of the analyzed areas generally corresponds to that declared by the supplier. The particles have an irregular shape, in some cases with prominent sharp edges, the particle size reaches 100 μm.

Table 1. Elemental composition of the ilmenite concentrate sample
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Fig. 2.
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Microphotographs of a prepared sample of ilmenite concentrate.

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Fig. 3.
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Microphotograph of a prepared sample of ilmenite concentrate with superimposed multilayer element distribution maps and distribution maps of individual elements. Color figure is available in the electronic version of the article.

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Lunar Dust Simulant LMS-1D

The LMS-1D lunar dust simulant used in the experiment was produced by ExolithLab (United States). The simulant replicates the mineralogical composition (Table 2), elemental composition (Table 3) and particle size distribution of the dust of the lunar seas. Elemental composition in Table 3 was obtained using XRF analysis by the manufacturer. The composition and particle size distributions are provided by the manufacturer. The particle size range is 0.04–32 µm with a median size of 4 µm. When preparing the sample for the experiment, 10% (by weight) of metallic magnesium (particle size no more than 75 μm) was added to LMS-1D. Such an additive is necessary to initiate microwave breakdown, since lunar regolith, and therefore its simulant, is mainly a composition of dielectric materials, without inclusions of neutral metals. It is worth noting that nanophase iron (Taylor et al., 2001; Basu, 2005), which is formed in lunar dust, in particular, by thermal reduction during the passage of a shock wave during micrometeorite impacts (Sorokin et al., 2020b), is absent in the simulant used. And nanophase iron is a characteristic product of space weathering of lunar regolith. Its presence could lead to the occurrence of a low-threshold microwave breakdown in the simulant without the need to add metal powder. After preparing a mixture of LMS-1Dc 10% magnesium metal powder, the resulting sample was ground in a mortar to homogenize and break up agglomerates. We obtained SEM microphotographs of several areas (Fig. 4), superimposed on them with multilayer element distribution maps and individual element distribution maps (Fig. 5). The elemental composition was within the limits stated by the manufacturer. Added particles of neutral magnesium are reliably detected in micrographs. The particles of the lunar dust simulant have an irregular shape, in some cases with prominent sharp edges; the particle size reaches 100 μm. Small-sized particles predominantly form agglomerates and also cover the surface of larger particles, which makes it difficult to identify and describe the shape of the latter. The sample does not contain spherical glass particles, the presence of which is characteristic of lunar regolith (Slyu-ta, 2014; Popel et al., 2020).

Table 2. Mineralogical composition of the lunar dust simulant LMS-1D
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Table 3.   Elemental composition of the lunar dust simulant LMS-1D, obtained using X-ray fluorescence analysis by the manufacturer (ExolitLab)
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Fig. 4.
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Microphotographs of a prepared sample of lunar dust simulant LMS-1D with the addition of 10% metallic magnesium.

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Fig. 5.
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Microphotograph of a prepared sample of lunar dust simulant LMS-1D with the addition of 10% metallic magnesium with superimposed multilayer element distribution maps and distribution maps of individual elements. Color figure is available in the electronic version of the article.

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Substance of the Tsarev Meteorite

The mineralogical and elemental composition of the Tsarev meteorite, which is a chondrite of petrographic type L5, was studied previously (Semenova et al., 1984; Migdisova et al., 1982; Barsukova et al., 1982; Oshtrakh et al., 2008, 2013a, 2013b; Maksimova et al., 2014, 2017, 2018). The main phases are olivine, orthopyroxene, troilite, clinopyroxene, the metal phase is represented mainly by an alloy of iron, nickel and cobalt.

The 102-g sample we were provided with was precleaved into several pieces which were then ground in a Fritsch Pulverisette 7 ball mill (in a tungsten carbide beaker using tungsten carbide balls). Grinding was carried out at 600 rpm 2 times for 10 min.

The particle size of the meteorite substance after grinding, measured using the dynamic light scattering method, lies in the range of 0.2–19.7 μm. SEM microphotographs (Fig. 6) generally confirm these dimensions. The particles are irregular in shape, in some cases with prominent sharp edges. No spherical glass particles were found in the sample. Larger particles are mainly agglomerates of smaller particles.

Fig. 6.
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Microphotographs of a prepared sample of the Tsarev meteorite substance obtained after grinding in a ball mill.

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In samples of the substance examined with SEM and XRD, no individual particles of iron or other metal were found. Element distribution maps are presented in Fig. 7, and the elemental composition for a separate sample is presented in Table 4. The composition was within the expected limits; however, it was not possible to determine the proportion of cobalt in the sample within the error limits of the method. The presence of nickel and high iron content indicate that, although individual metal particles cannot be detected in micrographs, iron–nickel alloys may be present in the sample. This gave us the opportunity to assume that the substance of the Tsarev meteorite itself will be sufficient to initiate microwave breakdown, and the addition of additional metal powder will not be required.

Fig. 7.
figure 7

Microphotograph of the substance of the Tsarev meteorite obtained after grinding in a ball mill, and a map of the distribution of elements. Color figure is available in the electronic version of the article.

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Table 4.   Elemental composition of a sample of the Tsarev meteorite
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Our studies of the phase composition of a sample of meteorite material in (Akhmadullina et al., 2023) revealed only the main phases in the form of olivine and orthopyroxene. The detection of other phases by X-ray phase analysis (XRD) is hampered by their small quantity (content less than 3% by volume) and low crystallinity for a number of phases.

EXPERIMENTAL RESULTS

Ilmenite Concentrate

The working gas in the experiment was atmospheric-pressure air. The microwave radiation power was 300–400 kW, and the pulse duration was 6–8 ms. The sample was exposed to five microwave pulses with a total energy of 11.4 kJ. The discharge in the powder sample of the ilmenite concentrate was initiated almost immediately at the front (~100 μs) of the microwave pulse. Photographs of the sample before and after exposure to the discharge are shown in Fig. 8.

Fig. 8.
figure 8

Photos of samples placed on a quartz substrate inside a quartz tube: (a) ilmenite concentrate before exposure to the discharge; (b) LMS-1D lunar dust simulant with the addition of metallic magnesium powder before exposure to the discharge; (c) substance of the Tsarev meteorite before the impact of the discharge; (d) ilmenite concentrate after exposure to the discharge; (e) LMS-1D lunar dust simulant with the addition of metallic magnesium powder after exposure to a discharge; (f) substance of the Tsarev meteorite after exposure to the discharge. Color figure is available in the electronic version of the article.

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The development of a discharge in a powder sample is accompanied by a radial expansion of the plasma formation above the sample to dimensions on the order of the width of the microwave beam. This is a longer process (0.5–1 ms) compared to the primary breakdown. During the microwave pulse, the plasma–dust formation has the highest temperature and glow intensity (Fig. 9, frame at 5.35 ms); the scattering of particles into the reactor volume is visually observed. After the end of the microwave pulse (Fig. 9, frame at 18.3 ms), the glow intensity of the plasma–dust formation begins to decrease. Under conditions of atmospheric pressure of the working gas, the plasma concentration existing during the microwave pulse (~1014 cm–3, was previously measured from the Stark broadening of the hydrogen line Hα in experiments with the addition of a small percentage of hydrogen-containing impurities) should decrease by several orders of magnitude over a time of about 1–2 ms (Reiser, 2009). Therefore, the high-speed camera frame at 18.3 ms (Fig. 9) already corresponds to a gas and dust cloud, and the glow is caused by thermal radiation of heated gas and individual dust particles.

Fig. 9.
figure 9

Footage of the development of a microwave discharge in a sample of ilmenite concentrate. The frame start time (milliseconds) from the moment of breakdown is indicated on each frame in the upper left corner. The frame exposure time is 100 μs. Microwave power 300 kW, microwave pulse duration 8 ms.

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In the emission spectra (Fig. 10) recorded by a survey spectrometer (Avantes AvaSpec-3648-U80, spectral range 371–920 nm, resolution 0.36 nm), the line of sight of which is directed from bottom to top and passes through the entire layer of the powder sample, lines of neutral iron Fe I, neutral titanium Ti I, titanium ion Ti II, molecular bands TiO, FeO, as well as impurity lines (Na, Li, K). This spectrum makes it possible to determine the temperature of thermal radiation along the continuum in the Wien region of the spectrum (Fig. 11) (Voronova et al., 2021). It is obvious that thermal radiation with such a line of sight will be dominated by radiation from a solid body, rather than from plasma or gas in a plasma–dust cloud. Since the energy in a pulse of microwave radiation (up to 3 kJ) is fundamentally not enough to heat the entire mass (1 g) of the sample to experimentally measured temperatures of 1500–3000 K, this temperature characterizes the thin surface layer in which the main interaction of sample particles occurs with plasma and microwave radiation.

Fig. 10.
figure 10

Emission spectrum in an experiment with the creation of a plasma–dust cloud in a powder sample of ilmenite concentrate during the duration of a microwave pulse.

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Fig. 11.
figure 11

Dynamics of surface temperature of a powder sample of ilmenite concentrate. The duration of the microwave pulse is indicated by a gray rectangle.

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The rotational temperature of the gas component of the plasma–dust cloud was determined from the γ‑TiO molecular band (Hermann et al., 2001) in the spectrum obtained with the AvaSpec-ULS2048CL-EVO spectrometer (210952U1: range 520–739 nm, resolution ~0.25 nm), which has the best resolution, and whose line of sight passed parallel to the surface of the powder sample. The calculation of rotational temperatures is hampered by the presence of a thermal continuum, so the calculated value has a significant error Tr = 5500 ± 1500 K.

More detailed spectra were taken with an AvaSpec-ULS4096-CL-2-EVO spectrometer (2109527U2: range 219–381 nm, spectral resolution ~0.1 nm; 2109528U2: range 379–521 nm, spectral resolution ~0.1 nm). The electron temperature was calculated from the relative intensity of pairs of metal lines (two pairs of Fe I lines 396.23 and 407.15 nm, 396.23  and 438.35 nm) under the assumption that there is an equilibrium between the population of some electronic levels of elements in the gas phase and the temperature of the electron gas (Lochte-Holtgreven, 1971), using

$$\frac{{{{I}_{1}}}}{{{{I}_{2}}}} = \frac{{{{A}_{1}}{{g}_{1}}{{\lambda }_{2}}}}{{{{A}_{2}}{{g}_{2}}{{\lambda }_{1}}}}{\text{exp}}\left( { - \frac{{{{E}_{1}} - {{E}_{2}}}}{{kT}}} \right),$$
(1)

where I is the line intensity, A is the transition probability, g is the degeneration coefficient, λ is the wavelength, E is the energy of the upper transition level, T is the temperature, k is the Boltzmann constant. Indices 1 and 2 refer to two different lines of the same element.

The resulting temperature of 5000 K turns out to be less than the average value of the rotational temperature, which may indicate some errors in the assumptions of the calculation methods used or uneven temperature distribution in the plasma–dust cloud.

To obtain SEM microphotographs (Figs. 12, 13) and maps of the distribution of elements (Figs. 14, 15) by XRD, samples of the ilmenite concentrate substance after exposure to a microwave discharge were taken both from the bulk of the substance on a quartz disk (Fig. 12), and from the inner walls of the quartz tube (Fig. 13). In the sample from the bulk of the substance, particles with an angular shape are predominant, as in the original sample, but a significant number of spherical particles also appeared. Particle spheroidization is a characteristic process of plasma exposure (Samokhin et al., 2019; Károly and Szépvölgyi, 2005). Spherical particles do not have a smooth surface and a porous structure. A number of smaller (~1 µm) particles settled on large particles (>10 µm). Judging by the element distribution maps (Figs. 14, 15), there was no noticeable change in the elemental composition compared to the original sample. Even spherical particles do not have a different distribution of elements from other particles. Local measurements of elemental composition (Table 5) show some scatter in the content of Fe, Ti and O.

Fig. 12.
figure 12

Microphotographs of ilmenite concentrate after exposure to a microwave discharge. The sample is taken from the bulk of the substance on a quartz disk.

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Fig. 13.
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Microphotographs of ilmenite concentrate after exposure to a microwave discharge. The sample was taken from the inner walls of a quartz tube.

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Fig. 14.
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Multilayer maps of the distribution of elements in samples of ilmenite concentrate after exposure to a microwave discharge: on the left—a sample from the bulk of the sample; on the right is a sample from the inner walls of a quartz tube. Color figure is available in the electronic version of the article.

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Fig. 15.
figure 15

Microphotograph of a sample of ilmenite concentrate after exposure to a microwave discharge with superimposed multilayer element distribution maps and distribution maps of individual elements. Color figure is available in the electronic version of the article.

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Table 5.   Content of oxygen, iron and titanium in a sample of ilmenite concentrate before and after (samples No. 1–4) exposure to a microwave discharge
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Lunar Dust Simulant LMS-1D

The working gas in the experiment was argon at atmospheric pressure. The microwave radiation power was 400 kW and the pulse duration was 6 ms. The sample was exposed to five microwave pulses with a total energy of 7.2 kJ. A discharge in a powder sample of the lunar dust simulant LMS-1D (with the addition of a 10% mass fraction of metallic magnesium powder) was initiated at the front of a microwave pulse. Photographs of the sample before and after exposure to the discharge are shown in Fig. 8. The dynamics of discharge development in a powder sample of a lunar dust simulant is hardly any different from a sample of ilmenite concentrate: at the initial stage, radial expansion of the plasma formation occurs; the highest radiation intensity and temperature of the plasma–dust formation are observed during the microwave pulse (Fig. 16, frame at 5.35 ms), when the scattering of particles into the reactor volume is also visually observed; after the end of the microwave pulse (Fig. 16, frame at 18.3 ms), the intensity of the glow of the plasma–dust formation begins to decrease, as do the temperatures.

Fig. 16.
figure 16

Footage of the development of a microwave discharge in a sample of the lunar dust simulant LMS-1D. The frame start time (milliseconds) from the moment of breakdown is indicated on each frame in the upper left corner. The frame exposure time is 100 μs. Microwave power 400 kW, microwave pulse duration 6 ms.

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In the emission spectra (Fig. 17), recorded with a survey spectrometer, the line of sight of which passes from bottom to top through the entire layer of the powder sample, lines of neutral iron Fe I, titanium Ti I, potassium K I and its regular companion rubidium Rb I, sodium Na I, iron ion Fe II, molecular bands of MnO oxides. Moreover, the lines of alkali metals are highly intense. There are no bands of neutral magnesium or its ion in the survey spectrum, however, in the experiment, magnesium lines were recorded in the ultraviolet region using an AvaSpec-ULS4096-CL-2-EVO spectrometer. The dynamics of the sample surface temperature, determined from the continuum in the Wien region of the spectrum, is shown in (Fig. 18). The electron temperature, calculated from the relative intensity of a number of iron lines (the method is described below), was ~5500 K. Due to the absence of TiO emission bands, despite the presence of titanium in the LMS-1D composition in the form of ilmenite, it was impossible to determine the rotational temperature of the gas using the method used for the ilmenite concentrate sample.

Fig. 17.
figure 17

Emission spectrum in an experiment with the creation of a plasma–dust cloud in a powder sample of the lunar dust simulant LMS-1D with the addition of metallic magnesium powder during the duration of a microwave pulse.

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Fig. 18.
figure 18

Dynamics of the surface temperature of a powder sample of a lunar dust simulant LMS-1D with the addition of metallic magnesium powder. The duration of the microwave pulse is indicated by a gray rectangle.

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Based on a number of lines of iron Fe I (Table 6), the temperature was calculated based on formula (1), reduced to the form

$$~\ln \frac{{I\lambda }}{{gA}} = - \frac{E}{{kT}} + K,$$
(2)

where all parameters, except temperature, refer to one line, and the value K is the same for all lines of one source (in our case, neutral iron Fe I). Each line corresponds to a point in coordinates \(\ln \frac{{I\lambda }}{{gA}}\) and E. These points must lie on a straight line, the angle of inclination of which depends only on temperature. Specific values for each frame were calculated using the least-squares method.

Table 6.   Lines of neutral iron Fe I observed in the emission spectra of the plasma–dust cloud for both the Tsarev meteorite material sample and the LMS-1D lunar dust simulant
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Figure 19 shows microphotographs of a sample of the lunar dust simulant LMS-1D with the addition of metallic magnesium powder after exposure to a microwave discharge. Samples in Fig. 19 were taken from the inner walls of the quartz tube. Among the deposited particles there are many very fine dust and particles that do not differ in shape from the original ones. Spherical particles are also found, but less frequently than in the ilmenite concentrate sample. Moreover, quite large (~100 μm) spherical particles are also detected. On the element distribution maps (Fig. 20), it can be noted that, as in the original sample, the main constituent elements are distributed almost evenly, with the exception of magnesium, some of which was added in the form of metal powder. Local measurements of the elemental composition in different areas of the sample and for particles of different shapes clearly indicate an increase (up to two times) in the proportion of magnesium compared to the composition of the initial sample of the lunar dust simulant LMS-1D with the addition of metallic magnesium powder (Table 7), which, apparently, is associated with the spraying and subsequent deposition on the surface of other particles of metallic magnesium added in the form of powder to the simulant. Most likely, magnesium is present in an oxidized state. You can also note the formation of agglomerates of small particles or settling of small particles on large particles, in some cases to the point of completely covering the surface of large particles.

Fig. 19.
figure 19

Microphotographs of the LMS-1D lunar dust simulant with the addition of metallic magnesium powder after exposure to a microwave discharge. The sample was taken from the inner walls of a quartz tube.

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Fig. 20.
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Microphotograph, with a superimposed multilayer map of the distribution of elements, and maps of the distribution of individual elements in a sample of lunar dust simulant LMS-1D with the addition of 10% metallic magnesium after exposure to a microwave discharge. Color figure is available in the electronic version of the article.

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Table 7.   Elemental composition of the lunar dust simulant LMS-1D with the addition of metallic magnesium powder before and after (samples No. 1–6) exposure to a microwave discharge
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Substance of the Tsarev Meteorite

The working gas in the experiment was atmospheric-pressure air. As part of an additional experiment, titanium plates were placed in a quartz tube (Fig. 8). Their long side was located perpendicular to the electric field strength vector of linearly polarized microwave radiation. The power of microwave radiation increased from pulse to pulse from 200 to 400 kW, and the pulse duration from 4 to 6 ms. The sample was exposed to 17 microwave pulses with a total energy of 39.2 kJ. The discharge in a powder sample of the Tsarev meteorite substance was initiated with a delay of 1–2 ms from the beginning of the front of the microwave pulse. Photographs of the sample before and after exposure to the discharge are shown in Fig. 8.

The development of a discharge in the material of the Tsarev meteorite (Fig. 21) has the same stages as the two samples given above: radial expansion of the plasma formation; the stage with the highest radiation intensity and temperature of plasma–dust formation during a microwave pulse; the stage of cooling and expansion after the end of the microwave pulse. The lower intensity and duration of glow in the frames of Fig. 21, compared to Fig. 9 and Fig. 16 may be associated with the use of significantly shorter exposure times.

Fig. 21.
figure 21

Footage of the development of a microwave discharge in a sample of material from the Tsarev meteorite. The frame start time (milliseconds) from the moment of breakdown is indicated on each frame in the upper left corner. Frame exposure time 0.6 μs. Microwave power 400 kW, microwave pulse duration 6 ms.

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In the survey radiation spectrum (Fig. 22), for the direction of receiving radiation from bottom to top through the entire layer of the powder sample, there are many lines of neutral iron Fe I, lines of neutral titanium Ti I, calcium Ca I, lithium Li I, potassium K I, rubidium Rb I, sodium Na I, calcium ion Ca II, and molecular bands of oxides MgO and CaO. The work (Akhmadullina et al., 2023) also detected lines of the iron ion Fe II in the ultraviolet region. The spectra we obtained in the experiment have similar features with the spectra of meteors (Borovička, 1994; Vojáček et al., 2015), with the spectra of an expanding gas cloud after an impact event (Schultz and Eberhardy, 2015) and with spectra when simulating an impact event using laser ablation (Ferus et al., 2018, 2019; Hawkes et al., 2008; Křivková et al., 2021). The similarity lies in the presence of a large number of iron and calcium lines (Tables 6, 8). Among the most significant differences, it is worth highlighting: the presence of an intense thermal continuum, the absence of lines of doubly ionized iron and other metals. A detailed comparison of the spectra deserves a separate study.

Fig. 22.
figure 22

Emission spectrum in an experiment with the creation of a plasma–dust cloud in a powder sample of the Tsarev meteorite substance during the duration of a microwave pulse.

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Table 8.   Lines of neutral calcium Ca I and calcium ion Ca II observed in the emission spectrum of a plasma–dust cloud for a sample of the Tsarev substance
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The sample surface temperature values are shown in (Fig. 23). The electron temperature, calculated from the relative intensity of pairs of calcium Ca I lines (lines 422.673 and 430.253 nm), which have a significant difference in the energy of the upper level and are present in several recording time windows during one discharge, was ~7900 K. The dynamics of changes in electron temperature are given in Table 9 for calculations using the above Ca I lines (TCa) and a number of Fe I lines (TFe), (the method is given above). The calculation was carried out for those moments in time indicated in Table 9 that were both within the duration (≤4 ms) of the microwave pulse, and after its end (>4 ms). The spectrum exposure time is 0.5 ms, and the time given in Table 9 corresponds to the beginning of the spectrum accumulation time interval.

Fig. 23.
figure 23

Dynamics of surface temperature of a powder sample of meteorite substance. The duration of the microwave pulse is indicated by a gray rectangle.

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Table 9.   Dynamics of electron temperature changes
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An SEM analysis of samples from the Tsarev meteorite after exposure to a microwave discharge is shown in Fig. 24. The sample taken from the inner walls of the quartz tube contains mainly spherical particles. It is worth noting the presence of spherical particles, the sizes of which (~100 μm) are larger than the sizes of the particles of the original substance. Which indicates the probable merging of microdroplets of the melt formed under the influence of a microwave discharge. The sample also contains porous particles and particles of complex shape. On the surface of large particles, submicron particles and small spherical particles adhering to them are observed. The elemental composition of the spherical particles is close to each other (Fig. 25), but local increases in the concentrations of nickel, aluminum, sulfur, and sodium within individual particles can be noted. The elemental composition of a separate sample of the material from the Tsarev meteorite after exposure to a microwave discharge is presented in Table 10 along with the elemental composition of the original sample for ease of comparison. The XRF we carried out in (Akhmadullina et al., 2023) did not show the appearance of new phases or changes in the ratio of the main phases (olivine main phase, orthopyroxene secondary) after exposure of the sample to a microwave discharge.

Fig. 24.
figure 24

Microphotographs of the substance of the Tsarev meteorite after exposure to a microwave discharge. The sample was taken from the inner walls of a quartz tube.

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Fig. 25.
figure 25

Microphotograph with a superimposed multilayer map of the distribution of elements, and maps of the distribution of individual elements in a sample of the Tsarev meteorite substance after exposure to a microwave discharge. Color figure is available in the electronic version of the article.

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Table 10.   Elemental composition in the initial sample of the substance of the Tsarev meteorite and in the sample after exposure to a microwave discharge according to local measurements
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CONCLUSIONS

In experiments on microwave discharge in powder samples, plasma–dust clouds were obtained from substances relevant for the cosmic processes of the formation of a multiphase plasma–dust medium. The dynamics of the development of plasma–dust clouds had characteristic stages: discharge initiation; radial expansion of the plasma formation above the surface of the samples; maintaining the maximum temperature of the plasma and gas during the duration of the microwave pulse with the ongoing evaporation of the substance and the emission of sample particles into the volume; cooling and scattering after completion of the microwave pulse. In the emission spectra of plasma–dust clouds, lines of neutral atoms, lines of ions and molecular bands of oxides of substances included in the original samples were recorded. The measured electron temperatures reached 7000 K, gas temperatures 5500 K, and powder sample surface temperatures 2000 K.

The occurrence of a low-threshold microwave discharge in a powder sample of ilmenite concentrate and material from the Tsarev meteorite without adding metal powders to them is interesting in itself. This is a consequence of the sufficient intrinsic content of conducting phases in these samples for the occurrence of a discharge of this type. This fact may be necessary to take into account, for example, in applied problems of sintering lunar regolith by irradiation with microwave radiation (Taylor and Meek, 2005; Meurisse et al., 2017). When a certain threshold is exceeded (10–20 kW/cm2) based on the wave intensity and sufficient conductive phase content, a discharge can occur. Moreover, vacuum conditions should not be an obstacle for the discharge to occur, since it will develop in the cloud of its own vapors of the heated regolith. In the future, it would be interesting to conduct such an experiment under high vacuum conditions.

The result of the impact of the microwave discharge on all samples was a change in the morphology of the particles (spheroidization of the particles occurred) and, to some extent, in the sizes of the particles (individual, larger spherical particles appeared than were in the original samples). In the case of the LMS-1D lunar dust simulant sample, fewer spherical particles were observed in the samples than for the other two samples. The appearance of large submicron-sized particles on the surface was observed. At the same time, no obvious changes in the elemental composition determined by XRD for all samples were detected.

The experimental approach used in the work can be used to study the prebiological synthesis of organic substances under conditions of plasma–dust clouds with the important role of dust particles as crystallization centers and/or as a catalyst (Parmon, 1999; Peters et al., 2023; Saladino et al., 2013, 2018; Snytnikov, 2010; Snytnikov et al., 2002). To do this, it is necessary to use either minerals characteristic of space objects or the substance of space objects directly as powder samples. The working gas can be a mixture of gases assumed for the location of the process (for example, a model of the Earth’s primary atmosphere).

Plasma–dust methods turn out to be effective for studying the possibility of processes of synthesis of organic compounds in the conditions of the early Earth, in particular, during the passage of meteoric bodies in the atmosphere. During such a passage, a meteoroid trail is formed containing various phases of substances, including a dust component. Organic matter is quite easily synthesized on solid catalysts. To do this, they must contain, for example, iron, nickel and silicon, which are present in meteoric matter. Also, as a result of the development of waves and instabilities in the dusty plasma of meteoroid tails, dust particles can concentrate in local areas, which increases the likelihood of chemical reactions occurring (Morozova and Popel, 2020, 2021a, 2021b, 2022a, 2022b, 2023). The energies for excitation of vibrational degrees of freedom at temperatures reached in meteor trails make it possible for the development of free radical chemical processes and the occurrence of a wider range of chemical reactions (Gurel and Gurel, 1983). Also in protoplanetary disks, one can expect the development of processes of synthesis of organic compounds in areas of dust concentration, especially in the case of its charging and the presence of electrons and ions in the surrounding plasma, especially during outbursts of protostars (Morozova and Kuznetsov, 2023; Morozova et al., 2022c; Snytnikov al., 2002).