Study of Iron and Stony Meteorite Ablation Based on Simulation Experiments in an Arc Heater

To observe meteorite ablation, simulation experiments were conducted on the L5 ordinary chondrite and IAB-MG iron meteorites in an arc-heated facility and three flight conditions were reproduced. To mimic the high heating rates and the significant shear stress that meteorites experience during Earth entry, the samples were machined into 9° spherical cones with a 20 mm nose radius. High-quality video, the surface temperature, a time-resolved spectrum, and infrared video were recorded. The atom species were determined via spectroscopy to analyze the ablation products. Due to the electrode erosion and dissociation of air, the atomic lines of copper, nitrogen, and oxygen were detected in all the tests. Although the copper atom is a pollutant to the flow field, the five copper lines were used to determine the flow-field temperature. The ablation rates and effective heat of ablation of both the samples were measured under different conditions. The results indicate that shear stress is the dominant factor influencing meteorite ablation. Furthermore, the diversity between stony and iron meteorites suggests that the mass loss of stony meteorites depends on the fragmentation of the main body and that of iron meteorites depends on the shearing loss of the molten layer. Then, the fusion crusts were analyzed, the microstructures of the samples were obtained, the crust thicknesses were measured, and the elemental distribution of the stony meteorites was determined via energy dispersion spectroscopy. The study results explain the differences in the ablation and recrystallization process between stony and iron meteorites.


Introduction
Due to the extremely high entry speed, the surface of meteoroids rubs violently against the Earth's atmosphere, yielding an ultrahigh-temperature and high-pressure environment.In this environment, the meteoroid experiences severe ablation, which leads to a series of physical phenomena, such as explosion (1908 Tunguska explosion; Chyba et al. 1993), fragmentation (Silber et al. 2018), and photothermal radiation (Love & Brownlee 1991;Liu et al. 2018).Basic data for modeling the meteoroid behavior can be simply obtained by directly observing the meteoroid falling (Jenniskens 2004).To date, numerous observational projects and methods have been funded to search for and monitor near-Earth objects in orbit from space and from the ground (Mainzer et al. 2015;Harris & D'Abramo 2015).Moreover, researchers have comprehensively tracked and studied recent meteoroid falls (Jenniskens et al. 2009;Schulte et al. 2010).However, owing to the randomness of these events and extremely high falling speeds (10-20 km s −1 ), the direct survey of meteoroid falls is extremely difficult and quantitative parameters, such as ablation rate, emission spectrum, and surface temperature, are difficult to obtain.
The pieces of meteoroids remaining after entry ablation and that fall to the ground are called meteorites.Meteoroids have kinetic energy that is equal to hundreds or even thousands of kilotons of trinitrotoluene, which can significantly damage objects and life on the ground (Popova et al. 2013;Kartashova et al. 2018).
Furthermore, meteorites bring interstellar materials to Earth, such as amino acids, which have been speculated to have resulted in the origin of life by some researchers (Park & Brown 2012).The survival of organic matter during the entry process implies that a meteorite retains its original characteristics even though its surface experiences erosion and ablation.Thus, the remaining pieces could be used as test samples in ground experiments after the removal of the outer ablated layer.
Simulation tests of meteorite and analog materials, such as basalt, have been conducted by many researchers since the middle of the 20th century.Ground tests provide more details than observations.Moreover, they yield qualitative and quantitative parameters since the experiments have sufficient time for conducting measurements and are repeatable.Thomas & White (1953) measured the spectrum of a stony meteorite in a ballistic facility and compared it with that of observed meteorites.Shepard et al. (1967) conducted the first simulation experiments on meteorite analogs in the Arcjet facility at the NASA Ames Research Center.Laser-induced evaporation of the meteorite test samples has also been studied (Milley et al. 2007;White & Stern 2017).Comprehensive data on the ablation process, surface temperature, and surface morphology after ablation have been obtained in several experiments (Bronshten 1983).Numerical models have been established to predict ablation (Johnston & Stern 2017;Bariselli et al. 2018).Recently, using advanced measurement techniques and employing the powerful simulation capabilities of novel test facilities, simulation tests have been conducted for several meteorite materials.Loehle et al. (2017) tested an H4 chondrite meteorite and two analog samples (basalt and argillite) in the PWK1 plasma wind tunnel at the Institut für Raumfahrtsysteme; they designed the test conditions to duplicate the environment of a meteoroid fragment flying at a Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
height of 80 km and a speed of 10 km s −1 .They obtained experimental data using five different diagnostic methods.Furthermore, they conducted detailed analysis of the spectral data, determined the vibrational and rotational temperatures of CN radicals, and compared the experimental and observational spectra.Agrawal et al. (2018) compared several different materials, including an H5 chondrite meteorite (Tamdakht), an IAB-MG iron meteorite (Campo Del Cielo), and two analog materials (basalt and fused silica).They simulated a 30 m diameter meteoroid moving at an altitude of 65 km with a velocity of 20 km s −1 .They machined the test models into 45°s pherical cones with a 0.635 cm nose radius.All tests were sustained for only a few seconds (planned for 5 s); the tests of the chondrite and iron meteorite models were stopped at 2 and 2.5 s, respectively, because of the unexpectedly high ablation and melting rate.Mass loss from the shearing of the molten layer and spallation of fragments was observed.The effective heat of ablation was also estimated numerically.Recently, Helber et al. (2019) conducted tests on the H5 ordinary chondrite meteorite and basalt model under different conditions at the Von Karman Institute Plasmatron facility.Samples were cut into cylinders of 16 mm diameter and 6 mm length and were embedded in a hemispherical holder composed of a corkcomposite ablative material to realize the one-dimensional (1D) heat conduction approach.Atom, ion, and molecule components in the flow field were determined.A 1D ablation model was established and used to calculate the stagnation temperature.Comparison with the test data showed that heterogeneous exothermic reactions play an important role in the ablation reaction.
In our previous study (Luo et al. 2020), carbon steel and basalt were selected as the analogs of iron and stony meteorites, and the test conditions duplicated the ablation environment experienced by a meteorite fragment in the Chelyabinsk event at low altitudes (velocity 6 km s −1 , height 17 km, and diameter 1 m).The ablation process of the test samples was clearly recorded, and the melt flow of two different materials, the spallation of fragments, and basalt vaporization were observed.The model morphologies before and after ablation were exhibited through three-dimensional scans, and the flow temperature was determined by emission spectroscopy.Furthermore, the effective heat of ablation of basalt was estimated.
Based on previous studies, experimental parameters, such as model shape, test conditions, and duration, were designed for the ablation tests.This study aimed to have not only sufficient time for measurements but also sufficient sample left for posttest analysis.The L5 ordinary chondrite (NWA 13132) and IAB-MG iron (Campo Del Cielo) meteorite samples were tested in the arc-heated facility at the Hypervelocity Aerodynamic Institute, China Aerodynamics Research and Development Center (HAI-CARDC).This study aimed to provide qualitative and quantitative experimental data via ground simulation tests of meteorite ablation to validate and correct the model of meteorite ablation.In the experiments, three flight conditions were duplicated.The ablative movements of the two samples were illustrated through full-color/infrared video images, the evolution of surface temperature/emission intensity, and recession of stagnation.To obtain quantitative parameters for analysis of the mechanisms, the flow temperature near the stagnation area was also estimated.This study mainly focused on comparing the ablative effects between fusion and shearing (corresponding to ablative results under different test conditions).Moreover, several post-test analyses were conducted.To our knowledge, this study is the first to analyze the fusion crusts of artificially formed meteorite samples.The microstructures and elemental contents were studied using optical and electron microscopes, and the thickness of the fusion crust was measured to reconstruct the ablation/recrystallization process.
Sections 2 and 3 introduce the test samples, experimental facility, and measurement methodology.Section 4 presents the test results of the two different meteorites, illustrating the measurement data.The effects of the test conditions are shown in Section 5. Post-test analysis of the fusion crusts is discussed in Section 6. Section 7 provides a brief conclusion.

Test Model and Experimental Setup
The heating environment was provided by a segmented archeater.To record and analyze the ablation process, measuring devices such as a high-quality full-color camera, an infrared camera, a two-color pyrometer, and a spectrometer were employed.L5 ordinary chondrite and IAB-MG iron meteorites were chosen as samples corresponding to basalt and carbon steel, respectively, and the samples were machined into the shape of a spherical cone.

Selection and Fabrication of the Test Model
Meteorites can be classified into two typical types: stony and iron.Our previous tests (Luo et al. 2020) of basalt and carbon steel exhibited obvious differences in the heating and mass loss behaviors between the two types of meteorite, signifying that the heat and mass transfer mechanisms of stony and iron meteorites during Earth entry are totally different.In this study, stony and iron samples were obtained from an L5 ordinary chondrite meteorite and an IAB-MG iron meteorite, respectively.The L5 ordinary chondrite meteorite was named Northwest Africa 13132 (NWA 13132) by the International Society for Meteoritic and Planetary Science.It was collected from Niger in 2017.The iron meteorite was named Campo Del Cielo, and was obtained from the meteorite that landed in Argentina about 4000 yr ago (dozens of meters in diameter).Part of the meteorite was used in the arcjet ablation tests of Agrawal et al. (2018).The compositions of the main elements in NWA 13132 and Campo Del Cielo are shown in Table 1 (Agrawal et al. 2018;Li et al. 2021).
In previous studies, the test samples were machined into different shapes.Loehle et al. (2017) and Helber et al. (2019) employed cylindrical models in their experiments.In Loehle et al.ʼs experiments, the model was fully exposed to the flow field.In contrast, Helber et al. used a hemispherical corkcomposite sample holder to insulate the test sample from the side-wall heating and obtain a shape of known geometry to enable comparisons with the numerical results, which provided a 1D heat conduction approach but significantly reduced the mass loss of the sample due to the shearing of the flow.In Agrawal et al.ʼs (2018) experiment, the test samples were machined into spherical cones with 0.635 cm nose radius, and a large cone angle of 45°was adopted to provide high heating rates and significant shear stress at model stagnation.However, severe ablation occurred during model stagnation, which led to a large variation of the test conditions between the stagnation and other areas of the model.Considering all types of models, the test models in this study were machined into spherical cones with 20 mm nose radius and 9°cone angle; Figure 1 displays the model configuration.A water-cooled sample holder was used, and a high silica glass fixture was installed in front of the holder to insulate the heat conduction from the test sample.Additionally, two thermal protective cones were set in front of the holder.This configuration was employed to protect the sample holder and reduce heat loss.

Experimental Setup and Measurement Methods
The test facility is a 20 MW segmented arc-heater with a 3 m test chamber that is connected to the atmosphere by a diffuser.An axisymmetric nozzle with a 60 mm exit was used to create a high-enthalpy supersonic flow.The measurement instruments that were installed on the side of the test chamber include a fullcolor camera, two pyrometers, an infrared camera, and a spectrometer.The full-color camera recorded details of the dynamic movements during sample ablation, the two ISR12-LO pyrometers (0.8-1.05 μm, 1300-3600 K) measured the evolution of surface temperature at the nose and body of the test models, and the infrared camera captured the temperature distribution at the model surface (ImagelR8325, 640 × 512 pixels −10 to 2500 °C, 3.7-4.8μm spectral range).Figure 2 displays the experimental setup.
The optical emission spectrum during model stagnation was measured using an ultraviolet-visible (UV-VIS) spectrometer (Ocean Optics HR4000, 200-1100 nm, and 0.75 nm spectral resolution).The spectrometer was connected to a reflective collimator by an optical fiber (200-1100 nm and 100 μm core diameter); the viewing area of the collimator was 12 mm in diameter, and the distance from the collimator to the model surface was about 2 m.A light source (LDLS, EQ99FC) with a spectral range of 250-2200 nm was used for the absolute irradiance calibration.

Test Conditions and Reconstruction of Flight Condition
The fundamental principle of the duplication of flight condition is to match the total enthalpy of the boundary layer edge h 0 , the stagnation pressure P s , and the radial velocity gradient in the radial direction at the wall in the experiment to the flight conditions (local heat transfer simulation methodology; Kolesnikov 1993).
To verify the test conditions, calibration probes with the same shape as the test models were used to obtain the stagnation heat flux and pressure.The stagnation pressure was directly verified based on the probe results.The stagnation enthalpy of the model was calculated based on the measured values of the stagnation heat flux and pressure as well as the theoretical calculation of the heat transfer at catalytic stagnation in laminar equilibrium on a hemispherical body.The velocity gradient at the stagnation point of a hemispherical body can be theoretically calculated by determining the stagnation enthalpy and Mach number (ASTM Standard E 637-98).The total enthalpy of the flow field was determined via the sonic throat method.In the supersonic arc-heater, based on the condition that the flow is choked at the nozzle throat (its velocity reaches the speed of sound at the nozzle throat), the total enthalpy h 0 can be calculated based on the known effective area of the throat A eff , the gas flow rate G t , and the chamber pressure P 0 (Park et al. 2006): Three test conditions with higher stagnation heat flux and pressure (compared to Loehle et al. 2017 andHelber et al. 2019) were chosen to recover the massive ablation when the total enthalpy was restricted by the capabilities of the facility.In this study, conditions I-III correspond to the flight conditions of a 1 m diameter object with a speed of 5.35, 4.51, and 3.97 km s −1 at an altitude of 15.1, 14.5, and 12.1 km, respectively.At these altitudes, there are only few seconds left before the remaining fragments hit the ground; these pieces are also slowed down by the atmosphere, and our experiments are aimed at duplicating these few seconds.The conditions are illustrated in Figure 3 in comparison with the test conditions of our former study and the fireball trajectory of the meteoroid fragment in the Chelyabinsk event after the explosion of the  main body (Borovička et al. 2013).For convenience, the stagnation pressure, stagnation heat flux, and total enthalpy under the test conditions are listed in Table 2.

Experimental Results
This section illustrates the test results of the stony (NWA 13132) and iron (Campo Del Cielo) meteorites.Furthermore, the ablation behaviors of the two samples are discussed.The progress of the ablation processes is clearly demonstrated using color/infrared images, the evolution in surface temperature, and emission spectra.The test samples in each experiment were exposed to a high-enthalpy flow field for 4 s, which is an effective measurement duration.

Measurement Results of Stony Meteorite Ablation
Before the experiments, the microstructure of the stony meteorite was analyzed to verify the meteorite type.Spherulites were observed in the samples taken from different parts of the meteorite using a scanning electron microscope (SEM).Most of the spherulites were in porphyritic and fire bar shapes (Figure 4), which mainly comprised olivine  , n = 25) and low calcium pyroxene (Fs 21.1-24.9Wo 1.0-2.0, n = 16) according to the component analysis.The minerals were nonuniformly distributed in the substrate material.The iron in the analyzed samples was between L group and LL group, with the majority belonging to the L group.Therefore, the stony meteorite (NWA 13132) employed herein could be defined as an L5 ordinary chondrite meteorite.
Images of the ablation process under condition III captured from a high-resolution video are shown in Figure 5, where the 4 s period is divided into six parts with 0.5 s intervals.The beginning of ablation is represented as 0 s, which is distinguished from the video frame by frame.The images show that after the commencement of the test, a molten layer immediately appeared on the sample head.Then, a liquid layer rapidly covered the entire model surface in 0.5 s.Thereafter, small segments started to split from the sample.A large part of the sample fell off the main body.Toward the end of the test, ablation continued along the fractured surface, forming large grooves on the model surface.The images show that the model shape changed significantly due to ablation.The main body lost large amounts of material due to fragmentation and spalling; therefore, the mass loss due to the shearing of the molten layer was negligible.The overview of the test model (NWA 13132) before and after ablation is shown in Figure 6.The cone-shaped sample transformed into an irregular configuration that was pitted with grooves and holes.In Figure 6(a), minerals can be clearly distinguished on the sample surface, which were uniformly distributed inside the main body as small particles.After ablation, the sample (Figure 6(b)) was covered with pits.Two obvious fissures were observed at the sample edge, where large pieces of the material cracked.This implies that the test sample experienced significant thermal stress during ablation.In conclusion, the mass loss of a stony meteorite occurs due to the disintegration of the main body.

Emission Spectroscopy
The time-resolved spectroscopic signatures of the ablation products and plasma were captured using a spectrometer with a 20 Hz sampling frequency.Emission spectroscopy was mainly performed to determine the contents of the test samples and ablation products during the test.Figure 7 illustrates the spectrum during the middle of the test.The absolute irradiance of the spectrum was calibrated with a UV-VIS-NIR (250-2200 nm) standard light source.Several intense spectral lines were distinguished in the spectrum.By matching the wavelength of the spectral lines with the typical statistics in the National Institute of Standards and Technology (NIST) database, nine atom species were identified.The baseline of the spectrum exhibited the shape of black/graybody emission; thus, it was fitted according to the Planck black/graybody emission theory (red line in Figure 7).The intensity of the Planck curve was determined based on the emissivity and temperature.The emissivity was assumed as 0.85, according to the emissivity of the Tamdakht H5 stony meteorite used in Agrawal et al. (2018).Then, by fitting with the Planck curve, the black/graybody temperature of the model at 2 s was obtained as 2300 K.This value was compared with the surface temperature measured by the pyrometer.
Among the atom species determined, iron, nickel, chromium, silicon, sodium, and potassium were identified as the ablation products.In addition, strong copper, oxygen, and nitrogen lines were detected (Figure 7).Based on our experience, the copper atoms stemmed from electrode erosion, which is unrelated to ablation of the model.The oxygen and nitrogen stemmed from the oxide and nitride in the sample or from the dissociation of air.Although the copper atoms are a contamination to the flow field, the copper atomic line can be used to calculate the temperature of the thermal equilibrium flow.Based on the assumption of local thermal equilibrium (LTE), the flow temperature around the stagnation area was calculated; the calculations and results are presented in Section 5.The atom species determined by spectroscopy were compared to the contents in Table 1; the spectral lines of silicon, iron, and nickel were captured, and no lines of aluminum and calcium were detected.Helber et al. (2019) claimed that the absence of the aluminum lines (part of the sample elemental composition) was due to the high melting temperature (1653 K).According to Table 1, the proportions of aluminum and calcium in the test sample were  relatively low (Al 1.17% and Ca 1.47%).Furthermore, the high melting temperature resulted in fewer contents in the flow field.Thus, the aluminum and calcium atomic lines were not detected.Figure 8 presents the temporal emission profiles of the main radiating species.
As shown in Figure 8, the spectral line intensity fluctuated violently during ablation.Furthermore, the line intensity of all the species increased rapidly after the model was moved into the high-temperature flow.Except for sodium, the emission maintained its maximum value until the test was stopped.This phenomenon conforms to the melting process seen in the video: the vaporized ablation products enter the flow field and are excited by the high temperature, and then the atoms at the excited energy level spontaneously jump to lower energy levels and emit optical radiation.The same behavior was observed for the basalt model in our previous experiments (Luo et al. 2020).However, due to the heat insulation of the sample holder, the emission intensity in Helber et al.ʼs (2019) experiment decreased after reaching its maximum value and again increased after 8 s due to mechanical fracturing.

Surface Temperature
Two pyrometers were used to measure the surface temperature at the nose and rear of the test model; the viewpoints are shown in Figure 2. On the right side of the test section, an infrared camera was set to measure the temperature distribution.The signal from the spectrometer was collected on the same side as the infrared camera.The evolution of the Planck temperature was obtained by fitting the continuous spectrum baseline.Figure 9 illustrates the results of the measurements of the evolution of surface temperature.The temperatures at the model nose and rear increased rapidly to approximately 2250 and 2200 K, respectively, and then remained constant until the end of the test.Due to the variation of emissivity during ablation, the result of the Planck fit was higher than that of the pyrometers and exhibited a decreasing tendency.
The infrared images corresponding to the ablation status in Figure 5 are shown in Figure 10.The sample surface was entirely covered by high-temperature melt in less than 1 s after exposure.As the sample spalled at 2 s, the temperature was relatively low inside the test sample, signifying that a  temperature gradient existed from the surface to the inside of the sample.

Measurement Results of Iron Meteorite Ablation
Elemental analysis of the iron meteorite samples was conducted by electron microscopy before the tests.The samples mainly comprised kamacite, taenite, and schreibersite.The samples contained four major elements, with over 90% being iron as well as nickel, phosphorus, and cobalt.Figure 11 displays the ablation process of the iron meteorite (condition III), and comprises six images at time points corresponding to those for the stony meteorite.The iron meteorite before ablation looked like an ordinary metal (Figure 12) with a highly reflective surface and no embedded mineral particles.After ablation, the model maintained its conical shape but its length decreased and its conical angle increased.A layer of fusion crusts that was loosely connected to the main body formed on the sample surface.
The color video showed that the ablation process of the iron meteorite was different from that of the stony meteorite.First, the fusion starting time of the iron meteorite was much later  than that of the stony meteorite due to the higher thermal diffusivity of iron.Then, several parts on the sample nose began to melt after 0.5 s (orange areas in Figure 11 (0.5 s)).Thereafter, a liquid layer formed on the sample head and some of the liquid flowed to the rear along the streamline.The sample body remained dark in color until 3.5 s.Based on the entire experiment, the sample experienced massive ablation and mass loss without fragmentation and spalling of the main body.In conclusion, the mass loss of an iron meteorite during ablation is dominated by the melting, vaporization, and shearing loss of the molten layer.

Emission Spectroscopy
Figure 13 displays the emission spectrum of the iron sample at 2 s after exposure.The atom species were determined from the spectral lines.The black/graybody (with 0.3 emissivity) temperature was obtained as 2020 K through a Planck fit.Emissions of the ablation products, such as iron, nickel, and cobalt, were obtained.Alkali elements, such as lithium, sodium, and potassium, were also observed in the spectrum and their emissions were intense.Moreover, several other elements, such as chromium, phosphorus, and calcium, were observed, but they are not shown in Table 1 due to their minimal contents.

Surface Temperature
The measurement results of the pyrometers and Planck fit are illustrated in Figure 15.
The temperature determined through the Planck fit is only shown for a particular period of the test because the baseline in the spectrum of the iron samples could not always be distinguished.Initially, the temperature was relatively low; thus, the continuous radiation from the blackbody emission was in the shape of a wake.In the later period of the test, due to the significant recession of the sample, the radiation in the viewing area of the spectrometer was dominated by emission from the high-temperature flow, which resulted in the disappearance of the Planck curve.The evolution of temperature was confirmed with infrared thermal imaging (Figure 16).
In the overview of the evolution of temperature, the surface temperature of the iron meteorites increased to the maximum value (approximately 2050 K) in approximately 1.5 s, which is considerably slower than that for the stony meteorites (less than 0.5 s).The temperature recorded by pyrometer 1 (at the model nose) decreased after 1.5 s.The reason for this variation was assumed to be the recession of the sample because of the restricted viewing area of the pyrometer.Thereafter, the  recession processes of the stony and iron meteorites were compared (Figure 17).A difference of only about 1 mm in recession was observed between the two samples, signifying that recession does not significantly influence the detection of temperature.
As shown in Figure 16, the temperature at the sample surface was relatively low initially while the sample holder was already heated to about 1800 K; this result is opposite to our expectations.The heat from the flow was believed to transfer from the front of the model to the rear; hence, the temperature at the stagnation was expected to be higher.However, since the nozzle exit (60 mm) was larger than the test sample, the hightemperature turbulent flow over the sample edge touched and heated the sample holder, becoming a dominant driving source of the temperature of the sample holder.The evolution of temperature also indicates that the sample was first overheated to a temperature considerably higher than its fusion point, and then the liquefaction and evaporation absorbed a large amount of energy to make the system approach thermal equilibrium.This is why the nose temperature decreased at 1.5 s.

Ablation under Different Test Conditions
The ablation experiments on the stony and iron meteorites were conducted under three conditions.Section 4 illustrated the measurement results under test condition III and analyzed and compared the ablation behaviors of the two meteorite samples.In this section, the results of the ablation rate, flow, and surface temperatures are given, and the effects of the condition parameters on meteorite ablation are discussed.

Ablation Rate
The average mass loss and recession of the stagnation per second are defined as the mass ablation rate (R m ) and stagnation line-ablation rate (R l ), respectively.They were determined by measuring the changes in mass and length of the model before and after the tests and dividing by 4 s (time of one test).Tables 3 and 4 present the ablation rates of the stony and iron meteorites, respectively, under the corresponding test conditions.According to our prior assumption, higher enthalpy/heat flux ought to result in higher ablation.However, both mass and stagnation line-ablation rates under condition III were higher than those under conditions I and II.The ablation rates of the stony meteorite increased with the stagnation pressure.Stagnation pressure is relate to the total pressure of the flow field.According to calculation results, the total pressure is proportional to static pressure perpendicular to the interface between the flow field and sample, so the friction force between the flow and sample surface depends strongly on pressure; in other words, higher pressure leads to higher shear stress.
For the iron meteorite, the ablation rates decreased from condition I to II with increasing stagnation pressure but sharply increased from condition II to III.This suggests that ablation is sensitive to the stagnation pressure.Furthermore, the lower the enthalpy, the higher the ablation.However, after carefully observing the video of the ablation process, the shearing effect was determined to occur mainly on the liquid layer on the  sample surface.In other words, the massive ablation caused by the shearing stress only occurred after the material melted, which implies that ablation occurs due to a combination of fusion and shearing.In conclusion, the test results show that shear stress is the dominant factor influencing meteorite ablation under the considered test conditions.

Flow and Surface Temperatures
The flow temperature is a key parameter for the heat transfer from the test gas to the sample.However, it is very difficult to determine via traditional methods in high-temperature environments.Herein, the method of temperature determination based on optical emission spectroscopy was employed.The flow around the model nose was assumed to be in LTE, and the spectral emissions from the atoms were used to calculate the flow temperature under different test conditions.
Based on the theory of the atomic spectrum (Lei et al. 2020), the intensity of the atomic spectral lines is related to the upper where E, g, A, λ, and I are the upper level energy, statistical weight, transition probability, wavelength, and intensity of the spectral line, respectively; k is the Boltzmann constant; T is the temperature; N is the total number of states; and U is the partition function.For the different spectral lines of one specific atom, ( ) / N U ln in Equation ( 2) is a constant.Subsequently, the flow temperature can be determined from the slope (−1/kT) of the plot of ln(Iλ/gA) versus e.
As shown in Figures 7 and 13, different spectral lines of copper, iron, nitrogen, and oxygen were captured.However, the iron spectral lines were too concentrated to identify and their wavelength and intensity were relatively low.Moreover, very few spectral lines of nitrogen and oxygen were obtained (one oxygen line and two nitrogen lines in Figure 7, and two oxygen lines and four nitrogen lines in Figure 13).Five copper lines were selected to calculate the flow temperature (510.554, 515.323, 521.820, 570.024, and 578.213 nm), and the spectral parameters were obtained from the NIST database (Table 5).Table 6 illustrates the flow temperature under the different conditions, with an uncertainty of about 300 K.
The flow temperature gradually decreased from condition I to III, but it was still considerably larger than the fusion   temperature of the sample material.This could be the reason for the complete melting of the outer parts of the samples during the tests.Based on the significant fusion process, the liquid layer attached to the sample surface could be easily sheared off.Therefore, the effect of the shearing of the liquid layer on mass loss is bigger than that of evaporation due to high enthalpy.
Figure 18 depicts the evolution of surface temperature of the stony and iron meteorites under the different test conditions.The surface temperatures were slightly different for the two samples, but the variations in stagnation heat flux and total enthalpy were relatively large (30% in heat flux and 45% in total enthalpy between conditions I and III).The surface temperature results illustrated that the test sample melted significantly and then a constant thermal transfer occurred from the test gas to the liquid molten layer and finally to the solid body; the high-temperature melt covering the sample surface reached the boiling point, causing a relatively constant variation of the surface temperature in the tests on the stony meteorites.The reasons for the decrease in temperature of the iron meteorites are discussed in Section 4.2.2.

Effective Heat of Ablation
The effective heat of ablation is a parameter for characterizing sacrificial thermally resistant materials, and is defined as follows (ASTM standard): where q hw is the hot-wall heat flux, q cw is the cold-wall heat flux, h e is the enthalpy at the edge of the boundary layer, h hw is the hot-wall enthalpy, h cw is the cold-wall enthalpy, and m is the mass loss rate.The effective heat of ablation describes the energy cost to ablate a unit mass of material; hence, if the effective heat of ablation is lower, the material is easier to ablate under the corresponding condition.
In our tests, h cw is considerably lower than h e , and the enthalpy at the edge of the boundary layer near the stagnation point can be approximated as total enthalpy h 0 .Thus, Equation (3) can be simplified as where h hw is determined based on the model stagnation temperature.The effective heat of ablation for the stony and iron meteorites under different test conditions is shown in Table 7.The results under test condition I for both samples and under test conditions II and III for the iron meteorite are close to 2 MJ kg −1 in Agrawal's experiment (Agrawal et al. 2018) and 2.6 MJ kg −1 in our previous work (Luo et al. 2020), and smaller values were observed for the stony meteorite under test conditions II and III.Generally, the effective heats of ablation for both samples in this test are much smaller than the evaporative ablation enthalpy (ASTM Standard), signifying that the mass loss of meteorites under this condition is dominated by the fragmentation and shearing loss of the molten layer.
Table 7 shows that the value decreases from the test condition I to test condition II and condition III for both samples.This suggests that with increasing stagnation pressure, the test sample is easier to ablate and mass loss occurs easily, but the total enthalpy and stagnation heat flux increase.According to the above analysis and Section 5.1, the results of the effective heat of ablation indicate that higher stagnation pressure could cause higher shearing loss of the material for the iron meteorite and make disintegration easier for the stony meteorite.This is because fragmentation and shearing loss of Furthermore, Table 7 shows that the decrease in the effective heat of ablation is much more significant from condition I to condition II than from condition II to condition III, but the increase in stagnation pressure is contradictory.Condition III has relatively low total enthalpy, stagnation heat flux, and flow temperature.This result implies another key factor in meteorite ablation: the molten layer.This phenomenon can be explained as follows.When the total enthalpy and stagnation heat flux are low, the temperature decreases sharply from the model surface to the interior.The molten layer is relatively thin and less material can be sheared out from the main body.
In conclusion, meteorite ablation occurs due to the combined action of melting and shearing.Moreover, higher stagnation pressure causes massive material loss due to fragmentation and  shearing loss of the molten layer.Under our test conditions, these are the dominant factors influencing the mass loss of stony and iron meteorites.

Post-test Diagnosis of the Fusion Crust
In natural environments, the fusion crusts of meteorites form after landing (Sears et al. 2001).They essentially stem from the cooling of high-temperature melts on the surface.They can be used to study the crystallization or glass-forming process of natural materials (Duczmal-Czernikiewicz & Michalska 2018).Moreover, the contents of the fusion crust can be used to estimate a meteorite's age (Tobase et al. 2016).Previous studies on fusion crusts inspired us to determine the thermal dynamic characteristics of the meteorites' main body by investigating the surface layer.Additionally, since the outer sphere is in direct contact with the flow, the thickness of the fusion crusts could reflect the shearing process.
In our tests, the reserved model was covered by a layer of fusion crusts.As shown in Figure 19, the fusion crusts and main body for both the samples were distinguished by a clear boundary.However, the layer of fusion crusts on the iron sample was like a shell, which could be easily removed.In contrast, the layer of fusion crusts on the stony sample was tightly connected to the bulk.This difference implies that the iron sample cooled much faster than the stony one.The thicknesses at different locations were measured; the thickness of the fusion crusts on the stony sample was more even than that on the iron sample.Furthermore, the layer of fusion crusts on the nose of the iron sample was much thinner than that on the rear.This indicates that the shearing effect on the iron sample was larger than that on the stony sample.
Figure 20 displays the detailed structures of the fusion crusts on the main body interface for the stony meteorite; the images were obtained using a microscope.In the figure, the boundary between the bulk and fusion crusts appears to have vanished, and instead a porous layer can be observed near the sample surface.Since the underside of the porous layer corresponds to the inside of the sample, the porous layer could be the fusion crust.Figure 21 presents the elemental distribution obtained by energy dispersion spectroscopy, which corresponds to the data in Table 1.
Figure 22 shows the microstructures of the fusion crusts of the stony and iron samples measured by the SEM.For the stony sample, a porous structure is observed in the image with 250× magnification.In images with lager magnifications, the surface is observed to be covered by grains that are smaller than 1 μm.In contrast, the fusion crust of the iron meteorite exhibits no porous structure, and it comprises numerous flaky grains with many small gaps between them.Figures 22(d) and (f) show that the width of the gaps is less than 1 μm and that the grains have a lamellar shape.

Conclusion
In this study, simulation tests of the L5 ordinary chondrite (NWA 13132) and IAB-MG iron (Campo Del Cielo) meteorites were conducted under three test conditions to obtain comprehensive observations on meteorite ablation.The test samples were designed to reproduce high heating rates as well as significant shear stress.Experimental data from the highresolution video, infrared images, evolution of surface temperature, and emission spectroscopy were collected.The ablation rates and flow temperature under different conditions were determined.After the tests, the thickness of the layer of fusion crusts of both the samples was measured and the microstructures were characterized.
The elemental content of both samples was verified through emission spectroscopy.The main ablation products in both meteorites were metal elements, such as iron, nickel, chromium, sodium, and potassium.A few nonmetal elements such as silicon were found in the stony meteorite and phosphorus was found in the iron meteorite.The emission spectroscopy also indicated that these elements will evaporate or sublimate from the material and transform into the flow field in the form of atoms due to the extremely high temperatures.In this experiment, elements heavier than iron, such as nickel and cobalt, appeared in the flow field during ablation.Thus, other elements that are much heavier than nickel and cobalt could similarly fall to the Earth from outer space.Some measurement results were contradictory to the traditional viewpoint, such as higher enthalpy resulting in lower ablation, the stagnation temperature being lower at the nose of the iron meteorite than at the rear, and the temperature decay observed at the nose of the iron meteorite during ablation.These phenomena were explained based on the test data, but the explanations are not sufficient for describing the entire ablation process.However, this study clearly shows that shear stress could be the dominant factor affecting meteorite ablation under the studied conditions.Moreover, the diversity between the stony and iron meteorites suggests that the mass loss of stony meteorites is controlled by the fragmentation of the main body while that of the iron meteorites depends on the shearing loss of the molten layer.

Figure 1 .
Figure 1.Diagram of the model configuration.

Figure 3 .
Figure 3.Comparison of the test and flight conditions for the Chelyabinsk event.

Figure 5 .
Figure 5. Ablation process of NWA 13132 in condition III.

Figure 6 .
Figure 6.Test model of NWA 13132 (a) before and (b) after ablation.

Figure 8 .
Figure 8. Evolution of emission intensity of atom species: (a) sodium, potassium, oxygen, and nitrogen and (b) copper, iron, nickel, chromium, and silicon.

Figure 9 .
Figure 9. Evolution of surface temperature of NWA 13132 during ablation.

Figure 10 .
Figure 10.Distribution of surface temperature of NWA 13132.

Figure 14
Figure14presents the evolution of the emission intensity during the tests.Almost every element exhibited an obvious upward trend, which conforms to the delay in sample melting.Emissions of the ablation products, such as iron, nickel, and cobalt, were obtained.Alkali elements, such as lithium, sodium, and potassium, were also observed in the spectrum and their emissions were intense.Moreover, several other elements, such as chromium, phosphorus, and calcium, were observed, but they are not shown in Table1due to their minimal contents.

Figure 11 .
Figure 11.Ablation process of Campo Del Cielo in condition III.

Figure 12 .
Figure 12.Test model of Campo Del Cielo (a) before and (b) after ablation.

Figure 13 .
Figure 13.Emission spectrum of Campo Del Cielo at 2 s.

Figure 14 .
Figure 14.Evolution of the emission of the ablation products of Campo Del Cielo: (a) sodium, potassium, magnesium, phosphorus ions, and lithium and (b) iron, nickel, cobalt, chromium, and calcium.

Figure 15 .
Figure 15.Evolution of the surface temperature of Campo Del Cielo during ablation.

Figure 16 .
Figure 16.Distribution of the surface temperature of Campo Del Cielo.

Figure 17 .
Figure 17.Recession process of stony (NWA 13132) and iron (Campo Del Cielo) meteorites during ablation in test condition III.

Figure 18 .
Figure 18.Evolution of surface temperature of (a) stony and (b) iron meteorites under different test conditions.

Figure 19 .
Figure 19.Images of the fusion crust of (a) stony and (b) iron meteorites after ablation in test condition III.

Table 1
Average Composition of the Main Elements in NWA 13132 and Campo Del Cielo (as Mass Fractions)

Table 2
Test Conditions (P s , Stagnation Pressure; q s , Stagnation Heat Flux; and h 0 ,

Table 7
Effective Heat of Ablation (h eff ) under Different Test Conditions