# A Thermal Origin to the Asymmetry of the Permanent Dust Cloud at the Moon

Sébastien Verkercke, Cem Berk Senel, Robert Luther, Menelaos Sarantos, Elena Martellato, Gareth S Collins, Lisa Krämer Ruggiu, Steven Goderis, Özgür Karatekin, Philippe Claeys

► To cite this version:

Sébastien Verkercke, Cem Berk Senel, Robert Luther, Menelaos Sarantos, Elena Martellato, et al. A Thermal Origin to the Asymmetry of the Permanent Dust Cloud at the Moon. Journal of Geophysical Research. Planets, 2025, 130 (10), pp.e2025JE009259. 10.1029/2025je009259. hal-05322647

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Distributed under a Creative Commons Attribution 4.0 International License
JGR Planets

RESEARCH ARTICLE

10.1029/2025JE009259

Sébastien Verkercke and Cem Berk Senel contributed equally to this work.

**Key Points:**

* The lunar regolith's temperature influences the ejecta mass caused by a micro-meteoroid impact, certainly caused by the thermal softening

* This thermal dependency can explain the asymmetry of the dust cloud at the Moon that was observed by LADEE

* Our simulations reproduce the LADEE measurements for a regolith's porosity >80%, giving an indirect remote sensing of the soil's porosity

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

S. Verkercke, sebastien.verkercke@latmos.ipsl.fr

**Citation:**

Verkercke, S., Senel, C. B., Luther, R., Sarantos, M., Martellato, E., Collins, G. S., et al. (2025). A thermal origin to the asymmetry of the permanent dust cloud at the Moon. *Journal of Geophysical Research: Planets*, 130, e2025JE009259. https://doi.org/10.1029/2025JE009259

Received 13 JUN 2025

Accepted 2 OCT 2025

**Author Contributions:**

**Conceptualization:** Sébastien Verkercke, Cem Berk Senel, Menelaos Sarantos

**Data curation:** Sébastien Verkercke, Cem Berk Senel

**Formal analysis:** Sébastien Verkercke, Cem Berk Senel, Robert Luther, Menelaos Sarantos, Gareth S. Collins

**Investigation:** Sébastien Verkercke, Cem Berk Senel, Robert Luther

**Methodology:** Sébastien Verkercke, Cem Berk Senel, Menelaos Sarantos

**Resources:** Cem Berk Senel

**Software:** Cem Berk Senel

© 2025 The Author(s). This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

<figure><img src="image_2.png" /></figure>

A Thermal Origin to the Asymmetry of the Permanent Dust Cloud at the Moon

Sébastien Verkercke¹ ID, Cem Berk Senel²,³, Robert Luther⁴ ID, Menelaos Sarantos⁵ ID, Elena Martellato⁶ ID, Gareth S. Collins⁷ ID, Lisa Krämer Ruggiu² ID, Steven Goderis² ID, Özgür Karatekin³, and Philippe Claeys²,⁸ ID

¹LATMOS/UVSQ, Paris, France, ²Archaeology, Environmental Changes & Geochemistry (AMGC) Research Unit, Vrije Universiteit Brussel, Brussels, Belgium, ³Reference Systems & Planetology Department, Royal Observatory of Belgium, Brussels, Belgium, ⁴Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany, ⁵Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA, ⁶INAF—Astronomical Observatory of Padova, Padova, Italy, ⁷Department of Earth Science and Engineering, Imperial College London, London, UK, ⁸Department of Earth, Ocean and Atmospheric Sciences, Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, BC, Canada

**Abstract** The Moon's surface, lacking an atmosphere, is continually bombarded by high-speed micro-meteoroids, creating a highly porous regolith composed of very fine grains. This regolith's porosity decreases with depth due to compression. Besides creating vapor and melt, micro-meteoroid impacts eject lunar dust, redistributing regolith grains, which can travel ballistically around the Moon. The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, which orbited the Moon from 2014 to 2015, discovered a permanent asymmetric dust cloud with a higher dust density at dawn compared to that at dusk. In addition, the dust cloud at dawn exhibits an asymmetry with a higher density on the dayside, which is attributed to different populations of impactors with varying orbits. Moreover, the inferred ejecta yield was found to be much smaller than expected. Numerical modeling is useful for studying these phenomena, as current experimental capabilities cannot reproduce the mass and velocity combinations typical of actual micro-meteoroid impacts. This study utilizes the iSALE-2D shock physics code to investigate the ejecta mass yield and velocity distribution caused by typical micro-meteoroid impacts as a function of the lunar regolith's porosity and temperature. Findings suggest that (a) the dust cloud asymmetry at the Moon may have a thermal contribution, (b) the predicted ejecta velocity distribution differs from what is assumed to interpret LADEE measurements, and (c) the ejecta yield could serve as an indirect measurement of the regolith structure.

**Plain Language Summary** The Moon's surface is continuously impacted by small dust particles, which result in a very porous surface composed of very fine grains. These fine grains can be partially ejected by the impacting particles, forming a dust cloud around the Moon. This dust cloud was observed between 2014 and 2015 by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, which discovered that the lunar dust cloud is the densest around the morning side of the Moon, with a tilt on the dayside. We used a shock-physics model to simulate the impact of micro-metric dust particles on the Moon surface. Our simulations show that the temperature of the surface can influence the amount of dust that is ejected by an impact. We also find that our simulations reproduce the measurements of LADEE when considering a very porous surface.

1. Introduction

Being an airless body, the Moon's surface is exposed to micro-meteoroid impacts (Janches et al., 2018; Marchi et al., 2005; Pokorny et al., 2019). These impacts form a highly porous regolith covering the lunar surface of very fine grains (<1 mm) (McKay et al., 1991; Szabo et al., 2022, 2023; Verkercke et al., 2023). Its porosity is believed to range between 0.7 and 0.99 (Veverka et al., 1988) and decreases with depth under the compression due to its weight (Mitchell & De Pater, 1994). Micro-meteoroids also eject lunar dust upon impact, which redistributes the regolith grains at the lunar surface (Gault et al., 1974; Szalay & Horányi, 2016). This turnover of the surface allows for fresh material to be exposed to irradiation, which can affect the lunar exosphere (Costello et al., 2020, 2021; Leblanc et al., 2022). The porous nature of the regolith allows for volatiles to diffuse the first few centimeters of the subsurface, leading to the formation of enriched layers (Leblanc et al., 2023; Reiss et al., 2021; Teolis et al., 2023; Verkercke et al., 2024). Micro-meteoroid impacts could penetrate the regolith to these depths, releasing large amounts of volatiles in the exosphere. Conversely, the ejected dust can cover some regions when
**Validation:** Sébastien Verkercke, Cem Berk Senel, Robert Luther, Menelaos Sarantos, Elena Martellato,

Gareth S. Collins, Steven Goderis, Philippe Claeys

**Visualization:** Sébastien Verkercke, Cem Berk Senel

**Writing – original draft:**

Sébastien Verkercke

**Writing – review & editing:**

Sébastien Verkercke, Cem Berk Senel, Robert Luther, Menelaos Sarantos, Elena Martellato, Gareth S. Collins, Lisa Krämer Ruggiu, Steven Goderis, Özgür Karatekin, Philippe Claeys

falling back to the surface, trapping volatiles momentarily (Hurley et al., 2012). Finally, micro-meteoroid impacts can represent the largest disturbance for permanently shadowed regions, which are never exposed to direct sunlight and exhibit very low temperatures, constituting cold traps for volatiles (Brown et al., 2022; Hurley et al., 2012; Moon et al., 2021; Prem et al., 2018), yet are accessible by meteoroids (Pokorny et al., 2020).

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, which orbited the Moon for six months between 2014 and 2015 (Horányi et al., 2014), carried the Lunar Dust Experiment (LDEX), an impact ionization dust detector (Elphic et al., 2014). It discovered the presence of a permanent asymmetric dust cloud (Bernardoni et al., 2021; Horányi et al., 2015; Szalay & Horányi, 2016) formed by ballistic ejection of lunar material upon the impact of incident meteoroids. While a significantly higher dust density was observed at dawn compared to dusk (expected as it is the leading hemisphere), LDEX data showed a tilt in the cloud density oriented toward the dayside. A density peak of the ejecta appeared to be located around 7 Local Time (LT) with respect to 5 LT (Horányi et al., 2015; Szalay & Horányi, 2016). Dynamical models of the meteoroid complex successfully accounted for the local time dependence of LDEX ejecta data from midnight to sunrise but underestimated the ejection from the lunar dayside (Janches et al., 2018; Pokorny et al., 2019). A day-night asymmetry in ejecta production was attributed to either a temperature dependence of the ejecta yield (ejecta mass normalized by the impactor mass; Janches et al., 2018) or the existence of an additional high-speed meteoroid population, betameteoroids, that exhibit hyperbolic trajectories preferentially impacting the dayside of the dawn terminator (Bernardoni et al., 2021; Horányi et al., 2015; Szalay et al., 2020). This inferred population would contribute ~0.5–0.75 tons/day, which is a considerable fraction of the better-established meteoroid populations from short and long-period comets (e.g., Janches et al., 2018; Pokorny et al., 2019). Later, it was suggested that the higher yield observed on the dayside by LDEX might have been caused by incident solar wind ions reflected as neutral energetic atoms (Bernardoni et al., 2023).

The studies conducted after Janches et al. (2018) and Pokorny et al. (2019) disregarded the possibility of a thermal dependence on the ejecta mass, since no physical mechanism had been proposed to link the temperature of the lunar surface to its impact ejecta yield (Bernardoni et al., 2021). However, it is known that the crater formation and size caused by km-size impactors depend on the temperature of the soil (Potter & Head, 2017). Hence, thermal variations across lunar regolith might drive the dayside-nightside asymmetry in ejecta production measured by LDEX.

Our current understanding of micro-meteoroid impact effects remains constrained by difficulties in replicating these high-energy phenomena under laboratory conditions. The impactors' high velocities (10–60 km/s, depending on the population (Marchi et al., 2005, Robertson et al., 2021) make present-day experiments rely on pulsed laser irradiation to simulate the short-duration, high-temperature events associated with micro-meteoroid impacts (Schmidt et al., 2019; Thompson et al., 2020). While reproducing the temperature spikes caused by impacts, such a method does not induce any deformation of the surface due to shocks (Schmidt et al., 2019). Moreover, the experimental pulses used in laboratory simulations have a maximum energy threshold that falls significantly short of the actual energy carried by meteoroids larger than approximately 0.1 µm in diameter. Similarly, dust accelerators encounter difficulties in achieving hypervelocity speeds for larger particles (>1 µm) (Shu et al., 2012; Thomas et al., 2017).

Numerical modeling provides essential insights into the physics of micro-meteoroid impacts that remain beyond the reach of current experimental capabilities. In this work, we use shock physics modeling to quantify the ejecta mass of micro-meteoroid impacts, along with the velocity and angular distribution of ejecta at the Moon as a function of the lunar regolith's porosity and temperature. One objective of this study was to investigate whether the asymmetry in the lunar dust cloud observed by the LDEX measurements could be attributed, at least in part, to surface temperature effects. This work also aims to determine if the dust measurements around airless bodies, such as the Moon, could be used as an indirect proxy for regolith porosity. In the impact simulations presented here, the regolith effect is studied over a range of porosities suggested by previous studies (Szabo et al., 2022; Verkercke et al., 2023; Veverka et al., 1988). We outline our methods in Section 2, present results and discussion in Section 3, and then summarize conclusions in Section 4.

## 2. Methods

To simulate micro-meteoroid crater formation and impact-generated ejecta, here we use the iSALE-2D shock physics code (Amsden et al., 1980; Collins et al., 2004; Wünnemann et al., 2006), which has been extensively
<table><caption>**Table 1**

*Parameters Defining the Characteristics of the Regolith and of the Micro-Meteoroid*</caption><thead><tr><th>Parameter</th><th>Value (Units)</th></tr></thead><tbody><tr><td><strong>Regolith</strong></td><td></td></tr><tr><td>φ</td><td>40%, 60%, 80%</td></tr><tr><td>ρ</td><td>2.86 (g/cm³)</td></tr><tr><td>c</td><td>8.37 (kPa)</td></tr><tr><td>T</td><td>90, 385 (K)</td></tr><tr><td>T<sub>m</sub></td><td>1,400–1,500 (K)<sup>a</sup></td></tr><tr><td><strong>Impactor</strong></td><td></td></tr><tr><td>d</td><td>100 (μm)</td></tr><tr><td>φ</td><td>10%</td></tr><tr><td>ν</td><td>40 (km/s)</td></tr></tbody></table>

*Note:* φ is the porosity, ρ is the density, c is the cohesion, T is the surface temperature, T<sub>m</sub> is the melting temperature of the regolith, and d and ν are respectively the diameter and the velocity of the impactor. <sup>a</sup>Chen et al., 2017.

utilized in modeling impacts and collisional processes. Regarding the model setup, the computational domain consists of two materials: a micro-meteoroid projectile and a lunar target. The projectile is approximated to be a regolith spherical grain with a porosity of 10%. The mesh resolution was set to 10 cells per projectile radius (CPPR), with a cell size of 5 µm. The regolith target is modeled as a half-space with axial symmetry, measuring 3,000 × 4,000 µm in radial and vertical directions, respectively. The right, bottom, and top boundaries are assigned to free-slip, no-slip, and outflow boundary conditions, respectively. The basalt ANEOS equation of state (EOS) and the Lundborg strength scheme (Lundborg & Stitzel, 1968) are employed for both the projectile and target. To model the porosity compaction of the simulated materials, the ε-α model (Wünnemann et al., 2006) is utilized. The lunar regolith is composed of very fine grains that form a very porous structure (>75%) (Hapke & van Hoen, 1963; McKay et al., 1991; Szabo et al., 2022, 2023; Verkercke et al., 2023; Veverka et al., 1988). It has been shown that the surface porosity can reduce the crater growth when formed by a large impactor (Wünnemann et al., 2006). To the best of our knowledge, this effect has not yet been investigated for micro-meteoroid impacts on the lunar surface. Three values of the regolith porosity φ were tested, with φ = 40%, 60%, 80%, covering the range proposed by previous studies (Hapke & van Hoen, 1963; McKay et al., 1991; Veverka et al., 1988). Notably, the detailed

porosity settings of the projectile and the target (e.g., transition distension, compaction efficiency, and sound speed ratio) follow the corresponding values reported for a regolith material as a function of the porosity (Luther et al., 2022). The regolith temperature could influence crater formation and the resulting yield ejected during a micro-meteoroid impact. This study aims to determine if such influence is important enough to induce a noticeable yield difference as observed by the LDEX. To study the potential thermal influence on the crater formation and ejecta, we simulated three porosities defined above while imposing a surface temperature of either 385K, the typical dayside surface temperature of the Moon, or a typical lunar surface temperature before sunrise of 90K (Williams et al., 2017). The type of lunar soil considered also affects crater formation, and this work only considers regolith in Mare regions, assuming that the surface is composed of basalt, with a density ρ of 2.86 g/cm³ and a melting temperature T<sub>m</sub> of 1,435 K (Greeley & Spudis, 1978).

The velocity of the impactor is fixed at 40 km/s, close to the average velocity of the Halley Type and Oort Cloud Comet micro-meteoroids (HTC and OCC, respectively) impacting the Moon near dawn (Pokorny et al., 2019), as those will contribute the most to the ejecta (Borin et al., 2016; Marchi et al., 2005; Pokorny et al., 2019), with an impact angle simplified to be fully vertical. Regarding the impactor, we consider a single size of 100 µm diameter. Pokorny et al. (2019) found that the mass distribution of long-period comets (LPC) arriving on Earth is dominated by smaller grains, with characteristic sizes for HTC being approximately 100–200 µm and for OCC around 20 µm. Given that these two meteoroid populations dominate the ejecta rate, selecting a 100 µm particle size for our simulations is a representative and practical choice. The Poynting-Robertson drag is neglected. Table 1 summarizes the parameter matrix explored in this study.

In our iSALE-2D simulations, Lagrangian tracers were placed to monitor the mass, velocity, angle, and positions of the particles ejected from the crater. These tracers are uniformly distributed across the computation domain and tracked over time through the iSALE-2D model. In this approach, ejected tracers are identified as they cross a flat “ejecta line,” typically positioned one projectile diameter (e.g., 100 µm here) above the initial target surface (Luther et al., 2018; Raducan et al., 2019). The total ejecta yield is then computed by summing the mass of all ejected material across the simulated scenarios.

# 3. Results

## 3.1. Porosity Influence on the Impact Ejecta

A 100 µm micro-meteoroid launched normally toward the surface at 40 km/s was modeled for the three values of the regolith porosity φ presented in Section 2. Figure 1 presents the cumulative ejecta mass normalized by the impactor's mass as a function of the ejecta velocity normalized by the impactor velocity at 10 µs post-impact. All
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Cumulative distributions of the ejecta mass normalized by the impactor mass (yield) as a function of the normalized ejecta velocity, for a vertical impact on the lunar regolith at an initial temperature of 385 K, and at *t* = 10 µs post-impact. The three different curves (from top to bottom: blue, red, green) stand for the different regolith porosities (respectively φ = 0.4, 0.6, 0.8).</figcaption></figure>

three impacts occur on a regolith at 385 K. The blue, red, and green curves are the cumulated ejecta mass for the three porosities tested, 0.4, 0.6, and 0.8. One can observe that, with increasing porosity, the total mass ejected by the impact decreases. This dependency shows that an increase in porosity by a factor of 2 (from φ = 0.4 to 0.8) reduces the total ejecta mass by less than 2 (from about 850 impactor masses ejected to 550 impactor masses ejected). This can be attributed to a “cushioning” effect due to the regolith porosity, which would compress the regolith without ejecting matter. Figure 2 presents the total ejected mass of a regolith at 385 K as a function of its porosity (red line) at *t* = 10 µs, which exhibits a linear dependency. This behavior is expected as this tendency was observed for larger impactors on porous media (Greeley & Spudis, 1978; Housen & Holsapple, 2003).

The mean ejecta velocity is higher for low porosity regolith compared to very porous cases. Figure 3 compares the distributions of ejecta angles with respect to the surface for three porosities, as a function of the ejecta velocity and the pre-impact surface temperature. It is evident that higher porosity favors lower ejection velocity. However, the high velocity ejecta only represents a negligible fraction of the total ejecta mass, as shown in Figure 3. The ejecta angular distribution from the regolith with a porosity of φ = 0.8 shows a mean angle slightly closer to the normal compared to φ = 0.4. Changes in crater formation due to variations in porosity result in different ejection directions. Laboratory experiments also showed that higher porosity reduces the amount of material ejected beyond the crater rim (Housen & Holsapple, 2003), indicating slower ejecta with a more normal ejection angle. This is corroborated by the mean velocity and ejecta angle, represented by the black symbols in Figure 3.

## 3.2. Temperature Influence on the Ejecta

Previous research disregarded the possibility of a thermal influence on the ejecta of an impact, as no physical mechanism has been proposed to link the regolith temperature to its impact ejecta yield (Bernardoni et al., 2021). However, the crater formation and the ejecta are mostly constrained by the energy transferred to the surface (Grieve & Cintala, 1992; Stoppar et al., 2014). The equations of states defining the transition between solid and melt depend on the reference temperature before impact (Cintala & Grieve, 1998; Collins et al., 2013). Therefore, the pre-impact surface temperature may influence the crater formation and the ejecta produced. In particular, the difference between the initial temperature and the melting point represents the amount of energy needed to change the surface state, which in turn affects both the crater formation and ejecta properties.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Normalized total ejecta mass as a function of the porosity for a regolith at 90 K (in blue) and 385 K (in red), at $t = 10$ µs post-impact.</figcaption></figure>

To study the potential influence of the temperature on the ejecta and the crater formation, we simulated the same three cases as above, setting a surface temperature of 385 K. Figure 2 presents the total ejecta mass as a function of the porosity for three different regolith porosities (0.4, 0.6, 0.8) and two different temperatures, with a regolith at 90 and 385 K. These parameters are summarized in Table 1. For the three porosities tested, the increase in the surface temperature induces a slight increase in the total ejecta mass. This total ejecta mass increase ranges from 6.5% to 8.3% of the 90 K surface temperature total ejecta mass, for the respective porosities of $\rho = 0.4$ and 0.8. Such an increase can be attributed to the larger surface temperature, which reduces the energy required by the impactor to induce melting (Potter & Head, 2017). More specifically, this modest enhancement in ejecta is associated with thermal softening at higher temperatures, where higher temperatures cause the material to weaken internally, which reduces the yield strength of the regolith (Supporting Information S1). A lower strength tends to allow for a larger ejecta mass, resulting in the difference observed in Figure 2. Moreover, the absolute difference in ejecta mass between the two surface temperatures increases with increasing porosity. We also observed that the crater size increases as porosity and temperature increase. For a 100 µm meteoroid impacting a 90 K regolith, the ratio between the crater diameter and the impactor diameter increases from 21.5 to 24.3 in the porosity range tested here. For a 385 K regolith, this ratio increased from 21.9 to 24.9 on the same porosity range.

Figure 3 presents the ejecta angle as a function of the ejecta velocity out of the three regolith samples at 90 and 385 K. The highest temperature clearly shifts the velocity distribution to slightly higher values. This effect is more pronounced for the regolith with the highest porosity. At increased initial surface temperatures, thermal softening facilitates the ejection of material more easily with larger velocities (Potter & Head, 2017). The angular distribution is shifted to angles closer to normal as the surface temperature increases, as shown by the mean of the angular distributions in Figure 3. Here, this effect is more pronounced for the most porous regolith. This is likely because the initial temperature influences the crater formation, which in turn affects the ejecta trajectories by altering the crater's profile due to the surface's initial properties (Housen & Holsapple, 2003). However, it is interesting to see that the high velocity ejecta is only accounting for a very small fraction of the total ejecta mass.

It is important to note the correlation between the angular direction and speed of the ejecta seen in Figure 3. This relationship has not been considered in the analysis of LDEX data via plume emission models, which assumed the independence of speed and emission angle (Bernandoni et al., 2021).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Ejection angle as a function of the ejection velocity for (a, b) $\varphi = 0.4$, (c, d) $\varphi = 0.6$, and (e, f) $\varphi = 0.8$, at $t = 10$ µs post-impact. The left panels show the distribution of the ejecta out of a cold regolith, while the right panels show the distribution of the ejecta out of a cold regolith. The color bar shows the density of tracers. The red symbols show the mean ejection angle and velocity for the respective distributions.</figcaption></figure>

### 3.3. Comparing to LDEX Measurements

From Figure 1, one can observe that the normalized ejecta mass can vary from about 400 to 800, depending on the porosity and the temperature. According to LDEX measurements, Szalay et al. (2018) showed that the expected ejecta mass by micro-meteoroid impacts at the Moon was around 10, while Pokorny et al. (2019) suggested a ratio close to 1, much lower than what is predicted by our model. However, to be detected, the ejected grains must reach the altitude at which the LADEE orbits (between 3 and 250 km; Horányi et al., 2014). As the observations of the permanent asymmetric cloud are mostly reported for altitudes above 10 km, we will use this lower limit as the altitude a grain must reach in our simulation to be detected. Additionally, we tested two other spacecraft altitudes at 25 and 100 km to study the bias that the altitude can introduce in the measurements. These altitudes were chosen to better sample the altitude range of the spacecraft and as Horányi et al. (2015) analysis mostly used measurements above 25 km.

The maximum height reached by a grain depends only on its ejection angle and its initial velocity, assuming only lunar gravity and no additional effects such as radiation pressure. By only accounting for the ejecta that can reach LADEE's orbit at 10 km, our simulation predicts a detectable normalized ejecta mass that ranges between 27 and 73, depending on the regolith's temperature and porosity, representing only about 3%–9% of the total ejected mass. Figure 4 presents the detectable yield as a function of the porosity for each temperature. These results show that the ejecta mass resulting from an impact on a more porous regolith is more consistent with the ejecta mass derived from LDEX (Szalay et al., 2018). This is valid for the two temperatures tested and would be coherent with a highly porous surface as found at the Moon (McKay et al., 1974, 1991). For the considered impactor, the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Simulated yield that would reach LADEE's orbit at 10 km, as the observations of the permanent asymmetric cloud are mostly reported for altitudes above 10 km. The yield normalized is shown as a function of the porosity for an impact on a regolith at 90 K (blue) and at 385 K (red).</figcaption></figure>

maximum depth reached by the crater is about a millimeter. The top layer of the lunar regolith is expected to have a high porosity of about 90% as it would be structured as a “fairy castle” (Hapke & van Hoen, 1963; Szabo et al., 2022, 2023; Verkercke et al., 2023). If extrapolated to the linear trend exhibited in Figure 4, this would result in detectable normalized ejecta mass between 20 and 30 for a $\varphi = 0.9$, respectively, for surface temperatures of 90 and 385 K. This highlights that the impact yield could be used as an indirect observation of the regolith structure. When considering a 25 km altitude for the spacecraft, the yield decreases to a range of 11–40 (again depending on porosity and temperature). This represents only 2%–4% of the total mass ejected. Finally, when considering a 100 -km altitude for the spacecraft, the detectable yield decreases to 2–14, representing now only ~0.4%–1.5%. These yields agree with the yields suggested by Szalay et al. (2018) and Pokorny et al. (2019). Here, we do not account for any observational bias that could have reduced the ejecta mass derived by LDEX measurements (such as the limit of mass and speed to trigger detection, varying altitude, and others). This could further reduce the detectable yield but would still be greater than the 0.0025% of dust that LDEX measured, as reported by Pokorny et al. (2019).

Additionally, the comparison between Figures 2 and 4 shows that more than 95% of the total ejecta is undetected by LADEE at 10 km altitude. This is due to the large population of low-velocity ejecta that never reach the spacecraft's orbit. Figure 5 presents the simulated total ejecta velocity distribution function for a surface temperature of 385 K and a porosity $\varphi = 0.4$ (solid blue) and $\varphi = 0.8$ (dashed red). Their resemblance further shows that most of the ejecta has a low velocity and that the high velocity ejecta only represents a negligible fraction of the total ejecta mass, no matter the porosity of the regolith. The green curve shows the velocity distribution derived by Horányi et al. (2015). The solid and dashed black lines represent the velocity distributions reaching the LADEE orbit when considering an altitude threshold of 10 and 25 km, respectively, and normalizing these distributions by the maximum of the green curve. The total distribution predicted by our model is very different from the velocity distributions derived by Horányi et al. (2015) or Szalay et al. (2018) from 140,000 impacts detected by LDEX. Modeled ejecta speeds peak at 60 m/s, while Horányi et al. (2015) and Szalay et al. (2018) distributions peak at 450 m/s and 600 m/s. Regarding the predicted distribution reaching LADEE's orbit at 10 km, the peak is at 260 m/s. However, Horányi et al. (2015) analysis mostly used measurements above 25 km. When considering such a threshold, the peak of the velocity distribution is at 420 m/s, very close to Horányi et al. (2015) distribution, with a shape that is also similar for velocities above 400 m/s. For the slower ejecta (<400 m/s), one should keep in mind that the velocity distributions derived from LDEX observations assume that the dust motion
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Ejecta velocity distribution for a regolith of $\varphi = 0.4$ (blue) and $\varphi = 0.8$ (red, dashed) at a temperature of 385 K. The green curve is the ejecta velocity distribution derived from LDEX measurements (Horányi et al., 2015). The dashed black and solid black and gray curves show the distribution seen at LADEE's orbit for a regolith of $\varphi = 0.8$ with respective altitude thresholds of 10, 25 and 100 km. These distributions are normalized by the maximum value of the green curve.</figcaption></figure>

is 1D, so the angular distribution is decoupled from the speed distribution. Horányi et al. (2015) thus certainly overestimated the contribution of ejecta leaving the surface normally, whereas our simulation predicts no normal ejecta, which prevents the slower ejecta from reaching LADEE in our simulation, and makes our predicted distributions flat at these low velocities. When considering a 100 km altitude, the velocity distribution peaks at around 700 m/s, which does not match the distribution derived from LDEX (gray solid line in Figure 5). In our models, we also neglect any interaction of impact plumes with dust. Vapourized projectile and target material, which can reach high velocities, can interact with dust. Such an interaction can significantly modify the dust dynamics (e.g., Artemieva et al., 2013; Luther et al., 2019). Interactions between the dust and the ejecta would lead to collision and energy loss, thus reducing the velocity of the ejecta.

LDEX measurements showed a permanent asymmetry of the dust cloud around dawn, with a slight enhancement of the dust density over the dayside with respect to the nightside (Horányi et al., 2015). This enhancement represents 10%-25% density increase on the dayside relative to the nightside. Figure 2 presents our model's prediction of an 8.3% increase in total ejecta mass for warmer regolith compared to colder regolith, which is slightly below the LDEX measurements. Additionally, Figure 4 shows that accounting for the different velocity distributions from a hot and cold surface and for an LDEX altitude of 10 km can increase the detectable yield over warmer surface elements by 10%-20%, respectively for $\varphi = 0.4$ and 0.8, which is in the range of the dayside enhancement observed by LDEX. The increase in detectable yield over warmer surfaces can be as high as 25% when considering a 100 km altitude for LADEE. The reason for this increase with altitude can be attributed to the slightly faster and closer to the surface normal ejecta for hotter regoliths, especially for the fast ejecta tail seen in Figure 3. While this yield increase due to temperature is in the same order of magnitude as the 80% day-side enhancement suggested by Pokorny et al. (2019), our model predicts a smaller enhancement. This enhancement could also depend on surface composition, impactor size and speed, or local topography, but such analysis falls out of the scope of this study. Thus, our model suggests that the temperature difference between the day and night sides of the Moon could account for the observed asymmetry in the dust cloud, negating the need for an additional population of impactors with much larger ejecta yields (Bernandoni et al., 2021; Szalay et al., 2021).
4. Conclusions

Micro-meteoroid impacts play an important role in shaping the lunar regolith structure as well as in the formation of the Moon's exosphere (Gault et al., 1974; Leblanc et al., 2022; Marchi et al., 2005; Szalay & Horányi, 2016). Upon impact, material is ejected and forms a dust cloud, as observed by LDEX (Szalay & Horányi, 2016). The observations performed by LDEX showed that this dust cloud was permanent and peaked in the leading hemi-sphere near dawn. This dust cloud is asymmetric as a dayside enhancement of 10%–25% was observed during LDEX operation (Bernardoni et al., 2021; Horányi et al., 2015). Such an asymmetry was attributed to β-mete-oroids impacting the dayside (Bernardoni et al., 2021; Horányi et al., 2015). The possibility of a thermal influence on the ejecta yield was considered but largely dismissed as no physical mechanism was proposed to explain this link (Bernardoni et al., 2021). Using the iSALE-2D hydrocode, we performed a parametric study on the regolith properties, including the surface temperature, that could influence the ejecta created by a micro-meteoroid impact.

Consistent with previous work (e.g., Housen & Holsapple, 2012), our results confirm that the porosity of the regolith has a large influence on the ejecta mass. For an impactor of 100 µm and a velocity of 40 km/s, a porosity increase of the regolith induces a decrease in the total ejecta mass, which is expected from laboratory mea-surements. In particular, a porosity increase from $\varphi = 0.4$ to $\varphi = 0.8$ results in a yield decrease from ~850 to 550, as shown in Figure 1. The porosity also influences the ejecta velocity and angular distributions. It appears that higher porosities induce lower mean ejecta speed with an angular distribution that is more directed toward the surface normal, while lower porosities induce higher mean velocities and more glazing angles for the ejecta. This is attributed to the crater formation that is highly influenced by the porosity of the regolith (Housen & Holsap-ple, 2003). A larger porosity seems to cushion the impactor as the regolith can be compressed more easily, changing the crater's shape. The crater's form then influences the trajectory of the ejecta.

Results indicate that the ejecta mass, velocity, and angular distributions are influenced by the pre-impact surface temperature. For the same impactor mass and speed, the ejecta mass on a surface at 385 K is about 6.5%–8.3% larger than the ejecta mass of a surface at 90 K, as shown in Figure 2. This modest increase can be attributed to the thermal softening via reduced yield strength of the lunar regolith. The pre-impact temperature also influences the ejecta velocity and angular distributions, which thus differ between the day and the night side. In the case of higher initial temperatures, our model indicates that the ejecta have larger velocities with more grazing angles with respect to the surface, while lower temperatures induce slower ejecta oriented more normally to the surface (Figure 3).

Our simulations clarify the correlation between ejecta characteristics and surface conditions, thus revealing in-sights into the generation of the lunar dust cloud. The day side enhancement measured by LDEX can be explained by combining two effects: a marginally larger ejecta mass, combined with a larger fraction of the ejecta speed distribution reaching LADEE's orbit over warm regolith. This explanation accounts for the asymmetry without necessitating another population of meteoroids.

LDEX found unexpectedly low ejecta yields, of the order of 10 (Bernardoni et al., 2021; Szalay et al., 2019). Our simulations offer a potential explanation. We suggest that the ejecta speed distribution derived from LDEX data overestimates the fraction of ejecta mass reaching the LDEX orbit. By using the modeled speed distribution from iSALE and considering the fraction that reaches LDEX altitude, we discovered that the excess total ejecta mass measured by LDEX (~10) closely aligns with the ejecta yield computed in our simulation (<30) for regolith with porosity $\varphi > 0.8$ (Figure 4). This detectable yield also decreases with altitude. The alignment not only validates our modeling approach but also offers a refined understanding of ejecta dynamics influenced by regolith porosity. This insight may contribute to a more accurate interpretation of the lunar dust cloud observations. Furthermore, these findings suggest a need for revisiting current assumptions in meteoroid impact studies, emphasizing the intricate relationship between ejecta yield and surface properties.

The numerical approach adopted in this study can enhance the understanding of ejecta processes beyond the limitations of laboratory experiments, and should be extended to include various regolith compositions associated with different regions on the Moon, impactor sizes, and speeds. Relevant sizes could be as small as 0.5 µm (β-meteoroids) to 20 µm (OCC meteoroids) and as large as centimeter-sized grains to include particles from meteoroid showers. Relevant velocities would be 10–100 km/s to include β-meteoroids and Mercury impactors in preparation for measurements from the BepiColombo mission. Current experimental measurements of ejecta yield were obtained for a good set of sizes (34.2 µm–5.42 mm) but were constrained to low-speed impacts of
<12 km/s (Koschny & Grün, 2001). Janches et al. (2018) and Pokorny et al. (2019) concluded that matching LDEX observations with Earth data for short- and long-period cometary material at 1 AU necessitates either a steeper yield dependence on impactor speed or a weaker dependence on impactor mass. Extending the yield function with numerical simulations to more realistic mass and speed parameters could address key knowledge gaps and refine our understanding of ejecta yield as a function of meteoroid properties.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

Data are openly available at Verkercke and Senel. (2025). iSALE-2D Simulation Outputs—Micro-meteoroid impact on lunar regolith [Dataset]. In Journal of Geophysical Research: Planets. Zenodo. https://doi.org/10.5281/zenodo.17097787. The iSALE-2D code is distributed on a case-by-case basis to academic users in the impact community, strictly for non-commercial use. Scientists interested in using or developing iSALE may apply to use it via https://isale-code.github.io/access.html.

## Acknowledgments

S.V. acknowledges the support of ANR, France of the TEMPETE project (Grant ANR-17-CE31-0016) and the support of CNES, France for the BepiColombo mission. CBS and LKR acknowledge support from the Research Foundation Flanders (FWO) with Grant numbers: 12AM624N to CBS, and 1295024N to LKR. This research was supported by the International Space Science Institute (ISSI) in Bern, through ISSI International Team project #616 "Multi-scale Understanding of Surface-Exosphere Connections (MUSEC)." We gratefully acknowledge the developers of the iSALE-2D shock physics model (https://github.com/isale-code), including G. Collins, K. Wünnemann, D. Elbeshausen, T. Davison, B. Ivanov, and J. Melosh.

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