Lunar Dust Fountain Observed Near Twilight Craters

Abstract

Lunar horizon glows observed by the Apollo missions suggested a dense dust exosphere near the lunar terminator. But later missions failed to see such a high-density dust exosphere. Why the Apollo missions could observe so large number of dust grains remains a mystery. For the first time, we report five dust enhancement events observed by the Lunar Dust Experiment on board Lunar Atmosphere and Dust Environment Explorer mission, which happen near a twilight crater with dust densities comparable to the Apollo measurements. Moreover, the dust densities are larger on the downstream side of the crater and favor a higher solar wind temperature, consistent with an electrostatic dust lofting from the negatively charged crater floor. We also check the Apollo observations and find similar twilight craters, suggesting that the so-called dust exosphere is not a global phenomenon but just a local electrified dust fountain near twilight craters.

Key Points

• We find five dust enhancements events near twilight craters, with the in situ dust measurements of the LADEE mission
• The dust enhancement events can be caused by an electrostatic dust lofting from the leeward wall of a twilight crater
• The lunar horizon glow observed in the Apollo missions can be also related to a twilight crater

Plain Language Summary

With in situ dust measurements, we find that a shadowed crater near the terminator can dramatically change the surface electrical environment and bring a dense dust cloud surrounding the crater, which should be carefully assessed by engineers for future lunar explorations. Moreover, our findings have the general implications in studying the dust environment near large topographic features (mountains and deep craters) of all kinds of airless bodies.

1 Introduction

Lunar dust has been regarded as a major problem for lunar surface explorations since the Apollo era, as these very abrasive and fine dust grains are harmful for both the instruments and the human beings (Colwell et al., 2007). Lunar horizon glows (LHGs) observed in the Surveyor (Rennilson & Criswell, 1974) and the Apollo missions (McCoy, 1976; McCoy & Criswell, 1974) provided direct evidence for lunar dust levitation and transport near the terminator. LHG was interpreted as forward scattered sunlight from a cloud of lofted dust grains, with an altitude depending on the radii of the dust grains (r_d) that is less than 1 m for the Surveyor observations (r_d ≈ 5) and tens of kilometers for the Apollo observations (  μm). The question is how these dust grains can be launched from the lunar surface. Proposed mechanisms include electrostatic lofting and meteoroid or micrometeoroid impact. The former was preferred in the Apollo era, since the secondary ejecta from meteoroid impacts was too weak to explain the duration (up to 2.5 hr for the Surveyor observations; Rennilson & Criswell, 1974) and the dust abundance (about 10⁴ m⁻³ for the Apollo observations), (McCoy, 1976) of the observed LHGs. However, later studies found that the electrostatic force was insufficient to overcome the cohesive force (Hartzell & Scheeres, 2011) and that there was no correlation of the LHG with solar UV and solar plasma conditions (Glenar et al., 2011). Hence, it was suggested that the high-density dust cloud could be caused by a sporadic meteoroid impact with a saltation-like process (Glenar et al., 2011). Recent remote sensing measurements by Clementine (Glenar et al., 2014) and Lunar Reconnaissance Orbiter (Feldman et al., 2014) showed a very tenuous lunar dust exosphere with a dust density of less than 1 m⁻³. A high-density dust cloud was also absent in the in situ measurements by the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission (Szalay & Horányi, 2015; Xie et al., 2016), where the tenuous asymmetric dust cloud was interpreted as the impact ejecta of cometary grains (Horányi et al., 2015). As a result, it seems that the impact process is more important than the electrostatic process.

On the other hand, the electrostatic mechanism has been improved in recent years. Stubbs et al. (2006) proposed a dynamic fountain model, in which the like-charged dust grains with radii of 0.01–0.1 μm could be accelerated by the electrostatic force in the sheath region and reach maximum altitudes of 0.1–100 km via a ballistic motion. Such a model can explain the high-altitude dust exosphere observed in the Apollo missions. Farrell et al. (2010) suggested that the large-scale topographic features (mountains or craters) near the terminator could behave as an obstacle for the incoming solar wind and a strong negative potential would be established at the leeward wall, which might bring a locally enhanced dust activity near the obstacle. Wang et al. (2016) proposed a patched charge model, in which some of the secondary electrons could be reabsorbed by the surface patches inside the microcavity between neighboring dust particles, resulting in a very large charge on the cavity-side surface patch. Consequently, the particle-particle repulsive force became the dominant electrostatic force that could loft micron-sized dust particles to a height of more than 10 cm, which can explain the low-altitude LHG observed in the Surveyor missions. Recently, we found that the upper height of dust deposition on the rocks near the Chang′E-3 landing site was about 28 cm (Yan et al., 2019), consistent with the Surveyor observations. Moreover, no dust deposition could be visibly recognized on the flat top of these rocks, suggesting that the electrostatic dust lofting should be more important than the meteoroid impact near the lunar surface.

Currently, the reason for lunar dust transport is still under debate. The meteoroid impact mechanism has difficulty in explaining the long duration of LHGs observed by the Surveyor missions. Additionally, it is difficult to link the significant LHG seen by the Apollo 17 astronauts near the sunset terminator to a meteoroid stream, as the meteoroid stream should be more likely to meet the surface near the sunrise terminator. The electrostatic lofting mechanism needs to explain how the electrostatic force can overcome the cohesive force. It is found that the cohesive force can be greatly reduced for irregular-shaped and hydroxylated grains (Kimura et al., 2014). Meanwhile, the electrostatic force can be significantly enhanced for some local charging effects, such as the supercharging effect proposed by De and Criswell (1977) and the patched charge effect suggested by Wang et al. (2016). It is interesting that both the hydroxylation of grain surfaces and the strong electric field are more likely to be found near the terminator, implying that the electrostatic lofting of dust grains is more easily achieved near the terminator (Kimura et al., 2014). Once these small grains leave the surface, they feel the large-scale potential determined by the solar wind surface and their motions can be described by the dynamic fountain model (Stubbs et al., 2006). For a twilight crater, there is a miniwake downstream the leeward crater wall (Farrell et al., 2010), where the electron density is greatly reduced but the electron temperature is enhanced due to the filtration of low-energy electrons by the strong negative potential in the wake (Halekas et al., 2005). As a result, we can expect an increase in secondary electron production in the crater, especially at the leeward wall where the electron temperature can be enhanced up to 100 eV. According to the patched charge model (Wang et al., 2016), a larger secondary electron production will bring a more active dust transport for micron-sized grains at the leeward wall. The saltation caused by the falling micron-sized grains may help to release some submicron grains from the surface. These submicron grains can get up to tens of kilometers high because of the strong negative potential in the crater, resulting in a local dust cloud near the crater, which may be the reason for the infrequent LHG observed in the Apollo missions. In this work, we search for evidence of enhanced dust activity near a twilight crater (defined as a shadowed crater near the terminator), with both the LADEE and the Apollo measurements.

2 Dust Fountain Observed by LADEE

The Lunar Dust Experiment (LDEX) onboard the LADEE spacecraft is an impact ionization dust detector, which detects dust grains with radii larger than 0.3 μm as individual impact events and dust grains with radii smaller than 0.3 μm as an integrated current (Hornáyi et al., 2014; Szalay & Horányi, 2015). However, low-energy ions from picked-up lunar ionospheric ions (Poppe et al., 2016), the backscattered solar wind (Xie et al., 2016) and the negatively charged spacecraft (Xie et al., 2017) can also contribute to the integrated current. Specifically, the picked-up ions are more likely to appear on the lunar dayside, while the spacecraft becomes negatively charged in the shadowed region of the lunar night side. Here we focus on examining the LDEX current peaks observed near the terminator, which may be related to a local dust cloud caused by the electrostatically dust lofting from a shadowed crater. We select the measurements taken in a solar zenith angle range from 80° to 100°, which corresponds to ±10° about the terminator, where the measured currents should be less affected by either the picked-up ions or the spacecraft charging. We use the MATLAB findpeaks function with minimum peak distance of 10 to find the peaks of LDEX current. Meanwhile, we use the plasma measurements from ARTEMIS mission (Angelopoulos, 2011) to select the peaks measured in the solar wind. It was found that the LDEX current favored a smaller angle between the solar wind velocity and the lunar surface normal vector (Xie et al., 2016) and the current could be enhanced up to 1.9 times when the angle changed from 90° to 60°. As a result, we only consider the current peaks with a value 1.9 times larger than the value of the ambient current trough, to exclude the possible current enhancement caused by the change in local topography. In this way, we have identified 1,357 current peaks on 855 out of 1,491 orbits.

It is found that most current peaks can be caused by occasional solar wind disturbances. As shown in the Figure S1 in the supporting information, any enhancement in solar wind density or in the angle (θ_E) between LDEX normal and the solar wind convection electric field can bring a current enhancement, which is consistent with the previous work done by (Xie et al., 2016) who found that the LDEX current could linearly increase with solar wind density and the current could be significantly enhanced when θ_E > 90°. In addition, some current peaks may be associated with local magnetic anomalies. Lunar magnetic anomalies are thought to be able to prevent the incoming solar wind and reduce the solar wind flux on the lunar surface (Deca et al., 2016; Xie et al., 2015). As a result, they can change the flux of backscattered solar wind and lead to a disturbance in the LDEX current, as discussed by (Walker et al., 2017). A 450° spherical harmonic model of lunar crustal field has been recently developed by Tsunakawa et al. (2015), with a high spatial resolution of about 6 km at lunar surface. With such a crustal field model, we can distinguish whether a current peak is related to a strong magnetic anomaly. An example is shown in Figure S2, where the two current peaks observed near 24 January 2014 02:18 UTC show good correlation with the locations of strong magnetic anomalies. Additionally, there are a few current peaks might be caused by an enhanced micrometeoroid bombardments. As mentioned above, dust grains with radii larger than 0.3 μm are detected as individual impact events , which is efficient to capture a meteoroid stream (Szalay et al., 2016). Consequently, we use the combined data from both the LDEX current and the LDEX impact event to judge whether a current peak is caused by a meteoroid stream. An example is shown in Figure S3, where the current peak coincides with a burst of micrometeoroid impacts that possibly results from a meteoroid stream. We check all the 1,357 identified current peaks and find that about 90% of them can be related to the solar wind disturbances, local magnetic anomalies, or micrometeoroid bombardments. The remaining current peaks may be caused by a random or small-scale solar wind disturbance that is observed by LDEX but not seen by the ARTEMIS spacecraft, or some other reason such as a shadowed crater. We further check the locations of these current peaks and find that 17 of them are close to a distinct crater with a sharp wall and a diameter larger than 40 km. To eliminate the possible effect from a random solar wind disturbance, we select the current peaks can be repeatedly observed near the same crater. In this way, we finally obtain five current peaks (or events) close to two twilight craters, in which an obvious current peak can be found with neither SW disturbances (Figure S4) nor magnetic anomalies and micrometeoroid impacts.

As shown in Figure 1, Events 1–3 happen near the Copernicus crater, and Events 4–5 happen near the Plinius crater. For all events, the LADEE spacecraft moves westward across the terminator, similar to the direction of solar wind flow. We estimate the number density of dust grains with a characteristic radius of  μm and a mass density of  kg m⁻³ (Text S1). It is found that the number densities are ∼10 m⁻³ for Events 1–3 (Figure 1a) and ∼100 m⁻³ for Events 4–5 (Figure 1b). Known from Table S1, the spacecraft altitudes for Events 1–3 is about 60 km, which are higher than the altitudes of about 40 km for Events 4–5. In addition, we find that the number densities favor a higher SW electron temperature (Table S1), and higher dust densities are found on the downstream side of the crater (Figures 1a and 1b). Theoretical models have found that the leeward wall of a shadowed crater could receive extra SW electrons and become strongly negatively charged (Farrell et al., 2010; Zimmerman et al., 2011), and then the like-charged dust would be emitted antisunward from the crater surface to balance the electron current, which might be the reason for the asymmetric dust cloud observed by LADEE. We show the peak dust densities of the five events as a function of altitude in Figure 1c and compare them with the model densities inferred from the Apollo observations (Glenar et al., 2011; McCoy, 1976). It can be found that these five dust densities show an exponential decay with the altitude, consistent with a gravity-constrained altitude distribution. Moreover, the five dust densities are comparable to the densities predicted by the Apollo measurements, though they are scattered around the Apollo results within a factor of two, which implies that the dust clouds observed by LADEE LDEX may have similar source to those observed by the Apollo missions.

Figure 1

(a and b) The locations of dust enhancement events observed by LADEE LDEX near twilight craters superposed on the topographic map obtained from the SLDEM2015 model (Barker et al., 2015), where Events 1–3 are observed near the Copernicus crater, Events 4–5 are observed near the Plinius crater, the vertical purple bars show the locations of lunar terminator, and the colored dots indicate the trajectories of LADEE spacecraft that moves westward, with different colors to denote the number densities of 0.1 μm sized dust grains obtained at different moments. The maximum density of each event is marked by a red arrow with the event number as well as the UTC time when the density peaks, and the thick white arrows indicate the direction of solar wind flow. (c) shows the comparison of dust densities obtained by LDEX as a function of altitude with the Apollo measurements, where the green stars indicate the peak dust densities of the five dust enhancement events in LADEE mission, and blue and red profiles show the dust densities obtained from the models of McCoy (1976) and Glenar et al. (2014), respectively, based on the Apollo measurements.

To verify a dust cloud caused by electrostatic dust lofting from a twilight crater, we need to know the electric potential in the crater. We use lunar topographic data obtained from the SLDEM2015 model (Barker et al., 2015) and the SW data from the ARTEMIS mission (Angelopoulos, 2011). We then can calculate the surface potential of the crater according to surface charging models (Farrell et al., 2010; Stubbs et al., 2014). It is found that the potential is determined by both the solar wind parameters and the topographic conditions. A higher SW electron temperature and a steeper crater wall gives a stronger negative potential (Text S2). In this way, apart from the vertical electric field caused by the Debye shielding, we can also obtain a horizontal electric field caused by the topographic change (Text S3). As shown in Figure 2a, the potential is about −50 V at flat surfaces but can be less than −300 V at some steep slopes in the crater. Most steep slopes are located around the leeward wall of the crater, with a minimum potential of about −500 V. With these potentials, we can then calculate the electric field as well as the transport distance of the lofted grains with the dynamic fountain model (Stubbs et al., 2006). The lofting distance is proportional to , but inversely proportional to (Text S3), where ϕ_s is the surface potential. As a result, the dust grains from flat surfaces can only reach a height of less than 5 km, while dust grains from the leeward wall can be lofted to more than 100 km, higher than the height of LADEE orbit (Figure 2b). Moreover, when the dust grains get to the maximum height, they have been also horizontally transported with a distance comparable to the maximum height, because of the horizontal electric field. With such a large horizontal distance, the dust grains can fly across the crater (the diameter of Copernicus is about 93 km) and be observed at the downstream side of the crater.

Figure 2

The surface potential and the lofting distances of 0.1 μm sized dust grains for Event 3. (a) The surface potential near the Copernicus crater. The white arrows show the direction of solar wind flow. The black dotted arrows indicate the trajectories of dust grains lofted from the leeward wall. The translucent gray cloud shows the horizontal scope of the expected dust cloud surrounding the crater. (b) The dust lofting distance for dust grains with  μm. The red line indicates the crater elevation along the profile shown as red dotted line in the panel a. Green circles show the maximum upward distance (Z_max) of dust grains from different parts of the crater floor, and the blue circles indicate the horizontal distance (X_max) at the moment when the dust grain reaches the maximum height (Z_max). The purple dashed line shows the altitude of LADEE orbit during the period of Event 3.

Due to the low solar wind temperature, the minimum surface potential of Event 2 (Table S1) is −197 V, which is too weak to loft 0.1 μm sized dust grains to the LADEE orbit. Consequently, the observed dust current should be caused by dust grains with a smaller radius (0.05 μm or even smaller). Similarly, smaller grains are needed to explain the large number dust grains observed in Event 5 (Figure S5). In addition, we find that some dust grains from the leeward wall can move sunward with a negative X_max, defined as the horizontal distance when the dust grain reaches the maximum height, but the sunward distance for 0.1 μm sized dust grains (less than 10 km) is too small to reach the orbital positions of the upstream observations (Events 1 and 5). Again, smaller dust grains are needed to explain the upstream LADEE measurements. Since a smaller dust grain produces a lower plasma charge on the LDEX impact target, the number density shown in Figure 1 would be underestimated. The question is why an upstream dust cloud is absent in the Events 2 and 3. A suspected secondary current peak can be found near the upstream edge of the crater (Figure S4), which may refer to the upstream dust cloud. However, the current is so weak that we cannot distinguish it from the SW generated current background with the present method (Text S1).

3 Twilight Craters for the Apollo Observations

Its meaningful to check whether the LHG observed in the Apollo missions is also related to a twilight crater. LHG refers to excess brightness along the lunar limb in the presence of coronal and zodiacal light (CZL) background, which is most significant in the Apollo 15 sunset measurements but is absent in the Apollo 16 sunrise measurement (Glenar et al., 2011; McCoy, 1976). Moreover, significant LHG with streamers were seen by the Apollo 17 astronauts in orbit, in which the streamers were thought to be caused by dust scattering of sunlight shadowed by an irregular lunar limb (McCoy & Criswell, 1974). With the Apollo ephemeris support data held at the National Space Science Data Center, we can obtain the spacecraft and camera position information of each Apollo mission. First, we search for topographic features present in at the locations of the Apollo 15 and 16 coronal measurements, and find that they both have a crater which could potentially cause the horizon glow, which is about 30 km in diameter and located near the intersection between the terminator and the camera sightline toward the CZL. The crater for the Apollo 15 sunset measurement is the Lambert crater, which lies in the Mare Imbrium basin with a sharp rim edge and a terraced inner wall. The crater for the Apollo 16 sunrise measurement is the Mills crater, which is a more degraded crater on the far side with a worn rim edge and a flat interior floor. We can expect a stronger negative potential in the Lambert crater given its steeper wall, and that may be the reason why the horizon glow was observed in the Apollo 15 mission but not in the Apollo 16 mission.

In addition, we find a large crater called Tsiolkovskiy for the Apollo 17 streamer observation. As shown in Figure 3a, the crater is just beneath the Apollo 17 spacecraft when it is flying across the terminator from the nightside. Furthermore, the crater has a steep inner wall and a large horizontal scale (the diameter is about 180 km), which may bring a large-scale dust cloud according to the electrostatic dust lofting model. Fortunately, the SW conditions during this streamer observation can be obtained from the Apollo 12 ALSEP instrument (Neugebauer et al., 1972). It is found that the Moon is in the Earth's magnetosheath during this period, where the ambient plasma has a higher temperature than the upstream solar wind. We use the averaged SW conditions within 15 min before and after the orbital sunrise time (16 December 1972 21:46:24 UTC) to calculate the surface potential. As shown in Figure 3b, the potential can be less than −700 V at the leeward crater wall, which is strong enough to loft 0.1 μm sized dust grains to a height larger than 200 km (Figure 3c). Meanwhile, the dust grains can be transported horizontally with an X_max of as large as 200–300 km. Assuming a parabolic motion for the dust grains, the horizontal scale of the dust cloud should be larger than 400 km on either side of the crater, which is consistent with the results obtained from the sketches of the astronauts, where the horizon glow spreads over 30° on either side of the central bulge (Zook & McCoy, 1991).

Figure 3

The trajectory of the Apollo spacecraft and electrostatic dust lofting for the Apollo 17 streamer observation. (a) Geometric position of the streamer observation. The green crosses represent the locations of Apollo 17 spacecraft at different times, superposed by a red dot to denote the orbital sunrise time (16 December 1972 21:46:24 UTC) and a blue dot to indicate the time of 1 min before the orbital sunrise when the astronaut saw conspicuous streamers above the lunar limb (the solid brown line). The white rectangle indicates the region used to calculate the surface potential. The thick white arrow shows the direction of SW, followed by the averaged SW conditions, where N, V, and T indicate the number density, the speed, and the temperature of SW, respectively. (b) The surface potential in the Tsiolkovskiy crater. The red dashed line indicates the profile used to calculate the lofting distance of dust grains. (c) The dust lofting distance for dust grains with  μm. The figure format is the same as Figure 2b.

4 Summary and Discussion

With the LADEE LDEX measurements, we find five dust enhancement events, which happen near twilight craters with dust densities as large as 100 m⁻³ at the height of about 60 km and 1,000 m⁻³ at height of about 40 km. Such densities are comparable to the Apollo measurements. In addition, we calculate the surface potential in the crater and find that the crater floor can be negatively charged to as small as −300 V at the leeward wall. Such a negative potential is sufficient to loft 0.1 μm sized dust grains to tens of kilometers high, which can explain the observed dust enhancement events by LADEE LDEX. Furthermore, we find that the LHG observed in the Apollo missions also occurred near a twilight crater, with a spatial scale consistent with the value inferred from an electrostatic model As a result, we suggest that there is a local dust fountain near a twilight crater, caused by the electrostatic dust lofting, which may be the reason for the sporadic dust exosphere observed by the Apollo missions.

However, it is not easy to observe such a dust fountain. First, a steep crater wall and appropriate solar wind conditions are required to form an electron-rich region, which gives a strong negative potential to loft the like-charged grains. In practice, the electron-rich region can be formed only when the topographic elevation angle is larger than the SW deflection angle, which is rarely given for the degraded state of many craters. Second, there are some uncertain factors that can affect when a dust grain can leave the surface, such as the cohesive force or a random disturbance in solar wind. Despite these difficulties, we believe such local dust fountains are worthy of future study. The south polar region of Moon has been become a compelling destination for future exploration missions (e.g., the National Aeronautics and Space Administration's, NASA's, Artemis, Indian's Chandrayaan-3 and China's Chang'E-7 missions), due to the possible presence of water in the permanently shadowed craters as observed by Chandrayaan-1 (Pieters et al., 2009) and LCROSS mission (Colaprete et al., 2010). Nevertheless, our findings from twilight craters suggest that shadowed crater regions could contain greater dust densities than previously thought, and a potential hazard that should be carefully assessed for future mission.

Acknowledgments

This work is supported by the Key Research Program of the Chinese Academy of Sciences, Grant No. XDPB11 and by the National Key R&D Program of China, Grant No. 2020YFE202100. This work is also supported by the Science and Technology Development Fund (FDCT) of Macau (020/2014/A1, 008/2017/AFJ, and 0042/2018/A2) and the National Natural Science Foundation of China (Grants 41704167, 41941001, and 11761161001). We acknowledge the reviewers for the helpful suggestions on our work. We specially acknowledge J. R. Szalay and M. Hornyi for their pioneering work on lunar dust environment and for producing the LADEE LDEX data used in this paper. We also thank V. Angelopoulos for use of data from the ARTEMIS mission. L. Lei acknowledges the support of Beijing Municipal Science and Technology Commission (Grant Z181100002918003).

Open Research

Data Availability Statement

LDEX data are available through NASAs Planetary Data System (sbn.psi.edu/pds/resource/ldex.html). The ARTEMIS data are publicly available at http://artemis.ssl.berkeley.edu and NASAs CDAWeb. The Apollo ephemeris support data are available at NSSDC (http://apollo.sese.asu.edu/EPHEMERIS/). The Apollo 12 ALSEP data are available at NASAs CDAWeb (https://spdf.gsfc.nasa.gov/).