Impact Ejecta Environment of an Eccentric Asteroid: 3200 Phaethon

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Planetary and Space Science 165 (2019) 194–204

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Planetary and Space Science

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Impact ejecta environment of an eccentric asteroid: 3200 Phaethon

Jamey R. Szalay a,*, Petr Pokorný b,c, Mihaly Horanyi d,e,f, Diego Janches b, Menelaos Sarantos b, Ralf Srama g a Department of Astrophysical Sciences, Princeton University, 4 Ivy Ln., Princeton, NJ, 08540, USA b Space Weather Lab., Mail Code 674, GSFC/NASA, Greenbelt, MD, 20771, USA c Department of Physics, Catholic University of America, Washington, DC, 20064, USA d Department of Physics, University of Colorado Boulder, 392 UCB, Boulder, CO, 80309, USA e Laboratory for Atmospheric and Space Physics, 1234 Innovation Dr., Boulder, CO, 80303, USA f Institute for Modeling Plasma, Atmospheres, and Cosmic Dust, 3400 Marine St., Boulder, CO, 80303, USA g Institut fr Raumfahrtsysteme, University of Stuttgart, Stuttgart, Germany

ARTICLE INFO ABSTRACT

Keywords: Airless bodies in the solar system are continually bombarded by meteoroids, sustaining impact ejecta clouds. Moon While large bodies like the Moon retain a significant fraction of ejecta, small asteroids shed this material into the Interplanetary dust interplanetary dust complex. Measurements of the lunar impact ejecta cloud found it was sustained by the known Meteors sporadic meteoroid sources. Here, we extend those measurements using a model of the IDP environment at 1 au to investigate the structure of an ejecta cloud at an eccentric airless body, asteroid 3200 Phaethon. Due to Phaethon's large eccentricity, its ejecta cloud is highly asymmetric. At 1 au, the cloud is canted towards the asteroid's apex direction and its density varies by five orders of magnitude. Compared to a body in a circular orbit at 1 au, Phaethon's peak ejecta density at 1 au is approximately 30 times higher, largely due to enhanced ejecta production from meteoroids shed from Jupiter Family Comets. Such asymmetric ejecta production suggests Phaethon experiences significantly different meteoroid-specific space weathering processes than a body with a similar semimajor axis on a circular orbit. We estimate impact ejecta processes at Phaethon shed approximately 1 ton per year, which is not sufficient to appreciably contribute to the Geminids meteoroid complex. These estimates most likely represent a lower limit, as Phaethon is expected to have a higher ejecta yield than the Moon. Additionally, we calculate predicted impact counts for a dust detector on close flybys of Phaethon in preparation for the DESTINYþ mission and find the large majority of impacts would be detected on the morning-side sunlit hemisphere. These results suggest eccentric asteroids shed more material than those on near-circular orbits, and are suitable candidates for in-situ dust detection and chemical characterization due to their amplified asymmetric ejecta production.

1. Introduction of airless bodies. This ejecta cloud is dominantly composed of the airless body's surface material, as the ratio of ejecta mass to impactor mass is Impact bombardment can play a signiﬁcant role in space weathering typically on the order of 103 (e.g., Koschny and Grün, 2001; Krüger et al., the surfaces of airless bodies (e.g. Pieters and Noble, 2016; Altobelli 2001; Horanyi et al., 2015), and encodes its chemical and mineralogical et al., 2018). It can modify their surfaces through the removal of aged makeup. Measurements of the extended ejecta clouds of airless bodies space weathered grains due to impact events exposing fresh material, would garner critical compositional information about the surface of the impact melt accumulation on the outer rims of grains modifying their body without requiring the complexity of a sample return mission. spectral properties, the production and accumulation of nanophase ma- In this study, we focus on the impact ejecta environment of asteroid terials such as nanophase iron, or most likely a combination of all these 3200 Phaethon. It belongs to the Apollo class of Near Earth Objects various processes. Additionally, a direct byproduct of impact bombard- (NEOs), with orbital elements semi-major axis a ¼ 1:27 au, eccentricity ment is the existence of a permanently present ejecta cloud in the vicinity e ¼ 0:89, and inclination i ¼ 22∘. Phaethon is a unique asteroid as it is

* Corresponding author. E-mail address: [email protected] (J.R. Szalay). https://doi.org/10.1016/j.pss.2018.11.001 Received 18 June 2018; Received in revised form 26 September 2018; Accepted 7 November 2018 Available online 12 November 2018 0032-0633/© 2018 Elsevier Ltd. All rights reserved. J.R. Szalay et al. Planetary and Space Science 165 (2019) 194–204 understood to be the parent body of the Geminids meteoroid stream (Hughes, 1983), whereas most meteoroid streams are understood to originate from active comets. Phaethon has been observed to have weak, intermittent cometary-like activity (Jewitt and Li, 2010; Li and Jewitt, 2013; Jewitt et al., 2013), however this activity is not sufficient to sustain the dense Geminids meteoroid stream. Phaethon is also the target of the upcoming DESTINYþ mission (Sarli et al., 2018), which will carry a dust detector capable of chemical composition analysis (Krüger et al., 2017). Due to its large eccentricity, Phaethon provides an interesting and representative case study to compare an eccentric body's impact ejecta environment to a canonical circular orbiting body. By understanding the complex impacting meteoroid fluxes and subsequent impact ejecta generation, we can gain insight into how eccentric asteroidal surfaces evolve compared to those with low eccentricities. In this study, we use models of the IDP environment in the solar system to predict the impact ejecta cloud structure of Phaethon. We utilize measurements of the lunar ejecta cloud (Horanyi et al., 2015), which identified the Moon's equatorial ejecta cloud is sustained by the known sporadic meteoroid sources (Szalay and Horanyi, 2015a). The IDP model in this study matches the observed lunar impact ejecta cloud with reasonable accuracy (Janches et al., 2018). We build on previous work that extended the lunar ejecta measurements to NEOs on circular orbits at 1 au to predict the absolute number densities and three-dimensional structure of Phaethon's impact ejecta cloud. In Section 2, we estimate Fig. 1. Trajectory of Phaethon in ECLIPJ2000 coordinates. The blue section the meteoroid fluxes to Phaethon for different sporadic meteoroid source indicates its location in the date range of 15-Feb-2025 to 22-Feb-2025. (For fi populations at 1 au. Combining these fluxes with measurements of the interpretation of the references to colour in this gure legend, the reader is lunar ejecta cloud, we estimate Phaethon's absolute impact ejecta num- referred to the Web version of this article.) ber density distribution at 1 au in Section 3. We make predictions for dust detector measurements near Phaethon in Section 4. In Section 5,we et al., 2018). All quantities and maps are calculated and displayed in the extend predictions of IDP fluxes and ejecta production throughout Phaethon-centric Solar Ecliptic (PSE) reference frame. PSE is defined Phaethon's orbit and discuss implications for space weathering. We with a primary x-axis pointing from Phaethon to the apparent Sun, a conclude with a summary of our results in Section 6. secondary z-axis pointing in the direction of the ECLIPJ2000 north pole (specifically, the component of this vector perpendicular to the 2. Meteoroid fluxes to Phaethon Phaethon-Sun direction), and the y-axis completes the right handed frame. The maps are shown from the perspective of an outward view To predict the meteoroid environment at Phaethon, we employ a from the center of the object and the direction of the Sun is set to ∘ dynamical model that tracks the orbital evolution of thousands of IDPs 0 longitude following existing radiant map conventions in the literature from different sources in the solar system. All simulations of IDPs are (e.g., Campbell-Brown, 2008). In each Phaethon map, the apex direction performed by SWIFT_RMVS3 (Levison and Duncan, 2013) and include at (334 , 10 ) is shown with a small white dot. the effects of radiation pressure, PR drag and solar wind. Here, we use As shown in Fig. 2, Phaethon experiences a significantly modified IDP four different source populations to model the inner solar system mete- flux distribution compared to a body on a circular orbit at 1 au. To un- oroid environment: Jupiter Family Comets - JFCs (Nesvorny et al., derstand its complex IDP fluxes, we first describe the fluxes for a circular 2011b); Halley Type Comets - HTCs (Pokorný et al., 2014); Oort Cloud orbit, then discuss how these fluxes are modified due to Phaethon's large Comets - OCCs (Nesvorny et al., 2011a); and Main Belt Asteroids - ASTs eccentricity. For a body on an approximately circular orbit at 1 au, spo- (Nesvorný et al., 2010). With a set of individual IDP model trajectories, radic meteoroids are typically categorized into sub-groupings: a) helion/ flux and velocity maps to a given body can be calculated. The impactor anti-helion (HE/AH) meteoroids, shed from JFCs, on orbits that intersect fluxes are based on existing models validated with visual and radar ob- the airless body with different eccentricities and average impact speeds servations of meteoroid ablating in Earth's atmosphere (Nesvorny et al., of 20–25 km s 1 (Nesvorny et al., 2011a); b) northern and southern 2011a, b; Pokorný et al., 2014). It has been utilized successfully to toroidal (NT/ST) meteoroids, shed from high inclination HTCs (Pokorný describe impact related processes at Mercury (Pokorný et al., 2017) and et al., 2014), with average impact speeds of 30–40 km s 1; and c) apex the Moon (Janches et al., 2018). (AP) meteoroids, shed from retrograde HTCs and OCCs (Nesvorny et al., Phaethon's orbit in J2000 ecliptic coordinates (ECLIPJ2000 in the 2011b), with very large impacting speed 60 km s 1 (approximately NAIF/SPICE framework) is shown in Fig. 1. Near 1 au, Phaethon's he- twice the local orbital speed). Note, gravitational focusing is not incor- liocentric velocity varies from 32 to 35 km s 1 in the time range 15-Feb- porated into any of these maps due to the weak gravity of such small 2025 to 22-Feb-2025 and its velocity vector points 28 from the bodies. Phaethon-Sun direction. The orbital configuration of Phaethon near 1 au All these sources have been defined and discussed in the literature for has great consequences for its impacting meteoroid fluxes. a body at 1 au (Earth) in a nearly circular orbit with an orbital velocity of To understand the complex impactor distributions at Phaethon, we 30 km s 1 approximately perpendicular to the Sun-body direction (e.g., provide the 1 au circular orbit analogue for all maps. Figs. 2 and 3 show Campbell-Brown, 2008). Since Phaethon is plunging towards the Sun at Mollweide projection maps of the incident flux and average impactor 34 km s 1 at a direction 28 from the Phaethon-Sun vector near its speeds to both Phaethon and an airless body in a circular orbit at 1 au descending node, the flux distributions are significantly morphed from with 0 inclination. Each meteoroid population has been scaled to ab- the circular case. The HE source is both amplified in flux and shifted solute values using ratios from Carrillo-Sanchez et al. (2016), and we do downwards in PSE latitude due to Phaethon's negative vz speed compo- not incorporate additional uncertainties for the relative weights of nent. As Phaethon is moving at a speed larger than the average speed of JFC ¼ 34.6 t d 1, AST ¼ 3.7 t d 1 , and HTC¼OCC¼2.5 t d 1 (Pokorný the AH source, this source is essentially non-existent. The AST fluxes,

195 J.R. Szalay et al. Planetary and Space Science 165 (2019) 194–204

Fig. 2. Incident IDP ﬂux in Mollweide projections (in the PSE frame) for Phaethon and an NEA at 1 au on a circular orbit. The white dot in each of the Phaethon maps indicates the Phaethon apex direction.

which were small and symmetric for the circular case have been focused depends on the surface material, Fm is the mass flux, α ¼ 0:23 and β ¼ similarly, augmented by Phaethon's large velocity vector. 2:46 are experimentally determined constants, and φ is the angle relative The long period cometary grains undergo similar transformations. to the surface normal. Mass production Mþ is calculated for each source Since the AP source is primarily due to retrograde grains with very large population for both a circular orbit at 1 au and Phaethon's orbit. We note relative speeds, their radiants in the Phaethon maps are not modified as that the exponents in the Mþ equation were determined over a smaller severely. However, the toroidal source, which has lower impactor ve- range of impactor speeds and masses than considered here, in Earth's locities, are more susceptible to the Phaethon speed modulation and are gravity, and for solid materials instead of regolith (Koschny and Grün, shifted entirely to the sunlit/apex hemisphere. As shown in the last panel 2001). In lieu of additional experiments that have yet to be performed for of Figs. 2 and 3, the combined flux and speed distribution at Phaethon is a conditions more similar to those experienced by airless regolith bodies, transformed version of the circular case, due entirely to the difference in we use this relation. velocity vector of the airless body considered. Mþ is directly proportional to the impact ejecta density at the surface, þ n0 (Szalay and Horanyi, 2016a). We convert M for each source to n0 by 3. Structure of Phaethon's impact ejecta cloud at 1 au scaling the Mþ maps such that the value in the circular orbit Mþ map at 6 ∘ ∘ 1 LT (local time) in the equatorial plane or (270 ,0) PSE is n0 ¼ 1:3 10 To predict impact ejecta cloud densities at Phaethon, impact ejecta m 3 for grains sizes a 0:1 μm, following previous predictions for NEAs þ mass production M is calculated from the predicted IDP mass flux maps (Eq. 4, Szalay and Horanyi, 2016b). Since we scale Mþ in this way, the þ α β 3 via M ¼ CFmm v cos φ (Gault et al., 1974; Koschny and Grün, 2001; value of constant C is not important. While the lunar size distribution was Szalay and Horanyi, 2015a). Here, C is a proportionality constant that measured for grains as small as a0 ¼ 0:3 μm in radius, here we assume

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Fig. 3. Average speed distribution in the same conﬁguration as Fig. 2.

1 the observed cumulative ejecta size distribution function f ða > a0Þ ∝ they have average impact speeds near 50 km s and are present at much a2:7 observed at the Moon (Horanyi et al., 2015) holds for smaller radii larger fluxes than all other sources. As such, Phaethon's combined ejecta grains and that ejecta processes at Phaethon are similar to the Moon's. distribution peaks more sunwards and at lower latitude than its apex Fig. 4 shows predicted maps of n0.AsinFigs. 2 and 3, each source has direction. Additionally, due to the extreme range of impacting speeds and been appropriated scaled using ratios from Carrillo-Sanchez et al. (2016); fluxes to Phaethon, its ejecta cloud is much more asymmetric than the Pokorný et al. (2018). As with the flux and speed maps, the predicted circular case, where the peak in the predicted ejecta density is five orders impact ejecta yield maps are very different for Phaethon compared to the of magnitude larger than its minimum value. This is in stark contrast to circular case. For a circular orbit, impact ejecta production and the the circular case, which varies by about a factor of just 40 from peak apex structure of the ejecta cloud is governed largely by the low flux, high to minimum anti-apex ejecta density. speed retrograde AP grains from HTCs and OCCs. Since ejecta production To determine the spatial distribution of ejecta around Phaethon, we -1 has a power-law dependence (Mþ∝ vβ), the AP source's large impacting note that Phaethon's escape speed is on the order of a few m s . The speeds, even with low fluxes, generate large quantities of ejecta pro- ejecta speed distribution we employ here is that inferred from the lunar duction. Hence, the AP source causes the ejecta distribution to peak in the impact ejecta distribution, which is approximately gaussian shaped with -1 apex direction at the center of the map, as previously observed at the a high speed tail and peaks around 600 m s (Szalay and Horanyi, Moon (Szalay and Horanyi, 2015a). 2016a). Therefore, we assume all grains are unbound on escaping tra- For Phaethon, instead of the high speed and low flux AP HTC/OCC jectories. As such, the density will scale as n ∝ r 2, where r is the distance grains, the HE JFC grains are the dominant producer of impact ejecta, as from the center of the body, following similar previous analysis for NEAs

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Fig. 4. Predicted impact ejecta surface density based on lunar ejecta measurements in the same conﬁguration as Fig. 2.

(Szalay and Horanyi, 2016b). Using these assumptions, Fig. 5 shows the spacecraft is on a circular orbit when intercepting Phaethon and ignore three-dimensional density distribution of the ejecta cloud for both the relatively smaller z-component of the velocity vector. A circular body Phaethon and the circular case. As exemplified by this figure, the ejecta near 1 au will transit the Phaethon ejecta cloud between 27∘ to 31∘ from distribution is highly asymmetric and most dense towards the apex the Sun-Phaethon direction in the PSE X-Y plane, as shown in Fig. 5a with direction. the dashed example line at 29∘ through the center of Phaethon. We then offset the trajectory shown in Fig. 5a for impact parameters (measured 4. Predictions for DESTINYþ from the center of Phaethon) of b ¼ 10, 30, 100, & 300 km, both on the apex hemisphere (positive values) and anti-apex hemisphere (negative Impact ejecta carries valuable information about the composition of values). We evaluate these flybys at the two bounds of the Feb. 2025 Phaethon and IDP sources. A close flyby near Phaethon with a dust in- DESTINYþ flyby window from 15-Feb-2025 to 22-Feb-2025 (Sarli et al., strument capable of chemical composition measurements, like that of the 2018). Fig. 6 shows the flybys overlaid on the density contours at these upcoming DESTINYþ mission, may garner critical measurements that time boundaries. The right panels of this figure show the cumulative can readily be linked back to Phaethon's surface. In this section, we make impacts (m 2) and instantaneous encountered densities as a function of predictions for impact ejecta measurements for a transiting spacecraft. closest approach distance. Tables 1 and 2 list the total predicted counts 2 Due to the highly asymmetric nature of Phaethon's ejecta cloud, the per m for lower size cuts of a0 ¼ 50, 100, & 300 nm. flyby geometry plays an important role in determining the total counts a There are a few conclusions that can be made from these predictions. transiting dust detector could make. For simplicity, we assume the Due to the r2 dependence of the ejecta density distribution, there are

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Fig. 5. Density distribution of the predicted impact ejecta cloud for grains with a 0:1μm on 17-Feb-2025, in the PSE frame. White arrows indicate the direction of orbital motion for each body, the yellow arrow in the subsets show the direction to the Sun (þx), and the dashed arrow in row (a) indicates the relative ﬂyby angle DESTINYþ will encounter Phaethon with. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the Web version of this article.)

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Fig. 6. Predicted ejecta cloud structure and impact counts for 15-Feb-2025 and 22-Feb-2025 ﬂybys. Left: Density distribution [m 3] in the X-Y PSE plane and a selection of possible ﬂyby trajectories. Right: Cumulative impact counts (top) and instantaneous encountered density of grains with a 0:1μm.

Table 1 Table 2 Total impacts per m2 on apex (anti-apex) transits for selected ﬂyby distances Total impacts per m2 on apex (anti-apex) transits for selected ﬂyby distances

(from the center of Phaethon) on 15-Feb-2025. a0 is the minimum detectable (from the center of Phaethon) on 22-Feb-2025. a0 is the minimum detectable grain radius. grain radius.

a0 [nm] 10 km 30 km 100 km 300 km 500 km a0 [nm] 10 km 30 km 100 km 300 km 500 km 50 21000 6400 1800 (830) 520 (200) 280 (82) 50 36000 11000 3100 900 490 (10000) (3100) (17000) (5000) (1300) (310) (130) 100 3000 (1500) 990 (480) 280 (130) 80 (30) 43 (13) 100 5600 (2600) 1700 (770) 480 (200) 140 (48) 75 (20) 300 170 (82) 51 (25) 14 (7) 4 (2) 2 (1) 300 290 (130) 88 (40) 25 (11) 7 (2) 4 (1)

great benefits by performing close flybys, where the total count rate would maximize its total count rates by performing a very near flyby on scales approximately like N ∝ r1. The highly asymmetric structure also Phaethon's apex/dawn hemisphere, where the majority of detections dictates two important features in expected counts: a) the vast majority of would be collected before and up to close approach. counts will be collected on the þXPSE or dawn hemisphere; and b) flybys There are trends in the total expected impact counts throughout any transiting the apex or dawn-side hemisphere ( YPSE) will collect 2to3 given flyby window. Fig. 7 shows a selection of orbital parameters and times more counts than the corresponding flyby on the dusk-side ( þ the average ejecta surface density during the 2025 DESTINYþ flyby YPSE). As such, a transiting spacecraft equipped with a dust detector window. As Phaethon approaches the Sun during this time range, there

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Fig. 7. Orbital parameters and average ejecta surface density during the 2025 DESTINYþ ﬂyby window. Ejecta densities and corresponding expected measured counts are expected to increase by a factor of 1.7 over this range.

Ejecta density n0 maps in the stereographic projection, centered on Phaethon's apex direction, are shown at the top in a linear scale to emphasize the trend. Fig. 8. IDP flux distribution throughout Phaethon's orbit in the ECLIPJ2000 x; y are three factors which all lead to a monotonically increasing expected plane, with maps shown in the PSE frame. The location of the Sun is given by the ejecta density with time. First, as shown in the top panel of the figure, orange dot. (For interpretation of the references to colour in this figure legend, Phaethon is moving towards the Sun. Since the IDP density increases the reader is referred to the Web version of this article.) with decreasing heliocentric distance, this causes additional impact ejection yield. Additionally, Phaethon's orbit takes it from a high þz value towards the z ¼ 0 plane. Similarly, it will encounter larger IDP densities closer to the z ¼ 0 plane. Finally, Phaethon's orbital speed increases as it gets closer to the Sun, increasing by 3kms 1 during this window. Since ejecta production is highly dependent on impacting speed, this further enhances the ejecta density. All three factors conspire to increase the total ejecta density, and therefore the total predicted counts, by a factor of 1.7 during this time window. Therefore, flyby's at smaller heliocentric distance, near the z ¼ 0 plane, and with larger Phaethon orbital speeds will allow for enhanced total dust impacts for DESTINYþ. To determine bounds on the expected counts DESTINYþ would encounter, we can investigate the ejecta density over a range of sizes and for different ejecta mass indices. We have been operating under the assumption that the cumulative ejecta size distribution is a power law with an index of 2.7. However, measurements of the Galilean satellites indicated a shallower slope of 2.4 (Krüger et al., 2001). If the size index were in fact 2.4, this would reduce our estimates by a factor of 0.6. Furthermore, the flyby encounter speed is so fast that any modern impact ionization detector would be able to detect grains as small as a few 10s of nanometers. If the power law holds down to grains as small as 25 nm, this would increase our estimates for the 50 nm case by a factor of 6.5. However, even if the power law holds down to this small size, very small grains are both more susceptible to radiation pressure (Burns et al., 1979) and Lorentz force (Consolmagno and Jokipii, 1977) perturbations, which would modify the structure of the ejecta cloud discussed in this work.

Fig. 9. IDP average speed distribution throughout Phaethon's orbit. See Fig. 8 5. Orbital variation in meteoroid bombardment caption for additional details.

In the previous sections, we investigated the details of Phaethon's complex ejecta cloud structure at 1 au, both to put into context the roles same PSE reference frame as the previously discussed maps. Shown in the of the various IDP sources and to make predictions for DESTINYþ. In this center of the orbit is the map for a body in a circular orbit with semimajor section, we extend our analysis throughout Phaethon's orbit. Figs. 8 and 9 axis of 1.3 au, the same as Phaethon's. The maps on the descending show the incident IDP ﬂux and average speed to Phaethon at 16 true portion of the orbit, from aphelion to perihelion in a counter-clockwise anomalies ν ¼½0; 45; 90; 120; 140; 150; 160; 170; 180 in the direction, qualitatively resemble those discussed in the previous

201 J.R. Szalay et al. Planetary and Space Science 165 (2019) 194–204 section, as it focused on the state near the descending node. Namely, since Phaethon's apex direction is in its sunlit hemisphere, its orbital configuration dominantly enhances the helion JFC IDPs. Likewise, in the ascending portion from perihelion to aphelion, the configuration is reversed and the dominant fluxes come from the anti-helion direction. Fig. 10 shows the ejecta density in the same configuration as Figs. 8 and 9.Reflecting the trends in the flux and average speed maps, the impact ejecta densities peak near perihelion. The most dense portion of the ejecta cloud shifts throughout Phaethon's orbit. These ejecta cloud predictions can be used to estimate the mass loss from Phaethon due to impact bombardment. From the relation between ejecta flux from the surface of an asteroid and n0 (Szalay and Horanyi, 2016b), the average ejecta flux for particles with a 100 nm at Phaethon can be determined from the ejecta densities from Fig. 10. For example, at 1 au the average 2 1 ejecta flux is Fav 200 m s . By assuming the ejecta distribution a) follows the lunar observed power law distribution, b) has a size range of 3 a0 ¼ 0:1 μmtoa1 ¼ 100 μm, and c) has a mass density of 2:5 10 kg m 3, the average mass per particle is calculated to be m ¼ R av m1 mf ðmÞdm ¼ 6 1017 kg. Integrating over the entire surface, the m0 _ 2 average mass loss rate is M ¼ 4πR Favmav. Fig. 11 shows M_ in the top panel as a function of ν. Also shown with the horizontal dashed line in the top panel is the value for a body in a circular orbit at 1.3 au of 106 kg s-1. Integrating over a single orbital _ period of 1.4 years, the average mass loss per orbit is hMphai4 5 1 _ 10 kg s ¼ 40Mcir , a factor of approximately 40 higher than the circular case. This value is considerably smaller than that estimated for the Fig. 10. Impact ejecta distribution throughout Phaethon's orbit. See Fig. 8 mass loss during Phaethon's active periods of 3 kg s-1 (Jewitt et al., 2013), caption for additional details. which is not sufficient to supply the Geminids meteoroid stream. We therefore conclude that impact ejecta processes do not appreciably contribute to the Geminids meteoroid stream. While impact processes at Phaethon are not responsible for appreciably contributing to the Geminids, they do govern how its surface evolves over time. In addition to surface evolution due to the extreme heating Phaethon experiences at its close flybys to the Sun, impact bombardment can play an important role as it can eject an airless body's regolith, exposing fresh, unweathered material. From the total mass loss estimate, we can also estimate the total change is regolith depth lost as a function of time. Following the same assumptions as before, Phaethon losses on average approximately 200 μm per 103 years, compared to the 6 μm per 103 years for the circular case. Within the last thousand years, impact bombardment may be responsible for exposing the top 0.2 mm of new material on Phaethon's surface. Last, we turn to the apparent asymmetry between the flux and impact ejecta production on the descending and ascending arcs of Phaethon's orbit. The descending portion largely has enhanced IDP fluxes and subsequent impact ejecta production compared to the same true anomalies on the ascending portions. However, the peak in flux actually occurs after Fig. 11. Top: total mass loss rate for Phaethon throughout its orbit. Middle: ∘ perihelion on the ascending arc, near ν ¼ 45 . Fig. 11 illustrates why such distance from the z ¼ 0 ECLIPJ2000 plane. Bottom: heliocentric distance (solid) an asymmetry exists. Due to Phaethon's inclination and argument of and speed (dashed) of Phaethon. All quantities shown as a function of true perihelion (ω ¼ 322∘), it spends more time near the z ¼ 0 ecliptic plane anomaly ν. in the descending arc of its orbit. As the IDP density distribution in the solar system decreases with increasing distance from the z ¼ 0 plane, the periences significantly enhanced impact fluxes and speeds from the ascending arc experiences lower IDP densities and therefore lower fluxes populous JFC IDPs compared to a similar body in a circular orbit. and subsequent ejecta generation. Also due to Phaethon's orbital pa- Phaethon's low escape speed of a few m s-1 compared to the average rameters, its ascending node is slightly after perihelion at ν ¼ 38∘ at a impactor speeds 104 ms-1 suggests that the large fraction of impact heliocentric distance of 0.16 au. The two vertical dotted lines in Fig. 11 ejecta is shed upon creation. With lower and upper size limits of the size fl denote Phaethon's nodes. Hence, the peak in ux does not occur precisely distribution to a0 ¼ 50 100 nm and a1 ¼ 100 1000 μm, Phaethon at perihelion, but near where Phaethon transits through the ecliptic plane sheds an orbit averaged value of 6 106 to 8 105 kg s-1. This shortly after, as demonstrated by the various maps and in Fig. 11. range of impact-generated mass loss rules out impact ejecta as a significant source for the Geminids meteoroid stream. However, impact 6. Discussion and conclusions bombardment can still play an important role in space weathering the surface of airless bodies such as Phaethon. Independent of the size In this study, we investigated the response of asteroid 3200 Phaethon bounds, Phaethon sheds 40 times more material than its circular to meteoroid bombardment. Due to its large eccentricity, Phaethon ex- counterpart. Therefore, Phaethon, and by analogy other eccentric airless

202 J.R. Szalay et al. Planetary and Space Science 165 (2019) 194–204 bodies, will shed significantly more material than similarly sized bodies assumed Phaethon's surface response is identical to the Moon's, we would in circular orbits. Hence we would expect the total quantity of small expect that the impact ejecta densities calculated in this work represent a surface regolith to be more populous at low-eccentricity airless bodies. lower limit. Future measurements by the DESTINYþ mission would be Electrostatic mobilization may also shed a portion of regolith. At the able to constrain the total impact ejecta yield and provide a quantitative Moon, there was no evidence for large-scale electrostatic mobilization as comparison between the Moon's and Phaethon's yield. low as 3 km from the surface (Szalay and Horanyi, 2015b). A particle While the total magnitude of impact detections DESTINYþ detects which reaches its turning point at 3 km was ejected from the lunar would inform on the ejecta yield, the structure of the impact rate profile surface at 100 m/s. The absence of a population of electrostatically also encodes the response of Phaethon to its meteoroid environment. ejected dust at the Moon suggests a similar process at Phaethon should With a flyby close enough to ensure sufficient impact rate statistics, the also not be prominent. However, the lunar measurements do not pre- impact rate time series of Phaethon's ejecta cloud will allow us to test our clude the possibility that electrostatic effects could eject grains in the models of IDP fluxes at 1 au in a manner unique to dust detector mea- range of a few m s-1 to 100 m s-1, which would be on unbound trajec- surements. Thus, with a single flyby, DESTINYþ could probe the tories and could contribute to the total near-Phaethon dust environment. response of a small asteroid's surface to impact bombardment, garner For example, recent laboratory measurements suggest there may be a critical insight into Phaethon's chemical makeup, and investigate the large degree of small-scale electrostatic mobilization at the surfaces of structure of the zodiacal cloud at 1 au. airless bodies (Wang et al., 2016; Schwan et al., 2017). The extent to which these processes operate and their ability to eject large quantities of Acknowledgements unbound ejecta remains to be investigated. We also provided estimates for the total dust impacts a transiting LDEX data are available through NASA's Planetary Data System, spacecraft like the upcoming DESTINYþ mission would expect to sbn.psi.edu/pds/resource/ldex.html. JRS was supported by NASA’s encounter. Due to the highly asymmetric nature of the impact ejecta Lunar Data Analysis Program (LDAP), Grant 80NSSC17K0702. PP was cloud, caused by Phaethon's large eccentricity and abundance of JFC supported though NASA’s Solar System Observations (SSO), Solar System IDPs, the vast majority of detections would be made before and up to Workings (SSW), and LDAP programs. MH was supported by the Institute closest approach for a prograde spacecraft trajectory. As DESTINYþ will for Modeling Plasma, Atmospheres, and Cosmic Dust of NASA's Solar have a large relative flyby speed with respect to Phaethon, the dust de- System Exploration Research Virtual Institute. tector should be oriented in the spacecraft ram direction to maximize impact detections, as assumed in this work. Transits on the apex hemi- Appendix A. Supplementary data sphere would further experience approximately a factor of 3 enhance- ment in total impact detections compared to transits on the anti-apex Supplementary data to this article can be found online at https://doi. hemisphere. Due to the large flyby speeds, a dust detector onboard org/10.1016/j.pss.2018.11.001. DESTINYþ would be able to detect very small grains in the ejecta cloud, should they exist. Within the DESTINYþ encounter date range of 15-Feb- References 2025 to 22-Feb-2025 (Sarli et al., 2018), Phaethon's ejecta cloud density monotonically increases as a function of time and is most dense by a Altobelli, N., Fiege, K., Carry, B., Soja, R., Guglielmino, M., Trieloff, M., Orlando, T.M., : Srama, R., 2018. Space weathering induced via micro-particle impactspart 1: factor of 1 7 on 22-Feb-2025 compared to 15-Feb-2022. 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