Meteorite ejecta travel follows predictable ballistic trajectories in the atmosphere
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Retrieved studies discuss impact ejecta and ballistic modeling in various extraterrestrial and terrestrial contexts, but provide only partial coverage regarding predictable atmospheric ballistic trajectories for meteorite ejecta.
The Surface Dust Analyser (SUDA) is a mass spectrometer onboard the Europa Clipper mission for investigating the surface composition of the Galilean moon Europa. Atmosphereless planetary moons such as the Galilean satellites are wrapped into a ballistic dust exosphere populated by tiny samples from the moon's surface produced by impacts of fast micrometeoroids. SUDA will measure the composition of such surface ejecta during close flybys of Europa to obtain key chemical signatures for revealing the satellite's composition such as organic molecules and salts, history, and geological evolution. Because of their ballistic orbits, detected ejecta can be traced back to the surface with a spatial resolution roughly equal to the instantaneous altitude of the spacecraft. SUDA is a Time-Of-Flight (TOF), reflectron-type impact mass spectrometer, optimized for a high mass resolution which only weakly depends on the impact location. The instrument will measure the mass, speed, charge, elemental, molecular, and isotopic composition of impacting grains. The instrument's small size of 268 mm × 250 mm × 171 mm , radiation-hard design, and rather large sensitive area of 220 cm<sup>2</sup> matches well the challenging demands of the Clipper mission.
SUDA will measure the composition of such surface ejecta during close flybys of Europa to obtain key chemical signatures for revealing the satellite’s composition such as organic molecules and salts, history, and geological evolution. Because of their ballistic orbits, detected ejecta can be traced back to the surface with a spatial resolution roughly equal to the instantaneous altitude of the spacecraft. SUDA is a Time-Of-Flight (TOF), reflectron-type impact mass spectrometer, optimized for a high mass resolution which only weakly depends on the impact location. The instrument will measure the mass, speed, charge, elemental, molecular, and isotopic composition of impacting grains.
The compositional mapping technique relies on the fact that impacts of fast, typically 100 μ m interplanetary meteoroids on the moon’s surfaces produce ejecta particles which populate a tenuous, approximately spherically symmetric cloud around the moon (Krivov et al. 2003 ; Sremčević et al. 2003 , 2005 ). Information about the geological activities at and below Europa’s surface, in particular the material exchange between the interior and the surface, is likely contained in the types and amounts of inorganic and organic components embedded in the surface.
This method is readily available for the Europa Clipper mission, as the ejecta dust clouds have already been detected – albeit without compositional information – around all the icy moons of Jupiter by the Galileo dust detector (Krüger et al. 2003c , see Fig. 1 ). Fig. 1 Ejecta clouds around moons without an atmosphere are a common phenomenon in the solar system. Left: Europa’s ejecta cloud was present during all Galileo flybys (Krüger et al. 2003a ). Middle: Saturn’s moon Enceladus maintains an ejecta cloud beside the ice particle plume at its south pole (Kempf et al. 2010 ). Right: Earth’s Moon also maintains a pronounced ejecta cloud (Horányi et al.
Most SUDA detections originate from within the nadir-projected circle on the surface with a radius at least equal to the instantaneous altitude of the spacecraft (Postberg et al. 2011a ; Goode et al. 2021 ). As long as the radius of this circle is comparable to that of a geological feature at or near closest approach, SUDA’s data can
Used parameters: F i m p ∞ = 7.6 ⋅ 10 − 16 k g / m 2 s , K e / K i = 30 % , Ψ = 30 ∘ , G s i l = 0 % , γ = 2.4 , β = 3.0 , s m a x = 100 μ m . Reference trajectory: 21F31v6. The naming convention for the ejecta model parameters follows Spahn et al. ( 2006 ) t C A UTC N a C A km v C A km/s ( λ , ϕ ) C A ∘ Tar.
Middle: Instrument orientation during Europa flyby. The ejecta cloud particles arrive from the anti-apex direction of the spacecraft, which is aligned with the SUDA boresight (+x) at the time of closest approach. Throughout the flyby, the spacecraft controls its attitude such that the boresights of the remote sensing instruments (+y) are oriented in the nadir direction. As a consequence, the SUDA sensitive area is time dependent and is maximum at closest approach. Top: The impact rate of Europa ejecta depends strongly on the spacecraft altitude and follows a similar trend as the instrument’s sensitive area due to the nadir tracking.
The maximum of a Catalog per SUDA activity is 216 kB, with 212.7 kB available for event data. This is equivalent to 6645 PEs (28 bytes each) or 53 , 172 PNEs (4 bytes each). Assuming that 50% of the detections made during a typical SUDA Europa flyby are noise events, the Catalog can accommodate up to 5859 PEs (total 11 , 718 events), which exceeds the predicted maximum number of detectable ejectas per flyby of approximately 2100 (see Table 10 ) by a factor of approximately 2.8.
Given the pronounced dependence of the number of collected ejecta N e as well as of the spatial resolution of the resulting composition map on the flyby attitude, low-altitude flybys (i.e., those lower than 35 km ) of particular value for the implementation of the scientific objectives. It is only during low-altitude flybys that the spatial resolution of SUDA composition maps is sufficient to resolve Unique Geographical Locations (UGL) on Europa’s surface. UGLs are geologically young landforms on Europa’s surface, such as Thrace Macula (see Daubar et al. 2024 , this collection).
Table 11 Predictions for the Clipper flybys on Ganymede, based on the model given in Krivov et al. ( 2003 ). In total, SUDA will collect more than 200 samples from Ganymede’s surface. Given are the time at closest approach (C/A) t C A , the total number of ejecta ≥ 200 nm N , the spacecraft (S/C) altitude at C/A a C A , the S/C speed relative to Ganymede at CA v C A . SUDA will not perform measurement during G07 flyby because of the Sun Keep-out Zone violation.
Impact ejecta are important references to establish regional and global stratigraphy of planetary bodies. Canonical views advocate radial distributions of distal ejecta with respect to the source crater, and their trajectories are significantly deflected on fast-rotating bodies. The Hokusai crater on Mercury formed a peculiar ray that features a hyperbola shape, and the sharp swerve of orientation was interpreted as a sign of a faster planetary rotation in the near past. Here, we show that this ray was not caused by a hypothesized larger Coriolis force, but due to abruptly-steepened ejection angles. Heterogeneous shock impedances of pre-impact impactor and/or target, such as topographic undulations, affect local propagation paths of shock and rarefaction waves, causing sudden changes of ejection angles. Distal ejecta with non-radial distributions are an inherent product of planetary impacts, and their unobvious provenances could mislead stratigraphic interpretations and hamper age estimations based on spatial densities of impact craters.
Canonical views advocate radial distributions of distal ejecta with respect to the source crater, and their trajectories are significantly deflected on fast-rotating bodies. The Hokusai crater on Mercury formed a peculiar ray that features a hyperbola shape, and the sharp swerve of orientation was interpreted as a sign of a faster planetary rotation in the near past. Here, we show that this ray was not caused by a hypothesized larger Coriolis force, but due to abruptly-steepened ejection angles.
Impact cratering forms various forms of ejecta that can be deposited over far-flung areas, and those landed at more than about five radii of the source crater are termed as distal ejecta 1 . Distal ejecta are basic references used to apply the law of superposition on exterrestrial bodies, so regional and global stratigraphic systems can be established 2 . It is a conventional knowledge that ejection angles during impact cratering are usually 45° ± 15° (refs. 3 – 5 ), and distal ejecta generally follow radial distributions 1 .
Recent impact modeling revealed that topographic roughness of pre-impact target 6 , 7 , inelastic collisions of impact ejecta 8 – 10 , and oblique impact angles 11 could affect the overall radial distribution of impact rays. Meanwhile, it is well understood that the Coriolis force, especially on fast-rotating planetary bodies such as the Earth and Mars, possesses significant deflections on trajectories of distal ejecta, causing non-radial distributions 12 – 14 . On the airless planet Mercury, a hyperbola-shaped impact ray is prominent at the western hemisphere, which exhibits a sharp swerve in the orientation (Fig. 1 ).
In this work, using high-resolution and multi-band optical images returned by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft 17 , we show that the curved ray was formed by the late-Kuiperian-aged (≪300 Myr 18 ) Hokusai crater, which is located at the northern high latitude of Mercury (Fig. 1a ). Based on comprehensive modeling for ballistic trajectories of impact ejecta, this study further reveals that the sharp change of orientation was not caused by a poised faster spin speed of Mercury, but due to azimuthally non-uniform ejection angles.
Therefore, the curved ray was likely formed by downrange ejecta excavated by the Hokusai crater, with ballistic ranges of over 4500 km from the crater center (Fig. 1a ). Impact rays with non-radial distributions with respect to their source craters are common on planetary bodies, such as the Moon 8 , 22 , 23 , Mars 24 , and Mercury 20 . However, secondary craters (secondaries) are usually visible in such rays, and their morphology is indicative to the azimuth of landing of the impacting ejecta (Fig. 2 ).
Physical simulations for the formation of secondaries revealed that during low velocity and oblique impacts
Falsification of a recent faster rotation of Mercury By updating the ballistic trajectory model for impact ejecta on rotating planetary bodies 12 , this work modeled landing positions and flight times ( T ) for large numbers of ejecta particles (see Methods). The model did not include the effect of rotational dissociation 27 or mutual collisions 9 , 10 of ejecta during flight, which may change their post-ejection trajectories. Therefore, ejecta with the same azimuth of landing had the same azimuth of ejection.
For each input parameter, the best-fit value was derived based on comparison with the observed distribution patterns of distal ejecta. The effect of local topography on landing positions and processes that may change post-ejection trajectories of particles, such as mutual collisions and rotational separation during flight, are not included in the model. The effect of planetary rotation on ballistic trajectories of impact ejecta is considered in the model, and w is rotation angular velocity, r is the radius of the target body.
Afterward, the ellipse eccentricity ( e ) is determined as: 5 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$e={(1-{({v}_{0}r\sin {\theta }_{0})}^{2}/(aGM))}^{1/2}$$\end{document} e = ( 1 − ( v 0 r sin θ 0 ) 2 / ( a G M ) ) 1 / 2 The ballistic range of ejecta (Δ) is then calculated based on the ellipse equation r = a (1- e 2 )/(1+ e cos ν ), where ν is the true anomaly at the beginning of the trajectory.
Ranges of ejection locations from the transient crater of Hokusai The effect of different positions of ejection from a transient on ballistic trajectories of impact ejecta are considered in this work. The Hokusai crater on Mercury has a rim-to-rim diameter of D = 114 km, and the empirical relationship between D and the diameter of the transient crater D t 64 is used to estimate the diameter ( D t ) for its transient crater.
The Earth is constantly bombarded by meteoroids of various sizes. During hypervelocity collisions a large amount of energy is coupled to the Earth's atmosphere leading to disruption of decimeter to hundred meter-sized meteoroids. Smaller meteoroids may form meteorite strewn fields while larger initial bodies and high-strength iron meteoroids may form impact crater strewn fields. Impact crater strewn fields are ephemeral and none documented to date are older than about 63,500 years. Here we report on a newly discovered impact crater strewn field, about 280 Myr old, in tilted strata of the Rocky Mountains near Douglas, Wyoming. It is the oldest and among the largest of impact crater strewn fields discovered to date, extending for a minimum of 7.5 km along a SE-NW trajectory. The apparent width of the strewn field is 1.5 km, but the full extent of the crater strewn field is not yet constrained owing to restricted exposure. We probably see only a small section of the entire crater strewn field. The cascade of impacts occurred in an environment that preserved the craters from destruction. Shock lithification aided this process.
During hypervelocity collisions a large amount of energy is coupled to the Earth’s atmosphere leading to disruption of decimeter to hundred meter-sized meteoroids. Smaller meteoroids may form meteorite strewn fields while larger initial bodies and high-strength iron meteoroids may form impact crater strewn fields. Impact crater strewn fields are ephemeral and none documented to date are older than about 63,500 years. Here we report on a newly discovered impact crater strewn field, about 280 Myr old, in tilted strata of the Rocky Mountains near Douglas, Wyoming.
The Earth´s atmosphere protects us from the majority of hypervelocity collisions with such small bodies 2 . Once the aerodynamic stresses exceed the strength of a meteoroid it will disrupt while passing through the atmosphere 3 , 4 . Decimeter to meter-sized meteoroids mostly disintegrate into a number of small fragments that decelerate to terminal velocity and form a meteorite strewn field 5 . The Carancas impact event is an exception where a meter-sized chondritic meteoroid survived atmospheric entry and created a 14 m diameter crater 6 .
Decameter-sized objects disrupt violently at high altitudes and can create airbursts that damage infrastructures, but then slow down to terminal velocity when passing through the lower atmosphere. An example is the recent Chelyabinsk event 7 that formed a 60 km long and less than a kilometer wide meteorite strewn field. Perhaps the largest meteorite strewn field, Gibeon, Namibia, was formed by an iron octahedrite that dispersed over an elliptical area 275 kilometers long and 100 kilometers wide 5 . Atmospheric protection of Earth becomes less effective for more massive meteoroids, and especially for high strength iron meteoroids.
While brittle deformation is common in the samples, shock features are generally rare and restricted to single grains. Table 3 gives an overview of samples that contain shocked grains. Except for one sample, all other samples were taken in situ . The fist-sized samples were hammered from the quartzitic Casper sandstone. They were taken from different locations with respect to the given crater as detailed in Table 3 . Shocked grains were found in the consolidated ejecta flap downrange and crossrange, in the uplifted crater rim as well as in dikes and breccias of the crater rim (Table 3 ).
( f ) Concussion fracture (right) is massively decorated with fluid inclusion. Boehm lamellae on the left. ( g ) Grain with high density of fluid-decorated fractures. Note that the fractures end at the round shaped original grain surface and do not extend into the syntactic overgrowth seams suggesting that the impact occurred prior to diagenesis. ( h ) Indentation and interlocking of quartz grains led to shock lithification. Hertzian-type concussion fractures follow point-to-point contacts and stress chains through the grains. These fractures are tensile fractures and are filled with fluid inclusions.
(0001); {10–11}; {15–61} in situ SM-1 42°39′7.30″N 105°26′58.73″W WC_1a 42°39′07.02″N 105°26′59.22″W inner crater rim, crossrange Casper Sst. (0001) in situ SM-3/4 42°38′59.32″N 105°26′52.65″W CR_3_8 42°39′00″N 105°26′53.5″W ejecta flap downrange Casper Sst. (0001); {10–14} in situ SM-9 42°38′52.29″N 105°26′57.03″W SM_9_1 42°38′52.40″N 105°26′56.97″W dike, crater center Casper Sst. (0001) (0001) in situ SM-34 42°39′47.45″N 105°27′13.67″W SM_34_4 42°39′46.69″N 105°27′14.13″W breccia, crest Casper Sst. {51–61}? in situ SM-36 42°40′0.37″N 105°27′17.02″W SM_36_5 42°39′59.73″N 105°27′16.55″W dike, uprange Casper Sst.
Deformation affected rounded quartz grains but not the quartzitic overgrowth seams (Fig. 5g ). This implies that the impact occurred in weakly consolidated to unconsolidated sand prior to diagenesis. Like PDFs, all fractures are massively decorated with fluid inclusions suggesting that the pore space was filled with water during the impact. The presence of water made the sand target cohesive. Ballistic experiments in sand imply that water saturation increases the likelihood of creating a coherent overturned crater flap 29 as is apparent in the geomorphology of several of the Sheep Mountain craters (Figs 2c and 4 ).
The elliptical outline of craters 1–4 indicate a shallow impact angle of less than 15° where
However, the age of this buried crater, constrained by well logging, paleontology, and seismic surveying, is 190+/−20 Myr. So a relationship to the Douglas crater strewn field is ruled out if the age of Cloud Creek is correct. The current state of evidence indicates a large 7.5 × 1.5 km impact crater strewn field formed by a violent disruption of a single body of yet unknown size and composition during passage through the atmosphere. In case the strewn field is wider than a few kilometers alternative scenarios should probably be taken into account.
In-situ materials provided adequate shielding against the risk of meteorite ejecta for future lunar surface operations. Both regolith simulant and simulant-epoxy mixtures stopped simulated ejecta projectiles with velocities at or near lunar escape velocity. Those depth penetrations for those two in-situ related materials were shallow inside 3 cm and may prove to be influential in future mission planning of lunar surface operations. The lunar surface poses a potential risk from meteoroid ejecta for future lunar surface operations. The safety ramifications of long-term crewed missions for the Artemis program, NASA’s lunar program, are being investigated or reconsidered from the previous Apollo program. Recent studies observed ejecta debris from meteoroid impacts, traversing over greater distances than originally postulated in this current geological epoch. Two end-member ejecta types that are the most hazardous are: (1) iron lithic fragments, and (2) impact feldspar spheres. A series of hypervelocity impact tests using a two-stage light gas gun (2SLGG) was conducted against a set of proposed materials that are likely to be used for lunar surface operations.
The results of this study have several implications for developing protective measures against lunar ejecta traveling at hypervelocity speeds in lunar environments. The success of regolith and regolith-epoxy simulants in stopping projectiles highlights their potential for use in shielding structures and spacesuit design. Furthermore, the ability of even a single layer of EVA suit material to degrade or vaporize micrometeoroids suggests that multilayer structures could indeed offer effective protection against such lunar hazards. Conversely, the failure of materials like aluminosilicate glass, aerogel, and space shuttle tiles underscores the need for more robust solutions for larger, higher-energy impacts. Additional research should focus on optimizing material properties with in-situ regolith, designing or incorporating layered structures such a Whipple-shields to enhance impact resistance. These findings can inform the development of more reliable protective measures for lunar landing spacecraft, lunar habitats, and spacesuits, thereby ensuring the safety of future lunar missions.
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