Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth1 and detected by remote sensing on icy bodies in the outer Solar System2,3. The mineralogical evolution of these brines is well understood in regard to terrestrial environments4, but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission5. These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu's parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth's atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes2,3.

Models predict that more than half of all impacting meteoroids should be carbonaceous, reflecting the abundance of carbon-rich asteroids in the main belt and near-Earth space. Yet carbonaceous chondrites represent only about 4 of meteorites recovered worldwide. Here we analyse 7,982 meteoroid impacts and 540 potential meteorite falls from 19 global observation networks and demonstrate that intense thermal stress at low perihelion distances coupled with the filtering effect of Earth`s atmosphere explains this mismatch. Meteoroids repeatedly subjected to intense thermal cycling near the Sun fracture and weaken, removing the most friable objects even before atmospheric entry. Our data also show that tidally disrupted meteoroid streams produce especially fragile fragments that rarely survive to the ground. Consequently, compact, higher-strength, thermally cycled bodies dominate the meteorite record. These findings reconcile the predicted carbonaceous flux with its scarcity in collections, underscoring how orbital evolution and atmospheric filtering shape the materials that reach Earth`s surface.

CubeSat and small satellite resistojet heat exchanger designs are based on conventional concepts that have been used since the 1960s, being primarily limited to helical or twisted tape heat exchangers. The design flexibility enabled by new additive manufacturing technologies is yet to be properly harnessed. This study introduces a novel resistojet concept that incorporates a highly miniaturized lattice structure as the heat exchanger. A conjugate heat transfer analysis determined that the lattice with a diamond unit cell had superior thermal performance compared to the same lattice with a gyroid unit cell and increased the heat transfer rate by up to 11% compared to a helical heat exchanger of the same volume. Performance testing of the prototype thruster with integral diamond lattice indicated that specific impulses of up to 94 s were possible with a 30-Watt heater using nitrous oxide as the propellant. The prototype thruster weighed only 22 g and demonstrated a 67% reduction in the power required to achieve the same specific impulse as previous nitrous oxide resistojets designed for the small satellite platform. The development of highly miniaturized latticed resistojets is shown to be feasible and highly attractive for CubeSats, where mass and power are of the utmost importance.

Samples returned from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 mission revealed that Ryugu is composed of materials consistent with CI chondrites and some types of space weathering. We report detailed mineralogy of the fine-grained Ryugu samples allocated to our "Sand" team and report additional space weathering features found on the grains. The dominant mineralogy is composed of a fine-grained mixture of Mg-rich saponite and serpentine, magnetite, pyrrhotite, pentlandite, dolomite, and Fe-bearing magnesite. These grains have mineralogy comparable to that of CI chondrites, showing severe aqueous alteration but lacking ferrihydrite and sulfate. These results are similar to previous works on large Ryugu grains. In addition to the major minerals, we also find many minerals that are rare or have not been reported among CI chondrites. Accessory minerals identified are hydroxyapatite, Mg-Na phosphate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, manganochromite, eskolaite, ilmenite, cubanite, polydymite, transjordanite, schreibersite, calcite, moissanite, and poorly crystalline phyllosilicate. We also show scanning transmission electron microscope and scanning electron microscope compositional maps and images of some space-weathered grains and severely heated and melted grains. Although our mineralogical results are consistent with that of millimeter-sized grains, the fine-grained fraction is best suited to investigate impact-induced space weathering.

We have discovered a presolar olivine from ALH 77307 with the highest ^25 Mg isotopic composition measured in a silicate to date ( δ ^25 Mg = 3025.1‰ ± 38.3‰). Its isotopic compositions challenge current stellar models, with modeling of magnesium, silicon, and oxygen showing a closest match to formation in a supernova (SN) where hydrogen ingestion occurred in the pre-SN phase. Presolar grains within primitive astromaterials retain records of processes and environmental changes throughout stellar evolution. However, accessing these records has proved challenging due to the average grain size (∼150 nm) of presolar silicates, their sensitivity to extraction agents, and instrumental restrictions, limiting the range of isotopic and chemical signatures which can be studied per grain volume. Here, we present the first known detailed geochemical study of a presolar silicate from a hydrogen-burning SN, studied in 3D without contributions to the analysis volume and at unprecedented spatial resolutions (<1 nm), essential for constraining physical and chemical processes occurring within this recently proposed stellar environment. From our results, we infer either (i) condensation within an environment depleted of heavy elements compatible with the olivine lattice under the pressure and temperature conditions during condensation, or (ii) during periods of limited mixing either near the end of the pre-SN phase or from a collapse so rapid localized pockets of different gas compositions formed.

Understanding the tension between the dynamical and physical characteristics of solar system debris has been a goal of astronomers and planetary scientists for a long time. This study considered a large (>1400) dataset of orbits gathered from six years of fireball observations observed by the Desert Fireball Network. We focused on the meteoroids we detected originating from short-period comet orbits (2 < TJ < 3). We examined how durable they were as they went through the atmosphere and their orbital evolution over the previous ten thousand years. Our results show that almost all of the meteoroids we see in this size range are sourced from the main belt, not the Jupiter-family comet population. The fact that we do not see these objects shows that genetically cometary material in the centimeter size range does not last long in the inner solar system. Even when meteor shower debris is taken into account, the majority of material at centimeter to meter-scales on comet-like orbits is from the main belt.We worked with inclusive criteria to be considered cometary in origin. To be classified as cometary, a meteoroid must be at least a Type II according to the PE criterion and have a >50% probability of originating from an unstable orbit over the previous 10 kyrs. Of the 50 sporadic comet-like fireballs observed by the DFN since 2014, only 2 fulfilled this criterion (figure below). Using a Markov Chain Monte Carlo to draw samples from the posterior distribution, we found that sporadic JFC-like meteoroids in NEO space is 94.2% ± 3.2% from the main belt when considering an uninformed prior. This demonstrates that cometary debris has physical lifetimes in near-Earth space less than the decoherence lifetimes for a stream (

This study investigates the expected cosmic-ray exposure (CRE) of meteorites if they were to be ejected by a near-Earth object, that is, from an object already transferred to an Earth-crossing orbit by an orbital resonance. Specifically, we examine the CRE ages of CI and CM carbonaceous chondrites (CCs), which have some of the shortest measured CRE ages of any meteorite type. A steady-state near-Earth carbonaceous meteoroid probability density function is estimated based on the low-albedo near-Earth asteroid population, including parameters such as the near-Earth dynamic lifetime, the impact probability with the Earth, and the orbital parameters. This model was then compared to the orbits and CRE ages of the five CC falls with precisely measured orbits: Tagish Lake, Maribo, Sutter's Mill, Flensburg, and Winchcombe. The study examined two meteoroid ejection scenarios for CI/CM meteoroids: Main Belt collisions and ejections in near-Earth space. The results indicated that applying a maximum physical lifetime in near-Earth space of 2-10 Myr to meteoroids and eliminating events evolving onto orbits entirely detached from the Main Belt (Q < 1.78 au) significantly improved the agreement with the observed orbits of carbonaceous falls. Additionally, the CRE ages of three of the five carbonaceous falls have measured CRE ages one to three orders of magnitude shorter than expected for an object originating from the Main Belt with the corresponding semi-major axis value. This discrepancy between the expected CRE ages from the model and the measured ages of three of the carbonaceous falls indicates that some CI/CM meteoroids are being ejected in near-Earth space. This study proposes a nuanced hypothesis involving meteoroid impacts and tidal disruptions as significant contributors to the ejection and subsequent CRE age accumulation of CI/CM chondrites in near-Earth space.

Introduction: The Moon’s bombarded surface acts as a 4 Ga cratering record due to the lack of any significant crustal renewal or erosional processes. This extensive record is analysed by studying the spatial densities of craters on the Moon’s surface across different geological landscapes. These differences in relative cratering densities are quantified and linked to radiometrically dated samples collected during the Apollo and Luna missions. Through all this, one can derive a model age of a cratered surface [1]. This technique is referred to as crater counting. Over the years, this technique has grown to be utilised in dating the surfaces of rocky bodies throughout the solar system [1]. The accuracy of this technique still lies in the ability to detect and measure impact craters. The availability of the Lunar Reconnaissance Orbiter (LRO) Narrow-Angle Camera (NAC) images, with a 0.5-2m/px resolution [2], allows us to see craters at the metre-scale. The leading lunar crater database, consisting of ~1.3 million manually counted craters, is complete down to 1km [3]. To utilise the high-resolution NAC imagery, we need to count ever-smaller craters. The number of smaller craters scale as a power law, meaning there are tens of millions of craters 80%) (Tab. 1). True crater diameters across both terrains were mostly overestimated by the CDA (Tab. 1) but are still within the error margins (±20%) of human manual detections [8]. 18 ‘fresh craters’ (craters with clear and easily identifiable rims), with D≥25m, were chosen across both images (Tab. 1*). The CDA has excellent TP detection rates (100%) and diameter estimations (~±10%) for the fresh craters (Tab. 1*).Prospects of the Lunar CDA: With further analysis and evaluation of the lunar CDA across broader lunar terrains, we want to create an accurate small crater lunar dataset. This dataset will fill the gap for craters

Transmission electron microscopy analyses of Hayabusa2 samples show that Ryugu organic matter exhibits a range of morphologies, elemental compositions, and carbon functional chemistries consistent with those of carbonaceous chondrites that have experienced low-temperature aqueous alteration. Both nanoglobules and diffuse organic matter are abundant. Non-globular organic particles are also present, and including some that contain nanodiamond clusters. Diffuse organic matter is finely distributed in and around phyllosilicates, forms coatings on other minerals, and is also preserved in vesicles in secondary minerals such as carbonate and pyrrhotite. The average elemental compositions determined by energy-dispersive spectroscopy of extracted, demineralized insoluble organic matter samples A0107 and C0106 are C 100 N 3 O 9 S 1 and C 100 N 3 O 7 S 1 , respectively, with the difference in O/C slightly outside the difference in the standard error of the mean. The functional chemistry of the nanoglobules varies from mostly aromatic C=C to mixtures of aromatic C=C, ketone C=O, aliphatic (CH n ), and carboxyl (COOH) groups. Diffuse organic matter associated with phyllosilicates has variable aromatic C, ketone and carboxyl groups, and some localized aliphatics, but is dominated by molecular carbonate (CO 3 ) absorption, comparable to prior observations of clay-bound organic matter in CI meteorites.

Detection of extra-terrestrial geochemical components in melt generated during meteorite impact provides diagnostic evidence that can be used to confirm a hypervelocity impact event, and in some cases, classify the projectile. However, projectile contamination is often present at sub-percent levels, and can be difficult to detect. In contrast, meteoritic abundances in glass from small impact craters (<1 km diameter) formed by iron meteorites can be anomalously high, which has been attributed to glass originating from the projectile-target interface. Emulsion textures, immiscible liquids, metal spherules, and non-meteoritic siderophile element ratios have been cited as evidence that the projectile component is typically fractionated in impact glass. Here we present compositional data for impact glass from the Henbury crater field in Australia, where the largest crater is 145 m in diameter and the subgreywacke target rock and IIIAB iron projectile are geochemically distinct. Mixing models (Fe-Si, Ni-Co, Cr-Ir) and high platinum group element abundances indicate average projectile contributions ranging from 3 to 13 % in Henbury glass, comparable to ranges reported in glass from the Kamil (Egypt) and Wabar (Saudi Arabia) impact craters. However meteoritic siderophile element ratios (Fe:Ni, Fe:Co, Ni:Co) in Henbury glass appear nearly unfractionated, whereas Wabar and Kamil glasses have more fractionated ratios. Observed variations are attributed to fractionation of meteoritic Ni by formation of immiscible Ni-rich spherules during oxidation of meteoritic iron, and subsequent separation of Ni-rich spherules from glass during ejection. The Henbury glass sample analyzed is interpreted as an example of an interface melt that quenched prior to extensive oxidation and phase separation, and thus may represent one of the least fractionated samples of melt from the projectile-target interface described thus far, whereas Wabar and Kamil glasses record more evidence of fractionation processes. These results further highlight the influence of metal spherule formation on the composition of ejected glass from small impact structures formed by iron meteorites and provide new insights that explain textural features observed in natural impact glasses.

The Mighei-like carbonaceous (CM) chondrites have been altered to various extents by water-rock reactions on their parent asteroid(s). This aqueous processing has destroyed much of the primary mineralogy of these meteorites, and the degree of alteration is highly heterogeneous at both the macroscale and nanoscale. Many CM meteorites are also heavily brecciated juxtaposing clasts with different alteration histories. Here we present results from the fine-grained team consortium study of the Winchcombe meteorite, a recent CM chondrite fall that is a breccia and contains eight discrete lithologies that span a range of petrologic subtypes (CM2.0-2.6) that are suspended in a cataclastic matrix. Coordinated multitechnique, multiscale analyses of this breccia reveal substantial heterogeneity in the extent of alteration, even in highly aqueously processed lithologies. Some lithologies exhibit the full range and can comprise nearly unaltered coarse-grained primary components that are found directly alongside other coarse-grained components that have experienced complete pseudomorphic replacement by secondary minerals. The preservation of the complete alteration sequence and pseudomorph textures showing tochilinite-cronstedtite intergrowths are replacing carbonates suggest that CMs may be initially more carbonate rich than previously thought. This heterogeneity in aqueous alteration extent is likely due to a combination of microscale variability in permeability and water/rock ratio generating local microenvironments as has been established previously. Nevertheless, some of the disequilibrium mineral assemblages observed, such as hydrous minerals juxtaposed with surviving phases that are typically more fluid susceptible, can only be reconciled by multiple generations of alteration, disruption, and reaccretion of the CM parent body at the grain scale.

Samples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L3 edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe3+) than in Ryugu (<50% Fe3+). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

The Hayabusa2 mission from the Japan Aerospace Exploration Agency (JAXA) returned to the Earth samples of carbonaceous asteroid (162173) Ryugu. This mission offers a unique opportunity to investigate in the laboratory samples from a C-type asteroid, without physical or chemical alteration by the terrestrial atmosphere. Here, we report on an investigation of the mineralogy and the organo-chemistry of Hayabusa2 samples using a combination of micro- and nano-infrared spectroscopy. Particles investigated with conventional FTIR spectroscopy have spectra dominated by phyllosilicate-related absorption, as observed for samples of CI-chondrites, selected ungrouped carbonaceous chondrites, and selected hydrated micrometeorites. Ryugu samples show smaller sulfate-related absorption than CI-chondrites. Our samples that were only briefly exposed to the Earth atmosphere show absorptions related to molecular water, revealing fast terrestrial contamination of the spectral signature at 3 mu m. Overall, our FTIR data are in agreement with other work done on Ryugu samples, revealing a low degree of mineralogical variability across Ryugu samples. AFM-IR mapping of the grains shows the presence of a micrometer-sized organic globule in one of our analyzed grains. The AFM-IR spectra obtained on this globule are similar to IR spectra obtained on IOM suggesting that it is constituted of refractory organic matter. This globule may host silicate in its interior, with a different mineralogy than bulk Ryugu phyllosilicate. The shape, presence of peculiar silicate, and the nature of organic constituting the globule point toward a pre-accretionary origin of this globule and that at least part of Ryugu organics were inherited from the protosolar nebulae or the interstellar media. Altogether, our results show the similarities between Ryugu samples and CI chondrites.

. IntroductionWhen a meteoroid enters the atmosphere, it experiences aerodynamic drag and dynamic pressure. Shock waves can be generated by the hypersonic flight in the atmosphere, fragmentation/airburst and impact in the ground. The hypersonic projectile motion in the atmosphere causes the formation of a Mach cone [1-3]. The shock waves generated during this hypersonic entry propagate almost perpendicular to the trajectory. Fragmentation of the meteoroid creates shock waves that propagate omnidirectionally [1,2]. In large impact events, bolides and/or crater events, the first wave to arrive at the seismic station is the P wave generated directly under the terminal point of the trajectory [4]. After the P wave, air-coupled Rayleigh wave arrive. Airwaves generated by the Mach cone will arrive later as they travel at the speed of sound [1]. The airwave that originates from the point of the trajectory having the shortest distance to the seismic station arrives first and they show the strongest seismic signals in time series data [1,4]. In fireball events, airwaves are a dominant seismic signature [1,4].2. Aim and MethodologyIn this study, we searched for seismic signals from fireballs that have been observed by the Desert Fireball Network (DFN), over a 6-year observational period (2014-2019). The DFN is the world’s largest fireball camera network, located in the Australian outback and consisting of 52 observatories, covering an area of 3 million km2 aimed to detect fireballs, recover meteorites and to calculate their orbits [5,6]. We used processed trajectory data from the DFN [6], with seismic data acquired from the Australian National Seismograph Network (ANSN).The criteria that determined if a seismic signal in time series data could be confidently classified as a signal coming from a fireball event were that the amplitude of the signals must be similar or lower than previously confirmed seismic signals from fireballs, the signal must be within the calculated arrival times of the airwave (direct or ground-coupled Rayleigh wave), there must not be any earthquake activity at the same time, and there must not be any clear anthropogenic-related noise.We checked if a seismic station could encounter the planar wavefront from the Mach cone. If the shortest distance to the seismic station is perpendicular to the bright flight trajectory and arrival times for the airwaves fit, signals are classified as originating from the Mach cone. If the shortest distance is not perpendicular to the bright flight trajectory, any seismic signals (if they fit with calculated arrival times), are assumed to come from an omnidirectional source that could be caused by fragmentation along the trajectory.3. ResultsWeak and short seismic signals were found for 24 fireball events out of 995 surveyed within 200 km of a seismic station (corresponding to 2.4%). The observed seismic signals in our dataset correspond to airwaves (either as direct airwaves or ground-coupled Rayleigh waves). We found 13 fireballs for which we suspect the signals to have originated from the Mach cone traverse and for 11 fireballs we detected signals that might originate from an airburst. No surveyed fireballs were detected by more than one seismic station. The total of 18 out of 24 signals showed the highest peak in vertical component. The shortest distance between the bright flight trajectory to the seismic station is about 50 km. Fireballs for which seismic signals have been detected cover the complete range of impact angles.4. Discussion and ConclusionThe weak and short signals that we see in our data are likely direct airwaves, or ground-coupled Rayleigh waves generated by fireball events. In many cases it is not possible to distinguish whether the signal originated from the direct airwave or ground-coupled Rayleigh wave due to overlapping arrival time windows and background noise. The reason why we see signals of some fireballs and not others is probably due to distance, directionality, noise, wind and properties of the seismic station.We report possible detections of seismic signatures originating from 2.4% of surveyed fireballs observed by the DFN. Unlike other studies who used data from images, seismic stations and infrasound to calculate the orbit and energies of meteors, this study uses information about the trajectory and timing of fireballs observed by the DFN to search for seismic signals.The importance of this work is evident as these impact events occur on a daily basis, yet are rarely reported as seismic events because their impact energy is often not sufficient to cause quakes that are detectable by seismic stations. Furthermore, understanding frequent meteoroid encounters on Earth could help us make better predictions about what may be impacting Earth and other planetary bodies, such as Mars, in terms of small impact events.References[1] Edwards W. N. et al. (2008) Rev. Geophys., 46(4).[2] Tancredi G. et al. (2009) Meteoritics & Planet. Sci., 44, 1967-1984.[3] Tauzin B. et al. (2013) Geophys. Res., 40(14), 3522-3526.[4] Brown P. G. et al. (2003) Meteoritics & Planet. Sci., 38, 989-1003.[5] Devillepoix H. A. R. et al. (2018) Meteoritics & Planet. Sci., 53(10), 2212-2227.[6] Devillepoix H. A. R. et al. (2019) MNRAS, 483(4), 5166-5178.

Introduction: Martian meteorites are the only samples from the Red Planet available for in-depth laboratory analyses. More than 280 pieces of 152 unique samples, originating from at least 11 source craters, are curated in the world’s collections [1]. Ejection ages, based on cosmic ray exposure (CRE), vary from 0.7 to 20 Myr [2-4]. The ejection sites are still unknown, despite several previous propositions [5-7], motivated by the significance of establishing a link between the crystallization ages, and the chemical and mineralogical properties of these samples with surface geology.Secondary impact craters, the key to identify the meteorites ejection sites: The formation of an impact crater generates debris ejected with speeds above and below the escape velocity on Mars (5 km/s). The fraction of ejecta material with a velocity higher than the escape velocity may get through the Martian atmosphere and into the interplanetary space. Numerical simulations suggest that impact events capable of producing such fragments would form craters larger than ~3 km in diameter on the Martian surface [8-9]. Material with a velocity lower than 5 km/s falls back to the surface in a radial pattern or rays around the primary source crater and forms secondary craters with a maximum size of about 2 to 5% of the primary crater diameter [10-12]. These secondaries are shallower than those formed by primary impacts and are rapidly eroded. Typically, a secondary crater of 100 m in diameter would be completely erased in about 50 Myr. Therefore, the occurrence of radial patterns of small secondaries is a diagnostic feature of young primary craters [6,13-14]. The use of high-resolution imagery would allow the identification of such small craters, but manual mapping of the tens of millions of secondary impact craters constellating the surface of Mars is not feasible.Machine learning approach to identify small craters on high-resolution images: We adapted an automated Crater Detection Algorithm (CDA) [15] to identify craters smaller than 1 km in diameter across the entire surface of Mars. The algorithm was trained using High-Resolution Imagery System Experiment (HiRISE) images (25 cm/pixel) and applied on the global Context Camera (CTX) mosaic [16] (6 m/pixel), thus generating a database of more than 90 million detections (Figure 1).Figure 1: Crater Size-Frequency Distribution of the automatic crater database (in red) compared to the manual catalogue (in blue) from [11-12].We evaluated the performance of our model against ~1000 impact craters >60 m (10 pixels in diameter on CTX imagery) manually mapped on 6 different geological units, thus constituting the ground truth (GT). For craters > 100 m in diameter, it results in an average true positive detection rate (or recall) of more than 80% and an average precision of 96%. The diameter estimation derived from our model is within 25% of uncertainty compared to our ground truth (Figure 2), which is within the expected human performance [17].Figure 2: Comparison between the difference in the diameter of craters estimated by the CDA and the ground truth (black crosses). The kernel density estimator shows that detections larger than 100m are estimated within 25% of uncertainties compared to manually identified impact craters.The ejection site of a group of Martian meteorites: We analyzed the spatial and size distribution of craters smaller than 300 m by computing a crater density map from this dataset. The figure 3 presents the results obtained by combining three density maps with red, green, and blue channels corresponding to local crater density (in a 0.05° grid) for three diameter size ranges, in order of decreasing range of diameter, respectively 150 < D < 300 m, 75 < D < 150 m and < 75 m. Using this map, we identified 19 secondary ray systems associated with large and recent primary craters (Figure 3).Figure 3: Density map of craters < 300 m in diameter (89,054,458 entries). Colors indicate crater densities of specific diameter ranges (see text for details).Using crater counts on the ejecta blanket of each of those 19 impact craters and their surrounding terrains, we derived respectively the model age of those impacts and the age of the impacted material. Those results are compared with the ejection age of Martian meteorites as well as their crystallization ages. We found that two craters, both located in the Tharsis volcanic province, are the most likely source of depleted Shergottites launched ~1 Myr ago [3]. Considering the magmatic ages and the petrogenesis of these samples, this implies that a major thermal anomaly deeply rooted in the mantle under Tharsis was active over most of the geological history of the planet, and has sampled a depleted mantle that has retained geochemical signatures of Mars’ early history.References: [1] https://www.lpi.usra.edu/meteor/metbull.php. [2] Herzog, G.F., & Caffee, M.W. Treatise on Geochemistry, 1, 419-454 (2014). [3] Lapen, T.J., et al. Sci. Adv., 3(2), e1600922 (2017). [4] Udry, A., et al. J. Geophys. Res., 25, e2020JE006523 (2020). [5] Werner, S. C., et al., Science, 343, 1343–1346 (2014). [6] McEwen, A.S., et al. Icarus, 176, pp. 351-381 (2005). [7] Mouginis-Mark, P.J. & Boyce, J.M. Chemie der Erde - Geochemistry, 72, 1-23 (2012). [8] Artemieva, N., & Ivanov, B.A. Icarus, 1, 84-101 (2004). [9] Head, J.N., et al., Science, 298, 1752-1756 (2002). [10] Lagain, A., et al., in Reimold, W.U., and Koeberl, C., eds., Large Meteorite Impacts and Planetary Evolution VI, 550, p. 1-16 (2021). [11] Robbins, S.J., & Hynek, B.M. J. Geophys. Res., 117, E05004, (2012). [12] Robbins, S.J. & Hynek, B.M. J. Geophys. Res., 116, E10003 (2011). [13] Hartmann, W.K., et al., Icarus, 208, 621-635 (2010). [14] Tornabene, L.L., et al. J. Geophys. Res., 111, 1-25 (2006). [15] Benedix, G.K., et al. Earth. Sp. Sci., 7, e2019EA001005 (2020). [16] Dickson, J.L., et al., 49th Lunar and Plan. Sci. Conf. (2019). [17] Robbins, S.J., et al. Icarus, 234, 109–131 (2014).

Near-Earth objects (NEOs) are typically fiercely monitored due to the inherent danger of their close encounters. Encounters with more massive objects at distances of a few lunar distances (LD) are relatively commonplace. However, fireball and meteor observation networks from around the world have witnessed ‘grazing’ events occur on several occasions [1, 2, 3, 4, 5]. Grazing events are characterized by their low impact angle and their possible re-entry into interplanetary space. These fireballs display how there are likely many smaller objects, that cannot be detected telescopically, that encounter the Earth all the time. Close encounters can quickly scatter meteoroids into drastically distinct orbits. This process is exemplified by the grazing fireball event detected by the Desert Fireball Network (DFN) in 2017 [5]. During this event, a ≥ 0.3 m object grazed the atmosphere coming from an Apollo-type orbit and exited with a JFC-like orbit. In order to characterize the population of objects in this small size range, we utilized the data collected by the Desert Fireball Network (DFN). The DFN is a continental-scale photographic fireball monitoring network covering over 2.5 million square kilometers of the Australian outback. The Earth’s close encounter flux in the 0.01-100 kg range was estimated using the impact flux observed by the DFN. To do this, several inherent biases had to be taken into account. Some of these biases include: limiting sensitivity of the fireball observatories, seasonal and diurnal variations in the flux, and gravitational focusing. These biases were all taken into consideration. The size-range analyzed in the DFN dataset was cutoff at small-sizes in order to remove the excess of fast, small meteoroids. Whereas, the diurnal and seasonal effects on the average flux of the DFN were considered negligible [6]. Most importantly, gravitational focusing must be corrected for or the flux of slower asteroidal material would be overestimated. The flux enhancement factor was accounted for using the global average enhancement determined by Opik [7], and scaled accordingly based on close encounter ¨ distance. In total, the close encounter population was modeled using 2.3 million test particles. The close encounter simulations, based on the DFN orbital dataset, demonstrated a significant population of close encounters at the centimeter/meter scale. Most of these bodies are negligibly affected during their close encounters; however, many experience considerable orbital changes (Fig. 1). Since the most likely objects to encounter the Earth are those with orbits more similar to the Earth, many close encounters come from asteroid-like (TJ > 3) objects. During the encounter, objects either gain or lose energy resulting in an inverse change to the objects TJ value. In total there appears to be a net gain of objects flung from asteroidal to JFC-like orbits. These encounters are considerably rare (about 0.16% of the total flux within 1.5 LD); however, considering the vast number of objects predicted to have close encounters at these small sizes, the size of this scattered population is not insignificant.References: [1] Z Ceplecha. In: Bull. Astron. Inst. Czechoslov. 30 (1979), pp. 349–356. [2] J Borovicka and Z Ceplecha. In: A&A 257 (1992), pp. 323–328. [3] D. O. Revelle, R. W. Whitaker, and W. T. Armstrong. In: vol. 3116. 1997, pp. 156–167. [4] J.M. Madiedo et al. In: MNRAS 460.1 (2016), pp. 917–922. [5] Patrick M Shober et al. “Where Did They Come From, Where Did They Go: Grazing Fireballs”. In: The Astronomical Journal 159.5 (2020), p. 191. [6] I. Halliday and A.A. Griffin. In: Meteoritics 17.1 (1982), pp. 31–46. [7] E.J. Opik. ¨ In: Proc. R. Ir. Acad. 1951, pp. 165–199.