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Impact Cratering: A Geologic Process
Abstract
The mechanisms involved in the formation of impact craters are examined theoretically, reviewing the results of recent investigations. Topics addressed include crater morphology, stress waves in solids, the contact and compression stage, the excavation stage, and ejecta deposits. Consideration is given to the scaling of crater dimensions, the crater modification stage, multiring basins, cratered landscapes, atmospheric interactions, and the implications of impact cratering for planetary evolution. Extensive diagrams, graphs, tables, and images of typical craters are provided.
... where r is the spherical distance from the crater/basin center to the landing site and R t is the transient rim-to-rim radius of the crater/basin. According to Melosh (1989) and Xie, Liu, and Xu (2020), we consider R t = 0.84 R for simple craters and R t = 0.84R 0.15 SC R 0.85 for complex craters, where R is the final rim-to-rim radius measured from images, R SC = 9.5 km is the transition radius from simple to complex craters (Pike, 1980). For large basins with dedicated investigations of impact cratering process and ejecta thickness distributions, the transient rim diameters of Orientale, Schrödinger, and Imbrium are referred to the studies of Xie and Zhu (2016), Xu and Xie (2020), and Miljković et al. (2013), and the primary ejecta thickness of Orientale and Schrödinger basin are computed from individual measurements of Xie and Zhu (2016) and Xu and Xie (2020), respectively. ...
... After that, the ejecta from Eratosthenian and Copernican-aged craters deposited, creating an ejecta deposits layer of ∼34 cm thick above. In parallel, impact bombardments continuously pulverized and mixed the topmost layers at the CE-6 landing site, producing a loose, fine-grained regolith layer (Melosh, 1989). Since the thickness of regolith statistically increases with the surface age (Melosh, 1989;Xie et al., 2021), we estimate the median regolith thickness at the CE-6 landing site to be larger than that at the CE-5 landing site (∼2.0 ...
... In parallel, impact bombardments continuously pulverized and mixed the topmost layers at the CE-6 landing site, producing a loose, fine-grained regolith layer (Melosh, 1989). Since the thickness of regolith statistically increases with the surface age (Melosh, 1989;Xie et al., 2021), we estimate the median regolith thickness at the CE-6 landing site to be larger than that at the CE-5 landing site (∼2.0 Ga; Li et al., 2021;Che et al., 2021), which is ∼4-6 m (Qian et al., 2021). ...
Plain Language Summary
In June 2024, China's Chang'e‐6 probe successfully returned the first‐ever lunar farside samples of 1,935.3 g, from the Apollo basin within the South Pole Aitken (SPA) basin, the largest, deepest, and oldest impact basin on the Moon. The age and sources of the samples are vital in understanding the geological history of the landing region and the whole Moon. In this study, we obtain the model ages of the Chang'e‐6 mare unit by impact crater statistics and conduct a thorough survey of the foreign ejecta materials from nearby and distal crater ejecta. We find that the Chang'e‐6 scooped and drilled samples are mainly from the proximal ejecta of nearby small craters and the local regolith, both having a dominant composition of intermediate‐Ti mare basalts. The surface age of the Chang'e‐6 mare unit is ∼2.8 billion years from crater density measurements, which can facilitate the refinement of lunar chronology function. Foreign (non‐mare) materials likely account for a minor fraction (∼9.3%) of the returned samples, primarily from Chaffee S and Vavilov craters. The Eratosthenian‐aged Chaffee S crater may have delivered lunar upper mantle materials previously excavated by the SPA impact, which could potentially be recognized in the Chang'e‐6 samples.
... Impact cratering is arguably the most pervasive geologic process in the solar system [e.g., Melosh, 1989;Osinski and Pierazzo, 2012]. After passage of an impact-generated shockwave and the following rarefaction wave, the residual velocity of material sets up an excavation flow. ...
... Although the collapse of steep crater walls leads to production of a breccia lens, small craters known as simple craters maintain a bowl shape after collapse and the final crater typically has a depth-to-diameter ratio of 1:5 [Melosh and Ivanov, 1999]. At larger sizes, craters undergo floor failure, leading to relatively flat floored craters with central peaks and uplifted strata near their centers [Melosh, 1989]. These complex craters exhibit terraced rims and their depths depend weakly on crater diameter Clayton et al., 2013]. ...
... Since the transition from simple to complex structures is a function of surface gravity (g), with a roughly 1/g dependence, it occurs at different diameters on different planetary bodies [e.g., Melosh, 1989]. On the Moon, the simple-to-complex transition occurs at approximately 20 km [Pike, 1977a,b;1980]. ...
We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on lunar craters in the simple-to-complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block-Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce craters of the same diameter within a suite of simulations, ranging in diameter from 10-26 km, which straddles the simple-to-complex crater transition on Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block-Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of craters in the simple-to-complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple-to-complex craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of lunar craters. This scaling suggests that the simple-to-complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple-to-complex transition occurs at larger sizes on Mercury than Mars.
... For example, according to Pierazzo & Melosh (1999), the pressure for incipient melting is 46 GPa for granite and 135 GPa for dunite. Melosh (1989) lists pressures of incipient melting for limestone (66 GPa, but calcite decarbonation starts already at 45 GPa) and granite (78 GPa, i.e. different from the Pierazzo & Melosh (1999) value). For granite and dunite, the pressure for complete melting is only slightly above the one for incipient melting (Pierazzo & Melosh, 1999). ...
... It does not correspond to the full depth of the crater, because material below the maximum depth of excavation is displaced downwards instead (see e.g. figures 5.13 and 5.14 in Melosh, 1989). Melosh (1989) provided an estimate of the excavation depth H exc from the transient crater radius R: ...
... figures 5.13 and 5.14 in Melosh, 1989). Melosh (1989) provided an estimate of the excavation depth H exc from the transient crater radius R: ...
With the number of confirmed rocky exoplanets increasing steadily, their characterisation and the search for exoplanetary biospheres is becoming an increasingly urgent issue in astrobiology. We aim to investigate the possibility of characterising an exoplanet (in terms of habitability, geology, presence of life etc.) by studying material ejected from the surface during an impact event. For given parameters characterising the impact event, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica or ejected microorganisms could be detected. For a 20 km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust. The collisional evolution is best modelled by considering two independent dust populations, a spalled population consisting of non-melted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapour evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.
... The contribution of the thermal pressure component is dominant at this range or higher peak pressures (Supplementary Materials S2). At these peak pressures, the particle velocity behind the shock wave itself cannot be neglected, because it corresponds to a few km/s at a 30 GPa shock compression for granitic and basaltic rocks [e.g., Melosh, 1989]. Following this, fast adiabatic expansion of compressed materials to the free surface is expected to occur to relax the high-pressure state, because adiabatic expansion effectively reduces the thermal pressure (Supplementary Information S2). ...
... The shocked materials are accelerated or decelerated during the pressure release. The change in the absolute particle velocity during the pressure release u p_release can be calculated using the Riemann invariant along the isentrope, as follows [Melosh, 1989;Kurosawa et al., 2015]: ...
... If the travel direction of an expansion wave is opposite to that of a shock wave, then the absolute value of the particle velocity after the release is ~2u pH . This is widely known as the velocity doubling rule at free surfaces [e.g., Melosh, 1989]. Such situations correspond to the rear surface in a planar shock propagation. ...
Hypervelocity ejection of material by impact spallation is considered a plausible mechanism for material exchange between two planetary bodies. We have modeled the spallation process during vertical impacts over a range of impact velocities from 6 to 21 km/s using both grid- and particle-based hydrocode models. The Tillotson equations of state, which are able to treat the nonlinear dependence of density on pressure and thermal pressure in the strongly shocked matter, were used to study the hydrodynamic and thermodynamic response after impacts. The effects of material strength and gravitational acceleration were not considered. A two-dimensional time-dependent pressure field within a 1.5-fold projectile radius from the impact point was investigated in cylindrical coordinates to address the generation of spalled material. A resolution test was also performed to reject ejected materials with peak pressures that were too low due to artificial viscosity. The relationship between ejection velocity veject and peak pressure Ppeak was also derived. Our approach shows that late stage acceleration in an ejecta curtain occurs due to the compressible nature of the ejecta, resulting in an ejection velocity that can be higher than the ideal maximum of the resultant particle velocity after passage of a shock wave. We also calculate the ejecta mass that can escape from a planet like Mars (i.e., veject higher than 5 km/s) that matches the petrographic constraints from Martian meteorites, and which occurs when Ppeak from 30-50 GPa. Although the mass of such ejecta is limited to from 0.1-1 percent of the projectile mass in vertical impacts, this is sufficient for spallation to have been a plausible mechanism for the ejection of Martian meteorites. Finally, we propose that impact spallation is a plausible mechanism for the generation of tektites.
... When a small, simple crater forms into hard rock, regolith is produced by fragmentation of the target, and regolith is deposited in the crater's ejecta blanket and within its transient crater cavity [Shoemaker et al., 1967]. The regolith unit within the crater cavity, i.e., the breccia lens, is developed during the excavation stage, during which time a bowl-shaped transient crater gravitationally collapses, followed by a process that loose materials move down on the steep wall of the transient crater [Melosh, 1989] and change in volume via dilatancy [Collins, 2014]. ...
... Eventually, some crater sizes reach a situation that the total number of erased craters is equal to that of newly created craters. We call this condition crater equilibrium [Melosh, 1989]. ...
... Therefore, it is necessary to find the part of the ejecta blanket correctly. In literature, the ejecta thickness at the rim is usually considered to be a half of the total rim height [Melosh, 1989]. However, recent works using high-resolution images revealed that for lunar craters and martian complex craters, the ejecta deposit, which is considered to be a part of the regolith unit in this study, might be much thinner than thought [Sharpton, 2014;Sturm et al., 2016]. ...
Impact cratering is likely a primary agent of regolith generation on airless bodies. Regolith production via impact cratering has long been a key topic of study since the Apollo era. The evolution of regolith due to impact cratering, however, is not well understood. A better formulation is needed to help quantify the formation mechanism and timescale of regolith evolution. Here, we propose an analytically derived stochastic model that describes the evolution of regolith generated by small, simple craters. We account for ejecta blanketing as well as regolith infilling of the transient crater cavity. Our results show that the regolith infilling plays a key role in producing regolith. Our model demonstrates that, because of the stochastic nature of impact cratering, the regolith thickness varies laterally, which is consistent with earlier work. We apply this analytical model to the regolith evolution at the Apollo 15 site. The regolith thickness is computed considering the observed crater size-frequency distribution of small, simple lunar craters (< 381 m in radius for ejecta blanketing and < 100 m in radius for the regolith infilling). Allowing for some amount of regolith coming from the outside of the area, our result is consistent with an empirical result from the Apollo 15 seismic experiment. Finally, we find that the timescale of regolith growth is longer than that of crater equilibrium, implying that even if crater equilibrium is observed on a cratered surface, it is likely the regolith thickness is still evolving due to additional impact craters.
... With this in mind we allow that impactors create craters according to accepted crater scaling laws for D > D min (Melosh 1989). For smaller impactors, those in a size range corresponding roughly to the size of the imbricated surface regolith, we will set the diameter of the affected area to D. But what is the appropriate value for D min ? ...
... Hence we would expect porosities of the asteroids composing our sample of S-complex families to be some 7× higher than that of the Moon leading to smaller ejecta blankets on asteroids compared to the Moon. Melosh (1989) describes continuous ejecta blankets as typically coating the lunar surface out to over two crater rim radii from the center. His eq. ...
... We imagine a torus immediately outside the continuous ejecta disc with an area equivalent to integrating the patchy distal ray coverage. Using the lunar crater Timocharis (see Figure 6.2 in Melosh (1989)) as a prototypical case we estimate that replacing the rays with such a torus increases the continuous ejecta disc of ∼ 2.3 R c to about 2.7 R c yielding an area ∼ 38% larger than the continuous ejecta blanket alone. Converting radius to diameter and combining D e = (2.7 ± 0.5) D 1.006 c and eq. ...
We have extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages <10^6 years and to newly identified asteroid pairs with ages <5x10^5 years. We have identified three S-complex asteroid clusters with ages in the range 10^{5-6} years. The average color of the objects in these clusters agree with the prediction of Willman et al., 2008. SDSS photometry of the members of very young asteroid pairs with ages <10^5 years was used to determine their taxonomy. The average color of the S-complex pairs is PC_1=0.49+/-0.03, over 5-sigma redder than predicted by Willman et al., 2008. Therefore, the most likely pair formation mechanism is gentle separation due to YORP spin-up leaving much of the aged and reddened surface undisturbed. In this case our color measurement allows us to set an upper limit of ~64% on the disturbed surface portion. Using pre-existing color data and our new results for the youngest S-complex asteroid clusters we have extended our space weather model to explicitly include the effects of regolith gardening and fit separate weathering and gardening characteristic timescales of tau_w=960+/-160My and tau_g=2000+/-290My respectively. The first principal component color for fresh S-complex material is 0.37+/-0.01 while the maximum amount of local reddening is 0.33+/-0.06. Our first-ever determination of the gardening time is in stark contrast to our calculated gardening time of tau_g~270My based on main belt impact rates and reasonable assumptions about crater and ejecta blanket sizes. A possible resolution for the discrepancy is through a `honeycomb' mechanism in which the surface regolith structure absorbs small impactors without producing significant ejecta. This mechanism could also account for the paucity of small craters on (433) Eros.
... such that for small N , f overlap ∼ 0, corresponding to no overlapping craters, and for large N , f overlap ∼ 1, corresponding to a geometrically saturated surface (e.g., [53,54]) on which all craters will have at least one intersection with another crater. The SFD of terrestrial craters is an important parameter in this model; shallower distributions, with a greater proportion of large craters, will saturate a surface with fewer impactors. ...
... To understand, qualitatively, which comets are most sensitive to the stresses of atmospheric entry we follow a similar argument to Melosh [54]. As a comet enters the atmosphere, it will encounter a column-integrated atmospheric mass P surf /g sin θ per unit area (where P surf is the surface atmospheric pressure, g the gravitational acceleration, and θ the angle with respect to the local horizontal). ...
... We assume in our Monte Carlo model that a secondary impactor is able to drive subsequent prebiotic chemistry provided it is smaller than the initial cometary impactor, to prevent the destruction of ferrocyanide salts. This requirement may not however be strict enough, with peak pressures achieved during the initial compression stage of crater formation typically in the range 100-1000 GPa [54]. Corresponding temperatures are on the order of several thousand degrees, which in many instances exceeds the melting temperature of rock, and will therefore vaporise any ferrocyanide salts. ...
Hydrogen cyanide delivered by cometary impactors can be concentrated as ferrocyanide salts, which may support the initial stages of prebiotic chemistry on the early Earth. One way to achieve the conditions required for a variety of prebiotic scenarios, requiring for example the formation of cyanamide and cyanoacetylene, is through the arrival of a secondary impactor. In this work, we consider the bombardment of the early Earth, and quantitatively evaluate the likelihood of origins scenarios that invoke double impacts. Such scenarios are found to be possible only at very early times (4Gya), and are extremely unlikely settings for the initial stages of prebiotic chemistry, unless (i) ferrocyanide salts are stable on 1000yr timescales in crater environments, (ii) there was a particularly high impact rate on the Hadean Earth, and (iii) environmental conditions on the Hadean Earth were conducive to successful cometary delivery (i.e., limited oceanic coverage, and low (bar) atmospheric surface pressure). Whilst environmental conditions on the early Earth remain subject to debate, this work highlights the need to measure the typical lifetime of ferrocyanide salts in geochemically realistic environments, which will determine the plausibility of double impact scenarios.
... Quaternary sediments were assigned a low κ = 2.6 × 10 3 SI, as an estimate for coarse-grained glacial deposits (e.g., till, outwash) (Gravenor & Stupavsky, 1974). The initial thickness of the breccia layer was set using the scaling equation for a 1.2 km diameter simple impact crater (Melosh, 1989) and optimized using model inversion ( Table 2). The breccia magnetic susceptibility (κ = 0.003 SI) was assigned using the estimate of Suttak (2013). ...
... The breccia magnetic susceptibility (κ = 0.003 SI) was assigned using the estimate of Suttak (2013). Various crater geometric parameters (rim-to-rim diameter, D; true depth, d t ; apparent depth, d a ; rim height, H r and rim-to-basement depth, H) were estimated from the 2-D model profiles for comparison with predicted values obtained using the scaling equations of Melosh (1989) and Pilkington and Grieve (1992) (Table 2). ...
... In other locations, fold axes are more randomly oriented and are not related to the general trend of the rim (Figure 4b). (Collins et al., 2005;Melosh, 1989;Pilkington & Grieve, 1992) Note. The crater apparent depth and breccia thickness estimates were obtained using the online Earth Impact Effects Program (Collins et al., 2005, www.lpl.arizona.edu/ ...
The Charity Shoal structure is a circular, ∼1.2‐km‐diameter, bedrock‐rimmed shoal in eastern Lake Ontario with a ∼20‐m‐deep central basin. The structure has been proposed as a possible Middle Ordovician impact crater or volcanic intrusion. We conducted marine seismic and magnetic surveys (9‐km²) and 3‐D geophysical modeling to better resolve the Charity Shoal subsurface geology and possible origins. Three models were evaluated: (a) a buried (>450 m) impact structure in Mesoproterozoic basement, (b) a maar‐diatreme, (c) a cylindrical, zoned volcanic plug. Seismic profiles and multi‐beam bathymetry revealed >30 m of Quaternary sediments overlying Middle Ordovician (Trenton Group) carbonate bedrock and complex, 3‐dimensional folding and faulting of the structure rim. Magnetic surveys recorded an annular magnetic high (>600 nT) over the structure rim and a central magnetic low (∼500–600 nT) coincident with a ∼−1.7 mGal Bouguer gravity anomaly. The continuity of Trenton Group strata in seismic profiles rules out a previously proposed Mesozoic maar‐diatreme intruded into Paleozoic strata. The zoned volcanic plug model reproduced the annular magnetic anomaly but was incompatible with Bouguer gravity profiles. The magnetic anomaly was best reproduced by a simple impact structure seated in Mesoproterozoic basement at 450–500 m depth with a rim‐to‐rim diameter of ∼1.2 km and rim height of ∼10–20 m. A 100‐m wide and 50‐m‐deep channel in the Mesoproterozoic basement may record fluvial dissection of the southwestern rim. A buried (>450 m), simple impact crater is most compatible with all available geophysical data at Charity Shoal.
... Impact craters on the Moon are formed by the collision of an impacting small body with the lunar surface, and their morphology is governed by both the projectile (e.g., diameter, velocity, and impact angle) and target (e.g., strength, porosity, and gravity) properties (e.g., Bray et al., 2008;Cintala et al., 1977;Melosh, 1989;Silber et al., 2017). Therefore, studies on the morphology of lunar craters can help us to better understand the impact cratering process and the physical properties of both the impactors and lunar surface. ...
... For fresh craters on the Moon, there is a simple-to-complex transition in their morphology as the crater diameter increases (e.g., Chandnani et al., 2019;Krüger et al., 2018;Osinski et al., 2019Osinski et al., , 2023Pike, 1977). A simple crater has a smooth bowled shape, a complex crater develops more complicated features (e.g., scalloped rim, wall terraces, flat floor, and central peak), and a transition crater is an intermediate type between the two (Melosh, 1989). Conventionally, the diameter of transitional craters was found to be 15-25 km based on the breakpoints in the relations between morphometric parameters (e.g., crater depth and rim height) and crater diameter, which were obtained by directly measuring crater dimensions in the spatial domain (e.g., Du et al., 2019;Kalynn et al., 2013;Pike, 1977). ...
... For the rim flank and continuous ejecta, here we assume that both of them refer to the thick ejecta blanket emplaced near the crater rim. As the structural uplift under the ejecta drops to zero at around 1.7 crater radii (Melosh, 1989), we can suppose that the surface topography of the ejecta near two crater radii (i.e., the radius of the continuous ejecta) follows the thickness of continuous ejecta (McGetchin et al., 1973). For a 50-km-diameter crater, this assumption leads to a surface slope of only 0.1°at two crater radii with a baseline of the SLDEM data resolution. ...
The morphology of fresh lunar craters contains information about the physical properties of both the impactors and the lunar surface, and is therefore crucial to our knowledge of the impact cratering process. Spectral analysis is a powerful tool to study crater morphology, as it can reveal the topographic variation on different scales. In this study, we calculate the power spectral densities of the radial distance and elevation of the rim crest, floor, and rim flank outlines of fresh lunar craters. The resulting power spectral density can be decomposed into an average component and a natural variability component. For the average component, we derive the classic morphometric parameter‐crater diameter relations that are consistent with previous studies. For the natural variability component, we find that in general the spectral power increases with wavelength, which can be fitted by a piecewise function with four breakpoints. Among the four breakpoints, the power of the third breakpoint (i.e., the degree‐2 power) is of particular interest, as it determines the ellipticity of the outline. The power of the third breakpoint is found to have a diameter dependence with a peak at 20 km, which indicates that transitional craters are more elliptical than simple and complex craters. The diameter dependence of the power spectral density enables us to generate the synthetic outlines of a crater of a particular size, which can be used to develop a preliminary 3‐dimensional shape model for fresh lunar craters that is useful for improving Monte Carlo modeling of cratered surfaces on the Moon.
... Planetary surfaces are commonly peppered by nonenumerable impact craters that are formed by hypervelocity collisions of small celestial bodies. Prominent mass displacement occurs during impact cratering, and impact ejecta cover much larger areas than interiors of the parent craters 1 . Impact ejecta play a crucial role in both sculpting planetary surface topography and blending materials with diverse provenances, and they also serve as pivotal references in the construction of regional and global stratigraphy for extraterrestrial bodies 2 . ...
... Depending on various factors of impact ejecta, such as the velocity, spatial dispersion, thermophysical status, and local target properties upon landing, impact ejecta that not escape exhibit diverse depositional patterns 3 , e.g., as continuous ejecta deposits, secondary craters (i.e., secondaries), impact rays 4 , melt splashes 5 , etc. Referring to their occurrence positions around parent craters, impact ejecta on terrestrial bodies are classified as proximal and distal ejecta. Proximal ejecta are conventionally regarded as an equivalent of continuous ejecta deposits, which cover areas about 1-2 radii of the parent crater from the crater rim 1 . Ejecta forming continuous ejecta deposits are believed to be mostly composed of solid fragments that had lower velocities than the threshold velocity of forming secondaries 6 . ...
... (ref. 1). For comparison, L of the nearest secondaries cataloged in earlier studies is considerably larger than those observed in this study (Fig. 2a, b). ...
The minimum velocity (v) for impact ejecta to form secondary craters (secondaries) remains enigmatic, but it is a crucial parameter in untangling the fate of impact ejecta on planetary surfaces. By cataloging the distances (L) of the nearest secondaries from centers of various-sized (D) primary craters (primaries) on the Moon, Mars and Mercury, we find that v can be as small as ~25 m/s, and an unified power-law relationship of L = 1.86D0.93 (both in meters) works for both simple and complex craters, regardless of different surface gravity and target properties. This relationship also successfully predicts occurrences of secondaries formed by craters on Venus. The constraint on v explains the common concurrences of structural disturbances in crater walls and continuous ejecta deposits caused by landing of cogenetic ejecta, suggesting that ejecta forming self-secondaries do not need near-vertical ejection angles and tertiary craters should be abundant on terrestrial bodies.
... To extrapolate the Carbon Crater's diameter from the volume of dense rock equivalents above requires scaling assumptions regarding the Carbon Crater's shape. The scaling parameters selected for this stage are taken from the best-available, peer-reviewed sources, including Collins et al. (2005) and Melosh (1989). Figure 3 shows the two-stage crater estimation process. ...
... Known as a transient crater, it has an approximate parabolic shape as shown in Figure 3. The transient crater is assumed to be approximately parabolic (Melosh, 1989). The formula for the volume (Vtc) of a transient crater comes also from Melosh (1989): ...
... The transient crater is assumed to be approximately parabolic (Melosh, 1989). The formula for the volume (Vtc) of a transient crater comes also from Melosh (1989): ...
Equilibrium climate sensitivity modeling relies heavily on paleoclimate records that were affected by meteorite impacts and volcanic explosions. This paper conducts a thought experiment by re-assembling known data and theories to shed new light on anthropogenic carbon waste. Anthropogenic activity, meteorite impacts and volcanic explosions all extract material from the Earth’s crust and deposit it in the atmosphere. This thought experiment tests if the mass of anthropogenic greenhouse gas emissions from 1851 to 2021 is comparable to the mass of atmospheric emissions from historical planetary events. Approximately 3.60 trillion tons of anthropogenic carbon dioxide equivalents are converted into volume and then transformed into the shape of a meteorite impact structure named the “Carbon Crater.” The 35 km crater (bounded by 34%–68% confidence intervals of 25–45 km) would rank 23rd on the list of known impact structures. The 2.6 km projectile required to create the Crater would be a 10 on the 1–10 Torino hazard scale described as “. . .capable of causing global climatic catastrophe. . .” The estimated re-occurrence interval for a projectile of this size is 5.3 million years. An interval of this time in Earth’s recent history predates homo sapiens. The Crater’s volume is exponentially larger than the 1883 eruption of Krakatoa and ranks as an 8 (“mega-colossal”) on the 0–8 Volcanic Explosivity Index. The thought experiment indicates that the Carbon Crater’s scale is comparable with large historical biospheric events. These results likely confer few direct implications for the study of the paleoclimate in climate modeling but might change conceptual understandings of atmospheric human waste. Additional empirical research is required to understand the potential impact of the Carbon Crater on conceptual change.
... To avoid significant errors due to these problems, we calculated the azimuthal average and standard deviation of area 1-2 crater radii away from its center. This region is covered by continuous ejecta (Melosh, 1989;Moore et al., 1974), which allows us to treat it as a representative value of materials exposed by the fresh crater. ...
... The HCP exposures could be caused by the deposition of basaltic mare material ejected from the nearby small maria after the Roche/Edison formation. To model this effect, we integrate the empirical relationship between ejecta thickness and distance from a crater (McGetchin et al., 1973;Melosh, 1989) with one of the global lists of craters with a diameter of 1 km or larger . In this estimation, craters smaller than Roche/Edison are considered because they tend to be younger than the Roche/Edison formation. ...
... Thus, the ejecta from the Roche crater could contain a certain portion of LGA material. In fact, most of the HCP exposures discovered in our spectral analysis are located within the continuous ejecta region (Melosh, 1989;Moore et al., 1974). ...
Linear gravity anomalies (LGAs) on the Moon have been interpreted as ancient magmatic intrusions formed during the lunar expansion. The composition of such ancient subsurface intrusions may offer hints for the lunar thermodynamic state in the initial stage of lunar history. To pose a first compositional constraint on magmatism related to lunar expansion, this study analyzed the spectrum and gravity around craters on LGAs, such as Rowland, Roche, and Edison craters. Using reflectance spectra around the craters, we first surveyed non‐mare basaltic exposures. To test the LGA excavation scenario as a possible origin of the discovered exposures, we then compared the Gravity Recovery and Interior Laboratory data and post‐cratering gravity simulation with the iSALE shock physics code. Our spectral analysis reveals no basaltic exposure around the Rowland crater. Further, the observed termination of LGA at the crater rim contradicts the gravity simulation, which assumes that LGA predates the Rowland crater. These results suggest that LGA formation might postdate the Rowland formation and that lunar expansion lasted even after the Nectarian age. On the other hand, we found that both Roche and Edison craters possess basaltic exposures in their peripheries. Because the gravity reduction inside Roche crater can be reproduced in our simulation, the discovered basaltic exposures are possibly LGA materials ejected from these craters. The composition of those exposures shows that the LGA intrusions at the two locations are composed of low‐titanium magma, indicating that ancient magma during the expansion did not contain ilmenite‐rich melt, perhaps resulting from the low‐ilmenite content of the ancient upper mantle.
... Introduction. -A better knowledge of the impact of a solid object into a granular target has many applied interests from both geologist and ballistic point of view [1,2]. Since a few years, numerous studies have been carried out by physicists interested in the ejection process [3][4][5][6][7][8], the crater morphology [9][10][11][12] and the penetration dynamics [4,[13][14][15][16][17][18][19][20], searching for scaling laws for the crater size [9][10][11][12] and penetration depth [13][14][15][16][17][18][19][20][21][22][23]. ...
... For the ejection process, a spectacular thin granular jet raising very high can be observed after the impact on small grains of rather low packing fraction [3][4][5][6][7] whereas an opening granular corona is seen for larger grains of rather high packing fraction [8]: the effect of air is crucial in the former case [5,6] whereas it is negligible in the latter one. For the crater morphology, the scaling laws found for high energy impacts of planetary interest [1] stand for low energy impacts of small scale laboratory experiments [9][10][11][12], indicating some universal physical processes involved in the crater formation. For the penetration dynamics, the observed deceleration of the impacting sphere towards its final stop is usually explained by a complex drag force resulting from frictional and collisional processes and involving several terms: a linear depth dependent term [17] arising from solid friction and velocity-dependent terms of linear or quadratic form arising from the ballistic [14,17,18] or the fluid mechanics point of view [21]. ...
The penetration by a gravity driven impact of a solid sphere into a granular medium is studied by two-dimensional simulations. The scaling laws observed experimentally for both the final penetration depth and the stopping time with the relevant physical parameters are here recovered numerically without the consideration of any solid friction. Collisional processes are thus found as essential in explaining the physics of the qualitatively observed phenomena whereas frictional processes can only be considered as secondary effects in the granular penetration by impact.
... The thermodynamical behavior is modeled by means of the non-linear Tillotson (1962) equation of state (eos), applicable over a wide range of physical conditions (see also Melosh 1989). The Tillotson eos has two distinct analytical forms, covering different regions of density and (specific) internal energy e. ...
... For cold expanded states (e < ecv and < 0 ) a low-density pressure cutoff is applied by setting p = 0 for / 0 < 0.9 to avoid unphysical tension in states where the material rather forms droplets or fractures instead of remaining a continuum. The required material parameters for 0 , e 0 , e iv , ecv, A, B, a, b, α, β for basalt and water ice are from Benz and Asphaug (1999), and those for iron from Melosh (1989). To estimate the amount of vaporized material after a collision, we define vaporization as e ≥ ecv and < 0 , i.e. falling into region (2) of the Tillotson eos. ...
Collisions between large, similar-sized bodies are believed to shape the final characteristics and composition of terrestrial planets. Their inventories of volatiles such as water, are either delivered or at least significantly modified by such events. Besides the transition from accretion to erosion with increasing impact velocity, similar-sized collisions can also result in hit-and-run outcomes for sufficiently oblique impact angles and large enough projectile-to-target mass ratios. We study volatile transfer and loss focusing on hit-and-run encounters by means of Smooth Particle Hydrodynamics simulations, including all main parameters: impact velocity, impact angle, mass ratio, and also the total colliding mass. We find a broad range of overall water losses, up to 75% in the most energetic hit-and-run events, and confirm the much more severe consequences for the smaller body also for stripping of volatile layers. Transfer of water between projectile and target inventories is found to be mostly rather inefficient, and final water contents are dominated by pre-collision inventories reduced by impact losses, for similar pre-collision water mass fractions. Comparison with our numerical results shows that current collision outcome models are not accurate enough to reliably predict these composition changes in hit-and-run events. To also account for non-mechanical losses we estimate the amount of collisionally vaporized water over a broad range of masses, and find that these contributions are particularly important in collisions of ~Mars-sized bodies, with sufficiently high impact energies, but still relatively low gravity. Our results clearly indicate that the cumulative effect of several (hit-and-run) collisions can efficiently strip protoplanets of their volatile layers, especially the smaller body, as it might be common e.g. for Earth-mass planets in systems with Super-Earths.
... Successful hydrocode simulations of impact scenarios for the formation of the Pluto system involve an impactorto-total mass ratio, γ = 0.3 to 0.5, implying that precursor bodies range from 0.3 MT to 0.7MT (Canup 2011), where the total mass of the Pluto/Charon system, MT = 1.463 × 10 25 grams (Stern et al. 2015 (Canup 2011); for the Pluto/Charon system, vesc ∼ 1 km/s. The peak shock pressure at the point of impact, P ∼ ρv 2 imp ∼ 1 GPa (Melosh 1989), is barely high enough to melt ice (Stewart & Ahrens 2005;Barr & Citron 2011). Impact simulations show that the temperature rise in the interiors of both bodies ∆T ∼ tens of K (Canup 2005). ...
... Another possibility is that the material strength of ice, which has been ignored in all prior simulations of dwarf planet collisions, could modify how the bodies respond to compression during the collision. Strength effects are thought to be important if the yield stress of the material σY ∼ ̺gR, where g is the local gravity (Melosh 1989), a condition not met for rock or ice on the dwarf planets. However, material strength is known to change the partitioning of compressional deformation and heat in the early stages of the impact (Grady 1980), and can lead to radically different outcomes in laboratory (Stickle & Schultz 2011) and numerical (Schultz 2011) collisions between spherical objects. ...
Kuiper Belt objects with absolute magnitude less than 3 (radius 500 km), the dwarf planets, have a range of different ice/rock ratios, and are more rock-rich than their smaller counterparts. Many of these objects have moons, which suggests that collisions may have played a role in modifying their compositions. We show that the dwarf planets fall into two categories when analysed by their mean densities and satellite-to-primary size ratio. Systems with large moons, such as Pluto/Charon and Orcus/Vanth, can form in low-velocity grazing collisions in which both bodies retain their compositions. We propose that these systems retain a primordial composition, with a density of about 1.8 g/cm. Triton, thought to be a captured KBO, could have lost enough ice during its early orbital evolution to explain its rock-enrichment relative to the primordial material. Systems with small moons, Eris, Haumea, and Quaoar, formed from a different type of collision in which icy material, perhaps a few tens of percent of the total colliding mass, is lost. The fragments would not remain in physical or dynamical proximity to the parent body. The ice loss process has not yet been demonstrated numerically, which could be due to the paucity of KBO origin simulations, or missing physical processes in the impact models. If our hypothesis is correct, we predict that large KBOs with small moons should be denser than the primordial material, and that the mean density of Orcus should be close to the primordial value.
... Furthermore, Marinova et al. [79] found that across the full spectrum of simulated impact energies, velocities, and impact angles less than 45°, the mass that escapes is between 3 and 100 times greater than the mass that is placed into orbit; in contrast, higher impact angles can lead to an escape of material that is up to 1000 times greater than that which enters orbit. For small, head-on impacts, combining the ejection velocity relationship of Melosh [81] and the impact scaling relation of Wilhelms and Squyres [82] suggests a relationship of ∝ . , where is the maximum ejection velocity. ...
... , where is the maximum ejection velocity. It is significant to note that Melosh [81] demonstrated that, in the case of small impacts, the peak ejection velocity is proportional to the crater's diameter, while the ejection velocity decreases radially from the point of impact. This indicates that the material closest to the impact site is ejected at higher velocities, whereas material farther from the crater is ejected at lower velocities. ...
Meteorites have intrigued humanity for centuries, representing our enduring pursuit of knowledge and exploration of the cosmos’ enigmas. These celestial objects have not only influenced artistic expression and the formation of myths but have also fostered scientific inquiry. In this regard, meteorites are crucial to space research, offering valuable information about the early solar system, the formation of planets, and the development of organic compounds. Their analysis aids in deciphering cosmic processes and identifying resources that may support future space missions, making them essential for advancing planetary sciences. Meteorites are also cultural heritage items, with most known samples preserved in natural history museums. This paper deals with the Martian meteorites collected to date, focusing on NWA 16788, the largest individual Martian meteorite recovered so far.
... The spall zone is not identified on highly porous targets, such as a sintered glass bead target with 80% porosity, but occurs for those with 60% porosity (Michikami et al., 2007). The spallation occurs due to tensile failure when the compressive stress is reflected at the surface and becomes tensile stress (Melosh et al., 1989). It was reported that all microcraters larger than 50 μm on lunar rocks had spall (Hoerz et al., 1975). ...
... This is because the penetration depth of the impactor increases when the density ratio of the impactor to the target increases. Quantitatively, the equivalent depth of burial of an explosive for an impact has been shown to depend on the density ratio, as given by the following equation (Melosh, 1989): ...
Cratering on small bodies is crucial for the collision cascade and also contributes to the ejection of dust particles into interplanetary space. A crater cavity forms against the mechanical strength of the surface, gravitational acceleration, or both. The formation of moderately sized craters that are sufficiently larger than the thickness of the regolith on small bodies, in which mechanical strength plays the dominant role rather than gravitational acceleration, is in the strength regime. The formation of microcraters on blocks on the surface is also within the strength regime. On the other hand, the formation of a crater of a size comparable to the thickness of the regolith is affected by both gravitational acceleration and cohesion between regolith particles. In this short review, we compile data from the literature pertaining to impact cratering experiments on porous targets, and summarize the ratio of spall diameter to pit diameter, the depth, diameter, and volume of the crater cavity, and the ratio of depth to diameter. Among targets with various porosities studied in the laboratory to date, based on conventional scaling laws (Holsapple and Schmidt, J. Geophys. Res., 87, 1849-1870, 1982) the cratering efficiency obtained for porous sedimentary rocks (Suzuki et al., J. Geophys. Res. 117, E08012, 2012) is intermediate. A comparison with microcraters formed on a glass target with impact velocities up to 14 indicates a different dependence of cratering efficiency and depth-to-diameter ratio on impact velocity.
... Still others would like to know how far a warhead can penetrate the earth prior to detonation [6]. It's also interesting to consider the effect of impact on the medium itself: the nature of the granular splash [7,8] and the morphology of the resulting crater [9,10,11,12,13,14,15]. Our motivation is more general: to find a non-contrived situation permitting the unusual nature of granular mechanics to be both highlighted and characterized. ...
... Evidently, to within sta- tistical uncertainty, the penetration depth scales as the square-root of projectile density with a dynamic range of over one and one half decades. We find the same density dependence as the jet penetration formula [10]. Before closing this section, we offer an alternative means of analyzing penetration data in terms of total drop distance. ...
We present data for the penetration of a variety of spheres, dropped from rest, into a level non-cohesive granular medium. We improve upon our earlier work [Uehara {\it et al.} Phys. Rev. Lett. {\bf 90}, 194301 (2003)] in three regards. First, we explore the behavior vs sphere diameter and density more systematically, by holding one of these parameters constant while varying the other. Second, we prepare the granular medium more reproducibly and, third, we measure the penetration depth more accurately. The new data support our previous conclusion that the penetration depth is proportional to the 1/2 power of sphere density, the 2/3 power of sphere diameter, and the 1/3 power of total drop distance.
... Most studies on the subject distinguish between a near-field and a far-field and perform fits for these two domains; only Monteux and Arkani-Hamed (2016) have chosen to introduce an additional narrow mid-field. Such expressions for the decay of the impact shock can be used to estimate the heating of the target following the formalism developed by Gault and Heitowit (1963) or a variant thereof (Melosh, 1989), and they have been used to define the thermal perturbations ∆T caused by impacts in planetary bodies that influence the course of evolution of mantle (and maybe even core) convection (e.g., Reese et al., 2002;Watters et al., 2009;Roberts and Arkani-Hamed, 2012). Global dynamical evolution models of this type often do not require the highly detailed representation of impacts provided by computationally expensive fully dynamical impact simulations, and their substitution with simple and readily computable first-order representations such as those developed in the following are the principal motivation for this paper. ...
... The impedance-match solution for the peak pressure generated in the collision of two infinite colliding planes has been given by various authors. This brief summary essentially follows Melosh (1989Melosh ( , 2011. ...
New forms of empirical formulae that provide an approximate description of the decay of shock pressure with distance in hypervelocity impacts are proposed. These forms, which are intended for use in applications such as large-scale mantle convection models, are continuous and smooth from the point of impact to arbitrarily large distances, thereby avoiding the need to divide the domain into different decay regimes and yielding the maximum pressure in a self-consistent way without resorting to the impedance-match solution. Individual fits for different impact velocities as well as a tentative general fitting formula are given, especially for the case of dunite-on-dunite impacts. The temperature effects resulting from the shock are estimated for different decay models, and the differences between them are found to be substantial in some cases, potentially leading to over- or underestimates of impact heating and melt production in modeling contexts like mantle convection, where such parameterizations are commonly used to represent giant impacts.
... The larger the impact, the deeper materials it excavates and the more stratigraphic layers it penetrates. Melosh (1989) proposed an equation describing the relationship between the diameter and maximum excavation depth for simple craters [30]: ...
... The larger the impact, the deeper materials it excavates and the more stratigraphic layers it penetrates. Melosh (1989) proposed an equation describing the relationship between the diameter and maximum excavation depth for simple craters [30]: ...
The Chang’E-6 (CE-6) mission successfully returned 1935.3 g of lunar soil samples from the Apollo basin within the South Pole–Aitken basin. One of its scientific objectives is to investigate the subsurface structure and regolith thickness at the landing site. Using remote sensing datasets, we estimated the regolith and basalt thicknesses at the landing site by employing the crater morphology method and crater excavation technique. A total of 53 concentric craters and 108 fresh craters with varying excavation depths were identified. Our results indicate that the regolith thickness at the CE-6 landing site ranges from 1.1 to 7.0 m, with an average thickness of 3.5 m. Beneath the regolith, the basalt layer consists of high-Ti basalt overlaying low-Ti basalt, with a total thickness of approximately 64 to 82 m, of which the high-Ti basalt layer accounts for about 22 to 30 m. Based on the local geological history, we proposed a stratigraphy at the CE-6 landing site. These findings provide valuable geological context for interpreting the Lunar Penetrating Radar data and analyzing the returned samples.
... Knowledge on large impact crater formation in particular has gained significantly in importance in the Earth Sciences for understanding punctuated geological events entailing drastic changes of climate (Artemieva et al., 2017;Gulick et al., 2019), the evolution of life (Goderis et al., 2021;Kring and Bach, 2021;Lowery et al., 2021) and the generation of giant natural resource deposits (Lightfoot, 2017). Large impact crater formation is typically considered to be completed within a matter of minutes (Melosh, 1989). Accordingly, numerical models of large impact cratering are generally tailored to elucidating cratering mechanisms on such very short time scales (Melosh and Ivanov 1999;Wünnemann and Ivanov 2003;Ivanov 2005;Collins et al., 2012;Morgan et al., 2016). ...
... The mare basalts are frequently found within and adjacent to large lunar craters (Jozwiak et al., 2012). Isostatic re-equilibration of crust ( Fig. 3) below large craters is attributed to the mass deficit caused by ballistic ejection of target rocks during cratering (Melosh, 1989). This process requires the deeper parts of the crust to deform viscously. ...
Crater floor fractures are prominent post-cratering structural vestiges that are known from large impact craters on rocky celestial bodies. Two mechanisms have been proposed to explain the formation of crater floor fractures: emplacement of horizontal igneous sheets below crater floors and isostatic re-equilibration of crust underlying target rocks, i.e., crustal relaxation. Here, we use two-layer analogue experiments to model the deformation of lower and upper crust following crater formation, scaled to the physical conditions on Earth, to explore the structural and kinematic consequences of crustal relaxation. Specifically, the structural evolution of model upper crust was systematically analysed for various initial depths and diameters of crater floors, gleaned from previous numerical models for average continental crust. The analogue modelling results provide quantitative estimates of the duration, geometry and distribution of deformation zones in the upper crust and, for the first time, a quantitative relationship between the diameter, depth and fracture geometry of crater floors. The experiments also show that crater floor uplift is accomplished by long-wavelength subsidence of the crater periphery, which may operate on time scales of hundreds of thousands of years in nature. We conclude that patterns of natural crater floor fractures, including impact melt rock dikes known from the Sudbury and Vredefort impact structures, can be caused by long-term uplift of the crater floor, compensated by lateral crustal flow toward the crater centre.
... Distal ejecta is not homogeneous, but rather is distributed in ray patterns protruding 192 from the center of the crater (see Figure 4) (Melosh, 1989). The amount of ejecta, including 193 impact melts and glasses, is much higher along a ray than at a similar distance outside The primary limitations for this model arise from the model representation of ejecta. ...
... The composition and thermal history of ejecta can be cal-162 culated from the geometry of the streamtube. A significant fraction of a crater's ejecta163 consists of impact melt, and the amount of melt ejected increases with increasing crater di-164 ameter(Melosh, 1989). The percentage of ejecta that is impact melt increases with distance165 from the crater rim (Liu et al., 2022). ...
... In addition to topographic roughness, there is a rich legacy of crater-counting studies on planetary bodies (Gault, 1970;Melosh, 1989;Xiao & Werner, 2015). These studies generally focus on probability distributions of crater size for given areas, which can ultimately be used as a relative or absolute age-dating technique. ...
... Overprinting could be incorporated into the theory by removing some portion of craters of size R with a frequency that relates to that of all larger craters. There is a large body of research that investigates the probability functions of crater sizes around the lunar surface (Fassett, 2016;Gault, 1970;Melosh, 1989;Xiao & Werner, 2015) which largely suggest that crater sizes on the moon are distributed as a power-law with f(R) ∝ R 2 , where f(R) is the probability density function of crater sizes that are in statistical equilibrium. In particular, we note Gault's definition that equilibrium is a state achieved when the crater production and degradation processes are equal-regardless of the degradation process. ...
Earth's terrestrial surfaces commonly exhibit topographic roughness at the scale of meters to tens of meters. In soil‐ and sediment‐mantled settings topographic roughness may be framed as a competition between roughening and smoothing processes. In many cases, roughening processes may be specific eco‐hydro‐geomorphic events like shrub deaths, tree uprooting, river avulsions, or impact craters. The smoothing processes are all geomorphic processes that operate at smaller scales and tend to drive a diffusive evolution of the surface. In this article, we present a generalized theory that explains topographic roughness as an emergent property of geomorphic systems (semi‐arid plains, forests, alluvial fans, heavily bombarded surfaces) that are periodically shocked by an addition of roughness which subsequently decays due to the action of all small scale, creep‐like processes. We demonstrate theory for the examples listed above, but also illustrate that there is a continuum of topographic forms that the roughening process may take on so that the theory is broadly applicable. Furthermore, we demonstrate how our theory applies to any geomorphic feature that can be described as a pit or mound, pit‐mound couplet, or mound‐pit‐mound complex.
... This is true for cratering and for giant impacts, however for cratering, the infinite target means that impact angle does not matter very much. No matter what the angle, the projectile can not find its way beyond the target, and there is no abrupt transition to 'grazing' behavior until impact angles of around 75 degrees from vertical [Melosh, 1989]. However for giant impacts and other similar-sized collisions [Asphaug, 2010], even a close-to-head-on impact (less than 20 degrees) can allow for lopsided evolution of material. ...
Giant impacts have been suggested to explain various characteristics of terrestrial planets and their moons. However, so far in most models only the immediate effects of the collisions have been considered, while the long-term interior evolution of the impacted planets was not studied. Here we present a new approach, combining 3-D shock physics collision calculations with 3-D thermochemical interior evolution models. We apply the combined methods to a demonstration example of a giant impact on a Mars-sized body, using typical collisional parameters from previous studies. While the material parameters (equation of state, rheology model) used in the impact simulations can have some effect on the long-term evolution, we find that the impact angle is the most crucial parameter for the resulting spatial distribution of the newly formed crust. The results indicate that a dichotomous crustal pattern can form after a head-on collision, while this is not the case when considering a more likely grazing collision. Our results underline that end-to-end 3-D calculations of the entire process are required to study in the future the effects of large-scale impacts on the evolution of planetary interiors.
... As described by Korycansky et al. (2006), we use the Tillotson equation of state (EOS). The Tillotson EOS was derived for cases requiring high-velocity impact calculations, can describe the transition of shocked material into the vapor phase, but cannot represent a two-phase region, i.e., where a liquid and gas co-exist (Melosh 1989). The Tillotson EOS parameters used for the basalt and ice impactors are the same as those listed in Table 1 of Korycansky et al. (2006). ...
We have investigated the 2009 July impact event on Jupiter using the ZEUS-MP 2 three-dimensional hydrodynamics code. We studied the impact itself and the following plume development. Eight impactors were considered: 0.5 km and 1 km porous (\rho = 1.760 g cm^{-3}) and non-porous (\rho = 2.700 g cm^{-3}) basalt impactors, and 0.5 km and 1 km porous (\rho = 0.600 g cm^{-3}) and non-porous \rho = 0.917 g cm^{-3}) ice impactors. The simulations consisted of these bolides colliding with Jupiter at an incident angle of \theta = 69 degrees from the vertical and with an impact velocity of v = 61.4 km s^{-1}. Our simulations show the development of relatively larger, faster plumes created after impacts involving 1 km diameter bodies. Comparing simulations of the 2009 event with simulations of the Shoemaker-Levy 9 events reveals a difference in plume development, with the higher incident angle of the 2009 impact leading to a shallower terminal depth and a smaller and slower plume. We also studied the amount of dynamical chaos present in the simulations conducted at the 2009 incident angle. Compared to the chaos of the SL9 simulations, where \theta is approximately 45 degrees, we find no significant difference in chaos at the higher 2009 incident angle.
... with the density of the target, , in g/cm 3 , the speed of sound in the target, v B , in km/s, and the Hugoniot slope S h = (1 + γ)/2 being related to the Grüneisen parameter γ (Melosh, 1989, App. II). ...
Impactors of different types and sizes can produce a final crater of the same diameter on a planet under certain conditions. We derive the condition for such "isocrater impacts" from scaling laws, as well as relations that describe how the different impactors affect the interior of the target planet; these relations are also valid for impacts that are too small to affect the mantle. The analysis reveals that in a given isocrater impact, asteroidal impactors produce anomalies in the interior of smaller spatial extent than cometary or similar impactors. The differences in the interior could be useful for characterizing the projectile that formed a given crater on the basis of geophysical observations and potentially offer a possibility to help constrain the demographics of the ancient impactor population. A series of numerical models of basin-forming impacts on Mercury, Venus, the Moon, and Mars illustrates the dynamical effects of the different impactor types on different planets. It shows that the signature of large impacts may be preserved to the present in Mars, the Moon, and Mercury, where convection is less vigorous and much of the anomaly merges with the growing lid. On the other hand, their signature will long have been destroyed in Venus, whose vigorous convection and recurring lithospheric instabilities obliterate larger coherent anomalies.
... This function is determined by the two conditions that ! !" is one-half of M tot at φ = 1 by definition and that it is an almost linear function at φ << 1 based on the scaling law derived by experimental and analytical studies of a crater (e.g., Melosh, 1989). Equation (10) is also drawn in Fig. 13. ...
Disruptive collisions have been regarded as an important process for planet formation, while non-disruptive, small-scale collisions (hereafter called erosive collisions) have been underestimated or neglected by many studies. However, recent studies have suggested that erosive collisions are also important to the growth of planets, because they are much more frequent than disruptive collisions. Although the thresholds of the specific impact energy for disruptive collisions (Q_RD^*) have been investigated well, there is no reliable model for erosive collisions. In this study, we systematically carried out impact simulations of gravity-dominated planetesimals for a wide range of specific impact energy (Q_R) from disruptive collisions (Q_R ~ Q_RD^*) to erosive ones (Q_R << Q_RD^*) using the smoothed particle hydrodynamics method. We found that the ejected mass normalized by the total mass (M_ej/M_tot) depends on the numerical resolution, the target radius (R_tar) and the impact velocity (v_imp), as well as on Q_R, but that it can be nicely scaled by Q_RD^* for the parameter ranges investigated (R_tar = 30-300 km, v_imp = 2-5 km/s). This means that M_ej/M_tot depends only on Q_R/Q_RD^* in these parameter ranges. We confirmed that the collision outcomes for much less erosive collisions (Q_R < 0.01 Q_RD^*) converge to the results of an impact onto a planar target for various impact angles and that M_ej/M_tot = C * QR/QRD* holds. For disruptive collisions (Q_R ~ Q_RD^*), the curvature of the target has a significant effect on Mej/Mtot. We also examined the angle-averaged value of M_ej/M_tot and found that the numerically obtained relation between angle-averaged M_ej/M_tot and Q_R/Q_RD^* is very similar to the cases for 45-degree impacts. We proposed a new erosion model based on our numerical simulations for future research on planet formation with collisional erosion.
... All the protoplanets are assumed to be initially differentiated with a 30wt% iron core and 70wt% silicate mantle. In our SPH simulations, we use the Tillotson EOS (Tillotson, 1962) with the parameter sets of granite for the mantle and of iron for the core (Melosh, 1989). Although peridotite is more appropriate for the mantle materials, there is no available parameter set of peridotite for the Tillotson EOS. ...
The Earth was born in violence. Many giant collisions of protoplanets are thought to have occurred during the terrestrial planet formation. Here we investigated the giant impact stage by using a hybrid code that consistently deals with the orbital evolution of protoplanets around the Sun and the details of processes during giant impacts between two protoplanets. A significant amount of materials (up to several tens of percent of the total mass of the protoplanets) is ejected by giant impacts. We call these ejected fragments the giant-impact fragments (GIFs). In some of the erosive hit-and-run and high-velocity collisions, metallic iron is also ejected, which comes from the colliding protoplanets' cores. From ten numerical simulations for the giant impact stage, we found that the mass fraction of metallic iron in GIFs ranges from ~ 1wt% to ~ 25wt%. We also discussed the effects of the GIFs on the dynamical and geochemical characteristics of formed terrestrial planets. We found that the GIFs have the potential to solve the following dynamical and geochemical conflicts: (1) The Earth, currently in a near circular orbit, is likely to have had a highly eccentric orbit during the giant impact stage. The GIFs are large enough in total mass to lower the eccentricity of the Earth to its current value via their dynamical friction. (2) The concentrations of highly siderophile elements (HSEs) in the Earth's mantle are greater than what was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the Earth can contribute to the excess of HSEs. In addition, the estimated amount of iron-bearing GIFs provides significant reducing agent that could transform primitive CO2-H2O atmosphere and ocean into more reducing H2-bearing atmosphere. Thus, GIFs are important for the origin of Earth's life and its early evolution.
... Large craters may lack LE because ice was insufficiently thick, whereas small craters could lack LE because ice was too thick. If 10-km diameter LE craters require at least tens of meters of surface ice (Alexander et al., 2023), then the optimum excavation depth (Abramov et al., 2013;Melosh, 1989) is about an order of magnitude greater than the ice thickness. Further numerical modeling is necessary to test this hypothesis. ...
Martian layered ejecta craters are theorized to form by impacting into an ice‐rich crust. The inference that some equatorial layered ejecta craters are Amazonian indicates that ice has persisted in the tropics. However, the detailed spatial and temporal distribution and evolution of this ice remain unknown, which is critical to constraining Mars' global water cycle and climate change over eons. Here we estimate absolute model formation ages for layered and radial (ballistic) ejecta craters to constrain the spatial and temporal distribution of equatorial ice. The assumption is that radial ejecta form where volatiles are not present in significant quantities. Ages are derived from the density of smaller craters superposed on the ejecta blankets. We examined 73 craters in a 30° × 30° area centered at 15°S, 355°E, with 44 layered and 29 radial ejecta. Layered and radial ejecta craters are mixed over distances comparable to their diameters, which represents an unreasonably short length scale for ground‐ice emplacement. This, along with the lack of trend with age, supports the suggestion that intermittent low‐latitude surface ice—from excursions to high obliquity—could be responsible. Analysis also suggests an increasing proportion of layered ejecta craters with decreasing diameter for those older than 3.4 Ga. This trend would support the hypothesis of more ice being available in early martian history. Conversely, this could indicate that “armoring” preferentially preserves layered ejecta relative to radial ejecta.
... The excavation depth (D E ) here is expressed as the altitude above (or below) the areoid up to which a bolide had excavated down from the surface receiving the impact to form the transient crater (Dasgupta et al., 2022;. The parameter for each crater is estimated using the two formulae given below (Croft, 1985), (Croft, 1980;Melosh, 1989): ...
There is general acceptance among researchers regarding the presence of a cryosphere in the subsurface of Mars. The two main geomorphic indications assumed to support the existence of this cryospheric layer are layered ejecta rampart craters and outflow channels. In essence, a layered ejecta rampart crater is a crater with an ejecta blanket having an asymmetric lobate geometry surrounding it. While there are no outflow channels in the East Coprates Planum, which is located within Mars' equatorial region, it does have a population of layered ejecta rampart craters. Therefore, these craters are the only avenues to study the local cryospheric level at the time of impacts which likely led to their formation. Identification, mapping and dating of Single Layered Ejecta craters (SLE) and Multiple Layered Ejecta craters (MLE), the two types of layered ejecta rampart crater present in East Coprates Planum, was integral to this study. Excavation depths (with respect to the areoid) and ages for each crater was estimated. Statistical surfaces for SLE and MLE craters were drawn to assess the control of local topography on the local cryosphere. It has been estimated, by comparing excavation depths of SLEs and MLE with respect to their time of formation, that the bottom layer of the Martian cryosphere in the study area was at lower altitudes (from areoid) prior to ~2.65 Ga, than it was in ~0.97 Ga. This indicates a change in the Martian cryosphere over time within the study region, and the analysis of the crater excavation depths indicates that the cryosphere became thinner over time. Also, East Coprates Planum being a lower elevated region may have facilitated a greater inflow of groundwater from surrounding regions as compared to the adjacent higher elevated Thaumasia Minor region.
... The hiatus length for Danielson could accurately reflect a longer period between the formation of the lowest layers in Danielson crater, and younger layers that eventually aggregated up to the relevant level on the crater wall, but that would imply an extremely long depositional timescale. As these large impacts are somewhat closely spaced (Figure 5d), the craters could have resulted from atmospheric fragmentation of a bolide (Melosh, 1989). If the craters formed from a fragmentation of a larger bolide then they would have formed simultaneously, and the hiatus would be constrained from an inappropriate portion of the CSFD. ...
Crater counting is a widely applied methodology for dating large areas of planetary surfaces, but is difficult to apply the method to constrain the durations of stratigraphic unconformities. Unconformities with exhumed craters are thought to indicate long hiatuses that can only be indirectly dated through stratigraphic relationships with other surfaces with uniform exposure ages. On Mars, sedimentary deposits with prominent unconformities with exhumed craters are found in layered deposits in the Arabia Terra region as well as Gale crater within Mount Sharp. In this work, we present a Linear Crater Counting methodology and apply it to constrain these unconformities observed in Arabia Terra and in Mount Sharp. The method applies a linear sampling domain correction to conventional two‐dimensional crater size frequency distributions and Bayesian Poisson process statistics in order to constrain the likely durations of these unconformities. We found that unconformities in Arabia Terra were on the order of 0.1–1 Gyr in length and that the unconformity preserved at Mount Sharp is at least 0.2 Gyr in length given estimates of the ages of the host craters. Hiatuses of these lengths constrain the age of the overlying deposits to be Late Hesperian or Amazonian in age. Two utility plots are also provided, along with the derivation, for researchers to apply this method to dating arbitrary geologic contacts on Mars and to adapt it to other bodies.
... Because of cylindrical symmetry, the resulting particle motion in the rock will be radially directed and propagate outward as a stress wave. Mathematical analyses of stress wave propagation in solids (e.g., Kolsky, 1953;Melosh, 1988) have shown that the amplitude of a longitudinal wave (σr) is related to its particle velocity (Vp) and rock density (ρ) by the equation: ...
The generally accepted view in rock blasting is that the sources of energy for the fracture and movement of rock reside in the shock wave and gas action resulting from the explosion, and yet the mechanisms by which these sources interact with the rock have remained unclarified. It has also been noted that up to 50% of the work capacity of an explosive released in a blast cannot be accounted for by field measurements of energy partitioning. In this study, we describe a physical model that details the response of rock to both shock wave and gas action. An analytical model based on momentum conservation is derived to describe the dynamics of shock-driven expansion of the blasthole. Radial expansion of the hole is the key parameter that permits the derivation of the following characteristics of rock response to shock loads: hole expansion time; volume of displaced rock; energy consumed per unit volume; expansion energy efficiency; stress wave pulse length; gas pressure in enlarged hole. Soon after the completion of hole expansion, the shock wave degenerates to an elastic stress wave that runs through the burden. Blasthole expansions of between 50% and 300% of diameter are completed in under 1 ms and, depending on rock properties, consume 32% to 42% of the detonation energy or about 55% of the available mechanical (Gurney) energy. Gas pressure in the enlarged holes in five rock types is between 35 MPa and 650 MPa, and drives the mass movement of burden rock.
... Many prior studies also rely on pressurizing rocks, instead of subjecting them to the high-strain rate conditions expected during shock wave passage. Furthermore, very little data is available across all studies at pressures above 5 GPa, at which point the Hugoniot elastic limit (HEL) is exceeded for most host minerals (e.g., Melosh, 1989). ...
The formation of lunar crustal magnetic anomalies is not well understood, and most anomalies are not associated with any obvious geologic features. To investigate further, we studied lunar craters from 100 to 400 km in diameter (totaling 305 craters) that may have demagnetized the crust. We find that the four craters Chaplygin, Keeler, Gauss, and Fermi are highly likely to have demagnetized the crust, based on our statistical methods. We modeled the magnetic source of these craters as a simple hole in a thin magnetized plate, representing the destruction of a surficial magnetized layer (Hypothesis 1). Alternatively, we also simulated the impact demagnetization of deeper‐seated magnetism in the crust by shock and temperature (Hypothesis 2). Some interior magnetization remains unexplained under both hypotheses, but the destruction of a pre‐existing surficial layer of magnetized material is consistent with the location of the peak in each crater's magnetic field. We also find three of the craters are inversely correlated with remotely sensed iron, further supporting our interpretation that the craters demagnetized a surficial layer. The four craters are located on magnetized ejecta deposits from the South Pole‐Aitken, Orientale, and Crisium basins. Hence, these four craters further support the hypothesis that large provinces of magnetized material on the Moon arise from hot impact ejecta that cooled in a dynamo field.
... This raises uncertainties about whether differential weathering between these granitic types fully accounts for the YCS's formation. Another hypothesis considers the possibility of a meteorite impact, characterized by stages of contact and compression, excavation, and modification (Melosh 1989;Collins et al. 2020;Choi et al. 2022). During the excavation phase, a shock wave radiates through the target rock and diminishes outward, while the modification phase produces bowl-shaped ring structures of varying diameters surrounding the crater (Collins et al. 2020;Choi et al. 2022;Hildebrand et al. 1991;Pilkington et al. 1994;Pilkington and Hildebrand 2000). ...
To ascertain the origin of the Yangju-Circular-Structure (YCS), located north of Seoul, South Korea, extensive gravity surveys were conducted. A collaborative analysis, integrating geology, geomorphology, gravity field interpretation, and density modeling, yielded significant insights:
(1) The eastern region of the YCS, adjacent to the Dongducheon fault line, has been displaced approximately 3,000 m southward. This substantial shift provides crucial evidence of significant tectonic activity affecting the area. (2) Our analyses indicate that the formation of the YCS is unlikely to be a result of differential weathering processes. (3) The YCS features three distinct concentric circular structures with varying in diameter. (4) The subsurface structure beneath the YCS appears to be symmetrical, likely resulting from concentric energy waves caused by an external impact, such as a meteorite.
... The belt-like distribution and the higher albedo are the evidence. The discontinuous ejecta is contrasted to the continuous blanket, which concentrates near the crater rim and covers the preexisting terrain [50]. Discontinuous ejecta lies beyond a distance of four times of the crater radius and is radial with a higher albedo. ...
The samples from lunar farside have great significance for the study of the Moon, and even the solar system. Chang’e-6 landed successfully on the southern mare of the Apollo basin and returned ~2 kg of samples from lunar farside. To provide a better understanding for the background of the returned samples, we conducted detailed crater size-frequency distribution (CSFD) measurements in the Chang’e-6 landing region, the southern mare of the Apollo basin. The southern mare is divided into the western mare (W region) and the eastern mare (E region), and then subdivided into five subunits (W1, W2, W3, W4, W5) and three units (E1, E2, E3), respectively, according to the elevation, TiO2, and FeO abundances. Within the W2 and W5 region, more detailed subunits were separated out. The results show that the southern mare surface was active during two epochs, the Imbrian period and the Eratosthenian period. The basalt eruption lasted for ~1.7 Ga, from 3.28 Ga of the eastern mare to 1.54 Ga of the western mare. The W region is younger than the E region, while the three units of the E region have an age of ~3.2 Ga. The ages of the western mare basalts range from 2.98 Ga to 1.54 Ga, lasting for 1.4 Ga. It is worth noting that the age of the basalt at the Chang’e-6 sampling site is ~1.68 Ga, indicating the samples returned may include components with this very young age.
... 154 The mass mej of material ejected after a collision is smaller at a greater velocity vej of 155 ejection. According to formula (7.12.3) from (Melosh, 1989), mej = ρ R 3 Cej vej -ν (g R) 0.5ν , where R 156 is the radius of the crater, g is the gravitational constant, ρ is the density of the material, Cej is a 157 coefficient, and ν equals to 1.7 for water and to 1.2 for sand. According to (Svetsov, 2011), mej is 158 proportional to vej -1.65 . ...
During formation of the Earth and at the stage of the Late Heavy Bombardment, some bodies collided with the Earth. Such collisions caused ejection of material from the Earth. The motion of bodies ejected from the Earth was studied, and the probabilities of collisions of such bodies with the present terrestrial planets were calculated. The dependences of these probabilities on velocities, angles and points of ejection of bodies were studied. These dependences can be used in the models with different distributions of ejected material. On average, about a half and less than 10% of initial ejected bodies remained moving in elliptical orbits in the Solar System after 10 and 100 Myr, respectively. A few ejected bodies collided with planets after 250 Myr. As dynamical lifetimes of bodies ejected from the Earth can reach hundreds of million years, a few percent of bodies ejected at the Chicxulub and Popigai events about 36-65 Myr ago can still move in the zone of the terrestrial planets and have small chances to collide with planets, including the Earth. The fraction of ejected bodies that collided with the Earth was greater for smaller ejection velocity. The fractions of bodies delivered to the Earth and Venus probably did not differ much for these planets and were about 0.2-0.3 each. Such obtained results testify in favour of that the upper layers of the Earth and Venus can contain similar material. The fractions of bodies ejected from the Earth that collided with Mercury and Mars did not exceed 0.08 and 0.025, respectively. The fractions of bodies collided with Jupiter were of the order of 0.001. In most calculations the fraction of bodies collided with the Sun was between 0.2 and 0.5. Depending on parameters of ejection, the fraction of bodies ejected into hyperbolic orbits could vary from 0 to 1. Small fractions of material ejected from the Earth can be found on other terrestrial planets and Jupiter, as the ejected bodies could collide with these planets. Bodies ejected from the Earth could deliver organic material to other celestial objects, e.g. to Mars.
... The nature of this fluidization is poorly understood at present and several weakening mechanisms have been suggested. These include thermal softening 9,10 , interstitial fluid and melt fluidization 11 and Acoustic Fluidization 12,13 . The weakened and fluidized rock mass rebounds vertically and flows laterally into the transient crater, to form a gravitationally stable, shallower and broader crater with a central uplift at the end of the crater modification stage. ...
The Nadir Crater offshore West Africa is a recently proposed near K-Pg impact structure identified on 2D seismic. Here we present 3D seismic data that image this crater in exceptional detail, unique for any such structure, which demonstrates beyond reasonable doubt that the crater-forming mechanism was a hypervelocity impact. Seismic mapping reveals a near-circular crater rim of 9.2 km and an outer brim of ~23 km diameter defined by concentric normal faults. An extended damage zone is evident across the region, well beyond the perceived limit of subsurface deformation for impact craters, except in a ‘sheltered zone’ to the east. The paleo-seabed shows evidence for widespread liquefaction because of seismic shaking, and scars and gullies formed by tsunami wave propagation and resurge. Deformation within the ~425 m high stratigraphic uplift and annular moat allows us to reconstruct the evolution of the crater, with radial thrusts at the periphery of the uplift suggesting a low-angle impact from the east. Structural relationships are used to reconstruct the deformation processes during the crater modification stage, with the central uplift forming first, followed by centripetal flow of surrounding sediments into the evacuated crater floor in the seconds to minutes after impact.
... In this study, Al-Biruni is assumed to have formed as a complex crater, whereas Al-Biruni C is considered a simple crater. The crater diameter (D) is a fundamental morphometric parameter used to estimate the original crater depth (d) [31,32]. A newly compiled global crater database provides improved values for lunar crater morphometric parameters based on high-resolution remote sensing images and altimetry data [33]. ...
Surface unloading due to impact cratering results in lava filling the crater floor. Elevation differences in the crater floor, a common geological phenomenon on the Moon, represent direct evidence of cratering processes. However, few studies have been conducted on mare-filled craters on the Moon. Al-Biruni (81 km) is a farside impact crater with an inclined topographic profile on its floor. We quantitatively measure the morphology of Al-Biruni and model the basaltic lava emplacement to depict the cratering process. Differential subsidence due to melt cooling, wall collapse, impact conditions and structural failure were assessed as potential factors influencing the formation of the elevation differences on the floor. The results suggest that pre-impact topography is a plausible cause of the differences in floor elevation within Al-Biruni. Other factors may also play a role in this process, affecting lava flow by altering the topography of the crater floor after the impact. Thus, regardless of whether the lava inside the crater is impact-generated or comes from outside the crater, altering topography at different stages of the cratering process is an essential factor in creating the sloped terrain on the crater floor.
... The structure of the lunar regolith includes a surface layer of fine powdery material, an intermediate layer of larger particles and rock fragments, and a bottom layer of rocks and large particles [2]. The mixing rate of the lunar regolith is high at shallow depths due to small impacts and decreases with depth [74]. The density of the shallow lunar regolith is about 1.3-1.8 ...
Lunar exploration is of significant importance in the development and utilization of in situ lunar resources, water ice exploration, and astronomical science. In recent years, ground-based radar (GBR) has gained increasing attention in the field of lunar exploration due to its flexibility, low cost, and penetrating capabilities. This paper reviews the scientific research on lunar exploration using GBR, outlining the basic principles of GBR and the progress made in lunar exploration studies. Our paper introduces the fundamental principles of lunar imaging using GBR and systematically reviews studies on lunar surface/subsurface detection, the dielectric properties inversion of the lunar regolith, and polar water ice detection using GBR. In particular, the paper summarizes the current development status of the Chinese GBR and forecasts future development trends in China. This review will enhance the understanding of lunar exploration results using GBR radar, systematically demonstrate the main applications and scientific achievements of GBR in lunar exploration, and provide a reference for GBR radar in future lunar exploration missions.
... The size of the LCROSS crater (D ∼ 22 m) would suggest a maximum excavation depth of ∼2 m (Melosh, 1989), though the low density of the projectile could reduce this somewhat (Schultz, 2006). Models of impact gardening for areas in equilibrium would imply intense mixing of the upper tens of centimeters on ∼100 My timescales, with complete reworking of the excavated column taking ∼1 to 2 Gyr (e.g., Costello et al., 2021). ...
Plain Language Summary
The LCROSS experiment formed an impact crater in an area of permanent shadow on the Moon, striking the surface at 2.5 km/s with a 2,300 kg spent rocket body on 9 October 2009. The impact ejecta from this cratering event included detectable amounts of water and other volatiles, which is perhaps the most direct evidence for significant water deposits on the Moon. However, since the impact location is in permanent shadow (no direct solar illumination), it proved hard to observe definitively the crater that LCROSS formed. Here, we use data from Mini‐RF, which illuminated the surface with S‐band radar, combined with ShadowCam, which acquires images within permanent shadows, to find the probable LCROSS impact crater. The impact crater is 22‐m in diameter, a bit smaller than was inferred indirectly after LCROSS. We also present new evidence that the volatiles in the ejecta likely got there in the last 20% of lunar history, which is important for understanding their origin and evolution.
... However, these studies primarily concentrate on the crystallization at low cooling rates, according to the hypothetical cooling rate of the lunar mantle [45,49]. Unfortunately, slow heating and cooling rates may not accurately represent the properties and behavior of lunar regolith melts generated by small-to-moderate meteorite impacts [50]. These impact melts exhibit higher cooling rates compared to those of the lunar mantle, resulting in rapid solidification characteristics. ...
Melting and solidification of lunar regolith are pivotal for comprehending the evolutionary dynamics of lunar volcanism, geology, and impact history. Additionally, insights gained from these processes can contribute to the advancement of in situ resource utilization technologies, for instance additive manufacturing and resource extraction systems. Herein, we conduct the direct observation of the melting and rapid solidification of lunar particles returned by the Chang’E 5 mission. The melting temperature and melting sequence were obtained. Bubble generation, growth, and release were clearly observed, with a maximum bubble diameter of 5 µm, which is supposed to be according to the release of volatiles that embedded in the particles. During the solidification process, evident crystallization occurred with incremental crystal growth rate approximately of 27 nm/s. Scanning electron microscopy and energy-dispersive x-ray spectroscopy results verified that the Fe-rich mineral crystalizes first. These results would improve the understanding of the evolution of lunar volcanism, geology, and impact history.
... Equation (3) follows from our assumption that the bubble can never be super-saturated (e.g. Zel'dovich & Raizer 1967;Melosh 1989). The "excess" mass-what would supersaturate the bubble if it were a gas5-condenses into dust particles. ...
Vaporized metal, silicates, and ices on the verge of re-condensing into solid or liquid particles appear in many contexts: behind shocks, in impact ejecta, and within the atmospheres and outflows of stars, disks, planets, and minor bodies. We speculate that a condensing gas might fragment, forming overdensities within relative voids, from a radiation-condensation instability. Seeded with small thermal fluctuations, a condensible gas will exhibit spatial variations in the density of particle condensates. Regions of higher particle density may radiate more, cooling faster. Faster cooling leads to still more condensation, lowering the local pressure. Regions undergoing runaway condensation may collapse under the pressure of their less condensed surroundings. Particle condensates will compactify with collapsing regions, into overdense clumps or macroscopic solids (planetesimals). As a first step toward realizing this hypothetical instability, we calculate the evolution of a small volume of condensing silicate vapor -- a spherical test "bubble" embedded in a background medium whose pressure and radiation field are assumed fixed for simplicity. Such a bubble condenses and collapses upon radiating its latent heat to the background, assuming its energy loss is not stopped by background irradiation. Collapse speeds can range up to sonic, similar to cavitation in terrestrial settings. Adding a non-condensible gas like hydrogen to the bubble stalls the collapse. We discuss whether cavitation can provide a way for mm-sized chondrules and refractory solids to assemble into meteorite parent bodies, focusing on CB/CH chondrites whose constituents likely condensed from silicate/metal vapor released from the most energetic asteroid collisions.
Gezegenimizin doğal kaynakları sınırlıdır ve artan nüfus ve tüketim seviyeleri nedeniyle hızla tükenmektedir. Bu senaryo, gelecekte temel hammaddelerin kıtlığına ve ardından fiyat artışlarına neden olabilir. Bu nedenle alternatif kaynaklara olan talep her geçen gün artmaktadır. Ay ve Mars, Dünya'ya yakınlıkları ve bol mineral kaynakları nedeniyle dünya dışı madencilik için en uygun olanaklardır. Uzay madenciliği, Ay ve Mars gibi göksel gezegenlerden kaynakların çıkarılması, işlenmesi ve ticari ürünlere dönüştürülmesini kapsar. Ay'da su buzu, helyum-3, nadir toprak elementleri ve metaller gibi zengin kaynaklar bulunurken Mars'ta su buzu, demir, magnezyum, alüminyum ve diğer metaller bulunmaktadır. Uzaydan elde edilen kaynaklar Dünya'daki kaynak açığını kapatabilir ve teknolojideki ilerlemeleri teşvik edebilir. Ay ve Mars'ta madencilik yapmanın önündeki engeller teknolojik sınırlamaları, dünya dışı ortamın ağır koşullarını ve kaynak çıkarma ve işleme ile ilgili önemli masrafları kapsamaktadır. Bununla birlikte, bu engeller gelecekteki teknolojik yenilikler ve uzay sektörünün ilerlemesiyle aşılabilir.
Abstract: Hypervelocity impact of extraterrestrial materials is one of the key controlling factors in the evolution of the Earth system. Impact cratering produces widespread vaporized, molten and shock metamorphic materials. Tektites, part of distal impact ejecta that are located at more than five radii of the source crater, are quenched from vaporized and molten materials during flight. Tektites are faithful recorders of extreme high-temperature and high-pressure environments, and their magnetic signatures are key information for decoding impact cratering process. The Australasian strewn field (AASF) is the largest (~1×108 km2) and youngest (788,000 years ago) Cenozoic strewn field of tektites and microtektites on Earth, but its source crater is undiscovered yet. AASF tektites formed in an oblique impact from north to south, and the majority of AASF tektites are distributed in the downrange area, i.e., the Indochina Peninsula–Australia–Antarctica and their adjacent areas. South China is part of the uprange area of this strewn field and tektites from this area are insufficiently studied compared to those from the rest of the strewn field. Here, we present rock magnetism study of AASF tektites from Guangdong, Guangxi, and Hainan Provinces. The results show that AASF tektites from South China are dominated by significant paramagnetic signals, and weak ferromagnetic signals are detected. In the entire strewn field, splash-form tektites from South China exhibit the lowest natural remanent magnetization, and saturation isothermal remanent magnetization, and Muong Nong-type tektites from South China exhibit the lowest magnetic susceptibility. Crystallographic investigation of mineral inclusions reveals the presence of nanoscale magnetite particles in Muong Nong-type AASF tektites from South China, consistent with the detected signals of pseudo-single domain magnetite. This study suggests that observed heterogeneous magnetic properties are mainly caused by the different contents and sizes of magnetic particles, which can be explained by the different shock level and/or cooling history of the tektite-forming melts. Although magnetic properties of AASF tektites in different regions show large variations, individual specimens of AASF tektites from South China have relatively homogeneous magnetic properties, indicating that impact melt that formed each tektite specimen had similar compositions and experienced similar thermal history. This study demonstrates the feasibility of rock magnetic studies in untangling formation processes of AASF tektites, and it is an important reference to the search of the potential source crater.
The Chandrayaan-3 mission with the Vikram-lander and the Pragyan rover landed in the high latitude highland region near the south pole of the Moon. The landing site is located ~350 km from the South Pole-Aitken basin rim, an ancient and highly cratered terrain. This site has undergone the complex emplacement sequence of SPA basin ejecta followed by the nearby and distant impact basins and crater ejecta materials. To evaluate the source of individual basin and crater ejecta emplacement over this landing site, we carefully demarcated the nearby and distal basins and craters that could have contributed to the source regolith material. We found that the SPA basin is the major contributor, which deposited nearly ~1400 m of ejecta materials, and 11 other basins deposited ~580 m of ejecta. The other complex craters contributed up to ~90 m of ejecta. Meanwhile, secondary craters of a few km in diameter located adjacent to the landing site contributed to ~0.5 m ejecta, which are crucial target materials for the Pragyan rover insitu analysis. Pragyan rover images revealed the landing site is devoid of >1 m
boulders along the traverse revealing typical highland terrain. The Pragyan rover Navcam and Orbital High Resolution Camera regional images revealed linear distal ejecta rays possibly from the distant impacts as insitu evidence of foreign material at the CH-3 landing site. We found a semi-circular, heavily degraded structure encompassed around the landing site, which is interpreted as a buried impact crater ~160 km in diameter probably formed before the SPA basin. The erasure of pre-SPA basin craters is caused by both the direct burial by SPA basin ejecta, high seismic shaking during SPA formation, and then followed by various post-SPA craters and its associated some of the degradation processes. Overall, Chandrayaan-3 landed within an ancient region that hosts some of the most deeply excavated materials on the Moon.
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