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GRAIL gravity data shows small-scale gravity anomalies radiating out from the Orientale basin between distances of ∼550 km (average distance to the Cordillera ring) and ∼1000 km. These radial gravity lineations are sometimes associated with secondary crater chains or catenae, of which the specific details of formation and their relation to the basin ejecta remain unclear. However, many of the radial gravity anomalies have no clear topographic signature at all. Typical anomalies have an average width of about 20 km, with a magnitude of ± 20 mGal, and a length of 50–200 km. Here we use gravity inversions, hydrocode modeling, and observations to investigate the radial gravity anomalies in more detail. Density inversion models show that the gravity can be matched by solutions ranging from broad low-amplitude anomalies with density contrasts of ± 20 kg/m3 extending to depths of tens of kilometers, to shallow high-amplitude anomalies with density contrasts of about ± 200–400 kg/m3 confined to the top ∼1–2 km of the crust. Hydrocode models of the low-velocity secondary impacts of basin ejecta show that the impacting material does not disperse – as occurs for hypervelocity impacts – but instead remains as a thin layer within the secondary craters. This remaining secondary projectile material derived from Orientale ejecta can explain the linear gravity anomalies associated with secondary crater chains and catenae. However, this does not explain the majority of linear gravity features that are not associated with secondary crater chains. Therefore, we conclude that the majority of radial gravity anomalies represent the structure of the ejecta deposit. This density variability within the ejecta of basins likely contributes substantially to the density variability of the shallow upper crust as a whole.

We present a simple and efficient implementation of a viscous creep rheology based on diffusion creep, dislocation creep and the Peierls mechanism in conjunction with an elasto-plastic rheology model into a shock-physics code, the iSALE open-source impact code. Our approach is based on the calculation of an effective viscosity which is then used as a reference viscosity for any underlying viscoelastic (or even visco-elasto-plastic) model. Here we use a Maxwell-model which best describes stress relaxation and is therefore likely most important for the formation of large meteorite impact basins. While common viscoelastic behavior during mantle convection or other slow geodynamic or geological processes is mostly controlled by diffusion and dislocation creep, we showed that the Peierls mechanism dominates at the large strain rates that typically occur during meteorite impacts. Thus, the resulting visco-elasto-plastic rheology allows implementation of a more realistic mantle behavior in computer simulations, especially for those dealing with large meteorite impacts. The approach shown here opens the way to more faithful simulations of large impact basin formation, especially in elucidating the physics behind the formation of the external fault rings characteristic of large lunar basins., Comment: 20 pages 6 figures

Here, we develop scaling laws for (1) the distribution of impact-induced heat within the mantle and (2) shape of the impact-induced melt based on more than 100 smoothed particle hydrodynamic (SPH) simulations. We use Legendre polynomials to describe these scaling laws and determine their coefficients by linear regression, minimizing the error between our model and SPH simulations. The input parameters are the impact angle $\theta$ ($0^{\circ}, 30^{\circ}, 60^{\circ}$, and $90^{\circ}$), total mass $M_T$ ($1M_{\rm Mars}-53M_{\rm Mars}$, where $M_{\rm Mars}$ is the mass of Mars), impact velocity $v_{\rm imp}$ ($v_{\rm esc} - 2v_{\rm esc}$, where $v_{\rm esc}$ is the mutual escape velocity), and impactor-to-total mass ratio $\gamma$ ($0.03-0.5$). We find that the equilibrium pressure at the base of a melt pool can be higher (up to $\approx 80 \%$) than those of radially-uniform global magma ocean models. This could have a significant impact on element partitioning. These melt scaling laws are publicly available on GitHub ($\href{https://github.com/mikinakajima/MeltScalingLaw}{https://github.com/mikinakajima/MeltScalingLaw}$)., Comment: Published in Earth and Planetary Science Letters

Multiring basins dominate the crustal structure, tectonics, and stratigraphy of the Moon. Understanding how these basins form is crucial for understanding the evolution of ancient planetary crusts. To understand how preimpact thermal structure and crustal thickness affect the formation of multiring basins, we simulate the formation of lunar basins and their rings under a range of target and impactor conditions. We find that ring locations, spacing, and offsets are sensitive to lunar thermal gradient (strength of the lithosphere), temperature of the deep lunar mantle (strength of the asthenosphere), and preimpact crustal thickness. We also explore the effect of impactor size on the formation of basin rings and reproduce the observed transition from peak‐ring basins to multiring basins and reproduced many observed aspects of ring spacing and location. Our results are in broad agreement with the ring tectonic theory for the formation of basin rings and also suggest that ring tectonic theory applies to the rim scarp of smaller peak‐ring basins.

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., 32 pages, 13 figures, 2 tables, and accepted for publication in JGR-Planets

We used gravity mapping observations from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) to detect, characterize and validate the presence of large impact craters buried beneath the lunar maria. In this paper we focus on two prominent anomalies detected in the GRAIL data using the gravity gradiometry technique. Our detection strategy is applied to both free-air and Bouguer gravity field observations to identify gravitational signatures that are similar to those observed over buried craters. The presence of buried craters is further supported by individual analysis of regional free-air gravity anomalies, Bouguer gravity anomaly maps, and forward modeling. Our best candidate, for which we propose the informal name of Earhart Crater, is approximately 200 km in diameter and forms part of the northwestern rim of Lacus Somniorum, The other candidate, for which we propose the informal name of Ashoka Anomaly, is approximately 160 km in diameter and lies completely buried beneath Mare Tranquillitatis. Other large, still unrecognized, craters undoubtedly underlie other portions of the Moon’s vast mare lavas.

Significance A field of black glassy blobs, strewn across about 20% of Earth’s Eastern Hemisphere, resulted from the impact of a large meteorite about 790,000 y ago. The large crater from which these tektites originated has eluded discovery for over a century, although evidence has long pointed to a location somewhere within Indochina, near the northern limit of the strewn field. We present stratigraphic, geochemical, geophysical, and geochronological evidence that the ∼15-km diameter crater lies buried beneath a large, young volcanic field in Southern Laos., The crater and proximal effects of the largest known young meteorite impact on Earth have eluded discovery for nearly a century. We present 4 lines of evidence that the 0.79-Ma impact crater of the Australasian tektites lies buried beneath lavas of a long-lived, 910-km3 volcanic field in Southern Laos: 1) Tektite geochemistry implies the presence of young, weathered basalts at the site at the time of the impact. 2) Geologic mapping and 40Ar-39Ar dates confirm that both pre- and postimpact basaltic lavas exist at the proposed impact site and that postimpact basalts wholly cover it. 3) A gravity anomaly there may also reflect the presence of a buried ∼17 × 13-km crater. 4) The nature of an outcrop of thick, crudely layered, bouldery sandstone and mudstone breccia 10–20 km from the center of the impact and fractured quartz grains within its boulder clasts support its being part of the proximal ejecta blanket.

NASA's GRAIL mission employed twin spacecraft in polar orbits around the Moon to measure the lunar gravity field at unprecedentedly high accuracy and resolution. The low spacecraft altitude in the extended mission enables the detection of small-scale surface or subsurface features. We analyzed these data for evidence of empty lava tubes beneath the lunar maria. We developed two methods, gradiometry and cross-correlation, to isolate the target signal of long, narrow, sinuous mass deficits from a host of other features present in the GRAIL data. Here, we report the discovery of several strong candidates that are either extensions of known lunar rilles, collocated with the recently discovered “skylight” caverns, or underlying otherwise unremarkable surfaces. Owing to the spacecraft polar orbits, our techniques are most sensitive to east-west trending near-surface structures and empty lava tubes with minimum widths of several kilometers, heights of hundreds of meters, and lengths of tens of kilometers.

The abundance of moderately siderophile elements (“iron-loving”; e.g. Co, Ni) in the Earth's mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure, but it does match expectation for equilibrium at high pressure and temperature. Recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean; under these conditions, cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean. We have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius = 100 km that impact into magma oceans. The impact process strips away the silicate mantle of the planetesimal and then stretches the iron core, dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle. Lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core. The final displacement distance of initially closest tracer pairs gives a metric of core stretching. The statistics of stretching imply mixing that separates the iron core into sheets, ligaments, and smaller fragments, on a scale of 10 km or less. The impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments rain through the magma ocean and settle deeper into the planet. Our results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean.

Martian meteorites are currently the only rock samples from Mars available for direct study in terrestrial laboratories. Linking individual specimens back to their source terrains is a major scientific priority, and constraining the size of the impact craters from which each sample was ejected is a critical step in achieving this goal. During ejection from the surface of Mars by hypervelocity impacts, these meteorites were briefly compressed to high temperatures and pressures. The period of time that these meteorites spent at high pressure during ejection, or the ‘dwell time’, has been used to infer the size of the crater from which they were ejected. This inference requires assumptions that relate shock duration to impactor size, and the relation used by many authors is neither physically motivated nor accurate. Using the iSALE2D shock physics code we simulate vertical impacts at high resolution to investigate the dwell time that basaltic rocks from Mars (shergottites) spend at high pressure and temperature during ejection. Future simulation of oblique impacts will lead to more accurate dwell time estimates. Ultimately, we find that dwell time is insensitive to changes in impact velocity but for a given impact, dwell times are longer for material originating from greater depth and material that experiences higher shock pressures. Using our results, we provide scaling laws for estimating impactor size. During the formation of craters 1.9, 14, and 104 km in diameter, material capable of escaping Mars will have mean dwell times of 1, 10, and 100 ms, respectively.

We model the formation of lunar complex craters and investigate the effect of preimpact porosity on their gravity signatures. We find that while preimpact target porosities less than ~7% produce negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction of pore space due to shock wave compression. The central BA of craters larger than ~215 km in diameter, however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking differences between the gravity signatures of craters on the Earth and Moon are the result of the higher average porosity and variable porosity of the lunar crust.

A parameterized convection model and observations of the puzzling polygons of the Sputnik Planum region of Pluto are used to compute the Rayleigh number of its nitrogen ice and show that it is vigorously convecting, kilometres thick and about a million years old. NASA's New Horizons spacecraft has revealed fascinating details of the surface of Pluto, including a vast ice-filled basin known as Sputnik Planum, which is central to Pluto's geological activity. Much of the surface of Sputnik Planum, consisting mostly of nitrogen ice, is divided into irregular polygons that are tens of kilometres in diameter and whose centres rise tens of metres above their sides. Two papers in this issue of Nature analyse New Horizons images of this polygonal terrain. Both conclude that it is continually being resurfaced by convection, but arrive at contrasting models for the process. Alexander Trowbridge et al. report a parameterized convection model in which the nitrogen ice is vigorously convecting, ten or more kilometres thick and about a million years old. William McKinnon et al. — from the New Horizons team — show that 'sluggish lid' convective overturn in a several-kilometre-thick layer of solid nitrogen can explain both the presence of the cells and their great width. Pluto’s surface is surprisingly young and geologically active1. One of its youngest terrains is the near-equatorial region informally named Sputnik Planum, which is a topographic basin filled by nitrogen (N2) ice mixed with minor amounts of CH4 and CO ices1. Nearly the entire surface of the region is divided into irregular polygons about 20–30 kilometres in diameter, whose centres rise tens of metres above their sides. The edges of this region exhibit bulk flow features without polygons1. Both thermal contraction and convection have been proposed to explain this terrain1, but polygons formed from thermal contraction (analogous to ice-wedges or mud-crack networks)2,3 of N2 are inconsistent with the observations on Pluto of non-brittle deformation within the N2-ice sheet. Here we report a parameterized convection model to compute the Rayleigh number of the N2 ice and show that it is vigorously convecting, making Rayleigh–Benard convection the most likely explanation for these polygons. The diameter of Sputnik Planum’s polygons and the dimensions of the ‘floating mountains’ (the hills of of water ice along the edges of the polygons) suggest that its N2 ice is about ten kilometres thick. The estimated convection velocity of 1.5 centimetres a year indicates a surface age of only around a million years.

Positive free-air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or “mascon.” High-resolution lunar gravity data from the Gravity Recovery and Interior Laboratory spacecraft reveal that these mascons are part of a bulls-eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies, which in turn is surrounded by an outer annulus of positive anomalies. To understand the origin of this gravity pattern, we modeled numerically the entire evolution of basin formation from impact to contemporary form. With a hydrocode, we simulated impact excavation and collapse and show that during the major basin-forming era, the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours. With hydrocode results as initial conditions, we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model, focusing on the mare-free Freundlich-Sharonov and mare-infilled Humorum basins. By constraining these models with measured free-air and Bouguer gravity anomalies as well as surface topography, we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage. The key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses, enabling the inner basin to rise above isostatic equilibrium. Our calculations relate basin size to impactor diameter and velocity, and they constrain the preimpact lunar thermal structure, crustal thickness, viscoelastic rheology, and, for the Humorum basin, the thickness of its postimpact mare fill.

The extreme pressures reached during jetting, a process by which material is squirted out from the contact point of two colliding objects, causes melting and vaporization at low impact velocities. Jetting is a major source of melting in shocked porous material, a potential source of tektites, a possible origin of chondrules, and even a conceivable origin of the Moon. Here, in an attempt to quantify the importance of jetting, we present numerical simulation of jetting during the vertical impacts of spherical projectiles on both flat and curved targets. We find that impacts on curved targets result in more jetted material but that higher impact velocities result in less jetted material. For an aluminum impactor striking a flat Al target at 2 km/s we find that 3.4% of a projectile mass is jetted while 8.3% is jetted for an impact between two equal sized Al spheres. Our results indicate that the theory of jetting during the collision of thin plates can be used to predict the conditions when jetting will occur. However, we find current analytic models do not make accurate predictions of the amount of jetted mass. Our work indicates that the amount of jetted mass is independent of model resolution as long as some jetted material is resolved. This is the result of lower velocity material dominating the mass of the jet.

The Lunar Orbiter Laser Altimeter (LOLA) measures the backscattered energy of the returning altimetric laser pulse at its wavelength of 1064 nm, and these data are used to map the reflectivity of the Moon at zero-phase angle with a photometrically uniform data set. Global maps have been produced at 4 pixels per degree (about 8 kilometers at the equator) and 2 kilometers resolution within 20 deg latitude of each pole. The zero-phase geometry is insensitive to lunar topography, so these data enable characterization of subtle variations in lunar albedo, even at high latitudes where such measurements are not possible with the Sun as the illumination source. The geometric albedo of the Moon at 1064 nm was estimated from these data with absolute calibration derived from the Kaguya Multiband Imager and extrapolated to visual wavelengths. The LOLA estimates are within 2 sigma of historical measurements of geometric albedo. No consistent latitude-dependent variations in reflectance are observed, suggesting that solar wind does not dominate space weathering processes that modify lunar reflectance. The average normal albedo of the Moon is found to be much higher than that of Mercury consistent with prior measurements, but the normal albedo of the lunar maria is similar to that of Mercury suggesting a similar abundance of space weathering products. Regions within permanent shadow in the polar regions are found to be more reflective than polar surfaces that are sometimes illuminated. Limiting analysis to data with slopes less than 10 deg eliminates variations in reflectance due to mass wasting and shows a similar increased reflectivity within permanent polar shadow. Steep slopes within permanent shadow are also more reflective than similar slopes that experience at least some illumination. Water frost and a reduction in effectiveness of space weathering are offered as possible explanations for the increased reflectivity of permanent shadow; porosity is largely ruled out as the sole explanation. The south polar crater Shackleton is found to be among the most reflective craters in its size range globally but is not the most reflective, so mass wasting cannot be ruled out as a cause for the crater's anomalous reflectance. Models of the abundance of ice needed to account for the reflectance anomaly range from 3 to 14% by weight or area depending on assumptions regarding the effects of porosity on reflectance and whether ice is present as patches or is well mixed in the regolith. If differences in nanophase iron abundances are responsible for the anomaly, the permanently shadowed regions have between 50 and 80% the abundance of nanophase iron in mature lunar soil.

We present a model that describes the formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Using the iSALE hydrocode, coupled to the ANEOS equation of state for silica, we create high-resolution two-dimensional impact models to track the motion of impact ejecta. We then estimate the size of the ejecta products using simple analytical expressions and information derived from our hydrocode models. Ultimately, our model makes predictions of how the size of the ejecta products depends on impactor size, impact velocity, and ejection velocity. In general, we find that larger impactor sizes result in larger ejecta products and higher ejection velocities result in smaller ejecta product sizes. We find that a 10 km diameter impactor striking at a velocity of 20 km/s creates millimeter scale melt droplets comparable to the melt droplets found in the Chicxulub ejecta curtain layer. Our model also predicts that melt droplets, melt fragments, and accretionary impact lapilli should be found together in well preserved ejecta curtain layers and that all three ejecta products can form even on airless bodies that lack significant volatile content. This prediction agrees with observations of ejecta from the Sudbury and Chicxulub impacts as well as the presence of accretionary impact lapilli in lunar breccia.

[1] The Rheasilvia impact on asteroid 4 Vesta may have been sufficiently large to create disrupted terrains at the impact antipode. This paper investigates the amount of deformation expected at the Rheasilvia antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses. We find that the magnitude and mode of deformation expected at the impact antipode is strongly dependent on both the sound speed and porosity of Vesta's mantle, as well as the strength of the Vestan core. In the case of low mantle porosities and high core strengths, we predict the existence of a topographic high (a peak) caused by the collection of spalled and uplifted material at the antipode. Observations by NASA's Dawn spacecraft cannot provide definite evidence that large amounts of deformation occurred at the Rheasilvia antipode, largely due to the presence of younger large impact craters in the region. However, a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur.

[1] We numerically modeled the formation of Rheasilvia crater, an enormous impact basin centered on asteroid 4 Vesta's south pole. Using a trial and error method, our models were adjusted to produce the best possible fit to Rheasilvia's size and shape, as observed during the Vesta orbital stage of the Dawn mission. The final model yields estimates of the shock wave decay, escaped material volume, depth of excavation, and other relevant characteristics, to the extent allowed by the two-dimensional (axially symmetric) approximation of the Simplified Arbitrary Lagrangian Eulerian hydrocode. Our model results permit interpretation of the Dawn data on Vesta's shape, topographic crater profiles, and the origin of the Vestoid asteroid family as escaped ejecta from the Rheasilvia crater.

We have constructed a numerical model of spherule formation in an impact produced vapor plume. This model tracks the expansion of the vapor plume using a one-dimensional Lagrangian hydrocode coupled with the ANEOS equation of state for silica. We then include the equations for nucleation and growth as described by homogeneous nucleation theory to describe the process of spherule formation. We use this model to determine the number and size of the spherules that an impact creates. We also explore when and where spherules are formed in the vapor plume, and how this affects the size of the spherules. In general we find that smaller spherules form in the outer, faster moving, portions of the vapor plume at earlier times. This work also explores the effect of impactor size and impact velocity on the resultant spherule size. We report a simple linear dependence on impactor size and a complex dependence on impact velocity. We find that a 10 km diameter asteroid impacting at a velocity of ∼21 km/s creates spherules that are ∼250 μm in diameter which is comparable to the spherules found in the K/Pg boundary layer.

Visible images from the Mars Reconnaissance Orbiter have revealed more than 200 new impact sites on Mars (almost all in dust-mantled regions) containing 1–50 m diameter craters, often in clusters. We count approximately 65,000 small-scale slope streaks within 2 to 3 km of one such cluster and categorize them into four morphologically distinct types. Here we show that these slope streaks (interpreted as dust avalanches) are triggered by the impact event but, surprisingly, are not due to seismic shaking; instead, the dust avalanches are due to airblasts created by the supersonic meteor(s) before impact. Sixteen of the new impact sites are associated with high areal densities of dust avalanches. The observed dust avalanche frequency suggests that impact-generated airblasts constitute a locally important and previously unrecognized process for inducing slope degradation on Mars.

Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.

Impact geologists have long assumed that shock metamorphic features, such as planar fractures and Planar Deformation Features (PDFs) in quartz are reliable indicators of an extraterrestrial impact. A new paper by Chen et al. (2017, https://doi.org/10.1002/2017GL073843) now shows that such features might arise in terrestrial lightning strikes, thus raising the bar for identification of impact sites.

We performed the first global survey of lunar regolith depths using Lunar Reconnaissance Orbiter Camera (LROC) data and the crater morphology method for determining regolith depth. We find that on both the lunar farside and in the nearside, non-mare regions, the regolith depth is twice as deep as it is within the lunar maria. Our data compare favorably with previous studies where such data exist. We also find that regolith depth correlates well with density of large craters (>20 km diameter). This result is consistent with the gradual formation of regolith by rock fracture during impact events.

We describe improvements to the ɛ-α porous compaction model for simulating solar system impacts. To improve the treatment of highly porous materials, we modified the ɛ-α model to account for thermal expansion of the matrix during compaction. We validated the improved model by demonstrating good agreement between numerically computed Hugoniot curves for porous iron (up to initial porosities of ∼80%) using the improved ɛ-α model and experimentally-derived Hugoniot data. Moreover, we verified that the model improvements are easily implemented into a hydrocode and preserve the efficiency advantage of a strain-based compaction function. We used the improved ɛ-α porous compaction model in the iSALE hydrocode to reproduce 2-km/s porous-target laboratory impact experiments. The simulation results were in qualitative agreement with the experiments but produced craters that were consistently deeper and larger in volume than the experiments. The results of the hydrocode simulations and laboratory experiments show a reduction in crater efficiency with increasing porosity. This reduction is more dramatic if the impactor density and velocity are higher.

Hale crater, a 125 × 150 km impact crater located near the intersection of Uzboi Vallis and the northern rim of Argyre basin at 35.7°S, 323.6°E, is surrounded by channels that radiate from, incise, and transport material within Hale’s ejecta. The spatial and temporal relationship between the channels and Hale’s ejecta strongly suggests the impact event created or modified the channels and emplaced fluidized debris flow lobes over an extensive area (>200,000 km2). We estimate ∼1010 m3 of liquid water was required to form some of Hale’s smaller channels, a volume we propose was supplied by subsurface ice melted and mobilized by the Hale-forming impact. If 10% of the subsurface volume was ice, based on a conservative porosity estimate for the upper martian crust, 1012 m3 of liquid water could have been present in the ejecta. We determine a crater-retention age of 1 Ga inside the primary cavity, providing a minimum age for Hale and a time at which we propose the subsurface was volatile-rich. Hale crater demonstrates the important role impacts may play in supplying liquid water to the martian surface: they are capable of producing fluvially-modified terrains that may be analogous to some landforms of Noachian Mars.

We investigate the spatial distributions of boulders ejected from 18 lunar impact craters that are hundreds of meters in diameter. To accomplish this goal, we measured the diameters of 13,955 ejected boulders and the distance of each boulder from the crater center. Using the boulder distances, we calculated ejection velocities for the boulders. We compare these data with previously published data on larger craters and use this information to determine how boulder ejection velocity scales with crater diameter. We also measured regolith depths in the areas surrounding many of the craters, for comparison with the boulder distributions. These results contribute to understanding boulder ejection velocities, to determining whether there is a relationship between the quantity of ejected boulders and lunar regolith depths, and to understanding the distributions of secondary craters in the Solar System. Understanding distributions of blocky ejecta is an important consideration for landing site selection on both the Moon and Mars.

Major impact events have drastically altered the evolution of life on Earth. The reentry of ejecta formed from these events can trigger widespread chemical changes to the atmosphere on a global scale. This mechanism was proposed as a source of HCN during the Late Heavy Bombardment (LHB), 4.1 to 3.8 billion years ago. Significant concentrations of HCN in surface water could directly lead to adenine formation, a precursor for RNA. This work uses the Direct Simulation Monte Carlo (DSMC) method to examine the production of CN and HCN due to the reentry of impact ejecta. We use the Statistical Modeling in Low-Density Environment (SMILE) code, which utilizes the Total Collisional Energy (TCE) model for reactions. The collisions are described by the Variable Soft Sphere (VSS) and Larsen-Borgnakke (LB) models. We compare this nonequilibrium production to equilibrium concentrations from bulk atmospheric heating. The equilibrium HCN yield for a 1023 J impact is 7.0×104 moles, corresponding to a 2.5×1014 molecules p...

The thermodynamic properties of SiO2 are approximated over a range of pressures and temperatures important under the extreme conditions achieved in impacts at typical solar system velocities from 5 to about 70 km/s. The liquid/vapor phase curve and critical point of SiO2 are computed using the equation of state (EOS) program ANEOS. To achieve this goal, two shortcomings of ANEOS are corrected. ANEOS, originally developed at Sandia National Laboratories to describe metals, treats the vapor phase as a monatomic mixture of atoms, rather than molecular clusters. It also assumes a Morse potential for the expanded solid state. Neither of these assumptions is accurate for geologic materials, such as SiO2, that contain molecular clusters in the vapor phase and are better described by a Mie-type potential in the solid state. Using the updates described here, an EOS adequate for numerical hydrocode computations is constructed that agrees well with shock data at pressures up to at least 600 GPa and temperatures up to 50,000 K. This EOS also gives a good representation of the liquid/vapor transition at much lower pressures and temperatures. The estimated critical point parameters for SiO2 are Pc = 0.19 GPa, Tc = 5400K, ρc = 550 kg/m^3.

Numerical modelling of impact cratering has reached a high degree of sophistication; however, the treatment of porous materials still poses a large problem in hydrocode calculations. We present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation. In contrast to previous attempts (e.g., P-alpha model, snowplow model), our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure. Our new e-alpha model requires only four input parameters and each has a physical meaning. The model is simple and intuitive and shows good agreement with a wide variety of experimental data, ranging from static compaction tests to highly dynamic impact experiments. Our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation. We present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity-scaled source size.

Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 ± 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.

Unusual minerals observed in lunar craters were thought to originate from beneath the Moon’s surface. Numerical simulations show that rather than being vaporized, much of the impactor material can survive in the crater, implying that the unusual minerals come from the impactor and may not be indigenous to the Moon.

A 100 km deep liquid water ocean probably underlies the icy exterior of Jupiter’s satellite Europa. The long-term persistence of a liquid ocean beneat ha n ice shell presents a thermal conundrum: Is the temperature of the ocean equal to the freezing point of water at the bottom of the ice shell, or is it equal t o the somewhat warmer temperature at which water attains its maximum density? We argue that most of the ocean is at the temperature of maximum density and that the bulk of the vigorously convecting ocean is separated from the bottom of the ice shell by a thin “stratosphere” of stably stratified water whi ch is at the freezing point, and therefore buoyant. If Europa’s subsurface water ocean is warm, it could explain the widespread geologic evidence for app arent melt-through events observed on its surface and may constrain the overall age of its surface.

In this paper we present a new model for the formation of shatter cones. The model follows earlier suggestions that shatter cones are initiated by heterogeneities in the rock, but does not require the participation of an elastic precursor wave: the conical fractures are initiated after the passage of the main plastic compression pulse, not before. Numerical simulations using the hydrocode SALE 2D, enhanced by the Grady–Kipp–Melosh fragmentation model, support the model. The conditions required for the formation of shatter cones are explored numerically and are found to be consistent with the pressure range derived from both explosion experiments and the analysis of shock metamorphic features in impact structures. This model permits us to deduce quantitative information about the shape of the shock wave from the shape and size of the observed shatter cones. Indeed, the occurrence of shatter cones is correlated with the ratio between the width of the compressive pulse and the size of the heterogeneity that initiates the conical fracture. The apical angles of the shatter cones are controlled by the shape of the rarefaction wave.

Near field velocity gauge data from many underground nuclear tests show that the rise time of the stress wave in geologic materials is extremely long—up to 300 ms at ranges of a kilometer or more in alluvium. Steady wave analysis of this data shows that stress waves traveling in salt, granite, alluvium, basalt, and dry tuff also broaden as they propagate, reaching widths from 140 m in salt to nearly 400 m in alluvium. This broadening ceases when the stress in the wave drops below a threshold ranging from a few MPa for alluvium to ∼60 MPa for granite. The rise time and width of the stress wave is a unique function of particle velocity in each material studied: it does not depend on the yield of the explosion. Effective viscosities range from 5×104 Pa s in tuff to 3×105 Pa s in granite. These large viscosities permit realistic computer models of shock wave structure because they can stabilize numerical computations without artificial viscosity.

It has been proposed that the high concentrations of moderately siderophile elements (e.g. Ni and Co) in the Earth’s mantle are the result of metal–silicate equilibration at the base of a deep magma ocean that formed during Earth’s accretion. According to this model, liquid metal ponds at the base of the magma ocean and, after equilibrating chemically with the overlying silicate liquid at high pressure (e.g. 25–30 GPa), descends further as large diapirs to form the core. Here we investigate the kinetics of metal–silicate equilibration in order to test this model and place new constraints on processes of core formation. We investigate two models: (1) Reaction between a layer of segregated liquid metal and overlying silicate liquid at the base of a convecting magma ocean, as described above. (2) Reaction between dispersed metal droplets and silicate liquid in a magma ocean. In the liquid-metal layer model, the convection velocity of the magma ocean controls both the equilibration rate and the rate at which the magma ocean cools. Results indicate that time scales of chemical equilibration are two to three orders of magnitude longer than the time scales of cooling and crystallization of the magma ocean. In the falling metal droplet model, the droplet size and settling velocity are critical parameters that we determine from fluid dynamics. For likely silicate liquid viscosities, the stable droplet diameter is estimated to be ∼1 cm and the settling velocity ∼0.5 m/s. Using such parameters, liquid metal droplets are predicted to equilibrate chemically after falling a distance of

Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes. Earth’s impact history is inferred therefore either from estimates of the present-day impactor flux as determined by observations of near-Earth asteroids, or from the Moon’s incomplete impact chronology. Asteroids hitting Earth typically vaporize a mass of target rock comparable to the projectile’s mass. As this vapour expands in a large plume or fireball, it cools and condenses into molten droplets called spherules. For asteroids larger than about ten kilometres in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found. Here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers. The impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3.5 billion years ago than it is now. This conclusion is consistent with a gradual decline of the impactor flux after the Late Heavy Bombardment.

The origin of most chondrules (small, previously molten spherules inside meteorites) is shown to be impact jetting; chondrules form from the shock-melted material ejected from a protoplanet on impact, making meteorites a byproduct of planet formation. The origin of chondrules, the millimetre-scale, once-molten droplets found in most meteorites, has been an enduring mystery in the science of meteoritics. Using a hydrocode model and a dynamical accretion model to simulate protoplanetary impacts, Brandon Johnson et al. show that large-scale accretionary impacts can produce massive sprays of millimetre-scale molten droplets in sufficient quantities during the first 5 Myr of planetary accretion to explain the observed abundance of chondrules. This finding supports an impact origin for chondrules and implies that meteorites are a byproduct of planet formation rather than leftover building material. Chondrules are the millimetre-scale, previously molten, spherules found in most meteorites1. Before chondrules formed, large differentiating planetesimals had already accreted2. Volatile-rich olivine reveals that chondrules formed in extremely solid-rich environments, more like impact plumes than the solar nebula3,4,5. The unique chondrules in CB chondrites probably formed in a vapour-melt plume produced by a hypervelocity impact6 with an impact velocity greater than 10 kilometres per second. An acceptable formation model for the overwhelming majority of chondrules, however, has not been established. Here we report that impacts can produce enough chondrules during the first five million years of planetary accretion to explain their observed abundance. Building on a previous study of impact jetting7, we simulate protoplanetary impacts, finding that material is melted and ejected at high speed when the impact velocity exceeds 2.5 kilometres per second. Using a Monte Carlo accretion code, we estimate the location, timing, sizes, and velocities of chondrule-forming impacts. Ejecta size estimates8 indicate that jetted melt will form millimetre-scale droplets. Our radiative transfer models show that these droplets experience the expected cooling rates of ten to a thousand kelvin per hour9,10. An impact origin for chondrules implies that meteorites are a byproduct of planet formation rather than leftover building material.

We studied the effect of extreme acceleration and change in acceleration, or jerk, on bacteria to determine if they could survive impact ejection from a planet. Computer simulations based on the spallation model [H.J. Melosh, Icarus 59 (1984) 234–260; H.J. Melosh, Nature 363 (1993) 498–499] for ejecting material from planetary surfaces provided estimates for acceleration, rise time, and jerk for material accelerated to escape velocity. For ejection from Mars, the maximum acceleration predicted was 3×106 m/s2, or 3×105×g, with a rise time of 0.5 ms, and a corresponding jerk of 6×109 m/s3. We tested the resistance of Bacillus subtilis spores and Deinococcus radiodurans cells to high acceleration and jerk by (1) subjecting B. subtilis spores to the forces of an ultracentrifuge and (2) firing both bacteria from a rifle into a plasticene target. We measured the survival of B. subtilis spores at extreme acceleration in an ultracentrifuge operated at its highest speed, 100 000 rpm, corresponding to an acceleration of 4.27×106 m/s2, or 4.36×105×g. Approximately 107 spores were centrifuged in phosphate-buffered saline for 24, 48, 50 and 72 h. Spores were inactivated with simple exponential kinetics, and 65 h of centrifugation was required to inactivate 90% of the spore population. To test for resistance to jerk, spores of B. subtilis or cells of D. radiodurans were loaded into the rear cavities of lead pellets fired from a compressed-air pellet rifle into a target consisting of plasticene modeling clay, previously chilled to 4°C. The velocity of each pellet was measured using a chronograph and the depth of penetration of each pellet into the target was measured before removing the pellet from the clay using sterile forceps. Two different rifles were used, one with a measured pellet velocity of ∼100 m/s and the other with a velocity of ∼300 m/s. These correspond to estimated accelerations of 1.5×106 and 4.5×106 m/s2 and jerks of 1.5×1010 and 1.5×1011 m/s3, respectively. The percent survival for both organisms ranged from 40 to 100%. The samples in the ballistic experiments were subjected to jerks and accelerations 2.5–25 times larger than those estimated to prevail during ejection according to the computer simulation. We therefore conclude that acceleration and jerk are not important lethal factors during the ejection of viable microorganisms from planetary surfaces.

A seismic reflection and three-dimensional wide-angle tomographic study of the buried, V200-km diameter, Chicxulub impact crater in Mexico reveals the kinematics of central structural uplift and peak-ring formation during large-crater collapse. The seismic data show downward and inward radial collapse of the transient cavity in the outer crater, and upward and outward collapse within the central structurally uplifted region. Peak rings are formed by the interference between these two flow regimes, and involve significant radial transport of material. Hydrocode modeling replicates the observed collapse features. Impact-generated melt rocks lie mostly inside the peak ring; the melt appears to be clast-rich and undifferentiated, with a maximum thickness of 3.5 km in the center. fl 2000 Elsevier Science B.V. All rights reserved.