Morphology of craters generated by hypervelocity impacts of micron‐sized polypyrrole‐coated olivine particles

To understand the process of cosmic dust particle impacts and translate crater morphology on smoothed metallic surfaces to dust properties, correct calibration of the experimental impact data is needed. This article presents the results of studies of crater morphology generated by impacts using micron‐sized polypyrrole (PPy)‐coated olivine particles. The particles were accelerated by an electrostatic dust accelerator to high speeds before they impacted onto polished aluminum targets. The projectile diameter and velocity ranges were 0.3–1.2 μm and 3–7 km s^?1. After impact, stereopair images of the craters were taken using scanning electron microscope and 3‐D reconstructions made to provide diameter and depth measurements. In this study, not just the dimensions of crater diameters and depths, but also the shape and dimensions of crater lips were analyzed. The craters created by the coated olivine projectiles are shown to have complicated shapes believed to be due to the nonspherical shape of the projectiles.     

Stardust interstellar dust calibration: Hydrocode modeling of impacts on Al‐1100 foil at velocities up to 300 km s−1 and validation with experimental data

Abstract– We present initial results from hydrocode modeling of impacts on Al‐1100 foils, undertaken to aid the interstellar preliminary examination (ISPE) phase for the NASA Stardust mission interstellar dust collector tray. We used Ansys’ AUTODYN to model impacts of micrometer‐scale, and smaller projectiles onto Stardust foil (100 μm thick Al‐1100) at velocities up to 300 km s^?1. It is thought that impacts onto the interstellar dust collector foils may have been made by a combination of interstellar dust particles (ISP), interplanetary dust particles (IDP) on comet, and asteroid derived orbits, β micrometeoroids, nanometer dust in the solar wind, and spacecraft derived secondary ejecta. The characteristic velocity of the potential impactors thus ranges from <<1 to a few km s^?1 (secondary ejecta), approximately 4–25 km s^?1 for ISP and IDP, up to hundreds of km s^?1 for the nanoscale dust reported by Meyer‐Vernet et al. (2009) . There are currently no extensive experimental calibrations for the higher velocity conditions, and the main focus of this work was therefore to use hydrocode models to investigate the morphometry of impact craters, as a means to determine an approximate impactor speed, and thus origin. The model was validated against existing experimental data for impact speeds up to approximately 30 km s^?1 for particles ranging in density from 2.4 kg m^?3 (glass) to 7.8 kg m^?3 (iron). Interpolation equations are given to predict the crater depth and diameter for a solid impactor with any diameter between 100 nm and 4 μm and density between 2.4 and 7.8 kg m^?3.     

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild‐2

Abstract— Metallic aluminum alloy foils exposed on the forward, comet‐facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl‐shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild‐2, with a known and constant spacecraft‐particle relative velocity and effective surface‐perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well‐documented particle size range from 10 μm to nearly 100 μm. Light gas gun buckshot firings of these particles at approximately 6 km s^?1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild‐2, independent of the active impact detector instruments aboard the Stardust spacecraft.     

Micron‐scale hypervelocity impact craters: Dependence of crater ellipticity and rim morphology on impact trajectory,projectile size,velocity, and shape

The interstellar collector on NASA's Stardust mission captured many particles from sources other than the interstellar dust stream. Impact trajectory may provide a means of discriminating between these different sources, and thus identifying/eliminating candidate interstellar particles. The collector's aerogel preserved a clear record of particle impact trajectory from the inclination and direction of the resultant tracks. However, the collector also contained aluminum foils and, although impact crater studies to date suggest only the most inclined impacts (>45° from normal) produce crater morphologies that indicate trajectory (i.e., distinctly elliptical), these studies have been restricted to much larger (mm and above) scales than are relevant for Stardust (μm). It is unknown how oblique impact crater morphology varies as a function of length scale, and therefore how well Stardust craters preserve details of impactor trajectory. Here, we present data from a series of impact experiments, together with complementary hydrocode modeling, that examine how crater morphology changes with impact angles for different‐sized projectiles. We find that, for our smallest spherical projectiles (2 μm diameter), the ellipticity and rim morphology provide evidence of their inclined trajectory from as little as 15° from normal incidence. This is most likely a result of strain rate hardening in the target metal. Further experiments and models find that variation in velocity and impactor shape complicate these trends, but that rim morphology remains useful in determining impact direction (where the angle of impact is >20° from normal) and may help identify candidate interstellar particle craters on the Stardust collector.     

Preparation of large Stardust aluminum foil craters for analysis

Over the last decade, silica aerogel tracks and aluminum foil craters on the Stardust collector have been studied extensively to determine the nature of captured cometary dust grains. Analysis of particles captured in aerogel has been developed to a fine art, aided by sophisticated preparation techniques, and yielding revolutionary knowledge of comet dust mineralogy. The Stardust foil craters can be interpreted in terms of impacting particle size and structure, but almost all studies of composition for their contents have relied on in situ analysis techniques or relatively destructive extraction of materials. This has limited their examination and interpretation. However, numerous experimental hypervelocity impact studies under Stardust-Wild 2 encounter conditions have shown that abundant dust components are preserved in foil craters of all sizes. Using some of these analogue materials, we have previously shown that modern, nondestructive scanning electron microscope imaging and X-ray microanalysis techniques can document distribution of dust remnants both quickly and thoroughly within foil craters prior to any preparation. Here we present findings from our efforts to quantify the amount of residue and demonstrate a simple method of crater shape modification which can bring material into positions where it is much more accessible for in situ analysis, or safe removal of small subsamples. We report that approximately 50% of silicate-dominated impactors were retained as impact crater residue; however, <3% of organic impactors remained in the craters after impact.     

HOLLOW LUNAR SPHERULES AND MICROCRATERING

Since thin-walled hollow glass spherules exist in the lunar regolith and perhaps as a component of cosmic dust, laboratory simulations of impacts by and upon such spherules were done to determine identifying features of the resulting craters and perforations. The targets were soda-lime glass, stainless steel, and hollow glass beads. Craters were generated in the first two targets by the normal impact of thin-walled hollow glass spheres with masses and velocities between eight and 240 pg and 1.8 and 10 km/s, respectively. With increasing impact velocity, the crater morphology in glass progresses as follows: 1, a dent; 2, a narrow lip around the depression; and 3, spallation around the pit that may carry away all of part of the lip. The craters differ from those formed by solid spherical projectiles in that the central pit is an annular rather than a cup-shaped depression. The craters in steel display a typical outer lip and an additional concentric inner lip which is subdued to an annular mound as the impact velocity increases. In both targets, shattered remnants of the projectiles remain in the craters at low impact velocities. At higher velocities, melting of the projectile material occurs. The annular features distinguish these craters from craters generated by solid spheres or irregular projectiles', and the existence of such a crater morphology on a surface exposed to cosmic dust would indicate the presence of thin-walled hollow spherules. Contrary to common opinion, hollow spheres do not adequately simulate cratering by low density materials because of the mass distribution. Penetrations of thin-walled hollow glass beads by high velocity, solid, micrometer-size spheres are characterized by inward and outward flowing lips that show asymmetries dependent on the angle of impact. The morphology is sufficient to discriminate against other mechanisms that cause perforations in the one to 10 μm size range in hollow lunar spherules. The identifying lip may break away by fragmentation in the impact of larger size projectiles.     

FIB‐TEM analysis of cometary material in 10 Stardust foil craters

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft's encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet's fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.     

The effect of the oceans on the terrestrial crater size‐frequency distribution: Insight from numerical modeling

Abstract— On Earth, oceanic impacts are twice as likely to occur as continental impacts, yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size‐frequency distribution. Despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments, detailed numerical modeling, and interpretation of geological and geophysical data, no definitive relationship between impactor properties, water depth, and final crater diameter exists. In this paper, we determine the relationship between final (and transient) crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models. This relationship applies for normal incidence impacts of stoney asteroids into water‐covered, crystalline oceanic crust at a velocity of 15 km s^?1. We use these relationships to construct the first estimates of terrestrial crater size‐frequency distributions (over the last 100 million years) that take into account the depth‐area distribution of oceans on Earth. We find that the oceans reduce the number of craters smaller than 1 km in diameter by about two‐thirds, the number of craters ?30 km in diameter by about one‐third, and that for craters larger than ?100 km in diameter, the oceans have little effect. Above a diameter of ?12 km, more craters occur on the ocean floor than on land; below this diameter more craters form on land than in the oceans. We also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact‐related resurge feature, or disturbance on the seafloor, instead of a crater.     

Impact History of Eros: Craters and Boulders

Preliminary measurements of craters and boulders have been made in various locations on Eros from images acquired during the first nine months of NEAR Shoemaker's orbital mission, including the October 2000 low altitude flyover. (We offer some very preliminary, qualitative analysis of later LAF images and very high-resolution images obtained during NEAR's landing on 12 February 2001). Craters on Eros >100 m diameter closely resemble the saturated crater population of Ida; Eros is more heavily cratered than Gaspra but lacks the saturated giant craters of Mathilde. These craters and the other large-scale geological features were formed over a duration of very roughly 2 Gyr while Eros was in the main asteroid belt, between the time when its parent body was disrupted and Eros was injected into an Earth-approaching orbit (probably tens of Myr ago). Saturation equilibrium had been expected to shape Eros' crater population down to very small sizes, as on the lunar maria. However, craters <200 m diameter are instead progressively depleted toward smaller sizes and are a factor of ∼200 below empirical saturation at diameters of 4 m. Conversely, boulders and positive relief features (PRFs) rise rapidly in numbers (differential power-law index ∼−5) and those <10 m in size dominate the landscape at high resolutions. The pervasive boulders and minimal craters on Eros is radically different from the lunar surface at similar scales. This may be partly explained by a major depletion of meter-scale projectiles in the asteroid belt (due to the Yarkovsky Effect: Bell 2001), which thus form few small craters and destroy few boulders. Additionally, the small size and low gravity of Eros may result in redistribution or loss of ejecta due to seismic shaking, thus preferentially destroying small craters formed in such regolith. Possibly Eros has only a patchy, thin regolith of mobile fines; the smaller PRFs may then reflect exposures of fractured bedrock or piles of large ejecta blocks, which might further inhibit formation of craters <10 m in size. Eros may well have been largely detached dynamically and collisionally from the main asteroid belt for the past tens of Myr, in which case its cratering rate would have dropped by two orders of magnitude, perhaps enhancing the relative efficacy of other processes that would normally be negligible in competition with cratering. Such processes include thermal creep, electrostatic levitation and redistribution of fines, and space weathering (e.g., bombardment by micrometeorites and solar wind particles). Combined with other small-body responses to impact cratering (e.g., greater widespread distribution of bouldery ejecta), such processes may also help explain the unexpected small-scale character of geology on Eros. If there was a recent virtual hiatus in cratering of Eros (during which only craters <∼300 m diameter would be expected to have formed), space weathering may have reached maturity, thus explaining Eros' remarkable spectral homogeneity compared with Ida.     

SIMS studies of Allende projectiles fired into Stardust‐type aluminum foils at 6 km/sec

Abstract— We have explored the feasibility of C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) on residues of Allende projectiles that impacted Stardust‐type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild‐2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF‐SIMS. Crater sizes were between 3 and 190 μm. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF‐SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock‐forming elements in the Allende projectiles are well‐characterized by the TOF‐SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.     

Microscopic impactor debris in the soil around Kamil crater (Egypt): Inventory,distribution, total mass,and implications for the impact scenario

We report on the microscopic impactor debris around Kamil crater (45 m in diameter, Egypt) collected during our 2010 geophysical expedition. The hypervelocity impact of Gebel Kamil (Ni‐rich ataxite) on a sandstone target produced a downrange ejecta curtain of microscopic impactor debris due SE–SW of the crater (extending ~300,000 m², up to ~400 m from the crater), in agreement with previous determination of the impactor trajectory. The microscopic impactor debris include vesicular masses, spherules, and coatings of dark impact melt glass which is a mixture of impactor and target materials (Si‐, Fe‐, and Al‐rich glass), plus Fe‐Ni oxide spherules and mini shrapnel, documenting that these products can be found in craters as small as few tens of meters in diameter. The estimated mass of the microscopic impactor debris (<290 kg) derived from Ni concentrations in the soil is a small fraction of the total impactor mass (~10 t) in the form of macroscopic shrapnel. That Kamil crater was generated by a relatively small impactor is consistent with literature estimates of its pre‐atmospheric mass (>20 t, likely 50–60 t).     

Crater dimensions from apollo data and supplemental sources

A catalog of crater dimensions that were compiled mostly from the new Apollo-based Lunar Topographic Orthophotomaps is presented in its entirety. Values of crater diameter, depth, rim height, flank width, circularity, and floor diameter (where applicable) are tabulated for a sample of 484 craters on the Moon and 22 craters on Earth. Systematic techniques of mensuration are detailed. The lunar craters range in size from 400 m to 300 km across and include primary impact craters of the main sequence, secondary impact craters, craterlets atop domes and cones, and dark-halo craters. The terrestrial craters are between 10 m and 22.5 km in diameter and were formed by meteorite impact.     

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

Abstract— The known encounter velocity (6.1 kms^?1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild‐2 fall within a range that allows simulation in laboratory light‐gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre‐existing major‐ and trace‐element composition of the foil, geometrical issues for energy dispersive X‐ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.     

Growth of planetesimals by impacts at ∼25 m/s

We study central collisions between millimeter-sized dust projectiles and centimeter-sized dust targets in impact experiments. Target and projectile are dust aggregates consisting of micrometer-sized SiO₂ particles. Collision velocities range up to 25 m/s. The general outcome of a collision strongly depends on the impact velocity. For collisions below 13 m/s rebound and a small degree of fragmentation occur. However, at higher collision velocities up to 25 m/s approximately 50% of the mass of the projectile rigidly sticks to the target after the collision. Thus, net growth of a body is possible in high speed collisions. This supports the idea that planetesimal formation via collisional growth is a viable mechanism at higher impact velocities. Within our set of parameters the experiments even suggest that higher impact velocities might be preferable for growth in collisions between dusty bodies. For the highest impact velocities most of the ejecta is within small dust aggregates about 500 μm in size. In detail the size distribution of ejected dust aggregates is flat for very small particles smaller than 500 μm and follows a power law for larger ejected dust aggregates with a power of −5.6±0.2. There is a sharp upper cut-off at about 1 mm in size with only a few particles being slightly larger. The ejection angle is smaller than 3° with respect to the target surface. These fast ejecta move with 40±10% of the impact velocity.     

Chandrayaan-1 observation of distant secondary craters of Copernicus exhibiting central mound morphology: Evidence for low velocity clustered impacts on the Moon

Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20-1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30-335 m diameter and 10-40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters.     

On the Shape of an Impact Crater at High-Speed Impacts

This paper presents the results of an experimental study on the geometric parameters of craters that originated at the impact interaction of polyethylene projectiles with a massive organic-glass target. The impactor speed ranged from 2.7 to 6.28 km/s. The shapes of the resulting craters are determined. Using statistical analysis and scaling theory, we obtained the dependence of the diameter and depth of the crater on defining parameters. The critical impact energy resulting in the catastrophic breakup of the target is estimated.     

Exposing metal and silicate charges to electrical discharges: Did chondrules form by nebular lightning?

In order to investigate the hypothesis that dust aggregates were transformed to meteoritic chondrules by nebular lightning, we exposed silicatic and metallic dust samples to electrical discharges with energies of 120 to 500 J in air at pressures between 10 and 10⁵ Pa. The target charges consisted of powders of micrometer-sized particles and had dimensions of mm. The dust samples generally fragmented leaving the major fraction thermally unprocessed. A minor part formed sintered aggregates of 50 to 500 μm. In a few experiments melt spherules having sizes ?180 μm in diameter (and, generally, interior voids) were formed; the highest spherule fraction was obtained with metallic Ni. Our experiments indicate that chondrule formation by electric current or by particle bombardment inside a discharge channel is unlikely.     

Impact microcrater morphology on Australasian microtektites

Abstract— Scanning electron microscopy of 137 Australasian microtektites and fragments from 4 sediment cores in the Central Indian Ocean reveals more than 2000 impact‐generated features in the size range of 0.3 to 600 μm. Three distinct impact types are recognized: destructive, erosive, and accretionery. A large variation in impact energy is seen in terms of catastrophic destruction demonstrated by fragmented microtektites through erosive impacts comprising glass‐lined pit craters, stylus pit craters, pitless craters, and a small number of accretionery features as well. The size range of observed microtektites is from 180 to 2320 μm, and not only are the smaller microtektites seen to have the largest number of impacts, but most of these impacts are also of the erosive category, indicating that target temperature is an important factor for retaining impact‐generated features. Further, microcratering due to collisions in impact‐generated plumes seems to exist on a larger and more violent scale than previously known. Although the microcraters are produced in a terrestrially generated impact plume, they resemble lunar microcraters in many ways: 1) the size range of impacts and crater morphology variation with increasing size; 2) dominant crater number densities in μm and sub‐μm sizes. Therefore, tektite‐producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials, and for the lunar rocks to qualify as reliable cosmic dust flux detectors, their tumbling histories and lunar surface orientations have to be known precisely.     

Investigating target versus impactor influences on Martian crater morphology at the simple‐complex transition

Comparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant. We have analyzed the Martian crater population within a narrow diameter range (7 km < crater diameter < 9 km) at the simple‐complex crater transition using three approaches. We looked for correlations of morphology with surface geology using a global crater database and global geologic map. We examined selected regions in detail with high‐resolution images to further understand the relationship between crater morphology and bulk target properties. Finally, we examined craters in close proximity to each other in order to hold target properties constant, so that we could isolate impactor effects on crater morphology. We found a strong correlation between target properties and interior crater morphology, and we found little evidence that impactor properties (other than impact angle) affect crater appearance. Central uplift and wall slumping are enhanced for less consolidated targets. Layered targets affected both the excavation and modification stages of complex crater formation; the resulting craters have pseudoterraces, flat floors, and central pits.     

Ejecta–megaregolith accumulation on planetesimals and large asteroids

Megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction, as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10. I have modeled global average ejecta accumulation as a function of the largest impact size, with no explicit modeling of time. In conjunction with an assumed cratering size‐distribution exponent b, the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith. The largest impactor mass ratio is a major fraction of the catastrophic‐disruption mass ratio, and in general the largest crater’s diameter is close to the target’s diameter. Total accumulation is roughly 1–5% of (and proportional to) the target’s radius. Global accumulations estimated by this approach are higher than in the classic Housen et al. (1979) study by a factor of roughly 10. This revision is caused mainly by higher (typical) largest crater size. For b ～ 2, the single largest crater typically contributes close to 50% of the total of new (nonrecycled) ejecta. Megaregolith can be destroyed by sintering, a process whose pressure sensitivity makes it effective at lower temperature on larger bodies. Planetesimals ～100 km in diameter may be surprisingly well suited (about as well suited as bodies two to three times larger in diameter) for attaining temperatures conducive to widespread melting. A water‐rich composition may be a significant disadvantage in terms of planetesimal heating, as the shallow interior may be densified by aqueous metamorphism, and will have a low sintering temperature.