Ejecta Emplacement Research Articles

Clustered impacts: Experiments and implications

Impact by clusters of projectiles rather than a single projectile can result from several processes: atmospheric breakup, tidal breakup, and ejecta from a large primary impact. Experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities (15 m/s to 6 km/s). At very low impact velocities (15–200 m/s), clustered impacts were produced by launching a grouped projectiles of aluminum shot, steel shot, iron filings, and sand. At moderate to high velocities (0.8–6 km/s), pyrex spheres were shattered above the target during passage through aluminum foil or paper, thereby forming a well‐defined cluster of fragments of overall radius rc. The ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion. Sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength. The experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body. “Tight” clusters (rc/rs < 3) displace a factor of 2 less than a single impactor of the same mass, “open” clusters (rc/rs ∼ 9) displace a factor of 5 less mass, and “dispersed” clusters (rc/rs > 20) displace a factor of 10 less mass. This reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio (diameter/depth) as large as 30 for open clusters. The size and velocity of the cluster (as well as the density and strength contrast between the impactor and target) can dramatically affect crater morphology. Open clusters impacting compacted pumice produce a flat, hummocky floor with an incipient multiring pattern, whereas a tight cluster impacting the same target produces a central floor mound. Oblique impacts by symmetrical clusters form a characteristic array of V‐shaped ridges pointing uprange. The apex angle of the ridges depends on the cluster dispersion and impact angle. Inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction. The amount of projectile material retained on the surfaces increases with decreasing impact angle (from the horizontal) and with increasing strength of the target relative to the projectile. Clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features. On this basis and on the basis of reasonable models of ejecta curtain structure, we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters. When the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain, then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90% primary material, Such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single, noninteracting impact events. Beyond the continuous ejecta facies, impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays.

Bunte Breccia of the Ries: Continuous deposits of large impact craters

The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion >100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only <0.15% (weight) of the total deposit consists of crystalline clasts larger than 1 cm that are derived from depths of >600 m; some 0.7% is composed of Triassic clasts, originating from 300 to 600‐m depths; Lower and Middle Jurassic horizons (approximately 300–150 m) constitute some 2.3%, and Upper Jurassic (0–150 m) makes up some 31.5%. In addition, the Bunte Breccia contains Tertiary materials in the form of >1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (<1 cm) accounting for the remaining 36.3%; these Tertiary materials constituted the immediate crater environment, i.e., a substrate, onto which the Ries ejecta were deposited. These substrate materials were thoroughly mixed into the continuous deposits. The ratio of “primary crater ejecta” to local substrate components decreases with increasing radial range. There is, however, no vertical stratification with regard to modal‐stratigraphic composition at any specific location; modal‐stratigraphic composition is highly variable on meter scales; Bunte Breccia appears to be a chaotic mixture resulting from a highly turbulent depositional environment. Also, the orientation of clasts larger than 1 cm is random. Detailed grain size data reveal progressively decreasing grain sizes with increasing radial range of both primary crater ejecta and local substrate materials. In addition, progressive comminution of primary ejecta related to increasing target depth is observed. Components shocked to >10 GPa constitute <0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte Breccia (⪖200 km³) is estimated to be of the order of 5 min only. These findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so‐called Z model, addressing material flow during various stages of crater formation. Qualitative to fair agreement of observations and predictions results. An initial crater radius of 6.5 km, an excavation depth of 1650 m, an excavation volume of 136 km³, and an associated transient cavity volume of aproximately 230 km³ appear to be reasonable estimates. Approximately 170 km³ of material was involved in slumping and restoration of the transient cavity for the above radius and Z=2.7. The modal composition of Ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large, complex planetary craters are predominantly derived from depths as small as one‐hundredth the apparent crater diameter. A number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias.

Martian rampart crater ejecta: Experiments and analysis of melt-water interaction

Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction.

Atmospheric effects on Martian Ejecta Emplacement

Analytical descriptions of crater growth and numerical calculations of aerodynamic drag have been combined to evaluate the possible effects of drag on impact crater ejecta emplacement on Mars. Below a critical particle size, ejecta deposition is severely restricted in range. This critical particle size increases with increasing crater size because ejection velocity at the same relative position within a crater increases with size and because aerodynamic deceleration increases as the square of the velocity. Numerical models of ejecta trajectories in the current Martian atmosphere under hydrostatic equilibrium reveal that craters of 1, 5, 10, and 20 km exhibit critical particle diameters of 0.4–0.8, 2–4, 5–10, and 10–20 cm. Ejecta approaching the critical particle size may impact within a crater radius of the excavation crater rim. Once velocities are reduced sufficiently, ejecta paths can be modified by turbulence and winds, thereby setting up conditions for ejecta flow from the rim region. However, ejecta larger than the critical particle size are relatively undecelerated and form secondary impact craters subsequently modified by the later arriving decelerated ejecta cloud. Consequently, ejecta emplacement will be multiphased, but the importance of this process depends on the distribution of ejecta sizes.

Emplacement of Fahrenheit craterejecta at the luna-24 site

The Luna-24 site is situated in Mare Crisium at a range of 18.4 km from Fahrenheit, an Eratosthenian-aged crater 6.4 km in diameter. Fahrenheit's ejecta deposits have been degraded to such an extent that secondary craters and rays cannot be unambiguously identified in the vicinity of the Luna-24 site. On the basis of an analogy between Fahrenheit and Lichtenberg B (a much younger crater of comparable size located in northern Oceanus Procellarum) Fahrenheit ejecta deposits near the sample site are inferred to have consisted of secondary crater clusters, subradially aligned secondary crater chains, and lineated terrain furrowed by fine-scale radial grooves. At the range of the Luna-24 site more than 80% of the mare surface should have been morphologically disturbed by the ballistic deposition of Fahrenheit ejecta. Blocks and fragment clusters of primary Fahrenheit ejecta ranging up to 5–20 m in diameter are inferred to have impacted the local surface at velocities of 165–230 m s−1 forming secondary craters ranging up to 100 m in diameter. The maximum depth of excavation of primary Fahrenheit ejecta deposited near the sample site is estimated to be at least 100 m. Primary Fahrenheit ejecta is expected to constitute a substantial fraction of the exterior deposits emplaced at the range of the Luna-24 site. Microgabbro and monomineralic fragments discovered in the Luna-24 drill core may have been derived from gabbroic rocks transported to the sample site by the Fahrenheit cratering event. This hypothesis is consistent with the widespread occurrence and characteristics of Fahrenheit ejecta anticipated in the vicinity of the Luan-24 site. Current interpretations of the drill core sample suggest that the Luna-24 regolith was deposited in its present configuration sometime during the last 0.3 AE implying that at least one local cratering event has occurred since the emplacement of Fahrenheit ejecta ∼2.0±0.5 AE ago.

Martian impact craters and emplacement of ejecta by surface flow

Several types of Martian impact craters have been recognized. The most common type, the rampart crater, is distinctively different from lunar and Mercurian craters. It is typically surrounded by several layers of ejecta, each having a low ridge or escarpment at its outer edge. Outward flow of ejecta along the ground after ballistic deposition is suggested by flow lines around obstacles, the absence of ejecta on top and on the lee side of obstacles, and the large radial distance to which continuous ejecta is found. The peculiar flow characteristics of the ejecta around these craters are tentatively attributed to entrained gases or to contained water, either liquid or vapor, in the ejecta as a result of impact melting of ground ice. Ejecta of other craters lacks flow features but has a marked radial pattern; ejecta of still other craters has patterns that resemble those around lunar and Mercurian craters. The internal features of Martian craters, in general, resemble their lunar and Mercurian counterparts except that the transition from bowl shaped to flat floored takes place at about 5-km diameter, a smaller size than is true for Mercury or the moon.

Stratigraphy of Ejecta from the Lunar Crater Aristarchus

Ejecta from lunar crater Aristarchus consist of mappable units that have different surface characteristics, lithologies, and geneses. Photogeologic mapping demonstrates that these units can be ordered into a stratigraphic succession representing stages in the emplacement of ejecta during a single impact event. Four main ejecta units are recognized: (a) a highly fractured rim unit consisting of an overturned flap of country rock, (b) a continuous ejecta blanket, (c) a zone of bright discontinous ejecta outside the continuous ejecta blanket associated with numerous secondary impact craters, and (d) a group of ejecta deposits on the rim which are genetically related to each other and include hummocky material (the hummocky rim unit), blocky lobes, and smooth flows. Other units, such as the ridged and leveed flows and the dark “lakes” or “playas,” are relatively younger than the ejecta and are only briefly discussed. The continuous ejecta blanket lies stratigraphically above the rim unit. To the east and south, the ejecta blanket has been stripped off the overturned rim unit by outward flowage from the crater during ejecta production, leaving parts of the rim eroded bare. Fall of large missiles to form the secondary craters and bright ejecta preceded the emplacement of the continuous ejecta. The general asymmetry of the continuous ejecta-blanket distribution appears to be related to the Aristarchus Plateau boundary fault, which may have controlled the way material was excavated. The hummocky rim unit only occurs on the northern and western rim and may be an overturned flap of premare Aristarchus Plateau bedrock not present near the surface on the southern and eastern rim of the crater. Blocky lobes and smooth flows mainly associated with the hummocky rim unit are interbedded, arguing that the smooth flows are part of the sequence of ejected rocks and are not younger volcanic flows.