Mark J. Cintala

Impact-induced thermal effects in the lunar and Mercurian regoliths

While the Moon and Mercury are similar objects in a geomorphologic sense, they also possess important differences, particularly in the context of small-scale impact phenomena. Not only is the surface of Mercury notably hotter than that of the Moon, but the impact flux is also more intense at Mercury due to higher impact velocities and a greater spatial density of micrometeoroids. By extrapolating the terrestrial micrometeoroid flux to the Moon and Mercury, it is found that the impact rate at Mercury is 5.5 times greater and the mean impact velocity more than 60 percent higher than at the Moon. A model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith. Although the surface temperature plays a role in determining the melt and vapor volumes, the difference in impact velocity appears to be markedly more important. “Average” craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size, because differences in the kinetic energy of the “average” impactors will be offset by the differences in gravitational acceleration. As a result, variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes. Each “average” impact on Mercury, however, will produce more than twice as much impact melt as its lunar counterpart, introducing more glass into the mercurian regolith. Although substantial quantities of impact vapor are produced on both planets, mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas; the resulting vapor coatings should be negligibly small for most purposes. A much larger agglutinate population should exist on Mercury, and individual agglutinates should be similar to those of the lunar highlands. Reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of Fe, perhaps similar to those of Apollo 16 light- and dark-matrix breccias. Laboratory studies have demonstrated that, as the magnetic fractions (which are correlated with agglutinate abundances) of soils derived from these breccias increase, the differences in 0.565-mm (visible light) reflectivity between the soils diminish. Should the regional variations in Fe-content of the mercurian crust be similar to those of these Apollo 16 samples, then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface. At the same time, the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses, seriously attenuating the Fe2+ crystal-field absorption near 0.9-mm. Supporting this picture are the most recently published spectral observations of Mercury, which provide little or no evidence of such an absorption feature.

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Dimensionality scaled penetration experiments: Aluminum targets and glass projectiles 50 μm to 3.2 MM in diameter

Summary Spherical soda-lime glass projectiles 50, 150, 1000 and 3175 μm in diameter (Dp) in aluminum targets (series 1100; “annealed”) of variable thickness T, were used to determine how the penetration-hole diameter (Dh) varied as a function of Dp/T at a constant impact velocity of 6 km/s. The target thickness ranged from infinite half-space geometries to 0.8 μm thick foils. Virtually identical morphologies characterize the penetration holes, no matter what projectile size, at equivalent Dp/T conditions. The relative hole diameter (Dh/Dp) decreases systematically with increasing Dp/T from Dh ≅ 4Dp for massive targets, to Dh = Dp for very thin foils. A modest dependence on the absolute projectile size is observed; comparatively small cracters, yet relatively large penetration holes are produced by the smallest (50 μm) impactors. Nevertheless, linear dimensional scaling seems suitable for first-order estimates of Dp from the measurement of Dh and T on space-exposed surfaces. The projectile fragments and the debris dislodged from the target were intercepted by witness plates that were located behind the target. The dispersion angle of this debris cloud depends on the thickness of the target. In addition, millimeter-sized impactors are collisionally fragmented with greater ease than small impactors. Furthermore, we observe that systematic changes in the specific energy of dislodged projectile and target material occur as a function of Dp/T. While linear scaling of target and projectile dimensions is a useful framework to explain many observations and associated shock processes, we suggest that consideration of the absolute and relative shock-pulse duration in the projectile (tp) and target (tt) may ultimately be the more useful approach. It implicitly accounts for all dimensions and, additionally, for specific impact velocities and pertinent material properties, via equations-of-state, for the impacting pair.

Cratering and penetration experiments in Teflon targets at velocities from 1 to 7 km/s

We conducted impact experiments into Teflon FEP targets of widely variable thickness to assist in the interpretation of craters and penetration holes in the ∼20 m 2 of thermal blankets that were exposed for 5.7 years in low-Earth orbit by the Long Duration Exposure Facility (LDEF). Soda-lime glass spheres of 3.2 mm diameter (D p ) were used as projectiles ranging in impact velocities (V) from ∼1 to 7 km/s. Teflon fails in a largely brittle fashion ; substantial spall zones were developed at the targets front side, and especially at the rear side, if penetrated. Crater diameter (D c ) varies with V 0.74 . Penetration-hole diameter (D h ) depends on specific target thickness (T) and can be larger than D c over a limited range of T (0.5 > D p /T 1, and systematically decreases as T decreases to eventually approach the condition were D h = D p at D p /T > 100. D h also increases with increasing V, yet the rate of increase depends on T, yielding a wide variety of velocity exponents that depend on D p /T. The velocity exponent is highest for massive targets and decreases with decreasing T to approach the condition of D h = D p at D p /T> 100, regardless of the velocity. The relationships of D h , T and V are sufficiently systematic that unique solutions for projectile dimensions are possible from the diameter measurement of any penetration hole in teflon targets of any thickness. This renders the calibration of individual penetration holes equivalent to that of individual impact craters. Thus, improved analysis of literally thousands of penetration holes and craters in the LDEF thermal blankets seems possible.

Multiple-mesh bumpers: A feasibility study

Discontinuous targets (meshes and grids) offer significant mass savings, on geometric grounds, relative to the continuous materials that are typically employed in collisional bumper designs. Laboratory experiments were conducted to compare single aluminum and multiple-mesh systems to those composed of continuous membranes for the ability to disrupt, disperse, and decelerate soda-lime glass and aluminum projectiles at velocities ranging from 1-6 km/s. Material thickness, mesh size and the number of bumper elements were systematically varied, resulting in specific areal masses of 0.016 to 1.6 g/cm 2 for the total bumper. Damage to individual target elements and to witness plates were used to characterize the performance of specific test articles.

Dimensionally scaled penetration experiments to extract projectile sizes from space exposed surfaces

Abstract Impact experiments were conducted which employed soda-lime glass projectiles (50, 150, 1000 and 3175 μm in diameter; D p ) and aluminum (1100 series) and Teflon (FEP) targets of variable thickness (T; ranging from thick infinite halfspace targets [D p /T p /T > 100]). The objectives of these impact experiments were determine, at constant impact velocity, the relationships between the diameter of the resulting penetration hole (D h ), the foil thickness (T) and the projectile size (D p ). We found that D h , and other morphologic features such as rim structures in aluminum or spall phenomena in Teflon exhibit a systematic relationship to the target thickness. This relationship is described by polynomial fits which permit unique solutions for unknown projectile sizes (D p ) from the measurement of T and D h on space-exposed surfaces.

Evolution of debris plumes as inferred from witness plates

Abstract Photographic overvies and first-order interpretations are presented of witness-plate damage patterns that were produced by 3.2 mm diameter (Dp) soda-lime glass projectiles penetrating targets of aluminum1110-0 and aluminum6061-T6, Inconel, lead, and teflon at 6 km/s, and of additional aluminum1100-0 targets penetrated at 2, 4, 5 and 6.7 lm/s. These targets varied systematically in thickness (T) from infinite halfspace to thin films (i.e.,0.1

Emplacement of Fahrenheit crater ejecta 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. Fahrenheits 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.

Penetration experiments in aluminum and teflon targets of widely variable thickness

A 5 mm light‐gas gun was used to fire spherical soda‐lime glass projectiles from 50 to 3175 μm in diameter (Dp), at a nominal 6 km/s, into aluminum (1100 series; annealed) and Teflon (TeflonTFE(R)). Targets ranged in thickness (T) from infinite halfspace targets (T≂cm) to ultra‐thin foils (T≂μm), yielding up to four orders of magnitude variation in absolute and relative (Dp/T) target thickness. This experimental matrix simulates the wide range in Dp/T experienced by a space exposed membrane of constant T that is being impacted by projectiles of widely varying sizes.Penetration hole size (Dh) decreases systematically with decreasing target thickness. Relative hole size (Dh/T) may be used to extract projectile diameter Dp from individual penetration holes in space‐exposed surfaces, provided one assumes an impact velocity. The condition of Dh=Dp mandates Dp/T≳50 in both targets. The ballistic‐limit thickness (TBL), at 6 km/s, occurs at Dp/T=0.29 for our aluminum, and at Dp/T=0.16 for the Teflon. While these ...