The role of impact cratering for Mars sample return

The preserved cratering record of Mars indicates that impacts play an important role in deciphering Martian geologic history, whether as a mechanism to modify the lithosphere and atmosphere or as a tool to sample the planet. The various roles of impact cratering in adding a broader understanding of Mars through returned samples are examined. Five broad roles include impact craters as: (1) a process in response to a different planetary localizer environment; (2) a probe for excavating crustal/mantle materials; (3) a possible localizer of magmatic and hydrothermal processes; (4) a chronicle of changes in the volcanic, sedimentary, atmospheric, and cosmic flux history; and (5) a chronometer for extending the geologic time scale to unsampled regions. The evidence for Earth-like processes and very nonlunar styles of volcanism and tectonism may shift the emphasis of a sampling strategy away from equally fundamental issues including crustal composition, unit ages, and climate history. Impact cratering not only played an important active role in the early Martian geologic history, it also provides an important tool for addressing such issues

Styles of ejecta emplacement under atmospheric conditions

Laboratory experiments provide essential first-order constraints on processes affecting ballistic ejecta and styles of ejecta emplacement under different atmospheric environments at planetary scales. The NASA-Ames Vertical Gun allows impacting different fine-grained particulate targets under varying atmospheric pressure and density, thereby helping to isolate controlling variables. Further analysis now permits characterizing distinct modes of emplacement that reflect the degree of ejecta entrainment within a turbidity flow created by ejecta curtain movement through the atmosphere

Atmospheric effects on oblique impacts

Laboratory experiments and theoretical calculations often use vertical impact angles (90 deg) in order to avoid the complicating effect of asymmetry. Nevertheless, oblique impacts represent the most likely starting condition for planetary cratering. Changing both impact angles and atmospheric pressure not only allows testing previous results for vertical impacts but also reveals phenomena whose signatures would otherwise be masked in the planetary cratering record. The laboratory studies were performed for investigating impact cratering processes. Impact angles can be increased from 0 to 90 deg in 15 deg increments while maintaining a flat target surface. Different atmospheres (nitrogen, argon, and helium) characterized the effects of both gas density and Mach number. Targets varied according to purpose. Because of the complexities in atmosphere-impactor-ejecta interactions, no single combination allows direct simulation of a planetary-scale (10-100 km) event. Nevertheless, fundamental processes and observed phenomena allow formulating first-order models at such broad scales

Crater ejecta morphology and the presence of water on Mars

The possible effects of projectile, target, and environment on the cratering process is reviewed. It is suggested that contradictions in interpreting Martian crater ejecta morphologies reflect over simplifying the process as a singular consequence of buried water. It seem entirely possible that most ejecta facies could be produced without the presence of liquid water. However, the combination of extraordinary ejecta fluidity, absence of secondaries, and high ejection angles all would point to the combined effects of atmosphere and fluid rich substrates. Moreover, recent experiments revealing the broad scour zone associated with rapid vapor expansion may account for numerous craters in the circumpolar regions with subtle radial grooving extending 10 crater radii away with faint distal ramparts. Thus certain crater ejecta morphologies may yet provide fundamental clues for the presence of unbound water

Non-random cratering flux in recent time

Proposed periodic cycles of mass mortality have been linked to periodic changes in the impact flux on Earth. Such changes in the impact flux, however, also should be recorded on the Moon. Previous studies have concluded that the impact flux on the Moon over the last 1 to 2 billion years has been reasonably constant, but sudden changes in the impact flux over time intervals as short as 30 my could not be detected in these studies unless the added crater population greatly exceeded the cumulative cratering record. Consequently this study focuses only on bright-rayed craters larger than 1 km thereby not only limiting the study to recent craters but also largely eliminating contamination by secondary craters. Preservation of ray patterns and other fine-scale surface textures in the ejecta provides first-order culling of craters younger than Tycho, i.e., about 100 my. Although a periodic change in the impact flux in the Earth-Moon system cannot yet be confirmed from the data, a non-random component appears to exist with an increased flux around 7 and 15 my. The concentrations in different quadrants of the lunar hemisphere would be consistent with a shower of debris generally smaller than 0.5 km

Floor-fractured crater models of the Sudbury structure, Canada

The Sudbury structure in Ontario, Canada, is one of the oldest and largest impact structures recognized in the geological record. It is also one of the most extensively deformed and volcanically modified impact structures on Earth. Although few other terrestrial craters are recognized as volcanically modified, numerous impact craters on the Moon have been volcanically and tectonically modified and provide possible analogs for the observed pattern of modification at Sudbury. We correlate the pattern of early deformation at Sudbury to fracture patterns in two alternative lunar analogs and then use these analogs both to estimate the initial size of the Sudbury structure and to model the nature of early crater modification at Sudbury

The gradational history of southern Ismenius Lacus

Two epochs of accelerated gradation affected the geomorphic evolution of southern Ismenius Lacus. These periods of enhanced gradational activity were likely related to periods of more clement climate induced by release of either recycled exogenic or juvenile endogenic atmospheric volatiles. Variations in the intensity and duration of gradation during the second epoch are indicated by the variability in the timing of cessation and degree of air fall deposit removal across the study area. Overall intensity of gradation decreased through successive epochs based on: (1) the decreasing diameter at which cumulative statistics from the respective surfaces cease to follow the expected production function; and (2) the decrease in size/increase in density of preserved valley networks incised into surfaces of differing age. A comparable decrease in valley density with time has been noted in the Isidis region

Seismic effects from major basin formation on the Moon and Mercury

Grooved and hilly terrains are reported which occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Order-of-magnitude calculations, for an Imbrium-size impact on the Moon, indicate P-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves are reported which would arrive subsequent to secondary ejecta impacts and would increase in magnitude as they converge at the antipode. Other seismically induced surface features include: subdued, furrowed crater walls produced by landslides and concomitant secondary impacts; emplacement and leveling of light plains units owing to seismically induced "fluidization" of slide material; knobby, pitted terrain around old basins from enhancement of seismic waves in ancient ejecta blankets; and the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region

Variation in multiring basic structures as a function of impact angle

Previous studies have demonstrated that the impact process in the laboratory varies as a function of impact angle. This variation is attributed to changes in energy partitioning and projectile failure during the impact and, in simple craters, produces a sequence of progressively smaller and more asymmetric crater forms as impact angle decreases from approximately 20 degrees. Variations in impact angle can produce differences in the appearance of multiring impact basins. Comparisons of Orientale to the more oblique impact structure at Crisium also suggests that these differences primarily reflect the degree of cavity collapse. The relative changes in massif ring topography, basin scarp relief, and the distribution of peripheral mare units are consistent with a reduction in degree of cavity collapse with decreasing impact angle. The prominent uprange basin scarps and the restriction of tectonically derived peripheral mare units along uprange ring structures also may indicate an uprange enhancement of failure during cavity collapse. Finally, although basin ring faults appear to be preferred pathways for mare volcanism, fault-controlled peripheral mare volcanism occurs most readily uprange of an oblique impact; elsewhere such volcanism apparently requires superposition of an impact structure on the ring fault

The spatial distribution and time evolution of impact-generated magnetic fields

The production of magnetic fields was revealed by laboratory hypervelocity impacts in easily vaporized targets. As quantified by pressure measurements, high frame-rate photography, and electrostatic probes, these impacts tend to produce large quantities of slightly ionized vapor, which is referred to as impact-generated plasma. Nonaligned electron density and temperature gradients within this plasma may lead to production of the observed magnetic fields. Past experiments were limited to measuring a single component of the impact-generated magnetic fields at only a few locations about the developing impact crater and consequently gave little information about the field production mechanism. To understand this mechanism, the techniques were extended to map the three components of the magnetic field both in space and time. By conducting many otherwise identical experiments with arrayed magnetic detectors, a preliminary 3-D picture was produced of impact-generated magnetic fields as they develop through time