Peter H. Schultz

Detection of Water in the LCROSS Ejecta Plume

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Several remote observations have indicated that water ice may be presented in permanently shadowed craters of the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to provide direct evidence (1). On 9 October 2009, a spent Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris, dust, and vapor. This material was observed by a second “shepherding” spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Near-infrared absorbance attributed to water vapor and ice and ultraviolet emissions attributable to hydroxyl radicals support the presence of water in the debris. The maximum total water vapor and water ice within the instrument field of view was 155 ± 12 kilograms. Given the estimated total excavated mass of regolith that reached sunlight, and hence was observable, the concentration of water ice in the regolith at the LCROSS impact site is estimated to be 5.6 ± 2.9% by mass. In addition to water, spectral bands of a number of other volatile compounds were observed, including light hydrocarbons, sulfur-bearing species, and carbon dioxide.

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

EPOXI at Comet Hartley 2

In situ observations show that comet Hartley 2 is an unusually hyperactive comet. Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.

Polar wandering of Mars

Abstract Most interpretations of Martian geology implicitly invoke a globe locked to its present spin axis. Thick (4–5 km) uncomformable deposits near the equator (the Mesogaea area southwest of Olympus, Mons), however, bear a striking resemblance to the present-day polar deposits in extent, appearance, and inferred processes. We propose that these and a similar antipodal deposit are remnants of a previous geographic polar location. Although crater statistics of the surface of these deposits may indicate a very young age, such a date reflects active aeolian reworking. Relative ages based on superposed craters in peripheral deposits suggest that these deposits predate the early stages of Tharsis volcanism but postdate Lunae Planum. Similar crater-count age dates of adjacent more eroded deposits indicate a sequence in formation, thereby revealing a possible polar wandering path. The youngest near-equatorial deposits occur southwest of Olympus Mons (15°W, 0°S); more eroded deposits with a slightly greater age occur east of Apollinaris Patera (180°W, 6°S). In the opposite hemisphere, these deposits are mirrored by similar deposits in Arabia and northwest of Schiaparelli, respectively. Very old and heavily eroded deposits also occur south of Elysium (210°W, 0°S) and are mirrored in the opposite hemisphere by the chaotic terrains of eastern Valles Marineris. The proposed polar wandering path provides a different perspective for interpreting the origin and evolution of volatile reservoirs such as the inferred source regions of major outflow channels. The more recent uncomformable deposits were eroded principally by sublimation of trapped volatiles and aeolian processes. Frozen volatiles in more ancient deposits (pre-Lunae Planum) were absorbed in the underlying regolith when higher lithospheric/atmospheric temperatures favored melting. Early polar wandering is proposed to be related to changes in the principal moments of inertia resulting from early epochs of widespread basin-filling flood basalts. However, the last major shifts are related to the formation of the Tharsis volcanic constructs and Olympus Mons. Although the total polar wandering path exceeded 120°, it was not a continuous process but occurred in short-lived shifts. These shifts are reasonably consistent with the observed distribution of compressional and extensional tectonic features, but the detailed record is complicated by preexisting impact basins, later volcanic/ tectonic events, and the global thermal history.

Exposed water ice deposits on the surface of comet 9P/Tempel 1

We report the direct detection of solid water ice deposits exposed on the surface of comet 9P/Tempel 1, as observed by the Deep Impact mission. Three anomalously colored areas are shown to include water ice on the basis of their near-infrared spectra, which include diagnostic water ice absorptions at wavelengths of 1.5 and 2.0 micrometers. These absorptions are well modeled as a mixture of nearby non-ice regions and 3 to 6% water ice particles 10 to 50 micrometers in diameter. These particle sizes are larger than those ejected during the impact experiment, which suggests that the surface deposits are loose aggregates. The total area of exposed water ice is substantially less than that required to support the observed ambient outgassing from the comet, which likely has additional source regions below the surface.

Floor-fractured lunar craters

Numerous lunar craters (206 examples, mean diameter = 40km) contain pronounced floor rilles (fractures) and evidence for volcanic processes. Seven morphologic classes have been defined according to floor depth and the appearance of the floor, wall, and rim zones. Such craters containing central peaks exhibit peak heights (approximately 1km) comparable to those within well-preserved impact craters but exhibit smaller rim-peak elevation differences (generally 0–1.5km) than those (2.4km) within impact craters. In addition, the morphology, spatial distribution, and floor elevation data reveal a probable genetic association with the maria and suggest that a large number of floor-fractured craters represent pre-mare impact craters whose floors have been lifted tectonically and modified volcanically during the epochs of mare flooding. Floor uplift is envisioned as floating on an intruded sill, and estimates of the buoyed floor thickness are consistent with the inferred depth of brecciation beneath impact craters, a zone interpreted as a trap for the intruding magma. The derived model of crater modification accounts for (1) the large differences in affected crater size and age; (2) the small peak-rim elevation differences; (3) remnant central peaks within mare-flooded craters and ringed plains; (4) ridged and flat-topped rim profiles of heavily modified craters and ringed plains; and (5) the absence of positive gravity anomalies in most floor-fractured craters and some large mare-filled craters. One of the seven morphologic classes, however, displays a significantly smaller mean size, larger distances from the maria, and distinctive morphology relative to the other six classes. The distinctive morphology is attributed, in part, to the relatively small size of the affected crater, but certain members of this class represent a style of volcanism unrelated to the maria - perhaps triggered by the last major basin-forming impacts.

Atmospheric effects on ejecta emplacement and crater formation on Venus from Magellan

The Venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation. This contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings. Seven general conclusions can be drawn. First, the dense lower atmosphere of Venus takes on the role of a low-density target for bodies smaller than about 4 km in diameter. Air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up. Second, dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient. Such a process may account for the formation of unusually small craters (1–3 km). Third, the dense atmosphere of Venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces. Such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor (i.e., comet versus asteroid) in distinctive run-out flows created before the crater has finished formation. Strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers. Fourth, ballistic ejection of excavated debris occurs from craters on Venus just as it does on planets without an atmosphere, thereby underscoring the fundamental mechanical transfer of energy from impactor to target. Fifth, ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta. The outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground-hugging debris flow outward without returning to the cavity. Flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes. Sixth, radar-dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft, perhaps analogous to terrestrial tektite strewn fields. And seventh, the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter-depth relations of craters. Hence surface ages may be significantly underestimated.

Seismic effects from major basin formations on the moon and mercury

Grooved and hilly terrains occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent extensive landslides and surface disruption produced by impact-generatedP-waves and antipodal convergence of surface waves. Order-of-magnitude calculations for an Imbrium-size impact (1034 erg) on the Moon indicateP-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves would arrive subsequent to secondary ejecta impacts beyond 103 km 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 perhaps the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region.

Atmospheric effects on ejecta emplacement

Laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere. Although small in scale, they can provide essential first-order constraints on the processes affecting late-stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces. The laboratory experiments involved impacting different fine-grained particulate targets under varying atmospheric pressure and density (different gas compositions). During crater formation, ballistic ejecta form the classic cone-shaped profile observed under vacuum conditions. As atmospheric density increases (for a given pressure), however, the ejecta curtain bulges at the base and pinches above. This systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere. Additionally, a systematic change in emplacement style occurs as a function of atmospheric pressure (largely independent of density): contiguous ejecta rampart superposing ballistically emplaced deposits (0.06 to 0.3 bar); ejecta flow lobes (0.3 to 0.7 bar); and radial patterns (>0.8 bar). Underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain, by changing target materials (including layered targets and low-density particulates), by varying atmospheric density, by changing impact angle, and by comparing the ejecta run-out distances with first-order models of turbidity flows. Three distinct ejecta emplacement processes can be characterized. Ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain. This style of “wind-modified” emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta. Such “eddy-supported flows” are observed to increase in run-out distance (scaled to crater size) with increasing atmospheric pressure. By analogy with turbidity flows, this scaled distance should increase as R1/2 for a given atmospheric pressure and degree of entrainment. Ejecta flows with much greater run-out distances develop as the turbulent power in atmospheric response winds increase. Such flows overrun and scour the inner ejecta facies, thereby producing distinct inner and outer facies. The degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain. Entrainment increases with increasing atmospheric density and ejection velocity (crater size) but decreases with ejecta density and size. The intensity of curtain-generated winds increases with ejection velocity (crater size). The dimensionless drag ratio characterizing the laboratory experiments can be applied to Mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer-scale events. For a given crater size (ejection velocity) and atmospheric conditions, a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water. Alternatively, the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history (massive basalts to fine-grained aeolian deposits). Scaling considerations further predict that ejecta run-out distances scaled to crater size on Mars should increase as R1/2; hence long run-out flows dependent on crater diameter need not reflect depth to a buried reservoir of water. On Venus, however, the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density. Under such conditions, ejecta run-out distances should decrease as R−1/2.

Effect of impact angle on vaporization

Impacts into easily vaporized targets such as dry ice and carbonates generate a rapidly expanding vapor cloud. Laboratory experiments performed in a tenuous atmosphere allow deriving the internal energy of this cloud through well-established and tested theoretical descriptions. A second set of experiments under near-vacuum conditions provides a second measure of energy as the internal energy converts to kinetic energy of expansion. The resulting data allow deriving the vaporized mass as a function of impact angle and velocity. Although peak shock pressures decrease with decreasing impact angle (referenced to horizontal), the amount of impact-generated vapor is found to increase and is derived from the upper surface. Moreover, the temperature of the vapor cloud appears to decrease with decreasing angle. These unexpected results are proposed to reflect the increasing roles of shear heating and downrange hypervelocity ricochet impacts created during oblique impacts. The shallow provenance, low temperature, and trajectory of such vapor have implications for larger-scale events, including enhancement of atmospheric and biospheric stress by oblique terrestrial impacts and impact recycling of the early atmosphere of Mars.