Geologic Map of Aphrodite Map Area (AMA; I-2476), Venus

We present a 1:10-M-scale geologic map of the Aphrodite Map Area (AMA) of Venus (0°N-57°S/60-80°E). Geologic mapping employed NASA Magellan synthetic aperture radar and altimetry data. The AMA geologic map, with detailed structural elements and geologic units covering over one eighth of Venus' surface, affords an important and unique perspective to test models of global-scale geologic processes through time. Geologic relations record a history inconsistent with global catastrophic resurfacing. The AMA displays a regional coherence of preserved geologic patterns that record three sequential geologic eras: the ancient era, the Artemis superstructure era, and the youngest fracture zone era. The ancient era and Artemis superstructure, with a footprint covering more than 25% of the surface, are recorded in the Niobe Map Area to the north. The latter two eras likely overlap in time. The fracture zone domain, part of a globally extensive province, marks the most spatially focused tectonomagmatic domain within the AMA. Impact craters are both cut by and overprint fracture zone structures. Twelve percent of AMA impact craters that occur within the fracture zone domain predate or formed during fracture zone development. This observation indicates the relative youth of the fracture zone era and is consistent with the possibility that this domain remains geologically active. The AMA records a rich geologic history of large tract of the surface of Venus and provides an important framework to formulate new working hypotheses of Venus evolution and contribute to planning future studies of the surface of planets.

A steeply-inclined trajectory for the Chicxulub impact

The environmental severity of large impacts on Earth is influenced by their impact trajectory. Impact direction and angle to the target plane affect the volume and depth of origin of vaporized target, as well as the trajectories of ejected material. The asteroid impact that formed the 66 Ma Chicxulub crater had a profound and catastrophic effect on Earth's environment, but the impact trajectory is debated. Here we show that impact angle and direction can be diagnosed by asymmetries in the subsurface structure of the Chicxulub crater. Comparison of 3D numerical simulations of Chicxulub-scale impacts with geophysical observations suggests that the Chicxulub crater was formed by a steeply-inclined (45-60° to horizontal) impact from the northeast; several lines of evidence rule out a low angle (<30°) impact. A steeply-inclined impact produces a nearly symmetric distribution of ejected rock and releases more climate-changing gases per impactor mass than either a very shallow or near-vertical impact.

Assessment of the probability of microbial contamination for sample return from Martian moons II: The fate of microbes on Martian moons

This paper presents a case study of microbe transportation in the Mars-satellites system. We examined the spatial distribution of potential impact-transported microbes on the Martian moons using impact physics by following a companion study (Fujita et al., in this issue). We used sterilization data from the precede studies (Patel et al., 2018; Summers, 2017). We considered that the microbes came mainly from the Zunil crater on Mars, which was formed during 1.0-0.1 Ma. We found that 70-80% of the microbes are likely to be dispersed all over the moon surface and are rapidly sterilized due to solar and galactic cosmic radiation except for those microbes within a thick ejecta deposit produced by natural meteoroids. The other 20-30% might be shielded from radiation by thick regolith layers that formed at collapsed layers in craters produced by Mars rock impacts. The total number of potentially surviving microbes at the thick ejecta deposits is estimated to be 3-4 orders of magnitude lower than at the Mars rock craters. The microbe concentration is irregular in the horizontal direction due to Mars rock bombardment and is largely depth-dependent due to the radiation sterilization. The surviving fraction of transported microbes would be only ∼1 ppm on Phobos and ∼100 ppm on Deimos, suggesting that the transport processes and radiation severely affect microbe survival. The microbe sampling probability from the Martian moons was also investigatesd. We suggest that sample return missions from the Martian moons are classified into Unrestricted Earth-Return missions for 30 g samples and 10 cm depth sampling, even in our conservative scenario. We also conducted a full statistical analysis pertaining to sampling the regolith of Phobos to include the effects of uncertainties in input parameters on the sampling probability. The most likely probability of microbial contamination for return samples is estimated to be two orders of magnitude lower than the 10^-6 criterion defined by the planetary protection policy of the Committee on Space Research (COSPAR).

Origin and implications of non-radial Imbrium Sculpture on the Moon

Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon. This pattern was coined the Imbrium Sculpture, and it was originally argued that it must have been formed by a giant oblique (~30°) impact, a conclusion echoed by later studies. Some investigators, however, noticed that many elements of the Imbrium Sculpture are not radial to Imbrium, thereby implicating an endogenic or structural origin. Here we use these non-radial trends to conclude that the Imbrium impactor was a proto-planet (half the diameter of Vesta), once part of a population of large proto-planets in the asteroid belt. Such independent constraints on the sizes of the Imbrium and other basin-forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of Jupiter and Saturn. Moreover, laboratory impact experiments, shock physics codes and the groove widths indicate that multiple fragments (up to 2% of the initial diameter) from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago.

Hydrogen cyanide production due to mid-size impacts in a redox-neutral N2-rich atmosphere

Cyanide compounds are amongst the most important molecules of the origin of life. Here, we demonstrate the importance of mid-size (0.1-1 km in diameter) hence frequent meteoritic impacts to the cyanide inventory on the early Earth. Subsequent aerodynamic ablation and chemical reactions with the ambient atmosphere after oblique impacts were investigated by both impact and laser experiments. A polycarbonate projectile and graphite were used as laboratory analogs of meteoritic organic matter. Spectroscopic observations of impact-generated ablation vapors show that laser irradiation to graphite within an N2-rich gas can produce a thermodynamic environment similar to that produced by oblique impacts. Thus, laser ablation was used to investigate the final chemical products after this aerodynamic process. We found that a significant fraction (>0.1 mol%) of the vaporized carbon is converted to HCN and cyanide condensates, even when the ambient gas contains as much as a few hundred mbar of CO2. As such, the column density of cyanides after carbon-rich meteoritic impacts with diameters of 600 m would reach ~10 mol/m(2) over ~10(2) km(2) under early Earth conditions. Such a temporally and spatially concentrated supply of cyanides may have played an important role in the origin of life.

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 approximately 12.9 ka, has been previously identified at approximately 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 approximately equal 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 approximately 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.

Impact craters in loose granular media

The craters formed by the impact of steel balls with a loose sand bed are experimentally studied. Both crater size and morphology depend strongly on the impact angle. Two scaling laws, corresponding to length and width, respectively, are proposed.

Understanding oblique impacts from experiments, observations, and modeling

Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.

Wind streaks on venus: clues to atmospheric circulation

Magellan images reveal surface features on Venus attributed to wind processes. Sand dunes, wind-sculpted hills, and more than 5830 wind streaks have been identified. The streaks serve as local "wind vanes," representing wind direction at the time of streak formation and allowing the first global mapping of near-surface wind patterns on Venus. Wind streaks are oriented both toward the equator and toward the west. When streaks associated with local transient events, such as impact cratering, are deleted, the westward component is mostly lost but the equatorward component remains. This pattern is consistent with a Hadley circulation of the lower atmosphere.