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Register a new userAn Exogenic Origin for the Volatiles Sampled by the LCROSS Impact
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Returning humans to the Moon presents an unprecedented opportunity to determine the origin of volatiles stored in the permanently shaded regions (PSRs), which trace the history of lunar volcanic activity, solar wind surface chemistry, and volatile delivery to the Earth and Moon through impacts of comets, asteroids, and micrometeoroids. So far, the source of the volatiles sampled by the Lunar Crater Observation and Sensing Satellite (LCROSS) plume (Colaprete et al. 2010; Gladstone et al. 2010) has remained undetermined. We show here that the source could not be volcanic outgassing and the composition is best explained by cometary impacts. Ruling out a volcanic source means that volatiles in the top 1-3 meters of the Cabeus PSR regolith may be younger than the latest volcanic outgassing event (~1 billion years ago; Gya) (Needham et al. 2017).
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The detection of olivine on Vesta is interesting because it may provide critical insights into planetary differentiation early in our Solar Systems history. Ground-based and Hubble Space Telescope (HST) observations of asteroid (4) Vesta have suggested the presence of olivine on the surface. These observations were reinforced by the discovery of olivine-rich HED meteorites from Vesta in recent years. However, analysis of data from NASAs Dawn spacecraft has shown that this olivine-bearing unit is actually impact melt in the ejecta of Oppia crater. The lack of widespread mantle olivine, exposed during the formation of the 19 km deep Rheasilvia basin on Vestas South Pole, further complicated this picture. Ammannito et al., (2013a) reported the discovery of local scale olivine-rich units in the form of excavated material from the mantle using the Visible and InfraRed spectrometer (VIR) on Dawn. Here we explore alternative sources for the olivine in the northern hemisphere of Vesta by reanalyzing the data from the VIR instrument using laboratory spectral measurements of meteorites. We suggest that these olivine exposures could be explained by the delivery of olivine-rich exogenic material. Based on our spectral band parameters analysis, the lack of correlation between the location of these olivine-rich terrains and possible mantle-excavating events, and supported by observations of HED meteorites, we propose that a probable source for olivine seen in the northern hemisphere are remnants of impactors made of olivine-rich meteorites. Best match suggests these units are HED material mixed with either ordinary chondrites, or with some olivine-dominated meteorites such as R-chondrites.
The Earth-Moon system is unusual in several respects. The Moon is roughly 1/4 the radius of the Earth - a larger satellite-to-planet size ratio than all known satellites other than Plutos Charon. The Moon has a tiny core, perhaps with only ~1% of its mass, in contrast to Earth whose core contains nearly 30% of its mass. The Earth-Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily explain all of these features has been the focus of decades of research.
The giant impact hypothesis for Moon formation successfully explains the dynamic properties of the Earth-Moon system but remains challenged by the similarity of isotopic fingerprints of the terrestrial and lunar mantles. Moreover, recent geochemical evidence suggests that the Earths mantle preserves ancient (or primordial) heterogeneity that predates the Moon-forming giant impact. Using a new hydrodynamical method, we here show that Moon-forming giant impacts lead to a stratified starting condition for the evolution of the terrestrial mantle. The upper layer of the Earth is compositionally similar to the disk, out of which the Moon evolves, whereas the lower layer preserves proto-Earth characteristics. As long as this predicted compositional stratification can at least partially be preserved over the subsequent billions of years of Earth mantle convection, the compositional similarity between the Moon and the accessible Earths mantle is a natural outcome of realistic and high-probability Moon-forming impact scenarios. The preservation of primordial heterogeneity in the modern Earth not only reconciles geochemical constraints but is also consistent with recent geophysical observations. Furthermore, for significant preservation of a proto-Earth reservoir, the bulk composition of the Earth-Moon system may be systematically shifted towards chondritic values.
The hypothesis of lunar origin by a single giant impact can explain some aspects of the Earth-Moon system. However, it is difficult to reconcile giant impact models with the compositional similarity of the Earth and Moon without violating angular momentum constraints. Furthermore, successful giant impact scenarios require very specific conditions such that they have a low probability of occurring. Here we present numerical simulations suggesting that the Moon could instead be the product of a succession of a variety of smaller collisions. In this scenario, each collision forms a debris disk around the proto-Earth that then accretes to form a moonlet. The moonlets tidally advance outward, and may coalesce to form the Moon. We find that sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early solar system and find that the planetary rotation is limited by impact angular momentum drain. We conclude that, assuming efficient merger of moonlets, a multiple impact scenario can account for the formation of the Earth-Moon system with its present properties.
The discovery of a large putative impact crater buried beneath Hiawatha Glacier along the margin of the northwestern Greenland Ice Sheet has reinvigorated interest into the nature of large impacts into thick ice masses. This circular structure is relatively shallow and exhibits a small central uplift, whereas a peak-ring morphology is expected. This discrepancy may be due to long-term and ongoing subglacial erosion but may also be explained by a relatively recent impact through the Greenland Ice Sheet, which is expected to alter the final crater morphology. Here we model crater formation using hydrocode simulations, varying pre-impact ice thickness and impactor composition over crystalline target rock. We find that an ice-sheet thickness of 1.5 or 2 km results in a crater morphology that is consistent with the present morphology of this structure. Further, an ice sheet that thick substantially inhibits ejection of rocky material, which might explain the absence of rocky ejecta in most existing Greenland deep ice cores if the impact occurred during the late Pleistocene. From the present morphology of the putative Hiawatha impact crater alone, we cannot distinguish between an older crater formed by a pre-Pleistocene impact into ice-free bedrock or a younger, Pleistocene impact into locally thick ice, but based on our modeling we conclude that latter scenario is possible.
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