¹S. Marchi,¹D.D. Durda,²C.A. Polanskey,³E. Asphaug,¹W.F. Bottke,³L.T. Elkins‐Tanton,⁴L.A.J. Garvie,⁴S. Ray,¹S. Chocron,⁴D.A. Williams
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE005927]
¹Southwest Research Institute, Boulder, CO, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
⁵Southwest Research Institute, San Antonio, TX, USA
Published by arrangement with John Wiley & Sons

The NASA Psyche mission will visit the 226‐km diameter main belt asteroid (16) Psyche, our first opportunity to visit a metal‐rich object at close range. The unique and poorly understood nature of Psyche offers a challenge to the mission as we have little understanding of the surface morphology and composition. It is commonly accepted that the main evolutionary process for asteroid surfaces is impact cratering. While a considerable body of literature is available on collisions on rocky/icy objects, less work is available for metallic targets with compositions relevant to Psyche. Here we present a suite of impact experiments performed at the NASA Ames Vertical Gun Range facility on several types of iron meteorites and foundry‐cast ingots that have similar Fe‐Ni compositions as the iron meteorites. Our experiments were designed to better understand crater formation (e.g., size, depth), over a range of impact conditions, including target temperature and composition.

We find that the target strength, as inferred from crater sizes, ranges from 700 to 1300 MPa. Target temperature has measurable effects on strength, with cooled targets typically 10‐20% stronger. Crater morphologies are characterized by sharp, raised rims and deep cavities.

Further, we derive broad implications for Psyche’s collisional evolution, in light of available low resolution shape models. We find that the number of large craters (>50 km) is particularly diagnostic for the overall bulk strength of Psyche. If confirmed, the number of putative large craters may indicate that Psyche’s bulk strength is significantly reduced compared to that of intact iron meteorites.

¹Debra L. Buczkowski,¹Kimberly D. Seelos,¹Christina E. Viviano,¹Scott L. Murchie,¹Frank P. Seelos,²Eric Malaret,²Christopher Hash
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006043]
¹Johns Hopkins Applied Physics Laboratory, Laurel, MD
²Applied Coherent Technology, Herndon, VA
Published by arrangement with John Wiley & Sons

The formation of widespread phyllosilicate‐bearing near‐surface layers on Mars have often been attributed to pedogenesis, a process of weathering basaltic soils by continued exposure to meteoric water percolating down from the surface which can result in layers of aluminum phyllosilicates forming over layers of iron‐magnesium phyllosilicates. We present evidence of an Fe/Mg‐smectite bearing layer stratigraphically above Al‐phyllosilicates in three circular features to the south of Coprates Chasma, suggesting that some process other than, or in addition to, a single pedogenic sequence must have been involved. A review of several formation mechanisms shows that all models require multiple episodes of aqueous alteration. In addition, only by invoking groundwater alteration in conjunction with pedogenesis can we reconcile the stratigraphic pattern of altered material exposed by these features.

¹N.E.B. Zellner
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006050]
¹Department of Physics, Albion College, Albion, MI, USA
Published by arrangement with John Wiley & Sons

Lunar impact glasses, formed during impact events when the regolith was quenched during the ejecta’s ballistic flight, are small samples whose information can lead to important advances in studies of the Moon. For example, they provide evidence that constrains both the compositional evolution of the lunar crust and the timing of the lunar impact flux starting at ~4000 million years ago. They are abundant in the lunar regolith and retain geochemical information that tells us where and when they formed. Thus they provide important details about areas of the Moon both sampled and not sampled by Apollo or Luna missions or lunar meteorites. Additionally, as a result of these glasses possessing a chemical memory of formation location and age, studies of lunar impact glasses provide a foundation on which to conduct studies of impact glasses from other planetary bodies. A summary of past and current lunar impact glass investigations, using glasses from the Apollo 11, 12, 14, 15, 16, and 17 regoliths, along with plans for future work, will be presented.

¹Marc D. Norman,²Fred Jourdan,¹Simeon S.M. Hui
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006053]
¹Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
²Department of Applied Geology and John de Laeter Centre, School of Earth and Planetary Sciences, Curtin University, Perth, WA, Australia
Published by arrangement with John Wiley & Sons

Lunar impact glasses are quenched droplets of melt that carry geochemical records of their target compositions, formation ages, and time‐integrated exposure in the upper layers of the lunar regolith. Here we present the first study to obtain major element, trace element, and Ar isotopic data for impact glasses from the Apollo 16 regolith sample 66031. Thirty particles were analysed with 27 of them yielding useable age information. The glasses have a wide range of major and trace element compositions, similar to that observed in lunar meteorites. Half of these glasses have compositions similar to Apollo 16 soils and are considered to be “locally derived”, whereas the others represent diverse source regions and are considered to be “exotic” particles that were delivered from a considerable distance to the landing site.
Almost 40% of the samples analysed for this study have formation ages younger than 500 Ma. Duplicate particles produced in single impact events contribute minimally to the age distribution, and diurnal or transient heating of the regolith does not appear to have had a significant effect on the 40Ar/39Ar ages. Rather, the ages reflect primarily the formation of these glasses by impact melting, with the distribution modified to some degree by preservation bias. As most of these glasses are likely formed by relatively small impactors, their age distribution cannot be compared directly with the crystalline lunar melt rocks to constrain the impact mass flux through time.