¹Eva L. Scheller,^1,2Bethany L. Ehlmann
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006190]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Jet Propulsion Laboratory, Pasadena, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

The western part of the Isidis basin structure hosts a well‐characterized Early Noachian to Amazonian stratigraphy. The Noachian Basement comprises its oldest exposed rocks (Early to Mid‐Noachian), and was previously considered a single LCP‐ and Fe/Mg‐smectite‐bearing unit. Here, we divide the Noachian Basement Group into 5 distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP‐bearing Plateaus Unit, Fe/Mg‐smectite‐bearing Mounds Unit), 2 geomorphological features (megabreccia and ridges), and a mineral deposit (kaolinite‐bearing bright materials), based on geomorphology, spectral characteristics, and stratigraphic relationships. Megabreccia contain four different pre‐Isidis lithologies, possibly including deeper crust or mantle materials, formed through mass‐wasting associated with transient crater collapse during Isidis basin formation. The Fe/Mg‐smectite‐bearing Stratified Basement Unit and LCP‐bearing Blue Fractured Unit likewise represent pre‐Isidis units within the Noachian Basement Group. Multiple Fe/Mg‐smectite‐bearing geological units with different stratigraphic positions and younger kaolinite‐bearing bright materials indicate several aqueous alteration episodes of different ages and styles. Units with slight changes in pyroxene spectral properties suggest a transition from low‐Ca pyroxene‐containing materials to those with higher proportions of pyroxenes higher in Ca and/or glass that could be related to different impact‐ and/or igneous processes, or provenance. This long history of Noachian and potentially Pre‐Noachian geological processes, including impact basin formation, aqueous alteration, and multiple igneous and sedimentary petrogeneses, records changing ancient Mars environmental conditions. All units defined by this study are available 20 km outside of Jezero crater for in‐situ analysis and sampling during a potential extended mission scenario for the Mars 2020 rover.

¹L.T.Elkins-Tanton et al. (>10)
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006296]
¹Arizona State University
Published by arrangement with John Wiley & Sons

Some years ago the consensus was that asteroid (16) Psyche was almost entirely metal. New data on density, radar properties, and spectral signatures indicate that the asteroid is something perhaps even more enigmatic: a mixed metal and silicate world. Here we combine observations of Psyche with data from meteorites and models for planetesimal formation to produce the best current hypotheses for Psyche’s properties and provenance. Psyche’s bulk density appears to be between 3,400 and 4,100 kg m‐3. Psyche is thus predicted to have between ~30 vol% and ~60 vol% metal, with the remainder likely low‐iron silicate rock and not more than ~20% porosity. Though their density is similar, mesosiderites are an unlikely analog to bulk Psyche because mesosiderites have far more iron‐rich silicates than Psyche appears to have. CB chondrites match both Psyche’s density and spectral properties, as can some pallasites, although typical pallasitic olivine contains too much iron to be consistent with the reflectance spectra. Final answers, as well as resolution of contradictions in the dataset of Psyche physical properties, for example, the thermal inertia measurements, may not be resolved until the NASA Psyche mission arrives in orbit at the asteroid. Despite the range of compositions and formation processes for Psyche allowed by the current data, the science payload of the Psyche mission (magnetometers, multi‐spectral imagers, neutron spectrometer, and a gamma‐ray spectrometer) will produce datasets that distinguish among the models.