¹Kathleen E. Vander Kaaden,²Francis M. McCubbin,^1,3Amber A. Turner,^1,4D. Kent Ross
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006239]
¹Jacobs, NASA Johnson Space Center, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
³Department of Geoscience, University of Las Vegas, Las Vegas, NV, USA
⁴University of Texas at El Paso‐CASSMAR, El Paso, TX, USA
Published by arrangement with John Wiley & Sons

The composition of a planet’s core has important implications for the thermal and magmatic evolution of that planet. Here, we conducted carbon (C) solubility experiments on iron‐silicon (Fe‐Si) metal mixtures (up to 35 wt% [~52 atom%] Si) at 1 GPa and 800–1800°C to determine the carbon concentration at graphite saturation (CCGS) in metallic melt and crystalline metal with varying proportions of Fe and Si to constrain the C content of Mercury’s core. Our results, combined with those in the literature, show that composition is the major controlling factor for carbon solubility in Fe‐rich metal with minimal effects from temperature and pressure. Moreover, there is a strong anticorrelation between the abundances of carbon and silicon in iron‐rich metallic systems. Based on the previous estimates of <1–25 wt% Si in Mercury’s core, our results indicate that a carbon‐saturated Mercurian core has 0.5–6.4 wt% C, with 6.4 wt% C corresponding to an Si‐free, Fe core and 0.5 wt% C corresponding to an Fe‐rich core with 25 wt% Si. The upper end of estimated FeO abundances in the mantle (up to 2.2 wt%) are consistent with a core that has <1 wt% Si and up to 6.4 wt% C, which would imply that bulk Mercury has a superchondritic Fe/Si ratio. However, the lower end of estimated FeO (≤0.05 wt%) supports CB chondrite‐like bulk compositions of Mercury with core Si abundances in the range of 5–18.5 wt% and C abundances in the range of 0.8–4.0 wt%.

¹B. S. Prince,¹M. P. Magnuson,²L. C. Chaves,²M. S. Thompson,^3,4M. J. Loeffler
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006242]
¹Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
³Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA⁴Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ, USA
Published by arrangement with John Wiley & Sons

Here we present results from pulsed laser irradiation of troilite samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. We find that the spectral trends observed in directly irradiated samples and samples with a vapor‐deposited coating are different than those found in silicate minerals previously studied. For instance, direct laser irradiation causes our troilite samples to initially brighten, but continued irradiation causes darkening and a decrease in spectral slope. In contrast, our samples with a vapor‐deposited coating show a continuous increase in spectral slope and overall albedo as the deposit thickness increases. Observation using both digital imaging and electron microscopy of our directly irradiated samples leads us to conclude that topography effects likely become important after a relatively high number of laser pulses in our directly irradiated samples, causing the apparent darkening and decrease in spectral slope. Thus, we conclude that the spectral changes observed relevant to space weathering via micrometeorite impacts are an increase in spectral slope and an increase in the albedo of troilite. Future studies will investigate whether these trends are generally representative of other sulfide‐bearing minerals and of weathering trends in other components found in the asteroid regolith.

¹Roger R. Fu,^1,2Pauli Kehayias,¹Benjamin P. Weiss,³Devin L. Schrader,⁴Xue‐Ning Bai,^5,6,7Jacob B. Simon
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006260]
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
²Sandia National Laboratories, Albuquerque, NM, USA
³Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
⁴Institute for Advanced Study, Tsinghua University, Beijing, China
⁵Department of Physics and Astronomy, Iowa State University of Science and Technology, Ames, IA, USA
⁶JILA, University of Colorado Boulder and NIST, Boulder, CO, USA
⁷Department of Space Studies, Southwest Research Institute, Boulder, CO, USA
Published by arrangement with John Wiley & Sons

Theoretical investigations suggest that magnetic fields may have played an important role in driving rapid stellar accretion rates and efficient planet formation in protoplanetary disks. Experimental constraints on magnetic field strengths throughout the solar nebula can test the occurrence of magnetically driven disk accretion and the effect of magnetic fields on planetary accretion. Here we conduct paleomagnetic experiments on chondrule samples from primitive CR (Renazzo type) chondrites GRA 95229 and LAP 02342, which likely originated in the outer solar system between 3 and 7 AU approximately 3.7 million years after calcium aluminum‐rich inclusion formation. By extracting and analyzing 18 chondrule subsamples that contain primary, igneous ferromagnetic minerals, we show that CR chondrules carry internally non‐unidirectional magnetization that requires formation in a nebular magnetic field of ≤8.0 ± 4.3 μT (2σ ). These weak magnetic fields may be due to the secular decay of nebular magnetic fields by 3.7 million years after calcium aluminum‐rich inclusions, spatial heterogeneities in the nebular magnetic field, or a combination of both effects. The possible inferred existence of spatial variations in the nebular magnetic field would be consistent with a prominent role for disk magnetism in the formation of density structures leading to gaps and planet formation.