¹Philip T.Metzger,²W.M.Grundy,³Mark V.Sykes,⁴Alan Stern,⁵James F.Bell III,⁶Charlene E.Detelich,⁷Kirby Runyon,⁸Michael Summers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114768]
¹Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1 Building, Suite 214, Orlando, FL 32826-0650, USA
²Lowell Observatory, 1400 W. Mars Hill Rd., Flagtsaff, AZ 86001, USA
³Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, USA
⁴Southwest Research Institute, 1050 Walnut St, Suite 300, Boulder, CO 80302, USA
⁵Arizona State University, School of Earth and Space Exploration, Box 876004, Tempe, AZ 85287-6004, USA
⁶Department of Geological Sciences, University of Alaska Anchorage, 311 Providence Drive, CPSB 101, Anchorage, AK 99508, USA
⁷Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
⁸George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
Copyright Elsevier

We argue that taxonomical concept development is vital for planetary science as in all branches of science, but its importance has been obscured by unique historical developments. The literature shows that the concept of planet developed by scientists during the Copernican Revolution was theory-laden and pragmatic for science. It included both primaries and satellites as planets due to their common intrinsic, geological characteristics. About two centuries later the non-scientific public had just adopted heliocentrism and was motivated to preserve elements of geocentrism including teleology and the assumptions of astrology. This motivated development of a folk concept of planet that contradicted the scientific view. The folk taxonomy was based on what an object orbits, making satellites out to be non-planets and ignoring most asteroids. Astronomers continued to keep primaries and moons classed together as planets and continued teaching that taxonomy until the 1920s. The astronomical community lost interest in planets ca. 1910 to 1955 and during that period complacently accepted the folk concept. Enough time has now elapsed so that modern astronomers forgot this history and rewrote it to claim that the folk taxonomy is the one that was created by the Copernican scientists. Starting ca. 1960 when spacecraft missions were developed to send back detailed new data, there was an explosion of publishing about planets including the satellites, leading to revival of the Copernican planet concept. We present evidence that taxonomical alignment with geological complexity is the most useful scientific taxonomy for planets. It is this complexity of both primary and secondary planets that is a key part of the chain of origins for life in the cosmos.

¹Andrew D. Langendam,¹Andrew G. Tomkins,²Katy A. Evans,³Nicholas C. Wilson,³Colin M. MacRae,⁴Natasha R. Stephen,³Aaron Torpy
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13755]
¹School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, 3800 Australia
²Department of Applied Geology, Curtin University, Perth, Western Australia, 6845 Australia
³Microbeam Laboratory, CSIRO Mineral Resources, Clayton, Victoria, 3169 Australia
⁴Plymouth Electron Microscopy Centre, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA UK
Published by arrangement with John Wiley & Sons

Ureilite meteorites contain regions of localized olivine reduction to Fe metal widely accepted to have formed by redox reactions involving oxidation of graphite, a process known as secondary smelting. However, the possibility that other reductants might be responsible for this process has largely been ignored. Here, 17 ureilite samples are investigated to assess whether, instead of smelting involving only solid reactants, a CHOS gas/fluid could have caused much of the smelting. Features consistent with gas- or supercritical fluid-driven reduction were found to be abundant in all ureilites, such as fracture-focused smelting, plume-like reaction fronts, and addition of sulfur. Many of these are developed away from graphite. In some ureilites, it is clear that the redox process coincided with annealing, and we suggest that this was caused by enhanced diffusion facilitated by a higher density gas or fluid, rather than slow cooling, which requires elevated pressure. The C-CO and CH4-C-H2O buffers were modeled to examine their relative potential to drive reduction. This modeling showed that a CH4-rich fluid is able to produce the observed mineral compositions at elevated pressures. This result, coupled with the observed textures, is used to develop a likely series of reactions. We suggest that at higher pressures, a H2-CH4-H2S-S2-bearing fluid-like phase, and at lower pressures, an equivalent gas, were able to infiltrate grain boundaries and fine fractures. Sulfidation to form troilite may have acted to maintain highly reduced gas/fluid conditions. The presence of hydrocarbons in ureilites supports a role for reduction driven by CHOS gas/fluid.