¹Zhen Tiana,²Tomáš Magna,³James M. D. Day,⁴Klaus Mezger,⁵Erik E. Scherer,¹Katharina Lodders,⁶Remco C. Hin,¹Piers Koefoed,¹Hannah Bloom,¹Kun Wanga
Proceedings of the National Academy of Sciences of the United States of America (PNAS) 118, e2101155118 Link to Article [https://doi.org/10.1073/pnas.2101155118]
¹Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130;
²Section of Isotope Geochemistry and Geochronology, Czech Geological Survey, CZ-118 21 Prague, Czech Republic;
³Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093;
⁴Institut für Geologie, Universität Bern, 3012 Bern, Switzerland;
⁵Institut für Mineralogie, Universität Münster, D48149 Münster, Germany;
⁶Bristol Isotope Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom

The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent “Perseverance” and “Tianwen-1” missions, “follow the water,” and, more broadly, “volatiles,” has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average “bulk silicate” δ⁴¹K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na “volatility” ratio, and most notably, bulk planet H₂O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H₂O to enable habitability and plate tectonics, with mass exceeding that of Mars.