Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention

Abstract

Significance Using spacecraft data and elemental abundances derived from martian meteorites, earlier studies set a paradigm of a volatile- and water-rich Mars relative to Earth. Nevertheless, inherent difficulty in determining the volatile budget of bulk silicate Mars (BSM) makes it challenging to directly compare the extents of volatile depletions among differentiated bodies in the Solar System. This study provides an alternative for evaluating the nature of volatiles on Mars using potassium (K) isotopes. The K isotopic composition of BSM and the strong correlation between δ41K and planet mass reveals that the sizes of planetary bodies fundamentally control their ability to retain volatiles. This could further shed light on the habitability of planets and assist with constraining unknown parent body sizes. 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.