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

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 δ 41 K 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.