Mars habitability limited by its small size, isotope study suggests

Researchers measured the potassium isotope compositions of Martian meteorites in order to estimate the presence, distribution, and abundance of volatile elements and compounds, including water, on Mars, finding that Mars has lost more potassium than Earth but retained more potassium than the moon or the asteroid 4-Vesta; the results suggest that rocky planets with larger mass retain more volatile elements during planetary formation and that Mars and Mars-sized exoplanets fall below a size threshold necessary to retain enough water to enable habitability and plate tectonics.

from Latest Science News -- ScienceDaily https://www.sciencedaily.com/releases/2021/09/210920151949.htm