The tangent to this is - the reason that Mars doesn't have much of an atmosphere isn't that its not gravitationally massive enough to hold on to it, but rather

The tangent to this is - the reason that Mars doesn't have much of an atmosphere isn't that its not gravitationally massive enough to hold on to it, but rather that its core cooled and lost its magnetic shielding. The lack of a magnetosphere then allowed the solar wind to strip its atmosphere. https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space https://www.nasa.gov/press-release/nasas-maven-reveals-most-...