Why is Mars only about one tenth of Earth's mass?
In plain words
Standard models of planet formation build a Mars almost as big as Earth, yet the real Mars is about ten times lighter. Something must have removed or never supplied material near Mars's orbit.
Precise statement
Explain the mass of Mars (about 0.11 Earth masses) and its rapid formation (within about 10 Myr from Hf-W chronometry) in terms of the initial solid distribution or dynamical depletion: Jupiter's inward-outward migration, an early giant-planet instability, a narrow annulus of planetesimals, or pebble accretion. An answer is a scenario reproducing the masses, orbits and isotopic compositions of all terrestrial planets and the asteroid belt.
What would settle it
A formation model that reproduces terrestrial planet masses and orbits and the nucleosynthetic isotope differences between Earth and Mars, and fails in its competitors.