While the psychological toll of isolation and the complex ethical dilemmas of non-terrestrial settlement are absolutely critical and monumental challenges, the biggest and most fundamental scientific challenge of colonizing Mars would ultimately be survival. This is because creating a closed-loop, self-sufficient ecosystem is an unprecedented feat of scientific and engineering integration; it requires solving for extreme radiation shielding, generating sustainable power, and establishing a robust, reliable bioregenerative life support system that provides air, water, and food from local Martian resources with virtually no external resupply. If the core scientific puzzle of making life self-sustainable on a barren planet cannot be solved through innovative science and engineering, the subsequent challenges of psychology and ethics quickly become moot.