Regolith as Martian biofilter

Mars soil chemistry (soluble salts or compounds) can actively inhibit or kill even extremophiles, meaning planetary surfaces may sometimes self‑sterilize against Earth microbes — but those inhibitors can be removed by water, changing the risk profile. That makes contamination risk conditional: native regolith chemistry may help protect Mars now but human activities (e.g., bringing water) could negate that protection. — This reframes planetary protection from a binary (contaminated vs clean) to a conditional risk informed by specific regolith chemistries and human actions, with consequences for mission design, sample‑return rules, and colonization plans.