Early Martian Habitability and Directional Lithopanspermia Across the Solar System

Abstract Whether life arose independently on habitable worlds or dispersed through interplanetary exchange remains unresolved. Here we investigate early Mars as a potential biological nursery by integrating comparative planetary thermal evolution, subsurface aqueous habitability, impact-driven ejecta transport and probabilistic biological survival. Models spanning 4.4 billion years indicate potentially habitable pore-water environments on both Mars and Earth at 4.4 Ga, with mean eligible water temperatures of 35.84 °C and 42.34 °C, respectively, at 4.2 Ga. Simulated 10-Myr material-transfer fractions are 2.588% for Mars→Earth, 1.523% for Mars→Venus and 0.182% for Earth→Mars, yielding an inward-to-reverse transfer ratio of 14.23 (95% interval: 11.41–17.99). Conditional biological-establishment simulations yield a mean Mars→Earth probability of 0.2465%, although outcomes remain strongly dependent on uncertain survival parameters. Exploratory outward-transfer simulations identify rare Mars→Jovian-orbit encounters (0.07% within 0.05 AU), without establishing delivery to individual moons. A prior-dependent Bayesian comparison does not presently favor Martian biological ancestry over independent terrestrial origins. These findings identify directional asymmetries in modeled material exchange while demonstrating that successful biological dispersal remains unverified. Early Mars therefore represents a testable, rather than established, source of potential biological exchange across the Solar System.