Natural Lithopanspermia to Earth: Transport, Shielding, and Survival Limits for Solar-System and Extrasolar Donor Classes

Abstract

Natural panspermia is a transport-and-establishment hypothesis, not a theory of abiogenesis. The Earth-specific question is whether any nonterrestrial donor remains competitive with terrestrial origin once launch, planetary-system escape, transit, Earth interception, atmospheric entry, terminal loading, and post-delivery establishment are imposed. We formulate this problem as a donor-class dependent Earth-directed transport-survival kernel, supplemented by a minimum protected-depth envelope d_{min}(t_{flight}) and a survival-weighted buried-volume fraction F_{bur} for the low-shock spall population. The resulting hierarchy is strongly ordered under current transport and survival constraints. Hard panspermia is physically credible only on Solar-System scales: early Mars combines demonstrated lithic exchange with Earth, low escape speed, early aqueous habitability, and a rare 10^2-10^4 yr transfer tail, whereas the more common 10^5-10^7 yr martian transfer regime requires lightly shocked meter-class carriers. Beyond the Solar System, low-v_{inf} capture and long-duration survival fail simultaneously; even the Solar birth-cluster channel yields only ~3 x 10^{-5} f_{seed} expected Earth-seeding events, where f_{seed} is the conditional probability that a viable arrival establishes life on Earth. Indigenous terrestrial origin therefore remains the default inference, early Mars is the only quantitatively serious external hard-panspermia alternative, and extrasolar or intergalactic hard panspermia is not competitive for Earth's actual origin history. Soft panspermia is much more plausible as chemical enrichment of early terrestrial abiogenesis.