Natural Panspermia to Earth: Quantitative Limits on Donor Classes, Transport, Survival, and Establishment

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_{\mathrm{fl}})) and a survival-weighted buried-volume fraction (\Fbur) for the low-shock spall population. The resulting hierarchy is sharp. 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},\mathrm{yr}) transfer tail, whereas the more common (10^{5})--(10^{7},\mathrm{yr}) martian transfer regime requires lightly shocked meter-class carriers. Beyond the Solar System, low-(\vinf) capture and long-duration survival fail simultaneously; even the Solar birth-cluster channel yields only (\sim 3\times10^{-5} f_{\mathrm{seed}}) expected Earth-seeding events, where (f_{\mathrm{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.

Source: arXiv:2604.03916v1 - http://arxiv.org/abs/2604.03916v1 PDF: https://arxiv.org/pdf/2604.03916v1 Original Link: http://arxiv.org/abs/2604.03916v1