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

TL;DR

This paper evaluates the plausibility of natural lithopanspermia as a mechanism for seeding Earth with life, concluding it is only credible within the Solar System, especially from early Mars, and unlikely from extrasolar sources.

Contribution

It develops a quantitative framework for assessing transport and survival probabilities of life-bearing material from various donor classes, highlighting the limits of hard panspermia.

Findings

Early Mars is the only viable external source for hard panspermia.

Extrasolar panspermia is statistically insignificant for Earth's origin.

Soft panspermia likely contributed to chemical enrichment, aiding abiogenesis.

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.