3I/ATLAS: An Interstellar Crustal Fossil in the M-Relic (HLF) Framework for Small-Body Evolution

Abstract. 3I/ATLAS (C/2025 N1), the third confirmed interstellar object, exhibits a unique combination of dynamical, compositional, and morphological anomalies. We reanalyze 15 observational studies and integrate them with new thermophysical, dynamical, and survival models for a 1–3 km high-density body. The combined evidence reveals: (1) a high Ni/Fe ratio (>10), (2) CO₂‑dominated activity with low mass loss (<0.01 %), (3) narrow jets and low non‑gravitational acceleration (~5×10⁻⁷ m s⁻²), (4) weak bluing and low polarization, (5) stable rigid-body rotation, and (6) multi-modal surface heterogeneity including metallic and hydrated/mineralized domains. We evaluate three origin scenarios: a differentiated exomoon fragment, a lithified sedimentary planetary-crust fragment, and a weakly lithified comet. The first two satisfy all constraints; the cometary scenario, not quite. 3I/ATLAS is best explained as a high-strength, geologically processed crustal relic capable of surviving >10 Gyr of interstellar exposure. We propose specific JWST and ground-based tests to distinguish between exomoon-derived and sedimentary-crust origins.