Earth was born dry until a cosmic collision made it a blue planet

Origin of Earth’s Water and Theia Impact

• Thread centers on the claim that early Earth formed dry and was later supplied with volatiles (water, C, H, S) by a collision with a water‑rich protoplanet (“Theia”), which also formed the Moon.
• Supporters note this can explain isotopic similarities between Earth and Moon (suggesting a shared, sudden source) and fits with models where inner-system material was initially too hot to retain volatiles.
• Others push back that a single impact is not required: multiple smaller impacts and volcanic outgassing are widely discussed alternatives, and the paper’s “all at once” conclusion is hard for lay readers to see in the technical details.
• Skeptics point to Mars’ past oceans and icy moons (Titan, Europa) as evidence that large water inventories can arise without such a specific giant impact.

Volatiles, Atmospheres, and Planetary Dynamics

• Questions arise about how volatiles survived a global-melting impact instead of boiling off; replies mention that atmospheric escape is mainly governed by long‑term processes (solar wind, hydrodynamic escape), not just transient heating.
• Some argue that a giant impact should have made Earth’s orbit highly eccentric; counterarguments say that near‑circular orbits are what you get after many interactions and collisions, and Theia may have been on a very similar orbit to proto‑Earth.

Water, Biochemistry, and Alternative Life Chemistries

• One camp insists water is almost certainly required for life: it’s abundant, chemically versatile, and works uniquely well with carbon-based chemistry; silicon-based or solvent‑like methane life is viewed as physically implausible or at least far rarer.
• Others emphasize we only know one example of life and should not assume water is strictly necessary, though they often concede that non‑water biochemistries are speculative.

Drake Equation, Rarity of Life, and the Fermi Paradox

• Several comments argue that needing a finely tuned impact (plus other constraints: plate tectonics, magnetic field, fossil fuels, asteroid extinctions, etc.) would make Earth-like, intelligent‑life‑bearing planets extremely rare, potentially resolving the Fermi paradox.
• Others counter that even very low per‑planet probabilities are compensated by the enormous number of planets and galaxies, so life (and even intelligence) could still be common.
• There is extensive discussion of the Drake equation: what its terms mean, how strongly it embeds assumptions (e.g., planets, Goldilocks zones), and whether with only one data point any numerical estimate is meaningful. A Bayesian treatment is cited that allows a wide range of outcomes, including us being alone.

Colonization, Von Neumann Probes, and Limits

• One line of argument: if spacefaring civilizations were common, self‑replicating probes or large‑scale colonization should have visibly altered galaxies; the lack of such signatures suggests rarity of advanced civilizations.
• Counterpoints stress economics and politics (poor ROI, tiny time horizons), the hostility and scale of space, and likely logistic rather than indefinite exponential growth. Many doubt von Neumann probes are technically or socio‑economically realistic, even for advanced species.

Anthropic Views, Panspermia, and Philosophy

• Some invoke the anthropic principle: because observers can only arise on worlds where a long chain of “unlikely” events occurred, our perception of extreme fine‑tuning is biased and not surprising.
• Panspermia is mentioned: if water/ice-rich impactors are common carriers of organics, life might spread between worlds, making Earth’s “immigrant” life plausible.
• A few commenters express broad skepticism, calling the scenario highly speculative or “science fiction,” while others caution against dismissing complex models simply because they are unintuitive; the consensus in the thread is that the model is intriguing but far from definitively proven.