Origin of Earth's Water: Asteroids, Comets, and In-Situ Formation

This episode probes a fundamental question in planetary science: what is the source of Earth’s water? It traces the long-standing debate over whether water arrived via water-rich asteroids or comets beyond the snow line, or if Earth formed with water in a primordial magma ocean. Astrobiologist Michael Wang explains how isotopic ratios, especially the deuterium-to-hydrogen signature, help distinguish sources, while dynamical models show how giant planets could shuffle icy material inward. The show also discusses a bold in-situ formation hypothesis, backed by laboratory tests using a diamond anvil cell. Finally, it considers implications for water-rich worlds around other stars and how future telescopes might test these ideas.

Introduction and the big question

The episode frames a central puzzle in planetary science: where did Earth's water originate? It surveys three competing ideas—delivery from water-rich asteroids or comets beyond the snow line, and the possibility that water formed during Earth’s birth—setting up isotopic and dynamical clues used to evaluate these hypotheses.

"There was a big debate over whether it was asteroids that delivered that water or was it comets that delivered the water?" - Mike Wang, Astrobiologist and planetary scientist, Carnegie Science

Snow line, protoplanetary disks, and water delivery hypotheses

The discussion explains how planets form in a protoplanetary disk with a radial temperature gradient. Inside the snow line, materials are drier; beyond it, ices can condense and be incorporated into planetesimals. Earth, positioned inside this line in the early solar system, would originally be relatively dry, prompting questions about how its water was acquired. The narrative examines whether asteroids or comets supplied most of Earth’s water, and how the disk’s structure influenced delivery pathways.

"the biggest clue comes in what's called the deuterium to hydrogen ratio of the water" - Mike Wang

Asteroids and comets as sources

The episode discusses isotopic fingerprints, notably the D/H ratio, to compare Earth’s water with that found in comets and carbonaceous chondrites. It explains why comets, despite being icy, may not match Earth’s water signature, while certain asteroids, especially carbonaceous chondrites, appear more consistent with Earth’s D/H ratio, suggesting asteroidal delivery played a major role. Dynamical models show how Jupiter and Saturn could scatter these water-rich bodies inward, whereas transporting material from distant reservoirs like the Oort Cloud or Kuiper Belt remains less efficient.

"you form a terrestrial planet and along with that formation process, you create water" - Mike Wang

In situ water formation on a young Earth

A bold alternative proposes that water could form directly on Earth as it accreted. In a hypothetical early Earth with a magma ocean and a thick hydrogen-rich atmosphere, reactions between hydrogen gas and iron oxides could release oxygen into water. The discussion covers how this concept shifted thinking about water origin and what it would require to be viable on a planetary scale.

"This sounds intense. I would have loved to see this." - Regina Barber

Laboratory tests and future observations

The episode highlights a laboratory demonstration using a diamond anvil cell to simulate extreme conditions and test the in situ water-formation hypothesis. It notes that future measurements of isotopic ratios in asteroids, comets, and exoplanet atmospheres, along with advances in telescope capabilities, could help distinguish between these scenarios and reveal watery worlds beyond our solar system.

"we should be able to see signs of watery worlds out there with future telescopes" - Mike Wang

Broader implications

Beyond Earth, the show considers how these ideas inform our understanding of water-rich exoplanets, planetary formation theories, and the prevalence of habitable environments elsewhere in the galaxy. By integrating isotopic data, dynamical simulations, and laboratory experiments, researchers aim to build a cohesive picture of how planetary systems acquire and retain water.