Episode Overview

Podcast: Short Wave
Episode: Did Earth’s Water Come From Space?
Date: January 5, 2026
Host: Regina Barber
Guest: Michael Wong, Astrobiologist and Planetary Scientist, Carnegie Science, Washington, D.C.

This episode dives into one of planetary science's most enduring mysteries: where did Earth’s abundant water originate? Regina Barber and guest Michael Wong walk through the prevailing hypotheses, the science behind the debate, and new discoveries that are reshaping our understanding of water’s origin—not just on Earth, but possibly on other worlds across the galaxy.

Key Discussion Points & Insights

1. Traditional Theory: Water Delivered from Afar

[00:19–01:29]

• Initial Assumptions: Earth formed from dry, rocky material too close to the Sun to retain water.
  • "They didn't have a lot of water in them because they were too close to the sun and too hot to retain any H2O." — Michael Wong [00:47]
• Water needed to be delivered from farther out in the solar system, in the form of ice, likely via asteroids or comets.
• Primary Debate: Which brought the water—asteroids or comets?

2. The “Snow Line” and Planet Formation

[04:28–05:43]

• Protoplanetary Disk: Planets form in a disk of gas and dust with a temperature gradient—hotter near Sun, colder farther out.
• Snow Line: The specific distance from the Sun in the early solar system where water vapor could condense into ice, typically between Mars and Jupiter (roughly where the asteroid belt is today).
  • "Hot near the sun, colder, farther out... There's this point where... water can form ice. And we call that the snow line." — Michael Wong [05:03–05:20]
• Earth is interior to the primordial snow line, so it was thought to form dry.

3. Asteroids vs. Comets: Decoding the Evidence

[06:15–10:14]

• Deuterium-to-Hydrogen Ratio (D/H): A chemical “fingerprint” used to match water sources.
  • "The biggest clue comes in what's called the deuterium to hydrogen ratio of the water... comets and asteroids have very different ratios... and you can compare that to... Earth." — Michael Wong [06:27–07:47]
• Comets have a D/H ratio that does not match Earth's water, but a type of asteroid called carbonaceous chondrites does.
• Most of Earth’s water likely came from asteroids, not comets.
  • "Maybe as much as 10% of Earth's water, tops, comes from cometary material... but most of it actually comes from the asteroids." — Michael Wong [09:51]

4. Cosmic Pinball: How Water-Rich Material Moved Inward

[08:10–08:43]

• Dynamic Solar System: Early motion of gas giants (Jupiter, Saturn, Uranus, Neptune) stirred up and scattered water-rich asteroids inward toward Earth.
  • "Jupiter and Saturn just wreak havoc on the entire solar system... this causes a lot of chaos... sent this water-rich material into where Earth was forming." — Michael Wong [08:45–09:15]

5. Emerging Hypothesis: Water Formed on Earth

[10:14–12:49]

• Instead of delivery, maybe some water formed here.
• Original Notion Revisited: Early Earth's magma ocean and thick hydrogen atmosphere could chemically create water.
  • Hydrogen gas pulled from the nebula reacts with iron oxides in Earth’s magma, forming water.
  • "Those two components, iron oxides and hydrogen gas, can react together... to basically rip the oxygen off of the iron oxide and deposit it in hydrogen, therefore creating water." — Michael Wong [11:32–12:26]

6. Groundbreaking Lab Evidence

[12:52–13:52]

• A recent lab study recreated these ancient processes using a diamond anvil cell—a device that simulates the extreme pressures and temperatures found during planet formation.
  • "This paper shows that you actually can in the lab. And they used a very clever technique called a diamond anvil cell to do it." — Michael Wong [12:52]
• Diamonds squeeze materials to very high pressure; lasers heat them to mimic early Earth conditions.

7. What’s Next: The Data We Need

[14:00–14:42]

• Better D/H ratio measurements of asteroids and comets.
• Observations hunting for watery worlds around other stars (exoplanets) may help resolve which theory is right.
  • "If we do, then we can sort of distinguish between some of these hypotheses. Whether or not they had, you know, the conditions right for delivery from outside... versus if they all just seem to be intrinsically full of water." — Michael Wong [14:00–14:42]

Memorable Quotes & Timestamps

• On planets forming bone dry:

  • “We thought that Earth was like pretty bone dry to begin with, so it had to be delivered from somewhere else in the solar system.” — Michael Wong [00:34]

• On the snow line:

  • “And there's this point where all of a sudden you can condense H2O water, can form ice. And we call that the snow line.” — Michael Wong [05:20]

• On isotope ratios:

  • “You get a rough sort of estimate for, like, what the D to H ratio is for the Earth. You compare that to the D to H ratios for comets. Oops, they don't match at all.” — Michael Wong [07:47]

• On water’s turbulent journey:

  • “Jupiter and Saturn just wreak havoc on the entire solar system... We think that some of these tumultuous motions of the gas giants could have actually sent this water rich material into where Earth was forming.” — Michael Wong [08:45–09:15]

• On water forming here:

  • “If you go right to the beginning of planet Earth, you've got a magma ocean... the combination of hydrogen gas, H2 and this magma ocean... can react together... to basically rip the oxygen off of the iron oxide and deposit it in hydrogen, therefore creating water.” — Michael Wong [11:13–12:26]

• On simulated ancient chemistry:

  • “This paper shows that you actually can in the lab. And they used a very clever technique called a diamond anvil cell to do it.” — Michael Wong [12:52]

• On future discovery:

  • "We should be able to see signs of watery worlds out there with future telescopes... If we do, then we can sort of distinguish between some of these hypotheses." — Michael Wong [14:00–14:42]

Notable Moments with Timestamps

• [03:55]: How long have scientists debated this mystery?
• [06:15–07:47]: Introduction and significance of deuterium/hydrogen ratios.
• [10:14–12:26]: Explanation of the “water formed on Earth” hypothesis and early conditions.
• [12:52–13:52]: Breakthrough laboratory experiment using diamond anvil cell.
• [14:00–14:42]: What observations and data might help settle the debate in the future.

Conclusion

This episode highlights the dynamic, detective-like nature of planetary science: centuries-old questions, cutting-edge experiments, and surprising new theories are all in play. Whether Earth’s water came from cosmic delivery or its own fiery origins, the answer holds profound implications—not just for our home planet, but for worlds beyond.

Final words from Michael Wong:
"It's been an absolute joy. Thanks." — Michael Wong [14:44]