“So what about the article, Cal?”

“What article?”

“The article about NASA’s Psyche mission to Psyche, the article in the magazine that you came in here ranting about. What did it say?”

“Not much, actually. It was mostly gee‑whizzery about how the Psyche asteroid is solid metal and probably worth trillions because of its gold and stuff. It’s a new mag, probably desperate for eyeball grabbers so I’m not making bets on it but is any of that possible?”

Kareem, our geologist, takes the bait. “You guys know I specialize in old rocks because they tell us Earth’s early geochemistry. I want to identify when in our history liquid water gave life a chance to start up. That’s why I keep up with asteroid news. Asteroids are the oldest rocks around, far older than what we’ve been able to dig up from the ancient cratons in Canada and Australia. Cratonic rocks max out at around 4 billion years but asteroids and Earth as a planet go back a half‑billion years more. We’ve learned a lot from asteroid‑sourced meteorites, but they’re just a tease. The cooking they get on their way through the atmosphere can burn out part of any water they had. That’s why I followed the Hayabusa2 and OSIRIS‑REx missions so closely — they brought us fresh samples from asteroids that should date back to the Solar System’s birth.”

“How about comets, Kareem? They’re ice‑balls. Those gorgeous tails they spout when they warm up, they’re all water and CO₂ and like that. Earth coulda got our water from comets.”

“Good point, Al — sorry, I mean Cal — except for two things. First, asteroids are a lot closer to Earth than comets. The densest part of the asteroid belt courses twice as wide as Earth’s orbit, about a hundred million miles outward from us. Short‑period comets generally drop in from the Kuiper Belt, which is about fifteen times wider. Long‑period comets hang out a thousand times farther out.”

“Yeah, but they do head in our direction every so often and a billion years is a long time. What’s your second thing?”

“Isotopes. You know about light hydrogen and heavy hydrogen, right? They’re both hydrogen, one proton and one electron, but the heavy kind carries a neutron along with the proton in its nucleus. Their chemistry is the same unless speed is a factor. At any given temperature, the lighter atom moves about 40% faster than its heavier cousin. Water molecules containing only light hydrogens evaporate faster than their heavier neighbors because the speedy atoms are primed to rip their molecule loose from the surrounding liquid or ice.”

“Wait, water evaporates from ice?”

“Mm-hm, except technically it’s called sublimation when ice is involved. That was a crucial process in the Solar System’s history. Five billion years ago we were this big disk of gas and dust. When the Sun finally got dense enough to light up, its radiated heat energy baked volatile components like water and such out of the metals and silicates in the rocky inner system. That’s why Earth had to import our water once we cooled off. Volatility is relative, of course. Eventually the volatiles condensed back to solid form in the ice belts near and beyond Uranus and Neptune. That’s your cometary ice balls.”

“But now you’re gonna say that ancient ice evap–, sublimated, too.”

“Sure. It’s a continual process. Sometimes a released molecule docks back on again, but mostly not. Anyhow, the light water molecules happily bounced off into the Universe whenever they could. The heavy ones stayed put. Cometary ice gradually became roughly twice as heavy‑enriched as the rest of the Solar System including us.”

“So when you look at Earth water…”

“It can’t have come from comets which is why we’re looking at asteroids.”

“Ah, but does asteroid water match Earth’s?”

“Mostly, Sy. We’ve found a few meteorites with a high heavy‑hydrogen content, but so few that they’d be <ahem> swamped by the water from all the other meteorites. Most meteorite isotopes match what we have on Earth. You’re drinking asteroid water.”

~~ Rich Olcott