Scientists Plugged a Gap with Another Guess

Four and a half billion years ago, the early Earth was molten. A new study suggests incoming asteroids and comets contributed more heat to keeping it that way than anyone had calculated before.

The finding closes what researchers call a 500 million degree heat gap. This sounds like progress—it is actually an exposure.

Everyone agrees the early Earth was molten. What planetary scientists have been arguing about is why it stayed molten so long. Researchers constructed a thermal baseline for the Hadean Eon, that first 500 million years, using measurements of radioactive decay, models of how planets cool. Observations of meteorites and isotope ratios.

Then they looked at the geological record and noticed the crust took longer to solidify than their models predicted. So they created a gap—something was missing from their heat calculations. The gap itself became real, it acquired momentum, and papers were written to explain it. Now they have identified the culprit. Cosmic bombardment delivered more thermal energy than they had factored in.

But this raises a question that should unsettle you. What if the baseline model was never reliable? What if we were comparing observations to a theoretical construct that was never independently validated in the first place? When you work backward from an untested assumption, you can only discover problems that the assumption itself permits you to see. The 500 million degree discrepancy didn't exist as a fact—it existed because we measured reality against a model we had never proven worked.

When you work backward from an untested assumption, you can only discover problems that the assumption itself permits you to see.

We have not discovered that impacts mattered. We have discovered that our methods for assigning heat to early Earth were crude enough to accommodate almost any explanation. The real question is whether we were ever asking the right one.