Using Neutrinos to Measure Earth’s Composition

Pigeons flying about chaotically, rock-melting lasers, targeted earthquakes, worldwide electrical storms, multiple nuclear bomb explosions, and an endless supply of Hot Pockets. All these elements feature prominently in a movie named The Core. A team of crack scientists and engineers undertake a mission to the center of the Earth to make the core spin again before all life is destroyed. During the team’s journey, they encounter unexpected obstacles because, frankly, humans have never seen the inside of Earth.

In reality, the composition of Earth’s interior remains uncertain, but a sophisticated detector in Japan is helping remove that uncertainty.

Humans have drilled as deep as 40,000 feet into Earth’s crust. While that represents an impressive engineering accomplishment, the borehole manages to pierce roughly one-third of the continental crust. Furthermore, it does not even begin to sample Earth’s mantle, which ranges from 115,000 feet (35 km) down to 9 million feet below the surface. Consequently, determining the structure and composition of Earth’s interior is difficult.

Scientists do know the amount of heat escaping from Earth because these measurements are made from Earth’s surface. This heat comes from two sources. First, a substantial amount of energy accompanied Earth’s formation and the subsequent impact events (like the moon formation collision and the late heavy bombardment) and this energy heated Earth’s interior. Second, radioactive elements like uranium, thorium, and potassium (U, Th, and K) emit energy as they decay. Some combination of these two heat sources ensures that Earth’s interior remains hot enough so that plate tectonics (necessary for life) operates reliably. Scientists measure 44 TW (terawatts or 10¹²W) of heat escaping from Earth today.

Since it is impossible to sample rocks from deep inside Earth, a Japanese detector named KamLAND does the next best thing. It measures the neutrinos (technically, antineutrinos) emitted from U and Th as they decay (the neutrinos from K have too low of an energy as they decay for KamLAND to detect). Although each atom of U and Th (specifically ²³⁸U and ²³²Th) that decays emits a neutrino, detecting these neutrinos presents more of a challenge. First, neutrinos hardly interact at all. These tiny particles easily pass through miles of Earth without interacting so it takes a large number of neutrinos passing through an enormous detector to have a chance at measuring some of them. However, this weak interaction means that neutrinos produced deep inside Earth can make it to detectors on the surface. Second, background sources (like nuclear reactors and the Sun) make it difficult to find the neutrinos from ²³⁸U and ²³²Th decays inside Earth.

Nonetheless, the scientists working on KamLAND have accounted for all the backgrounds and convincingly detected a significant signal of geoneutrinos—those produced from radioactivity within Earth.¹ From the signal, they calculated the amount of ²³⁸U/²³²Th—and the subsequent heat generated from their decay—and found that about half of Earth’s heat budget (20TW from ²³⁸U/²³²Th plus another 4 TW from ⁴⁰K) comes from radioactive decay.

This discovery shows that Earth has not exhausted its primordial heat and that a substantial portion now, a few billion years after formation, comes from radioactive decay. Our sister planets, Venus and Mars, reveal how important a large heat supply is to plate tectonics. While both of these planets exhibited tectonic activity early in their history, their small size (meaning less primordial heat) and smaller quantity of long-lived radioisotopes like ²³⁸U/²³²Th resulted in cessation of persistent tectonic activity a couple of billion years ago. Consequently, any water they might have started with has disappeared.

In the past few years, astronomers have announced a number of planets as “habitable”––based on their size (in order to be rocky planets) and distance from the host star (so that liquid water might exist). While scientists expect a number of planets to meet these two characteristics, they also note that plate tectonics plays a crucial role in Earth’s habitability. Thus, to start with sufficient primordial heat, a habitable planet must be suitably large and it must experience numerous collisions. Further, the planet must also form and be enriched with long-lived radioisotopes to maintain the internal heat for billions of years.

As astronomers continue to locate planets outside the solar system, scientists also find more constraints that such planets must meet in order to potentially host life. Earth clearly meets all the criteria for life, but the growing list of requirements implies that, at least from a strictly naturalistic perspective, other habitats are either rare or nonexistent. Such rarity bolsters the notion that an intelligent Designer fashioned this planet for human habitation.