Where Did All the Mass Go?

Although over 10,000 asteroids have well-determined orbits, the combined mass of all other asteroids is not as great as that of the largest asteroid, Ceres. That makes the total mass of the asteroid belt only about 0.001 of the mass of the Earth. A frequently asked question is, if a major planet exploded, where is the rest of its mass?

Consider what would happen if the Earth exploded today. Surface and crustal rocks would shatter and fragment, but remain rocks. However, rocks from depths greater than about 40 km are under so much pressure at high temperature that, if suddenly released into a vacuum, such rocks would vaporize. As a consequence, over 99% of the Earth’s total mass would vaporize in an explosion, with only its low-pressure crustal and upper mantle layers surviving.

The situation worsens for a larger planet, where the interior pressures and temperatures get higher more quickly with depth. In fact, all planets in our solar system more massive than Earth (starting with Uranus at about 15 Earth masses) are gas giants with no solid surfaces, and would be expected to leave no asteroids if they exploded. Bodies smaller than Earth, such as our Moon, would leave a substantially higher percentage of their mass in the form of asteroids. But the Moon has only about 0.01 of Earth’s mass to begin with.

In short, an asteroid belt with a mass of order 0.001 Earth masses would be the norm when a terrestrial-sized planet or moon explodes. Meteorites provide direct evidence for this scenario of rocks either surviving or being vaporized. Various chondrite meteorites (by far the most common type) show all stages of partial melting from mild to almost completely vaporized. Indeed, it is the abundant melt droplets (called “chondrules”) that give chondrite meteorites their name.