The Asteroid Belt Is Vanishing

Between the orbits of Mars and Jupiter lies a ring-shaped region called the asteroid belt, home to the vast majority of our solar system’s space rocks. The asteroid belt is as old as the solar system itself, having formed from leftover material that failed to coalesce into a full-grown planet due to Jupiter’s gravitational influence.

Over the past 4.6 billion years, Jupiter has continued to shape the asteroid belt through gravitational resonances—regions where an asteroid’s orbit aligns with that of Jupiter, Saturn, or Mars. This can either fling an asteroid toward the inner solar system or outward toward Jupiter’s orbit. Meanwhile, those that don’t escape constantly collide with other asteroids in the belt, gradually being pulverized into dust. As such, the asteroid belt is slowly disappearing.

In a new study that has yet to undergo peer review, researchers led by planetary scientist Julio Fernández of Universidad de la Republica in Uruguay estimate the rate of the belt’s depletion. The findings, currently available on the preprint server arXiv, indicate that the collisionally active portion of the asteroid belt is losing about 0.0088% of its mass every million years.

A very slow vanishing act

The collisionally active portion of the belt refers to asteroids that are small enough to be involved in frequent collisions and dynamical ejections—basically everything except large, primordial bodies such as Ceres, Vesta, and Pallas. A loss of 0.0088% per million years may not sound like much, but over the eons, it adds up.

Fernández and his colleagues estimate that 3.5 billion years ago, the asteroid belt may have contained 50% more mass, with a loss rate double what it is today. That estimate correlates nicely with a more intense impact rate reflected in Earth and the Moon’s geologic records, according to the researchers.

Where do the lost asteroids end up? Well, according to the researchers’ calculations, about 20% escape into space, occasionally crossing into Earth’s orbit and even plummeting through our atmosphere as meteors. The other 80% gets ground into meteoritic dust that filters into the zodiacal cloud—a thick, pancake-shaped dust cloud that orbits the Sun within the inner solar system.

Will the asteroid belt completely disappear?

Previous research has estimated that the combined mass of all the asteroids in the belt today is roughly equivalent to just 3% of the Moon’s mass. Still, it would take many, many more years for the belt to disappear completely through pulverization and dynamical ejections alone. The death of the Sun—projected to occur in about 5 billion years—will destroy it sooner than that.

This study provides an answer to an arguably more important question: At what rate is the asteroid belt ejecting space rocks that could potentially impact Earth? Additionally, by extrapolating the rate of dynamical ejection back in time, the researchers present data that can help scientists better understand the impact history that shaped the planet’s surface.