Delivery: from asteroid families to Earth

Which asteroids are our meteorites actually coming from? Dynamics, spectroscopy and exposure ages are converging on answers.

Escape routes

Two dynamical routes dominate delivery from the main belt. The ν6 secular resonance at the inner edge of the belt, near 2.1 astronomical units, and the 3:1 mean-motion resonance with Jupiter near 2.5 astronomical units, which forms one of the Kirkwood gaps, both pump eccentricities until orbits cross those of the terrestrial planets.

Bodies are fed into these resonances mainly by the Yarkovsky effect, the drift caused by anisotropic thermal emission. Its rate depends on size, spin, thermal properties and distance from the Sun, and the related YORP effect changes spin rates and axis orientations over time.

Typical travel times from resonance to Earth impact are a few million years or less, but the slow Yarkovsky drift from source to resonance can take tens of millions of years. This helps explain the range of cosmic-ray exposure ages in stony meteorites.

Linking meteorites to asteroids

Reflectance spectroscopy measures how asteroids reflect sunlight at visible and near-infrared wavelengths. Absorption bands near 1 and 2 micrometres reveal olivine and pyroxene. For decades, a puzzle was that the common S-type asteroids looked redder and had weaker bands than ordinary chondrites in the laboratory.

The explanation is space weathering. Solar wind ions and micrometeorite impacts produce nanoparticles of iron in grain rims, reddening and darkening surfaces. Particles returned by Hayabusa from the S-type asteroid Itokawa showed such rims and matched LL chondrites, confirming the link.

Other links are well established: Vesta and the HEDs, and the CI-like nature of Ryugu and Bennu. In 2024, two studies combining dynamical modelling and spectroscopy argued that most ordinary chondrite falls derive from a few young asteroid families, with the Massalia family proposed for L chondrites and the Koronis and Karin families for H chondrites.

The flux through time

The meteorite flux has not been constant. The best example comes from the mid-Ordovician period, about 466 million years ago. Limestone beds in southern Sweden and elsewhere contain fossil meteorites and abundant chromite grains of L chondrite composition, recording a sharp increase in the arrival of L chondritic material after the break-up of the L chondrite parent body.

The argon-argon ages of many shocked L chondrites cluster around 470 million years, matching this event within uncertainty.

Researchers have also studied sediments of other ages for extraterrestrial chromite, finding that the mix of meteorite types arriving on Earth has changed over time. In some older sediments, types that are rare among today's falls appear to have been more common.

Today's falls, therefore, are a snapshot shaped by recent collisions in the belt, not a permanent average.

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