During the infancy of the solar system two basic ingredients were available for constructing solid objects such as planets, moons and protoplanets. One consisted of chondrule‑like grains that were flash‑heated to become millimetre‑sized rock particles. The other was a fine‑grained dust matrix rich in water ice and organic molecules.
Chondrules are thought to form in localized hot spots of the protoplanetary disk, where lightning discharges or shock waves briefly melt dust grains before they cool rapidly. This produces spherical, glassy particles that are denser and contain far fewer volatiles than the surrounding dust. Their formation leaves a high‑temperature chemical signature.
The matrix, by contrast, stays cold and preserves both ice and complex organics. It surrounds the chondrules and acts as a glue that helps the grains stick together into larger aggregates. Although it is rich in volatiles, its overall mass in the inner disk is comparatively low.
Meteorite analyses and astronomical observations reveal that the inner solar system shows a clear excess of chondrule‑rich material over matrix‑rich material. This imbalance suggests that early heating processes depleted the supply of icy dust, favouring the assembly of bodies that are depleted in volatiles. The resulting composition directly influences the makeup of the inner planets and the evolution of their atmospheres.