NASA's Lucy spacecraft, during a flyby of the asteroid Donaldjohanson last year, has returned data revealing evidence of chemical alteration by liquid water on the object's surface. The finding suggests the asteroid once harbored interior water ice that melted and reacted with rock, a process typically associated with bodies that formed further from the sun.
Donaldjohanson is a main-belt asteroid, but the new analysis indicates it originally coalesced in a colder, outer region of the solar system before gravitational interactions gradually nudged it inward to its current orbit. The water alteration signature, identified through spectral analysis, points to past hydrothermal activity that is rare among small asteroids.
The Lucy mission, launched in 2021, is designed to study Jupiter's Trojan asteroids, but the Donaldjohanson flyby served as a calibration target en route to its primary destinations. Scientists say the unexpected discovery adds complexity to our understanding of asteroid migration and the early solar system's dynamics.
This finding challenges the assumption that main-belt asteroids are all primitive, unaltered relics from the solar system's formation. Instead, Donaldjohanson appears to have experienced a more active history, possibly indicating that water-driven chemistry was more widespread among small bodies than previously thought.
The evidence, detailed in a recent paper, underscores Lucy's value even during secondary targets. With the spacecraft now heading toward the Trojan swarms, researchers anticipate further revelations about the solar system's evolution and the role of water in shaping planetary building blocks.