💧 Liquid water flowed on this asteroid

An asteroid bears traces of ancient liquid water, but it orbits in the inner part of the asteroid belt, where it is too hot for ice to survive. This discovery, made by NASA's Lucy probe during its flyby of asteroid Donaldjohanson in April 2025, calls into question our ideas about the formation and movement of these small celestial bodies.

Donaldjohanson, nicknamed DJ by the mission team, is a strangely shaped asteroid with two lobes connected by a narrower neck. It is actually a fragment of a much larger asteroid that broke apart 155 million years ago during a titanic collision. This remnant belongs to the Erigone family, named after the main asteroid that resulted.


Spectral analysis of DJ revealed the presence of ferrous phyllosilicates, minerals that form only in the presence of liquid water. According to Simone Marchi, a planetary scientist at the Southwest Research Institute, these phyllosilicates indicate that water was present and caused aqueous alteration, but it stopped prematurely. Several hypotheses explain this interruption: a lack of internal heat due to late formation, or simply less water available where the asteroid formed.


The presence of these hydrated minerals is a valuable clue for understanding the history of the Solar System. Primitive asteroids like DJ are veritable fossils, witnesses to the materials that formed the planets. Knowing where they formed and how they migrated can inform us about the origin of water and organic compounds on Earth, elements essential for the emergence of life.

The two-lobed shape of Donaldjohanson is shared by many small bodies in the Solar System, whether asteroids like Itokawa, Toutatis or Selam, or comets like 67P Churyumov-Gerasimenko. However, the formation mechanisms of this shape may differ: for comets, erosion by sublimation of gases hollows out the neck between the lobes, while for asteroids, it could be fragments that reassembled under gravity, forming what is called a contact binary.

After this successful flyby, Lucy continues on its way to Jupiter's Trojan asteroids, which it is expected to reach in 2027. The Trojans are even more primitive than DJ and contain more carbon, water, and volatiles. Only one of them, Eurybates, has a composition similar to that of DJ. Comparing these two objects will help understand how the giant planets migrated and scattered asteroids in the early Solar System.


The big question that remains is how many asteroids similar to DJ were thrown into the inner Solar System, potentially becoming sources of water and organic matter for early Earth. Lucy, by exploring the Trojans, hopes to provide answers about these ancient migrations and how our planet acquired its essential ingredients for life.