Asteroid Bennu: traces of salt water and organic molecules

Using advanced electron-matter interaction techniques such as cathodoluminescence, the researchers identified a wide variety of salts, including phosphates, carbonates, sulfates, chlorides and fluorides. The recognition of an evaporitic sequence testifies to the importance of brine circulation in the evolution of Bennu's parent body. In addition, the presence of complex organic compounds makes Bennu a promising candidate for the study of the origin of life. The return of samples and their meticulous preservation were essential to identify these salts, which rapidly degrade on contact with air, as described in an article published in the journal Nature on January 30, 2025.

Brines, aqueous environments with a high concentration of dissolved salts, offer conditions conducive to the emergence and development of life in the solar system, and are therefore prime targets for space exploration, as demonstrated by the Juice mission currently underway. Evaporation or freezing of these brines leads to the precipitation of various minerals (carbonates, sulfates, halides), forming deposits similar to those exploited on Earth for their resources of critical elements (lithium). While the processes by which minerals are formed from terrestrial brines are well understood, those operating in extraterrestrial environments are still largely unknown, not least because of the lack of samples.

A wide variety of salts in Bennu's complex mineral matrix

The OSIRIS-REx mission, by bringing back to Earth 120g of samples from the primitive asteroid Bennu, has opened up new perspectives in the study of the evolution of the Solar System's first small bodies. A battery of analyses using today's best microscopy and mass spectrometry techniques has revealed a wide variety of salts within Bennu's complex mineral matrix, composed of hydrated phyllosilicates, carbonates, magnetite and sulfides. These salts include phosphates, carbonates, sulfates, chlorides and sodium fluorides. Our electron microscopy and cathodoluminescence analyses reveal, for example, complex intercrossings between these different phases (dolomite, Fe-Mn magnesite, phosphate) as well as the existence of veins in the matrix.

The salts identified in the Bennu samples, notably carbonates, sulfates and chlorides, associated with mineralized veins, testify to a long history of aqueous alteration. These observations suggest that brines circulated within the asteroid's parent body, precipitating salts as they evaporated. Although many questions remain unanswered concerning the exact conditions of this alteration and its timing, the presence of complex organic compounds, associated with this evaporitic sequence and with clay and carbonate minerals, makes Bennu a promising candidate for the study of the origin of life.

The identification of these varied salts was only made possible by the return of samples and their rigorous preservation, as they are subject to alteration by the Earth's atmosphere. This discovery suggests that similar processes may have operated within icy bodies such as Ceres and Enceladus, where the presence of sodium carbonates, detected by spectrometry, bears witness to past aqueous activity.

Figure : a) Sodium carbonate veins in matrix rich in phyllosilicate (violet), magnetite, sulfide (red), and dolomite (green), b) Cathodoluminescence panchromatic image of a zoned dolomitee (D) with an iron-rich magnesite core (M), c) Sequence of mineral phase formation during brine evaporation on the Bennu parent body.

Research contact

Guy Libourel, Lagrange, Université Côte d'Azur, OCA, CNRS, libou@oca.eu, Co-I OSIRIS-REx and geographic coordinator (France-Europe)
Marc Portail, CRHEA, Université Côte d'Azur, CNRS, Marc.Portail@crhea.cnrs.fr
G.L & M.P would like to thank CNES, ANR, UniCA and the Doeblin federation for their financial support.

Further information

An evaporite sequence from ancient brine recorded in Bennu samples. T.J. McCoy & S.S. Russell et al, Nature, January 2025.