Analysis of material retrieved by JAXA’s Hayabusa-2 spacecraft indicates that liquid water traversed an ancient asteroid over a billion years post-formation. This ancient hydrological activity is theorized to have been initiated by an impact event, which provided the thermal energy necessary for ice sublimation and simultaneously created structural fissures, thereby facilitating aqueous migration.
This image of the asteroid Ryugu was captured by the Optical Navigation Camera – Telescopic (ONC-T) on JAXA’s Hayabusa-2 spacecraft on June 26, 2018, from a distance of 13.7 miles (22 km). Image credit: JAXA / University of Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / Aizu University / AIST.
Ryugu is classified as a near-Earth Cg-type asteroid and is associated with the Polana collisional family.
This celestial body, also designated 1999 JU3, was identified in May 1999 by astronomers engaged in the Lincoln Near-Earth Asteroid Research project.
Its dimensions approximate 900 meters (0.56 miles) in diameter, and it maintains an orbital path around the Sun from 0.96 to 1.41 astronomical units (AU), completing a revolution every 474 days.
Tsuyoshi Iizuka, a researcher from the University of Tokyo, and his colleagues stated, “While our comprehension of Solar System genesis is relatively robust, numerous lacunae persist.”
“A significant void in our scientific understanding concerns the origins of Earth’s substantial water inventory.”
“It has been a long-established tenet that carbonaceous asteroids, such as Ryugu, originating from the outer Solar System’s ice and dust, were instrumental in furnishing Earth with water.”
Dr. Iizuka further elaborated, “We ascertained that Ryugu retains an unaltered historical record of hydrologic activity, providing compelling evidence of fluid movement through its rocky constituents at a much later epoch than anticipated.”
“This discovery mandates a reassessment of our conception regarding the enduring sustenance of water within asteroids. The aqueous content persisted for an extended duration, rather than being depleted with the celerity previously conjectured.”
Within the scope of this investigation, the research team meticulously examined isotopes of lutetium (Lu) and hafnium (Hf). The radioactive decay of lutetium-176 to hafnium-176 functions as a geochronometer, enabling the measurement of geological processes.
The anticipated concentrations of these isotopes within the analyzed samples were expected to correlate with the asteroid’s age in a predictable manner.
However, the measured ratio of hafnium-176 to lutetium-176 significantly exceeded expectations.
This observation led the researchers to strongly infer that a pervasive fluid had effectively leached lutetium from the surrounding lithic materials.
Dr. Iizuka commented, “Our initial premise was that Ryugu’s geochemical signature would closely mirror that of certain meteoritic samples previously studied on Earth.”
“The findings, however, proved entirely divergent. This necessitated a rigorous process of eliminating alternative hypotheses, ultimately leading us to conclude that the Lu-Hf isotopic system had been perturbed by late-stage fluid percolation.”
“The most plausible inciting factor was a celestial impact event impacting Ryugu’s progenitor, a larger asteroid, which resulted in rock fracturing and the melting of subsurface ice, thereby permitting the infiltration of liquid water throughout the body.”
“The revelation was genuinely astonishing! This impact phenomenon may also bear responsibility for the fragmentation of the parent body, leading to the formation of Ryugu.”
One of the most significant implications of this research is the possibility that carbonaceous asteroids harbored and delivered substantially more water to Earth than previously posited.
It appears that Ryugu’s ancestral body maintained its icy reserves for upwards of a billion years, suggesting that analogous cosmic objects impacting a nascent Earth could have transported an estimated two to three times the volume of water predicted by prevailing models, thereby profoundly influencing our planet’s primordial oceans and atmosphere.
Dr. Iizuka remarked, “The notion that Ryugu-like entities preserved ice for such an extended period is truly remarkable.”
“It implies that the foundational constituents of Earth were considerably more hydrated than our current understanding suggests.”
“This necessitates a fundamental reconsideration of the initial conditions governing our planet’s hydrological cycle.”
“While it remains premature to offer definitive pronouncements, my research cohort and other scientific groups may build upon these discoveries to elucidate critical aspects, including the mechanisms and chronology by which Earth attained habitability.”
The findings have been published in the esteemed journal Nature.
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T. Iizuka et al. Late fluid flow in a primitive asteroid revealed by Lu-Hf isotopes in Ryugu. Nature, published online September 10, 2025; doi: 10.1038/s41586-025-09483-0
