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New results from the Hayabusa-2 space probe show that asteroids formed at the very beginnings of our Solar System retained substantial amounts of water for hundreds of millions of years, potentially delivering water to Earth and other planets for much longer than previously thought. The work by a large international team, including Professor Qing-Zhu Yin at the UC Davis Department of Earth and Planetary Sciences, was . 

Launched in 2014 by the Japanese space agency, reached the asteroid Ryugu, which orbits between Earth and Mars, in 2018. It landed rovers on the surface and used an impactor to make a crater and collect samples from beneath the surface. These samples were returned to Earth in 2020 and Japanese scientists have shared them with labs around the world. 

Ryugu is a kilometer-wide pile of rubble left over from the very earliest days of the Solar System. Formed from dust and ice in the outer reaches of the Solar System, many of these objects now reside in the main asteroid belt between Mars and Jupiter. About five million years ago, Ryugu migrated closer to the Sun. 

Measuring isotope ratios

One of the tools scientists use to study these asteroids is the ratio between certain isotopes that are decay products of another radioisotope. For example, lutetium-176 decays to hafnium-176 with a half-life of 37 billion years. This very long timescale means that the ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf ratio can be used to measure geologic processes over a very long time. 

But there¡¯s a problem. 

¡°It has been known for about two decades in the community that the slope of the isochron for the lutetium-hafnium clock, most often than not, tends to give too old an age compared to the expected age,¡± Yin said.

There is less lutetium in the samples than expected, making the slope of the ratio a bit too steep. Where did the lutetium go? 

In the new Nature paper, the authors considered alternative hypotheses for the missing lutetium. The most likely explanation, they concluded, is that at some point long after it formed, Ryugu was struck by another object, melting water ice inside the asteroid. 

This liquid water would have partly dissolved a mineral called apatite (also found in tooth enamel) which contains lutetium and other rare earth elements. This water would have evaporated from the asteroid into space but deposited the lutetium elsewhere out of the apatite. (Lutetium is a heavy element and unlikely to be lost into space itself.)

The researchers estimate that this collision happened about 3.5 billion years ago, a billion years after the formation of the Solar System (and Ryugu). That implies that asteroids similar to Ryugu were carrying around a lot more water for a lot longer than previously thought. This increases the possibilities of delivering abundant water to the surface of growing planets over a long time in the history of the Solar System, Yin said.