Today, the near-Earth asteroid known as Ryugu is bone dry. But new research suggests that the half-mile space rock may have once been flowing with liquid water — and crucially, at a period in the solar system's history far later than when that would have been thought possible.
The findings, published in a new study in the journal Nature, could add weight to the theory that soggy asteroids brought Earth its first stores of H2O billions of years ago.
"This changes how we think about the long-term fate of water in asteroids," said study lead author Tsuyoshi Iizuka, an associate professor in the department of earth and planetary science at the University of Tokyo, in a statement about the work. "The water hung around for a long time and was not exhausted so quickly as thought."
When the Sun was still young, it was surrounded by a so-called protoplanetary disc of floating material that would eventually give birth to the planets. The leftovers became asteroids. And many c-type, or carbonaceous, asteroids like Ryugu formed past or near a certain boundary called the frost line, where temperatures were low enough for water to condense into tiny grains of ice.
Until now, astronomers believed that liquid water activity only took place in the earliest moments of the solar system. In the vacuum of space, water, even sheltered in asteroids, should eventually evaporate or freeze, since the absence of atmospheric pressure drastically lowers its boiling point. What little water survives is believed to be trapped inside minerals, in molecular form.
The new findings suggest that may not always be the case. The work involved examining microscopic rock samples taken from the asteroid by the Japan Aerospace Exploration Agency (JAXA) probe Hayabusa2 in 2018 and 2019. In particular, the team measured the radioactive decay of the isotopes lutetium 176 and hafnium 176 to estimate the age of Ryugu — and it didn't add up, despite it normally being a reliable way of dating.
"We got ages of around 4.8 billion years for Ryugu samples, which are far older than the age of the solar system," Iizuka told New Scientist. "This means the clock is off in Ryugu samples."
And what could throw off the clock? According to the researchers, a liquid flushing out some of the lutetium isotopes. This points to flowing water being present on Ryugu — or the original rock that spawned it — roughly a billion years after it formed. By extension, that means there should have been a sizable store of frozen water that survived this entire lengthy period before thawing.
The team doesn't have a clear picture yet of what could've thawed the water ice, because heat from the Sun shouldn't penetrate deep enough into the asteroid. Their suspicion is that another object slammed into the asteroid, breaking it apart and heating it up.
The implications, in any case, are astounding. That leaves open at least a billion year window in which ice-packed rocks could've collided with Earth and deliver it water in substantial quantities — not just the tiny mounts trapped by minerals. This changes the picture of not just how our planet got its oceans, but the formation of other worlds, too.
"The idea that Ryugu-like objects held on to ice for so long is remarkable," Iizuka said in the statement. "It suggests that the building blocks of Earth were far wetter than we imagined.
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