Asteroid Ryugu Was Once Packed with Water, Offering Clues on Earth’s Habitability
Despite having such a dry and rocky exterior, asteroids used to be overflowing with water on the inside. The ancient asteroids of the early solar system even carried out the most important delivery in history: bringing water to our world.
A new study published in Nature has determined that the asteroids responsible for shaping the young Earth’s water systems were actually wetter than scientists once thought. The evidence comes straight from fragments of asteroid Ryugu, which have shown how water flowed through Ryugu’s parent asteroid more than a billion years after it first formed. This discovery has revealed that water was long-lived on ancient asteroids, potentially recasting ideas about Earth’s habitability.
Ryugu’s Waterlogged History
Ryugu is a carbonaceous (or carbon-rich) asteroid that likely formed right outside of Jupiter, like other asteroids of its kind. Since Ryugu’s orbit around the Sun crosses Earth’s orbit, scientists set out to make contact with the asteroid. After reaching the asteroid in 2018, the Hayabusa2 spacecraft led by the Japanese Aerospace Exploration Agency (JAXA) collected samples and brought them to Earth in 2020.
Five years of research have yielded several new insights about Ryugu’s birthplace and composition. Now, thanks to the asteroid’s samples, researchers have a better idea of how long water may have been stored in carbonaceous asteroids of the past.
“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” said author Tsuyoshi Iizuka, a professor at the University of Tokyo’s Department of Earth and Planetary Science, in a statement. “This changes how we think about the long-term fate of water in asteroids. The water hung around for a long time and was not exhausted so quickly as thought.”
Read More: Origins of Water on Earth May Not Have Started with an Asteroid Impact
Turning Back Time with Isotopes
To track down evidence of past water activity on Ryugu, the researchers had to measure isotopes of lutetium (Lu) and hafnium (Hf) from the asteroid’s fragments. The radioactive decay of these elements act as a dating tool, helping to establish the geologic processes that were occurring on asteroids billions of years ago.
After conducting experiments on samples smaller than a grain of rice, the researchers noticed that the ratio of the 176Hf isotope to the 176Lu isotope was far higher than anticipated, indicating that fluid was likely washing out the lutetium from the rocks containing it.
According to the researchers, a larger parent asteroid of Ryugu likely experienced a collision that caused buried ice to melt and become liquid water, which started to flow within the asteroid.
The Origins of Earth’s Water
Ryugu’s parent asteroid contained ice for over a billion years, meaning similar asteroids that hit Earth during the early stages of its formation may have held an estimated two to three times more water than previous models have surmised, according to the researchers.
“The idea that Ryugu-like objects held on to ice for so long is remarkable,” said Iizuka. “It suggests that the building blocks of Earth were far wetter than we imagined. This forces us to rethink the starting conditions for our planet’s water system. Though it’s too early to say for sure, my team and others might build on this research to clarify things, including how and when our Earth became habitable.”
Even with this newfound knowledge, researchers still don’t know the whole picture. They plan on examining phosphate veins within Ryugu samples to confirm a more accurate age of the fluid flow. They will also compare the results from Ryugu with samples from another near-Earth asteroid, Bennu, to see if there are similarities in water activity. These further studies, they hope, will shed light on how water first came to Earth and made it habitable.
Read More: Basic Building Blocks of Life Found on Asteroid Bennu
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