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Ryugu is a heap of space rubble that might unlock the mysteries of water on Earth

Early results from the Japanese Hayabusa2 mission are not just giving us insights into space rocks—they can tell us about our own planet’s history, too.
March 19, 2019
Japan Aerospace Exploration Agency (JAXA)

We traveled millions of miles from Earth to visit a pile of rubble in space. Luckily, Ryugu, the near-Earth asteroid visited by the Japanese Hayabusa2 probe, is far more interesting than that sounds. Not only is it giving us unique insights into how space rocks form, but it’s teaching us more about how water may have appeared on Earth.

The Hayabusa2 mission launched in 2014 and has taken a number of pictures and scans of the asteroid. It has also deployed hopping rovers on its surface and shot bullets into its rocks, which told us more about the geology of its surface. Now, three papers published in Science today have used this data to measure the asteroid’s density, mass, shape, and spin. The findings should help scientists better understand the rock samples Hayabusa2 is slated to bring back to Earth in 2020.

Here’s some of what we know so far:

  • The asteroid has a low density. This suggests it has a porous rubble-like interior.
  • It’s about 1 kilometer wide at its equator, with an approximate mass of 450 billion kilograms.
  • Ryugu was probably created from a much larger parent body.
  • The rocks that make up the asteroid created the shape of a spinning top during a time when the object was rotating at about twice its current rate.
  • A near-infrared spectrometer found hydrated minerals—minerals that have water as part of their chemical structure—on the surface of the rock, but less water than researchers expected. Ryugu has significantly less water than Bennu, a similar near-Earth asteroid currently being studied by NASA.

Other surprises emerged as well. “The biggest surprise to me was the fact that Ryugu’s surface is covered by the same color of boulders,” says one of the paper’s lead authors, Seiji Sugita. This probably means that Ryugu’s parent asteroid—the larger body from which it was created—had a uniform interior.

(Those surface rocks took other teams by surprise—but for other reasons. The craft’s landing on Ryugu was supposed to happen in October 2018, but gravel on the asteroid’s surface was bigger than expected. That called for some new Earth-based testing before the landing, which finally happened in February.)

Artists rendering of Hayabusa 2
Go Miyazaki | Wikimedia Commons

What the researchers are learning from studying Ryugu up close is also giving insights into other objects in our solar system, including Earth. In the past it’s been hypothesized that the quantity of hydrated minerals and organic materials in asteroids was mainly determined by the temperature in space where they were created.

In a cold place, ice and organics condense into small objects called planetesimals. Eventually, these combine with other small objects to form asteroids or even planets.

In warmer areas of space, water and organics do not condense as easily, meaning the planetesimals that come together to make asteroids there are less likely to contain water. “This may have influenced how much water and organics Earth received from the asteroid belt when life was born,” says Sugita.

Billions of years ago, Earth was hit by numerous asteroids, which may have transferred the water trapped in their rocks upon impact to make life as it is today possible. Sei-ichiro Watanabe, the author of one of the other papers, says carbon-rich asteroids like Ryugu are considered primary candidates for having delivered these water and organics to Earth. This makes the relative lack of water a head-scratcher.

The results of studying Ryugu could mean altering our models of the early solar system and adjusting our theories on the composition of rocks that brought water to Earth. According to Sugita, the observations of Ryugu suggest another reason why an asteroid might contain less in the way of water-bearing material or organics than we expect: radioactive heat during the rocks’ early years dehydrates them. But there is still a way to go in the mission before we get some concrete answers. Comparing the results from Ryugu and Bennu will help researchers come to more definite conclusions.

The next major step for the mission is an explosive one: it’ll be dropping a 2-kilogram (4.4-pound) device onto Ryugu on April 5. This will let the Hayabusa2 take samples from unexposed areas underground. It’ll require some quick flying so that the spacecraft is out of the way when the asteroid fragments go whizzing into space.

While all this data is boosting our understanding of asteroids, scientists will have to wait until 2020 before they actually get their hands on the tiny samples that Hayabusa2 managed to grab after bullets were fired into the rock. These tiny crumbs of Ryugu’s surface (an estimated 10 to 100 milligrams) should finally help resolve outstanding questions about this fascinating pile of space rubble.

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