Ryugu photographed from around 20 kilometres away

Ryugu is located millions of kilometres away from Earth, with the exact distance fluctuating as both orbit the Sun. Image © JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST

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Ryugu asteroid samples offer insight into early years of the solar system

Scientists have laid out the history of a near-Earth asteroid in unprecedented detail.

Museum researchers are among those analysing the most pristine asteroid samples ever returned to Earth to reveal what the early solar system could have been like. 

Researchers have caught a glimpse of the cosmos just two million years after the formation of the solar system

Samples returned from the asteroid Ryugu by the Hayabusa2 probe in 2020 have revealed hints about some of the events that took place as the Sun and its planets formed, after being analysed by an international team of scientists, including researchers from the Museum. 

They found that the asteroid was once part of a much larger body that formed relatively early in the history of the solar system and was chemically altered as melting ice reacted with the minerals it contained.

Dr Ashley King is a Museum researcher and UKRI Future Leaders Fellow who co-authored the analysis of Ryugu. He says, 'We found that Ryugu's parent asteroid formed in the cold, outer regions of the solar system where there was both carbon dioxide and water ice.'

'Those ices accreted into the asteroid, where they melted and then reacted with rocky materials to create abundant hydrated minerals. The original asteroid was then smashed apart, with some of the fragments accumulating into what we now know as Ryugu.'

The findings of the study were published in the journal Science

JAXA staff collect Ryugu's capsule from Australia

Ryugu's samples were returned to Earth by a capsule which landed in Australia. Image © JAXA

What is Ryugu, and how did it form?

Ryugu is a near-Earth asteroid first discovered in 1999. It is named after the undersea palace of the Dragon God in Japanese mythology, from which a fishermen retrieved a box with a great secret.

This name reflects it being the target of JAXA's Hayabusa2 probe, which carried out only the second-ever sample return mission from an asteroid. Over five grams of Ryugu was collected from the asteroid's surface, with the probe subsequently returning the samples to Earth in pristine condition.

Even before these samples returned, Ryugu was already starting to change our understanding of the structure of asteroids.

'Both Ryugu and Bennu, an asteroid visited by NASA, have turned out to be rubble piles,' Ashley explains. 'These are bodies formed by the accumulation of smaller rocks under their own gravity into larger aggregations.'

'We now believe these a really common type of asteroid formed from the remnants of the solar system's first generation of asteroids.'

The researchers believe that this first generation formed within two million years of the beginning of the solar system, which is estimated to have taken place around 4.6 billion years ago. 

As the samples only contain limited amounts of inclusions known as chondrules, which form under high temperatures, the scientists think that Ryugu's parent body accreted at more than 600 million kilometres from the Sun, in a region of the early solar system where water and carbon dioxide would have turned to ice. 

Radioactive elements within the asteroid gradually heated it up until the ice began to melt. The rock chemically reacted with the meltwater to form hydrous minerals, with the asteroid reaching its peak temperature around five million years later.

One billion years ago, a catastrophic impact broke Ryugu's parent asteroid apart. Using simulations based on the available evidence, the researchers calculated that the parent, likely around 50 kilometres in radius, was struck by a smaller asteroid moving at around five kilometres a second.

Some of the fragments of the parent, located on the opposite side from the impact site, were thrown together and later coalesced to form Ryugu. The rotation of the asteroid led to its spinning top-like shape, with centrifugal forces creating a bulge in the middle.

Other fragments may form part of the Eulalia or Polana asteroid families located in the asteroid belt. 

A sample of Ryugu on display in a sealed container

Small samples of the asteroid have been distributed to labs around the world. Image © 稲妻ノ歯鯨, licensed under CC BY-SA 4.0 via Wikimedia Commons

Why do scientists need to collect asteroid samples?

Understanding the composition of asteroids can help explain how the early solar system developed, and subsequently how the Earth formed. They may even help explain how life on Earth came about, with asteroids believed to have delivered much of the planet's water as well as compounds which form the building blocks of DNA.

While meteorites, the fragments of asteroids which fall to Earth, can provide some of these clues, every stage of their journey to Earth affects them. This affects how scientifically useful they can be.

'Meteorites are modified as they travel through space, and then develop a fusion crust as they are superheated during their passage through the Earth's atmosphere,' Ashley explains. 'They then sit on the ground until they're found, and for all that time they're being modified and changed by the environment. Collecting these samples from the asteroids themselves is the only way to prevent these changes.'

It can also be difficult to work out exactly where the meteorites have come from. Studying Ryugu, whose samples have been preserved in a protective atmosphere, can help provide a starting point for where to look.

'Ryugu's samples are very similar to CI chondrite meteorites that we already have on Earth,' Ashley says. 'These meteorites are really rare as there are only five on Earth. We're really fortunate that the type specimen, the Ivuna meteorite, is held in the Museum.'

'These chondrites are special because they are the most aqueously altered class of meteorite. They are absolutely full of minerals that formed through the reaction of water and rocks at the start of the solar system. Their chemistry is also thought to be similar to that of the early solar system, so despite being altered they're quite primitive in some respects.'

A picture of the Ivuna meteroite, showing its metallic, almost coppery colour.

It was thought that Ivuna was the most pristine material in the solar system until Hayabusa2 brought its sample back. ©The Trustees of the Natural History Museum, London

Collecting samples from the surface of Ryugu is already revealing new discoveries about asteroids. Unusual flat surfaces have been found in the samples, which aren't observed in meteorites. Ashley suggests these could be related to the final stages of alteration, or possibly impact processes on the asteroid changing its surface.

Evidence of these features can now be searched for in meteorites that have fallen to Earth, as Professor Sara Russell, a Senior Research Lead at the Museum, explains.

'Studies of asteroids from missions such as Hayabusa2 complement the work we are doing on our systematic meteorite collection,' Sara says. 'The Museum houses one of the oldest and best collections in the world, and our planetary materials team have been actively working on meteorites similar to Ryugu for many years.'

While scientists continue to pore over the collections, they're also looking ahead to the future. The NASA mission OSIRIS-REx is due to return from Bennu in 2023 with over 60 grams of the asteroid – more than 12 times the amount collected from Ryugu.

If the samples are returned successfully, this wealth of material will open up a variety of new options for scientists, including Ashley and Sara, to discover how our solar system came together.

'A sample return mission is a planetary mission that keeps on giving,' Sara adds. 'Part of the sample will be stored for centuries and will be used to answer questions that we haven't even thought of yet.'