Collecting and transporting back to Earth samples of other planets, moons, asteroids and comets is extremely difficult, costly and time-consuming. But as just-released papers based on Japan’s Hayabusa2 sample return mission to the asteroid Ryugu make abundantly clear, the results can be fabulous.
In a series of articles in the journal Science, scientists who studied the samples (which were returned to Earth in late 2020) and commentators marvel at the opportunity to study material that was formed as the solar system itself formed — more than 4.5 billion years ago.
The sample contains thousands of different organic (carbon-based) molecules of different kinds, including amino acids and a range of aromatic hydrocarbons. There are also many minerals formed in the presence of water.
This composition was not a big surprise based on other similar carbon-based meteorites that have fallen to Earth. But they were totally clean samples that were in no way contaminated by life and physical conditions on our planet. They also had not made the fiery passage through our atmosphere before landing and becoming a meteorite that someone may chance to find.
What they are, then, are pristine examples of the early solar system — solar system baby pictures — with the chemistry and physical thumbprints of the solar nebula and interstellar space from which our Sun and solar system were formed.
The return capsule brought back about 10 grams of the asteroid. That might not seem like a lot, but it was more than enough to learn a great deal about an important asteroid from an ancient asteroid family.
As Hiroshi Naraoka of Kyushu University and his colleagues conclude in their Ryugu paper, “Meteorites made of material similar to Ryugu may have delivered amino acids and other prebiotic organic molecules to the early Earth and other rocky planets — providing the building blocks of life.”
Ryugu provides the best chance to date to study what precisely could have been delivered.
The studies together tell the history of Ryugu, its history and its composition.
As described in an introduction to the Science papers, some of the organic molecules collected and brought to Earth formed in the interstellar medium, so they predate the emergence of the solar system.
That happened about 4.5 billion years ago from the collapse of a dense cloud of interstellar gas and dust. When this dust cloud collapsed, it formed a solar nebula – a spinning, swirling disk of material which became the solar system.
About 2 million years after the solar system began to form, material in its outer region collapsed under gravity to form a planetesimal — either Polana or Eulalia — which later became Ryugu’s parent body.
About 3 million after the planetesimal was formed, the decay of radioactive elements in the object raised the planetesimal’s internal temperature high enough to melt frozen water. Those water-based alteration reactions modified the mineralogy and formed additional organic molecules. The water reacted with the rock for a few million years, until the temperature fell again.
The planetesimals were located in a perhaps outer region of the asteroid belt that was especially rich in carbon. Meteorites with relatively high levels of carbon are called carbonaceous chondrites and are prized when found on Earth.
After about a billion years, an impact on the parent body kicked material into space and some of that rubble reaccumulated under gravity to form Ryugu.
Ryugu spent most of its lifetime in the main asteroid belt, between Mars and Jupiter. Only about 5 million years ago, gravitational forces caused Ryugu to migrate into its current near-Earth orbit.
When it is closest to Earth, Ryugu is half the distance from our planet to the Sun. At its furthest orbit, it is 2.5 times that Earth-Sun distance, called an “astronomical unit.”
The Science issue featuring Ryugu includes five studies that reports on the elements , minerals and organic molecules found, as well as some important isotope ratios. Together they provide the first detailed look at the makeup and likely history of a carbonaceous asteroid.
The findings show that organic molecules had abiotic origin; they likely arose from a water-rock reaction of Ryugu’s parent body.
The findings also show that these types of organic molecules can survive on the surfaces of carbonaceous asteroids, despite the harsh space environments, and that they can be transported throughout the solar system.
And if this can happen in our solar system, there is no reason to think that under the right conditions it couldn’t happen in other solar systems. The chemicals and laws of physics, after all, appear to be universal.
The successful Hayabusa2 mission followed the original Hayabusa mission to a different asteroid, Itokawa. That first Hayabusa mission — about which several popular Japanese movies were made — included many mission-threatening mishaps that were sometimes near miraculously resolved and the payload was dropped onto the desert of Western Australia. But the amount of rock dust collected was much smaller and the asteroid was not as ancient and potentially informative as Ryugu.
The Hayabusa2 payload was also dropped onto Western Australia, where teams from the Japanese space agency were spread out waiting to collect it.
The published results were embraced as proof that sample return missions are essential to better understand our solar system.
“Beyond the insights into Ryugu’s history, these studies emphasize the primary justification for sample return missions: Analyses in laboratories on Earth can provide far more detailed information than even the most advanced, space-qualified instrumentation,” writes Christopher Herd in a related Science Perspective.
Sample return is not an entirely new idea. Apollo astronauts, after all, brought back to Earth innumerable rocks from the moon. The crew of China’s Chang’e-5 mission to the moon have also more recently brought back lunar samples.
NASA’s OSIRIS-REx mission to the near-Earth asteroid Bennu, which launched in 2016, is scheduled to drop off its sample payload fall of this year. Bennu is also a carbonaceous asteroid.
The most ambitious sample return effort now in progress involves NASA and European Space Agency efforts to bring rock dust to Earth from Mars. The Mars rover Perseverance is now collecting samples in Jezero Crater — a once watery site with fossil rivers and deltas — and plans are to bring some of them to Earth in the early 2030s.
The architecture and some of the technology needed for this long-desired and much more challenging sample return effort are under development now, and include construction of a air-tight facility where scientists can safely examine the precious Mars samples.
In addition to providing a wealth of insights into the chemistry and geology of Mars, the sample will also be examined painstakingly for signs of earlier life, when Mars was wetter an warmer. That life-detection task was not part of the Ryugu mission, nor will it be for Bennu since asteroids are almost generally seen an unable to support life.
But Mars is now known to have been habitable in its early days so there is a possibility that microbial life once existed there. And it will almost surely take laboratories on Earth that can study returned samples with sophisticated technology to make any major breakthroughs on that front.