An artist’s illustration of the asteroid belt in the solar system

Samples and satellite observations hint at how raining meteorites may have made the planet habitable

Olivia Lee

A long time ago, a hard rain fell on the young Earth, still at the most tumultuous stage of its formation. Tumbling down from the skies were meteorites with origins in asteroids, their innards filled with such chemicals as ammonia and carbon dioxide, thus creating the right conditions for life.

A few billion years later – in 2003 by the Gregorian calendar – Hiroyuki Kurokawa, then studying to be a doctor, ambled into his high school bookstore. Browsing idly, he chanced upon a book on astronomy, Shigeru Ida’s book “The Strange Planets.” It reignited his dormant passion in the subject. Thanks to that chance encounter with a tome on a now relatively peaceful geologically Earth, we have added a new wrinkle in our understanding of the evolution and formation of planets. Now, Kurokawa and Ida both work at the new Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology.

Kurokawa studied, among other things, the big picture of how carbonaceous meteorites from the outer asteroid belt — a crowded region of rocks between Mars and Jupiter — kickstarted life on Earth. But somehow there appeared to be some discrepancy between the asteroids observed via telescope and their fallen counterparts.

The raw materials of life

Asteroids showed ammonia-bearing clay surfaces, a proxy for the existence of volatile elements, yet meteorite samples did not exhibit these features. Furthermore, these elements are only stable in an environment which the current asteroid belt does not support.

Those were two issues Kurokawa and his colleagues had on their hands.

Satellite image from AKARI
Satellite image from AKARI. (C)JAXA

The team studied light below the threshold for human vision to gauge the chemical properties of the asteroid’s surface. They got the data for this from Japan’s AKARI satellite to do this, a method called spectroscopy. Spectroscopy relies on splitting light into its constituent colors – such as through a prism. Depending on the chemicals in the source, different colors are absorbed. By analyzing the pattern of colors, astronomers can gauge the materials on the light source.

The team, from the ELSI, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Caltech, and the Japan Aerospace Exploration Agency (JAXA), also used computer models to simulate the chemical and water environment found in the AKARI data.

It took six months to get the spectroscopy analysis and models running right; interpreting the data took another six.

Dissecting an asteroid: A scenario for the formation and evolution of an asteroid with clear distinction between water-dominated and rock-dominated layers
A scenario for the formation and evolution of an asteroid with clear distinction between water-dominated and rock-dominated layers. Reproduced from Kurokawa et al. 2022 AGU Advances CC BY 4.0

Putting the pieces together

“These results are just a piece of a big jigsaw puzzle. We needed to think carefully about the broader picture of the history of the solar system or asteroids,” Kurokawa explains.

The AKARI data revealed ammonia-bearing minerals on many asteroid surfaces, but the computer models were unambiguous: to form these minerals required a water-rich environment with ammonia and temperatures lower than 70 degrees C (-94 degrees Fahrenheit) within the asteroid. This suggests to researchers that the asteroids must have formed in much colder environments, somewhere at or beyond Saturn’s orbit, before migrating to their current position.

When heading away from the sun, temperatures drop to those levels on asteroids only at about 5 astronomical units (700 million km, or 435 million miles). That is really close to the orbit of the Jupiter. Beyond that “snow line,” comets going away from the sun lose their tails as the water vapor that makes it up solidifies again. Most of the asteroid belt we see between Jupiter and Mars falls in a warmer patch.

“This study gave us the direct clue for that kind of migration history that was proposed before and gave a more direct constraint on temperature conditions for formation,” Kurokawa says.

This representation of Ceres' Occator Crater in false colors shows differences in the surface composition. These salts could have been crucial to life on earth
This representation of Ceres’ Occator crater in false colors shows differences in the surface composition. The bright areas are associated with salts. Pic courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The inside story

Why ammonia-bearing clays are found on asteroids but not meteoroids was only explained when the team figuratively dissected an asteroid. Considering a large asteroid, such as Ceres, can be more than 100 kms (62 miles) in diameter, Kurokawa explains that gravity would see denser, rocky material sinking to the center, and allow the less dense water to rise to the surface. “This meant that ammonia-bearing minerals could only form in the water-rich surface layers,” Kurokawa says.

“We hypothesized that in contrast to asteroid observations where data only covers the ammonia-bearing surface, meteorites would be fragments of the inner rocky section as the upper layer sublimates in space before reaching Earth. Constructing this scenario was the most challenging part of the research,” he says with some relief, explaining that he has spent three years on this paper.

The team hopes their trick of combining actual data with computer models will catch on in their community.

Kurokawa admitted his work has not always been readily accessible.

Hiroyuki Kurokawa enjoying a leisurely day when he is not in the lab
Hiroyuki Kurokawa enjoying a leisurely day outside the lab, Pic courtesy Hiroyuki Kurokawa

“In Japan (or maybe also in other places in the world), scientists are sometimes thought to be odd people working on strange things,” he said. “To be highlighted in media (newspapers, TVs, etc.) is an important opportunity to tell my families and friends what I am working on. Several years ago, when I appeared on a short TV show introducing my work, I received lots of messages from my friends who were surprised by it.”

The bigger picture

Discussing future plans, Kurokawa said he would love to see further collaboration between researchers in the field of protoplanetary discs, solar system exploration, and extrasolar planet formation. He plans to use the disparate pieces of data to put together the larger story.

As he put it: “My hope is to combine these different observations to understand the complete picture of how planetary systems form in our universe, and how life develops on other planets.”

Olivia Lee

Olivia Lee is a fish sex health specialist turned science communicator. She is passionate about using the power of stories to generate more interest and inclusion in STEM.

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