New Study Sheds Light on Solar System’s Great Divide

A new study, published in the journal Nature Astronomy, suggests that our young Solar System’s protoplanetary disk was divided into two regions: on the one side were terrestrial planets, such as Earth and Mars, made up of fundamentally different types of materials than the more distant Jovian planets, such as Jupiter and Saturn.

This is an artist impression of a protoplanetary disk around a young star. Image credit: A. Angelich / NRAO / AUI / NSF.

This is an artist impression of a protoplanetary disk around a young star. Image credit: A. Angelich / NRAO / AUI / NSF.

“The most likely explanation for that compositional difference is that it emerged from an intrinsic structure of this disk of gas and dust,” said Professor Stephen Mojzsis, a researcher in the Department of Geological Sciences at the University of Colorado, Boulder, and the Institute for Geological and Geochemical Research at the Hungarian Academy of Sciences.

The Great Divide — a term that Professor Mojzsis and his colleague, Dr. Ramon Brasser from the Earth-Life Science Institute at the Tokyo Institute of Technology, coined — does not look like much today.

It is a relatively empty stretch of space that sits near Jupiter, just beyond what astronomers call the main asteroid belt.

“But you can still detect its presence throughout the Solar System. Move sunward from that line, and most planets and asteroids tend to carry relatively low abundances of organic molecules,” the scientists said.

“Go the other direction toward Jupiter and beyond, however, and a different picture emerges: almost everything in this distant part of the Solar System is made up of materials that are rich in carbon.”

“This dichotomy was really a ‘surprise’ when it was first found,” Professor Mojzsis said.

Many scientists assumed that Jupiter was the agent responsible for that surprise.

The thinking went that the planet is so massive that it may have acted as a gravitational barrier, preventing pebbles and dust from the outer Solar System from spiraling toward the Sun.

But the study authors were not convinced; they used a series of computer simulations to explore Jupiter’s role in the evolving Solar System.

They found that while Jupiter is big, it was probably never big enough early in its formation to entirely block the flow of rocky material from moving sunward.

“We banged our head against the wall. If Jupiter wasn’t the agent responsible for creating and maintaining that compositional dichotomy, what else could be?” Dr. Brasser said.

Astronomers know that young stars are often surrounded by protoplanetary disks that, in infrared light, look a bit like a tiger’s eye.

“If a similar ring existed in our own Solar System billions of years ago, it could theoretically be responsible for the Great Divide,” Professor Mojzsis and Dr. Brasser said.

“That’s because such a ring would create alternating bands of high- and low-pressure gas and dust.”

“Those bands, in turn, might pull the Solar System’s earliest building blocks into several distinct sinks — one that would have given rise to Jupiter and Saturn, and another Earth and Mars.”

“In the mountains, the Great Divide causes water to drain one way or another,” Professor Mojzsis said.

“It’s similar to how this pressure bump would have divided material in the Solar System.”

“But there’s a caveat: that barrier in space likely was not perfect. Some outer solar system material may still have climbed across the divide. And those fugitives could have been important for the evolution of our own world.”

“Those materials that might go to the Earth would be those volatile, carbon-rich materials. And that gives you water. It gives you organics,” he said.

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R. Brasser S.J. Mojzsis. The partitioning of the inner and outer Solar System by a structured protoplanetary disk. Nat Astron, published online January 13, 2020; doi: 10.1038/s41550-019-0978-6

This article is based on text provided by the University of Colorado, Boulder.

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