If Earth was built close to the young Sun, where did its water come from? New isotope analysis rewrites part of Earth’s origin story and gives scientists a new mystery to solve

One of the longstanding theories behind Earth's formation has to do with the transfer of material from the cold parts of the Solar System.

The Earth had formed fairly close to the Sun, meaning that high temperatures in the young planetary disc would have made it impossible for water and other volatiles to stick around.

Hence, scientists started wondering whether the material containing abundant water that came from beyond Jupiter's orbit migrated to the Earth afterwards.


However, the new research conducted by planetary scientists at ETH Zurich is about to refute this theory.

Also Read | This strange world has no star to orbit, yet James Webb has revealed its changing weather from 20 light-years away

By looking at the isotopic data from 10 different isotope systems, Paolo Sossi and Dan Bower managed to demonstrate that the composition of the Earth can be explained solely on the basis of inner-Solar-System materials.

This conclusion was reached in their paper recently published in Nature Astronomy – namely, Earth formed from an inner-Solar-System reservoir instead of using the material that formed beyond Jupiter in significant quantities.

Since there may be no contribution of water-rich outer Solar System materials into the formation of the Earth, then the sources for the oceans on the planet should be located much closer to the young Sun.

Meteorites carry a record of where Earth's building blocks came from

Isotopes are varieties of an element which have a different number of neutrons. Isotope chemistry may provide evidence for the formation regions of planetary material, which allows comparing Earth with meteorites and other planets.

Meteorites are classified based on their material origin and divided into non-carbonaceous, originating closer to the Sun and being inner-Solar-System objects, and carbonaceous, having more carbon and hydrated compounds and originating farther from the Sun.

Also Read | 55 million light-years away, scientists spot something unusual: A strange galactic collision may have left this spiral galaxy with a centre that spins sideways

Previous estimations indicated that between 6% and 40% of the material of the Earth originates from the outer Solar System.

Paolo Sossi and Dan Bower from ETH Zurich made a comprehensive analysis by investigating 10 nucleosynthetic isotopic systems, whereas previous research was based on just a few of them. Earth isotopic data were compared with meteorite isotopic data as well as data for Mars and Vesta.

Results showed that Earth falls into the range of data characteristic of inner-Solar-System objects. It was found out that Earth's isotopic composition could be obtained without the need for material beyond Jupiter.

Nevertheless, Earth doesn't belong to any meteoritic group. It has a unique isotopic composition among inner-Solar-System objects.

Jupiter may have stopped distant material from reaching Earth

The result also offers a possible explanation for why the Solar System ended up with such a strong chemical divide.

Early in the Solar System's history, the young Sun was surrounded by a disc of gas and dust. Planets eventually formed from material within this disc.

Jupiter grew rapidly during this period. As its mass increased, its gravity could have opened a gap in the surrounding disc and restricted the movement of material between the outer and inner regions.

That makes Jupiter more than just the largest planet in today's Solar System. During planetary formation, it may have helped maintain the separation between two chemically distinct reservoirs of material.

The new isotope analysis suggests that this separation was highly effective. According to ETH Zurich, material originating beyond Jupiter probably contributed less than 2% of Earth's mass, and potentially none at all.

That is considerably different from scenarios in which significant quantities of outer-Solar-System material were needed to build Earth.

The study does not depend on a detailed physical model of how material moved through the young Solar System. Instead, the researchers say their conclusion emerges from the isotope data themselves. Their statistical approach used principal component analysis alongside Bayesian latent factor analysis to examine relationships across the different isotope systems.

That distinction is important because the exact dynamics of the early planetary disc remain difficult to reconstruct.

Earth's water is now the harder question

The new result creates an obvious problem.

Earth is covered by oceans, and water is also stored in minerals and deep inside the planet. Yet the region close to the young Sun was expected to have been relatively hot and dry compared with the outer Solar System.

If Earth did not need a substantial supply of water-rich material from beyond Jupiter, where did its water come from?

The researchers themselves identify this as the next major question to investigate.

ETH Zurich says Sossi and his team plan to examine how sufficient water could have existed in the hot inner Solar System to eventually produce Earth's oceans.

The new research therefore does not establish that Earth's water was present in its present form before the planet formed. It instead changes the setting in which scientists need to investigate the problem.

Water or other volatile ingredients could potentially have been incorporated into the inner-Solar-System material from which Earth grew, or arrived through processes that did not require large quantities of material from beyond Jupiter. Determining which processes were important remains an open research question.

ETH Zurich's planetary geochemistry research also notes that studies of Earth's volatile elements have indicated that the young Earth was initially relatively dry and that volatile material arrived later while the planet's core was still forming.

So the new study should not be read as proving that scientists have solved the origin of Earth's oceans.

Instead, it removes one commonly proposed route for delivering some of the material and leaves researchers with a narrower, more intriguing problem.

Mars and Vesta may help explain Earth's unusual composition

The analysis also points to a broader relationship between the rocky worlds of the inner Solar System.

Earth's isotopic composition follows a trend that includes material associated with Mars and the asteroid Vesta. This suggests that the rocky bodies forming in the inner Solar System may have shared a related compositional history.

The result may also provide a way to estimate the compositions of Mercury and Venus.

There is one major limitation: researchers do not currently have returned rock samples from either planet comparable to the meteorite material available for Earth, Mars and asteroid Vesta. That means the predicted compositions cannot yet be tested directly using samples from their surfaces.

If future missions return material from Mercury or Venus, those samples could provide an important test of the pattern suggested by the new analysis.

The implications also extend beyond our own planetary neighbourhood. Sossi and his colleagues want to explore whether similar processes could occur around other stars, where planets may form from separate reservoirs of material in a young planetary disc.

For now, the study leaves Earth with a rather unusual origin story.

The planet may have assembled from material that remained remarkably close to the young Sun, while Jupiter helped keep much of the outer Solar System's material at a distance.

That makes Earth's oceans even more interesting: the water that made our planet habitable may not have required the distant delivery route scientists once considered important.

The next challenge is finding out exactly how enough water survived, moved and became incorporated into a young Earth forming in a region that was expected to be comparatively dry.

That question is still open.

FAQ

1. Where did the material that formed Earth come from?
The new study concludes that Earth's building material can be explained by a single reservoir in the inner Solar System. The researchers found no need for a substantial contribution from material originating beyond Jupiter.

2. Does the study explain where Earth's water came from?
No. It creates a new question about Earth's water. If large amounts of water-rich outer-Solar-System material were not required to form Earth, researchers need to determine how enough water was present or delivered within the inner Solar System.

3. How did scientists determine where Earth's building material came from?
Researchers compared isotope variations across ten different isotope systems in Earth and various meteorite and planetary reservoirs. They used statistical techniques to identify the relationships between those isotope signatures.

4. Did Jupiter influence Earth's formation?
The study is consistent with the idea that Jupiter's rapid growth created a gravitational barrier between inner and outer Solar System material. The researchers' analysis suggests very little material from beyond Jupiter reached the region where Earth formed.