Earth formed over 4.5 billion years ago via accretion. Earth’s building blocks were chunks of rock of varying sizes. From dust to planetesimals and everything in between. Many of those chunks of rock were carbonaceous meteorites, which scientists think came from asteroids in the outer reaches of the main asteroid belt.
But some evidence doesn’t line up well behind that conclusion. A new study says that some of the Earth-forming meteorites came from much further out in the Solar System.
The nebular hypothesis is the widely-accepted explanation for the Solar System’s formation. It says that a mass of gas and dust collapsed gravitationally and formed a rotating disk. The Sun formed in the center of the disk, and everything else formed from what remained.
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A critical feature of the Solar System is the frost line. The frost line divides the Solar System into two regions. Beyond the frost line, it’s cold enough for volatiles to solidify into ice grains. Volatiles include water, ammonia, carbon dioxide, carbon monoxide, and methane. Inside the frost line, the Sun’s energy heats the surrounding material and breaks apart the volatiles. The Sun’s solar wind then pushes them away from the inner regions. Once past the frost line, they can solidify. As a result, the inner material is more dry and rocky, while the cold outer regions are icier.
The frost line hasn’t always been in the same place. The Sun pushed the frost line further outward as the Solar System evolved. That’s because initially, the Sun was less energetic than it is now. The solar nebula was also more opaque.
The Sun is made up of a representative sample of the materials in the solar nebula because it was the first body to form. But the planets aren’t. Their positions relative to the frost line and types of materials inside and outside of the frost line governed their formation. The inner planets, Mercury, Venus, Earth and Mars, are mostly rock (mainly composed of heavier elements, such as iron, magnesium and silicon), while the outer planets beyond the frost line are primarily composed of lighter elements, mainly hydrogen, helium, carbon, nitrogen and oxygen.
As Earth accreted, carbonaceous chondrites played a role in the planet’s formation. Astronomers think that carbonaceous chondrites (CC) come from the outer regions of the main asteroid belt. There are different families of CCs based on their composition, and each family has the same parent body. Individual CCs are fragments of the parent body resulting from impacts between objects in the asteroid belt.
But a new study says there might be more going on. The researchers behind it say that some of the carbonaceous chondrites came from asteroids that formed much further away in the outer Solar System, outside of the main asteroid belt and well beyond the frost line.
The study is “Distant Formation and Differentiation of Outer Main Belt Asteroids and Carbonaceous Chondrite Parent Bodies.” Researchers at the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology led the study, and assistant professor Hiroyuki Kurokawa is the lead author. The study is published in the journal AGU Advances.
The study focuses on the composition of asteroids in the asteroid belt between Mars and Jupiter. Observations of asteroids in the outer part of the belt reveal a reflectance feature that indicates water ice and/or ammonium clays (ammoniated phyllosilicates) on their surfaces. Those materials are only stable at lower temperatures and can’t easily form in their present location. Some evidence shows that these asteroids are the parent bodies of CCs. But what’s puzzling is that meteorites recovered on Earth generally lack the same feature.
This discrepancy is one of the asteroid belt’s puzzling features.
The new research suggests a solution for this puzzle. Some asteroids may have formed in the more distant reaches of the Solar System then were transported toward the inner Solar System by chaotic mixing processes. “Our results suggest that multiple large main-belt asteroids formed beyond the NH3 and CO2 snow lines (currently >10 au) and could be transported to their current locations,” the study says.
These asteroids were large enough to be differentiated, meaning they had cores and mantles with different compositions. The mantles were rich in water, and the cores were denser. “Ammoniated phyllosilicates form within the water-rich mantles of the differentiated bodies containing NH3 and CO2 under high water-rock ratios (>4) and low temperatures (<70°C),” the study explains.
CCs can come from the dense rocky cores of these asteroids. Since these cores are heavier and more solidified, they’re more likely to be sampled as meteorites. “CCs can originate from the rock-dominated cores, that are likely to be preferentially sampled as meteorites by disruption and transport processes,” the researchers write.
Asteroids form via accretion as planets do. That’s represented in 1 in the above figure. Some of them formed beyond the frost line, where they accreted NH3 and CO2 ice in addition to water ice. Large asteroids then differentiate into mantles and cores, shown in 2. Part 3 in the figure shows the differentiated asteroid after freezing. The hydrated regolith mantle lacks the reflectance features the CCs on Earth display. Part 4 shows how collisions can disrupt the parent asteroid. Fragmentation meteorites from the hydrated core have the same features as CCs on Earth.
If this study is accurate, it points out a quirk in Solar System formation. Astronomers think that Jupiter migrated to within 1.5 AU of the Sun, then migrated outward again to its present position. Saturn also underwent a migration. These movements are called the Grand Tack Hypothesis.
The migrations of the Solar Systems’ two most massive planets affected the asteroid belt. The asteroids were scattered, and many of them ended up in positions they didn’t form in. During those scattering events, there were collisions, creating CCs. Some of the CCs that formed Earth came from these collisions among asteroids originally from beyond the frost line.
This study is based on both observations and modelling. Many observations come from the AKARI satellite, led by JAXA, the Japan Aerospace Exploration Agency. AKARI was an infrared astronomy satellite that performed an all-sky survey in multiple infrared bands. AKARI produced a catalogue of more than 5,000 asteroids in infrared.
“Based on these results, we proposed that several, if not all, C-complex asteroids and CC parent bodies formed beyond NH3 and CO2 snow lines and differentiated,” the authors write in their conclusion. “The distant origin of C-complex asteroids is naturally expected from the modern planet formation theory which involves Solar System-scale migration of pebbles and planetesimals,” they explain.
Fortunately, scientists have more to go on than observations and modelling. Soon they’ll have pieces of asteroids to study.
Japan’s Hayabusa 2 asteroid sampling mission collected samples from the asteroid Ryugu. The spacecraft returned the asteroid samples in December 2020. NASA’s OSIRIS-REx asteroid sampling mission collected samples from the near-Earth asteroid Bennu. Those samples should be back on Earth by September 2023.
Scientists will be able to compare their predictions for these asteroids, based on modelling and observations, with the samples. The distant origins of these asteroids tell us that Hayabusa 2’s samples should contain ammoniated salts and minerals. OSIRIS-REx samples are another test of these predictions.
One of the questions in astronomy is if our Solar System is representative of other solar systems. Are formation processes similar for all systems? How similar? How different?
“Whether our solar system’s formation is a typical outcome remains to be determined, but numerous measurements suggest we may be able to place our cosmic history in context soon,” said lead author Hiroyuki Kurokawa.