The reconstruction of Earth’s formation history heavily relies on meteoritic evidence. Since the vast majority of meteorites originate from asteroids, comprehending asteroids, in turn, can illuminate the processes behind Earth’s genesis. Collectively, asteroids and meteorites represent the debris left over from the birth of rocky planets.
Asteroids and meteorites reside within the inner Solar System. Astronomers are keen to discern what proportion of Earth’s composition stems from inner Solar System material versus material sourced from the outer Solar System, the region beyond Jupiter. Because conditions vary drastically between these zones, the material formed in each area also differs. Pinpointing the relative contributions from each part of the Solar System will help explain not only how Earth came to be, but also the general principles of planetary formation. The carbon content in meteorites and asteroids plays a crucial role in addressing this.
Scientists speculate that a sizable fraction of Earth, ranging from 6% to 40%, might have been imported from the outer reaches of the Solar System. This influx could account for Earth’s acquisition of water. Bodies situated beyond Jupiter typically harbor more water ice. During Earth’s accretion, some of the gathered material came from past Jupiter’s orbit, or the Solar System’s snow line, bringing trapped ice water upon incorporation into Earth. This forms one hypothesis explaining our planet’s hydration.
A new study, detailed in the journal Nature Astronomy, examined the chemical makeup of known asteroids and meteorites to ascertain the split between inner and outer Solar System material contributing to Earth’s bulk composition. More broadly, it tests the concept of water delivery originating from beyond Jupiter. The paper is titled “Homogeneous Accretion of the Earth in the Inner Solar System.” The lead author is Paolo Sossi from the Institute of Geochemistry and Petrology, Department of Earth and Planetary Sciences, ETH Zurich.
“Meteorites are classified as carbon-bearing or carbon-lacking, meaning they likely represent bodies that formed in the inner or outer Solar System, respectively,” the researchers state in their publication. “Despite its location in the inner Solar System, Earth is thought to incorporate either a minor ($\sim6\%$) or a substantial amount ($\sim40\%$) of material from the outer Solar System.”
Much of deciphering the origin of ancient rocks in our Solar System—be they meteorites, asteroids, or Earth-like planets—boils down to isotopes. Isotopes are essentially different versions of the same chemical element, distinguished by having varying numbers of neutrons. For instance, oxygen possesses three stable isotopes: $^{16}\text{O}$, $^{17}\text{O}$, and $^{18}\text{O}$.
Isotopes exist in different relative abundances, and by measuring these isotopic ratios across various celestial bodies, scientists can establish connections between bodies sharing a common ancestry. Researchers have identified isotopic systems that link various inner Solar System object families together. However, a challenge remains.
Researchers have been investigating isotopic systems for many years. The main difficulty is that sometimes they reach ambiguous conclusions because they haven’t studied larger isotopic systems across a greater number of objects and object families. “The Earth’s origin remains uncertain,” the authors explain, “because the range of isotopic systems examined in previous research has been limited, often restricted to just one or two.”
In this current work, however, the team utilized available data to scrutinize ten isotopic anomalies. “Here we investigate the variations of ten nucleosynthetic isotopic anomalies across the planets and the parent bodies of meteorites,” the authors write.
The findings were unexpected, even to the researchers themselves. The outer Solar System may have contributed as little as 2% to Earth’s mass, or potentially zero percent. Equally surprising is that Earth’s composition does not match that of any chondrite, which constitutes the most common category of asteroids and meteorites. “Thus, Earth formed exclusively from inner Solar System material whose chemical composition was unaltered during accretion and was, on average, distinct from that of any chondrite,” the researchers explain in their paper.
“Our calculations clearly demonstrate: the building blocks of Earth originate from a single material reservoir,” Sossi declared.
Co-author Dan Bower, also from the ETH Zurich institution, added, “We were genuinely taken aback to find that Earth is entirely composed of inner Solar System material that is distinct from any combination of extant meteorites.”
What they are essentially examining is known as the BSE, or Bulk Silicate Earth, also referred to as Earth’s primitive mantle. Analyzing chondritic meteorites is an established method for probing the BSE because the mantle itself has undergone significant alteration since Earth’s formation.
“Our analysis shows that all elements, irrespective of their geochemical character or nucleosynthetic origin, share a common isotopic heritage within the BSE: it is an end-member of the NC (non-carbonaceous) group,” the researchers elaborate. “Therefore, the composition of the BSE is determined to be homogeneous with respect to isotopic anomalies.”
These results emerged from analyzing all this data using a powerful statistical methodology infrequently employed in geochemistry. “Our research is essentially a data analysis experiment,” Sossi commented. “We performed statistical calculations that are rarely seen in geochemistry, even though they are potent tools.”
This study also benefited from recent advancements in isotopic geochemistry. Tracing the history of Solar System bodies via isotopes was previously limited to oxygen isotopes. However, about 15 years ago, it became evident that isotopes of other elements, such as chromium and titanium, could also be utilized. This development has led to a deeper grasp of Solar System formation. Scientists have learned that meteorites fall into two broad groups: non-carbonaceous ones, formed in the inner Solar System, and carbonaceous ones, formed in the outer Solar System and possessing more water.
This work suggests that Earth might derive entirely from non-carbonaceous material originating in the inner Solar System. Only a very minor component could be carbonaceous and sourced from the outer reaches. Why such a stark division?
Jupiter may be responsible. It rapidly amassed mass and effectively fractured the protoplanetary disk surrounding the young Sun—the reservoir of material from which planets formed. Conceive of this as a barrier preventing material migration from the outer Solar System inward. Scientists are aware of this “Jupiter barrier,” but the extent of its effectiveness was unclear before this research.
“Our calculations are very robust and rely purely on the data themselves, rather than physical assumptions, as those are not yet fully understood,” Bower noted.
The findings appear applicable to Mars and the asteroid Vesta, one of the largest bodies in the main asteroid belt. They might also extend to Mercury and Venus, although we lack physical samples from those planets for direct confirmation. “Based on our analysis, we can theoretically predict the compositions of these two planets,” Sossi stated. This work implies that Venus and Mercury should exhibit even more extreme isotopic compositions than Earth.
Sossi maintains that these results shed new light on Earth’s formation, which is quite clear-cut. The findings also bear implications for the origin of water on Earth.
The source of our planet’s water remains a point of contention among scientists. One working hypothesis suggests a significant portion, or most of it, was delivered by comets or other icy bodies from the outer Solar System, where water ice was abundant. The inner Solar System was too warm for water ice to survive.
Another common hypothesis posits that water was intrinsically incorporated into Earth during its formation. This internal water formation theory suggests that chemical reactions between hydrogen and oxygen within Earth’s early mantle led to its abundance. The present findings lend support to this notion.
These results also raise new questions that the researchers themselves are eager to address. Foremost among these is the water conundrum. If it didn’t form internally and if the warm inner Solar System lacked sufficient water to account for Earth’s reserves, where did it come from? The researchers also aim to determine if these findings translate to the process of exoplanet formation in other solar systems.
These significant findings will not settle the ongoing debate regarding planetary formation and the provenance of Earth’s water. A single study cannot conclude such broad discussions. However, science advances incrementally, and this may prove to be a crucial step.
“Until then, however, Dan and I will have many spirited discussions about the material makeup of Earth and its planetary neighbors, because the scientific debate about Earth’s building blocks is far from over, despite the new discoveries,” Sossi concludes.
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