For years, planetary scientists have argued that some of the material that built Earth must have drifted in from beyond Jupiter, carrying water and other volatile ingredients with it. Estimates often put that outer Solar System share somewhere between 6 percent and 40 percent. <\/p>\n
A new analysis led by ETH Zurich<\/a> researchers Paolo Sossi and Dan Bower takes a very different view. After comparing Earth\u2019s isotopic makeup with that of meteorites, Mars and the asteroid Vesta, they argue that Earth formed almost entirely from material already present in the inner Solar System.<\/p>\n That does not mean the question is settled. Sossi himself says the debate over Earth\u2019s building blocks is \u201cfar from over.\u201d But the new study, published in Nature Astronomy<\/a>, sharply narrows the room for one popular idea, that large amounts of outer Solar System matter crossed Jupiter\u2019s orbit and became part of the growing Earth.<\/p>\n \u201cWe were truly astonished to find that the Earth is composed entirely of material from the inner Solar System distinct from any combination of existing meteorites,\u201d Bower said.<\/p>\n This is roughly what the formation of the Earth in our solar system might have looked like. The birth of two planets (light brown dots) in a protoplanetary disc around the young star WISPIT 2. (CREDIT: ESO \/ C. Lawlor, R. F. van Capelleveen et al. \/ Creative Commons By 4.0) A stronger look at an old puzzle<\/p>\n The argument turns on isotopes, atoms of the same element that carry different numbers of neutrons and therefore different masses. Scientists have long used isotopic signatures in meteorites to trace where Solar System material came from. Over time, that work revealed two broad families. Non-carbonaceous material formed in the inner Solar System<\/a>. Carbonaceous material, richer in water and carbon, came from farther out.<\/p>\n Earlier studies often focused on only one or two isotopic systems, especially oxygen, chromium and titanium. Sossi and Bower widened the lens. Using existing data on ten isotopic systems, they applied a probabilistic statistical approach to compare the bulk silicate Earth with known meteorite groups and other rocky bodies.<\/p>\n \u201cOur studies are actually data science experiments,\u201d Sossi said. \u201cWe carried out statistical calculations that are rarely used in geochemistry, even though they are a powerful tool.\u201d<\/p>\n Their conclusion is blunt. Earth\u2019s composition fits an inner Solar System source alone. The paper argues that carbonaceous material likely makes up less than about 0.1 percent of the bulk silicate Earth and less than 2 percent of the bulk Earth. In some cases, the contribution could be effectively zero.<\/p>\n Prediction of the isotopic compositions of Venus and Mercury in isotopic Euclidean distance space. (CREDIT: Nature Astronomy) Jupiter, the divider<\/p>\n That finding matters because the Solar System seems to have been split early into two reservoirs. Researchers think Jupiter\u2019s rapid growth opened a gap in the young protoplanetary disk<\/a> around the Sun, acting as a gravitational barrier between inner and outer material. What remained unclear was how leaky that barrier really was.<\/p>\n This study says it was barely leaky at all, at least for Earth-bound material.<\/p>\n The researchers found no isotopic evidence that Earth was built from a blend of inner and outer Solar System matter. Instead, they argue that Earth grew within a relatively static inner system, drawing on neighboring rocky material. They also report that Earth\u2019s composition lines up closely with material linked to Mars and Vesta.<\/p>\n That has a second consequence. If little or no outer Solar System matter reached Earth, then volatile elements such as water must already have been available in the inner Solar System. Sossi and his team plan to investigate that next.<\/p>\n Not every piece is in hand<\/p>\n The paper also pushes back on earlier models that treated Earth as a mixture of known meteorite families, sometimes with a small carbonaceous component and sometimes with a much larger one. The new statistical analysis suggests those mixtures are not needed once all ten isotopic systems are considered together.<\/p>\n At the same time, the study leaves open more than one path for how Earth got its present isotopic signature. One possibility is that Earth accreted material<\/a> that stayed isotopically similar over roughly 34 million years, the interval during which its core formed. Another is that Earth accreted distinct materials, but later mixing between core and mantle erased those differences. The authors say the first option appears more probable, though they do not claim to have closed the issue.<\/p>\n Results of the B-LFA and deterministic PCA. a\u2013d, All elements (a), heavy elements (b), iron-peak elements (c) and siderophile elements (d) for all 14 reservoirs. (CREDIT: Nature Astronomy) <\/p>\n There are other limits too. The researchers suggest Venus and Mercury may fall along the same inner Solar System trend seen for Earth, Mars and Vesta, and they use a model to predict where those planets should sit isotopically. But Sossi notes that this cannot be checked directly because scientists do not have rock samples from Venus or Mercury.<\/p>\n That makes the paper strongest where the data already exist, and more speculative where samples are missing.<\/p>\n A different picture of Earth\u2019s beginnings<\/p>\n