Science1 publisher3 min readPublished
ETH Zurich reanalysis caps Earth's outer Solar System material below 2 percent of its mass
Paolo Sossi and Dan Bower ran published meteorite data through ten isotope systems instead of the usual two, and found Earth's composition matches inner Solar System material alone, with under 2 percent from beyond Jupiter.
The Scientist · Science desk
What happened
- Paolo Sossi and Dan Bower of ETH Zurich report in Nature Astronomy that the material that formed Earth came entirely from the inner Solar System.
- Published proposals had put 6 to 40 percent of Earth's building material in the outer Solar System, a route invoked mainly to explain how the planet acquired water and other volatiles.
- The pair worked from existing meteorite measurements across ten isotope systems, where earlier provenance studies typically used only two.
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Why it matters
- constraint Any account of Earth's volatile budget now has to make water available in the inner disc, because the transport route across Jupiter's gap that most delivery models borrow is what this analysis rules out.
- contradiction The same result that argues for one source reservoir also says no mixture of known meteorites reproduces Earth, so the reservoir is inferred from how Earth fails to fit the samples we hold rather than identified in them.
- precedent A provenance claim built on two isotope systems is now harder to defend, and the ten-system test sets the standard later reanalyses of archival meteorite data will be measured against.
The signal comes from ratios of sibling atoms: same element, same proton count, different mass [10]. Those ratios record the region of the disc where a rock's material formed. For many years oxygen isotopes were the main way to read them; research in the early 2010s showed that chromium and titanium work too [10][11]. That widened the record enough to sort meteorites into two families. Non-carbonaceous meteorites formed only in the inner Solar System. Carbonaceous ones carry more water and carbon and came from farther out [12]. Earth's composition sits with the first family [13].
Published proposals had put 6 to 40 percent of Earth's building material in the outer Solar System [2]. Sossi and Bower cap it below 2 percent, and say the data are consistent with none at all [3]. The lowest previous figure is three times their ceiling [19]. The evidence base grew by a similar factor: ten isotope systems against the two that earlier work typically used, five times as many [5][20].
The water part is indirect. Isotope ratios of this kind trace where material formed [11]. The water conclusion runs through the meteorite record instead: the water-rich and carbon-rich samples are the outer ones [12]. A planet built only from inner material had to draw its volatiles from close to the young Sun, which is what the ETH Zurich team concludes [15]. The same analysis has Earth growing in a relatively stable environment, taking in smaller nearby bodies as it went [14].
Bower said the team was truly astonished to find that Earth is made up entirely of inner Solar System material, distinct from any combination of existing meteorites [7]. Earth's composition lies outside every mixture of the meteorites in the sample set, so the single reservoir is inferred from the way Earth fails to fit the samples we hold. Sossi put the conclusion more flatly: "Our calculations make it clear: the building material of the Earth originates from a single material reservoir" [6].
On method, Sossi said "Our studies are actually data science experiments," and that the team "carried out statistical calculations that are rarely used in geochemistry, even though they are a powerful tool" [8][9]. The ETH Zurich release does not name the technique [21]. Sossi also said the calculations rely on the data alone and not on physical assumptions [18].
Jupiter is the presumed cause of the split. As the planet grew, its gravity carved a gap in the protoplanetary disc and limited the inward movement of outer material, though how well that barrier held was not known [16]. On this analysis, very little from beyond it ever reached the region where Earth assembled [17].
What to watch
- Whether other groups reproduce the ten-system result from the same archival meteorite measurements once the Nature Astronomy paper's statistical method is examined in detail.
- Whether disc chemistry models can supply Earth's water from inner Solar System material alone, without transport across Jupiter's gap.
- Whether new meteorite classes or returned samples turn up material closer to Earth's composition than any mixture now in the collections.