The NC–CC isotopic dichotomy

Nucleosynthetic isotope anomalies split meteorites into two great families that formed in separate regions of the disc. This discovery reshaped ideas about Jupiter and planet formation.

Nucleosynthetic anomalies

The elements in the Solar System were made in many earlier stars, through different nuclear processes. If the dust that formed the Sun's disc was not perfectly mixed, different regions could inherit slightly different proportions of isotopes produced by different stellar sources. These small variations, measured in parts per ten thousand or less, are called nucleosynthetic isotope anomalies.

Unlike mass-dependent fractionation, they cannot be created or erased by ordinary geological processes on a parent body. They therefore act as genetic markers of where, or from what reservoir, a body's material came.

Key elements include titanium, chromium, nickel, molybdenum, ruthenium and tungsten, measured with high-precision multi-collector mass spectrometers and thermal ionisation mass spectrometers. Results are commonly reported in epsilon units, parts per ten thousand, relative to terrestrial standards.

Two reservoirs

In 2011, Paul Warren showed that when anomalies in titanium-50, chromium-54 and oxygen were plotted against each other, meteorites did not form a continuum. They fell into two distinct clusters.

The non-carbonaceous, or NC, group includes ordinary and enstatite chondrites, HEDs, angrites, aubrites, ureilites and many iron groups such as IAB, IIAB, IIIAB and IVA. Earth, the Moon and Mars also plot in the NC field.

The carbonaceous, or CC, group includes all carbonaceous chondrite groups, plus some achondrites and several iron groups, among them IIC, IID, IIF, IIIF and IVB. Material returned from the asteroids Ryugu and Bennu also belongs to the CC group.

Molybdenum isotopes have since shown the split especially clearly, because they separate the effects of different nucleosynthetic processes and allow both irons and stony meteorites to be compared.

What separated them?

Iron meteorites provide a clock. Hafnium-tungsten ages show that the parent bodies of many NC and CC irons formed their cores within roughly the first one to four million years. Since both reservoirs were producing bodies over this period without mixing, something must have kept them apart.

In 2017, Thomas Kruijer and colleagues proposed that the early formation of Jupiter's core was the barrier. A growing proto-Jupiter would open a pressure maximum in the gas disc that traps drifting pebbles, holding CC material in the outer disc and NC material inside. On this view, Jupiter's core reached around 20 Earth masses within about a million years.

Alternatives exist. A pressure bump in the disc not caused by Jupiter, or changes over time in the material falling onto the disc, could also produce the dichotomy. The debate continues, but the existence of two reservoirs is now widely accepted.

Implications for Earth

Earth's composition lies within the NC field, but not exactly matching any single meteorite group. Mass-balance studies of molybdenum and other elements suggest Earth accreted mostly NC material, with a contribution of CC material that may have arrived relatively late in its growth.

That CC contribution matters because CC bodies are rich in water and volatile elements. Estimates of its size vary, but several studies suggest it could have supplied a significant part of Earth's water and volatiles.

The dichotomy also tells us that the asteroid belt today contains material from both reservoirs, implying that CC bodies formed beyond Jupiter were later scattered inwards, perhaps during the growth or migration of the giant planets. Meteorites, therefore, record not only local history but the large-scale dynamics of the young Solar System.

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