Tag: Planetary accretion

A hint of proto-Earth that predates Moon formation by giant impact  

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Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.

Geochemists have gradually built a model of the proportions of the 92 naturally occurring elements that characterise the Solar System. It is based on systematic chemical analysis of meteorites, especially the ‘stony’ ones. One hypothesis for Earth formation is that the bulk of it chemically resembles a class of meteorites known as C1 carbonaceous chondrites. But there are important deviations between that and reality. For instance the relative proportions of the isotopes of several elements in meteorites have been found to differ. Because nuclei of all the elements and their individual isotopes have been shown to form in supernovae through nucleosynthesis, such instances are known as ‘nucleosynthetic anomalies’. An example is that of the isotopes of potassium (K), which was investigated by a team of geochemists from the Carnegie Institution for Science in Washington DC, USA and the Chengdu University of Technology, China led by Nicole Nie  (Nie, N.X. et al. 2023. Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae. Science, v.379, p, 372-376; DOI: 10.1126/science.abn1783).

A measure for the magnitude of this nucleosynthetic anomaly  is the ratio between the abundance in a sample of potassium’s  rarest (40K) and its most common isotope (39K), divided by the ratio in an accepted standard of terrestrial rock. Since isotopically identical samples would yield a value of 1, the result has 1.0 subtracted from it to emphasise anomalies. Samples that are relatively depleted in 40K give negative values, whereas enriched samples give positive values. This measure is signified by ε40K, ε being the Greek letter epsilon. The authors found significant and variable positive anomalies of ε40K in carbonaceous chondrite (CC) meteorites, compared with non-carbonaceous (NC) meteorites. They also found that ε40K data in terrestrial rocks are quite different from those of CC meteorites. Indeed, they suggested that Earth was more likely to have formed from NC meteoritic material. Clearly, there seems to be something seriously amiss with the hypothesis that Earth largely accreted from C1 carbonaceous chondrites.

The correlation between ε40K and ε100Ru in meteorites (EC – enstatite chondrites, OC – ordinary chondrites; CC – carbonaceous chondrites), Earth and a geochemically modelled proto-Earth. Credit: Da Wang et al., Fig 2

Three of the authors of Nie et al. and other researchers from MIT in Cambridge MA and Scripps Institution of Oceanography in San Diego CA, USA and ETH in Zurich, Switzerland have produced more extensive potassium isotope data to examine Earth’s possible discrepancy with the chondritic Earth hypothesis (Da Wang et al. 2025. Potassium-40 isotopic evidence for an extant pre-giant-impact component of Earth’s mantle. Nature Geoscience, v. 18, online article; DOI: 10.1038/s41561-025-01811-3). To better approximate the bulk Earth’s potassium isotopes they analysed a large number of terrestrial rock samples of all kinds and ages to compare with meteorites of different classes. Meteorites also have variable  nucleosynthetic anomalies for ruthenium-100 (ε100Ru). So, ε40K  and ε100Ru may be useful tracers with regards to Earth’s history. But, for some reason, the research group did not analyse ruthenium isotopes in the terrestrial samples.

Most samples of igneous rocks from different kinds of Phanerozoic volcanic provinces (continental flood basalts, island arcs, and ocean ridge basalts) showed no evidence of anomalous potassium isotopes. However, some young ocean-island basalts from Réunion and Hawaii showed considerable depletion in 40K. A quarter of early Archaean (>3.5 Ga) metamorphosed basaltic rocks from greenstone belts also showed clear 40K depletion. Yet no samples of granitic crust of similar antiquity showed any anomaly and nor did marine sediments derived from younger continental crust. Even the oldest known minerals – zircon grains from Jack Hills Western Australia – showed no anomalies. The authors suggest that both the anomalous groups of young and very ancient terrestrial basalts show signs that their parent magmas may have formed by partial mantle melting of substantial bodies of the relics of proto-Earth. To account for this anomalous mantle Da Wang et al. suggest from modelling that proto-Earths 40K deficit may have arisen from early accretion of meteorites with that property. Later addition of material more enriched with that isotope, perhaps as meteorites or through the impact with a smaller planet that triggered Moon-formation. That cataclysm was so huge that it left the Earth depleted in ‘volatile’ elements and in a semi-molten state. It reset Earth geochemistry as a result of several processes including the mixing induced by very large-scale melting. No radiometric dating has penetrated that far back in Earth history. However, in February 2004, Alex Halliday used evidence from several isotopic systems (Pb, Xe, Sr, W) to show that about two thirds of Earth’s final mass may have accreted in the first 11 to 40 Ma of its history.

Curiously, none of the hundreds of meteorites that have been geochemically analysed show the level of 40K depletion in the terrestrial samples. Nicole Nie has comments, “… our study shows that the current meteorite inventory is not complete, and there is much more to learn about where our planet came from.”

I’m persuaded to write this by ‘Piso Mojado’. And today – 23rd October – is the anniversary of the Creation of Earth, Life and the Universe in 4004 BCE, according to Archbishop James Ussher (1581-1656) by biblical reckoning, which always tickles me!

See also: Chu, J. 2025. Geologists discover the first evidence of 4.5-billion-year-old “proto Earth”. MIT News, 14 October 2025.

Origin of life: some news

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For self-replicating cells to form there are two essential precursors: water and simple compounds based on the elements carbon, hydrogen, oxygen and nitrogen (CHON). Hydrogen is not a problem, being by far the most abundant element in the universe. Carbon, oxygen and nitrogen form in the cores of stars through nuclear fusion of hydrogen and helium. These elemental building blocks need to be delivered through supernova explosions, ultimately to where water can exist in liquid form to undergo reactions that culminate in living cells. That is only possible on solid bodies that lie at just the right distance from a star to support average surface temperatures that are between the freezing and boiling points of water. Most important is that such a planet in the ‘Goldilocks Zone’ has sufficient mass for its gravity to retain water. Surface water evaporates to some extent to contribute vapour to the atmosphere. Exposed to ultraviolet radiation H2O vapour dissociates into molecular hydrogen and water, which can be lost to space if a planet’s escape velocity is less than the thermal vibration of such gas molecules. Such photo-dissociation and diffusion into outer space may have caused Mars to lose more hydrogen in this way than oxygen, to leave its surface dry but rich in reddish iron oxides.

Despite liquid water being essential for the origin of planetary life it is a mixed blessing for key molecules that support biology. This ‘water paradox’ stems from water molecules attacking and breaking the chemical connections that string together the complex chains of proteins and nucleic acids (RNA and DNA). Living cells resolve the paradox by limiting the circulation of liquid water within them by being largely filled with a gel that holds the key molecules together, rather than being bags of water as has been commonly imagined. That notion stemmed from the idea of a ‘primordial soup’, popularised by Darwin and his early followers, which is now preserved in cells’ cytoplasm. That is now known to be wrong and, in any case, the chemistry simply would not work, either in a ‘warm, little pond’ or close to a deep sea hydrothermal vent, because the molecular chains would be broken as soon as they formed. Modern evolutionary biochemists suggest that much of the chemistry leading to living cells must have taken place in environments that were sometimes dry and sometimes wet; ephemeral puddles on land. Science journalist Michael Marshall has just published an easily read, open-source essay on this vexing yet vital issue in Nature (Marshall, M. 2020. The Water Paradox and the Origins of Life. Nature, v. 588, p. 210-213; DOI: 10.1038/d41586-020-03461-4). If you are interested, click on the link to read Marshall’s account of current origins-of-life research into the role of endlessly repeated wet-dry cycles on the early Earth’s surface. Fascinating reading as the experiments take the matter far beyond the spontaneous formation of the amino acid glycine found by Stanley Miller when he passed sparks through methane, ammonia and hydrogen in his famous 1953 experiment at the University of Chicago. Marshall was spurred to write in advance of NASA’s Perseverance Mission landing on Mars in February 2021. The Perseverance rover aims to test the new hypotheses in a series of lake sediments that appear to have been deposited by wet-dry cycles  in a small Martian impact crater (Jezero Crater) early in the planet’s history when surface water was present.

Crystals of hexamethylenetetramine (Credit: r/chemistry, Reddit)

That CHON and simple compounds made from them are aplenty in interstellar gas and dust clouds has been known since the development of means of analysing the light spectra from them. The organic chemistry of carbonaceous meteorites is also well known; they even smell of hydrocarbons. Accretion of these primitive materials during planet formation is fine as far as providing feedstock for life-forming processes on physically suitable planets. But how did CHON get from giant molecular clouds into such planetesimals. An odd-sounding organic compound – hexamethylenetetramine ((CH2)6N4), or HMT – formed industrially by combining formaldehyde (CH2O) and ammonia (NH3) – was initially synthesised in the late 19th century as an antiseptic to tackle UTIs and is now used as a solid fuel for lightweight camping stoves, as well as much else besides. HMT has a potentially interesting role to play in the origin of life.  Experiments aimed at investigating what happens when starlight and thermal radiation pervade interstellar gas clouds to interact with simple CHON molecules, such as ammonia, formaldehyde, methanol and water, yielded up to 60% by mass of HMT.

The structure of HMT is a sort of cage, so that crystals form large fluffy aggregates, instead of the gases from which it can be formed in deep space. Together with interstellar silicate dusts, such sail-like structures could accrete into planetesimals in nebular star nurseries under the influence of  gravity and light pressure. Geochemists from several Japanese institutions and NASA have, for the first time, found HMT in three carbonaceous chondrites, albeit at very low concentrations – parts per billion (Y. Oba et al. 2020. Extraterrestrial hexamethylenetetramine in meteorites — a precursor of prebiotic chemistry in the inner Solar System. Nature Communications, v. 11, article 6243; DOI: 10.1038/s41467-020-20038-x). Once concentrated in planetesimals – the parents of meteorites when they are smashed by collisions – HMT can perform the useful chemical ‘trick’ of breaking down once again to very simple CHON compounds when warmed. At close quarters such organic precursors can engage in polymerising reactions whose end products could be the far more complex sugars and amino acid chains that are the characteristic CHON compounds of carbonaceous chondrites. Yasuhiro Oba and colleagues may have found the missing link between interstellar space, planet formation and the synthesis of life through the mechanisms that resolve the ‘water paradox’ outlined by Michael Marshall.

See also: Scientists Find Precursor of Prebiotic Chemistry in Three Meteorites (Sci-news, 8 December 2020.)