Hydrogen and how the Earth formed

A third piece with hydrogen as its focus in a couple of months? Well, from a galactic perspective there’s a lot of it about. Modern cosmology suggests that only 4.6% of the energy in the universe consists of elemental atoms made of protons, neutrons and electrons, dwarfed by dark energy and dark matter that are something of mystery. But of the more familiar energy equivalent, tangible matter (as in E=mc2), 74% of the universe is hydrogen, 24% is helium and the other 92 elements amount to just 2%. That tiny proportion of heavier elements was created by nucleosynthesis within stars from the two products of the Big Bang (H and He). Nuclear fusion reactions formed those with atomic numbers (protons in their nuclei) up to that of iron (26), whereas the heavier elements were created through neutron- and proton capture when the largest stars destroyed themselves cataclysmically as supernovae. Yet the planet whose surface we inhabit contains only minute amounts of helium and elemental hydrogen. Of course water at and beneath the surface, in the form of atmospheric vapour and locked within minerals retains some of the cosmically available hydrogen. But current estimates suggest that hydrogen accounts for a mere 0.03% of Earth’s mass. Despite the fact that some forms of radioactive decay generate alpha particles that become helium it forms a vanishingly small proportion of terrestrial mass.

The solar system formed around 4.6 billion years ago by a complex gravitational accretion of the gas and dust of an interstellar cloud: mainly H and He. Its dynamic collapse resulted in gravitational potential energy being transformed into heat: in the case of the Sun, sufficient to set off self-sustaining nuclear fusion. As a body grows in this way so does its gravity and thus the speed needed for matter to escape from its pull (escape velocity). As temperature increases so does the speed at which atoms of each element vibrate; the lower the atomic mass the faster the vibration and the greater the chance of escape. So the ‘blend’ of elements that an astronomical body retains during its early evolution depends on its gravity and its surface temperature. The Sun is so massive that very little has escaped its pull, despite a surface temperature of about 5 to 6 thousand degrees Celsius. Its composition is thus close to the cosmic average. Those of the giant planets Jupiter, Saturn, Uranus and Neptune are not far short because of their large gravities and low surface temperatures. Even today, the smaller Inner Planets are unable to cling on to elemental hydrogen and helium and nearly all that is left of the matter from which they formed is the 2% of heavier cosmic elements locked into solids, liquids and gases.

Processes in the early solar system were far more complicated than they are today. In the mainly gaseous disc, from which the solar system evolved, gravity dragged matter towards its centre. That eventually ignited nuclear fusion of hydrogen to form our star. More remote from its gravitational pull vortices aggregated dust into bodies known as planetesimals that in turn accreted to larger protoplanets. Solar gravity dragged gas from the inner solar system leaving rocky protoplanets, whereas gas was able to be attracted to the surface of what became the gas giants where their gravity outweighed that of the far-off Sun. This was complicated by a sort of Milankovich Effect on steroids in which protoplanets continuously changed their orbits and underwent collisions. The best known of these was between the protoEarth and a Mars-sized body that formed the Earth-Moon system, both bodies having deep magma oceans as a result of the huge energy focussed on them by the collision. What may have happened to the protoplanet that became Earth before the Moon-forming collision has been addressed by three geoscientists at the University of California Los Angeles and the Carnegie Institution for Science Washington DC, USA (Young, E.D. et al. 2023. Earth shaped by primordial H2 atmospheres. Nature, v. 616, p. 306–311; DOI: 10.1038/s41586-023-05823-0 [PDF request to: eyoung@epss.ucla.edu]).

A thick hydrogen-rich atmosphere’s interacting chemically with a protoplanet (left). A possible later stage (right) where iron oxide in the magma ocean of the Early Hadean after Moon formation oxidises a hydrogen atmosphere to form surface water (Credit: Sean Raymond 2023, Fig 1)

The focus of the work of Edward Young, Anat Shahar and Hilke Schlichting is directed at the possibility that the Earth-forming protoplanets originally retained thick hydrogen atmospheres. They use thermodynamic modelling of the equilibrium between hydrogen and silicate magma oceans that had resulted from the energy of their accretion. The authors’ main assumption is that insufficient time had elapsed during accretion for the protoplanets to cool and crystallise: a distinct possibility because loss of accretionary heat by thermal radiation would have been ‘blanketed’ by actively accreting dust and gas in orbit around the growing protoplanets. Effectively, the equilibrium would have been chemical in nature: reactions between highly reducing hydrogen and oxidised silicate melts or even vaporised rock evaporated from the very hot surface. The authors suggest that protoplanets bigger than Mars (0.2 to 0.3 times that of Earth) could retain a hydrogen-rich atmosphere long enough for the chemical reactions to come to a balance, despite high temperatures. There would have been no shortage of hydrogen at this early stage in solar system evolution: perhaps as much as 0.2% percent the mass of the Earth surrounding a protoplanet about half its present size.

Two outcomes may have emerged. Reaction between hydrogen and anhydrous silicates could produce H2O in amounts up to three times that currently in the Earth’s oceans, some locked in the magma ocean, some in the dense atmosphere. A by-product would have been iron oxide, giving the current mantle its oxidising properties known from the geochemistry of basaltic magmas.  Hydrogen might also have dissolved in molten iron alloys, thereby contributing to the nascent core. That second outcome would help explain why the modern core is less dense than expected for iron-nickel alloy, both solid and liquid. In fact densities calculated by geophysicists from the speeds of seismic waves that have travelled through the core are 5 to 10% percent lower than expected for the alloy. So the core must contain substantial amounts of elements with low atomic numbers.

Several other possibilities have been suggested to account for Earth’s abundance of water. Two popular ideas are comets arriving in the ‘settled’ times of the Hadean or by original accretion of hydrous chondrite meteorites, whose hydrogen isotope proportions match those of ocean water. Hydrogen as the light element needed in the core is but one possibility along with oxygen, sulfur and other ‘light’ elements. Also, the oxidising potential of the modern mantle may have resulted from several billion years of wet lithosphere being subducted. To paraphrase Sean Raymond (below), ‘other hypotheses are available’!

See also: Raymond, S.N. 2023. Earth’s molten youth had long-lasting consequences. Nature (News & Views), v. 616, p. 251-252; DOI: 10.1038/d41586-023-00979-1 [PDF request to: rayray.sean@gmail.com]

A glimpse of the Hadean

There is something deeply unsatisfying, even untidy, about a geoscientific history from which the first half billion years is more or less a blank. Every likely stone has been turned and every isotope hurled as a curve-ball through a mass spectrometer in the quest for either direct evidence of Hadean events or an acrid whiff that lingers in later matter. All, that is, except for one…

Formed in a proposed supernova that likely helped trigger formation of the Sun and Solar System, 150Gd quickly decayed to produce 146Sm, which itself had a half-life of about 68 Ma. That is too short for any significant trace of that radioactive rare-earth element to remain in terrestrial rocks, but its daughter isotope 142Nd bears witness to its former existence. Checking the proportion of 142Nd against the heavier 144Nd is a means of assessing isotopic fractionation according to atomic mass between a solid source of a magma, and between residual magma and solids that crystallised from it.

A popular and well-supported view of the Hadean is that shortly after accretion of the Earth a stupendous impact left a deep ‘ocean’ of magma and flung off mass that produced the Moon. Solidification of that ocean, which would have involved denser minerals sinking and lighter ones rising to higher levels, has been suggested to have resulted in differentiation of the mantle into two portions, one enriched, the other depleted; an event on which the entire later geochemical history of our planet has depended. Should either part of the mantle melt again, the igneous rocks that would result should carry a neodymium isotope signature of one or the other. Little sign of either emerges from studies of igneous rocks younger than 2.5 Ga, but older rocks from Greenland that go back to 3.8 Ga demonstrate that almost all of them melted from the Hadean depleted mantle. Without rocks carrying 142Nd/144Nd ratios signifying the other side of the more ancient mantle division, an enriched source, the grand idea was flawed. But this one-sidedness appears now to have been balanced by other Archaean igneous rocks (Rizo, H. et al. 2012. The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks. Nature, v. 491, p. 96-100).

3.8 billion year-old Amitsoq gneisses, West Greenland (Image credit: Stephen Moorbath, via Royal Society)

The analysed rocks are interesting for another reason, for they are 3.4 Ga old vertical sheets of basalt or dykes that cut through the more ancient west Greenland crust. They are the first evidence of a brittle crust that cracked under tension to be followed by mantle-derived magma. Some members of the Ameralik dyke swarm show just the isotopic signature predicted for the enriched member of the postulated fundamental mantle division. However, for some yet to be recognised reason, few post-Archaean rocks show any sign of widespread mantle heterogeneity. Such matters could be addressed with any confidence only after mass spectrometry allowed precise discrimination between isotopes of a whole variety of both common and rare elements. That was not so long ago, so a rich trove of future revelations can be anticipated.