What followed the Giant Impact (read Lord Mayor’s Show)?

The dominance of the Lunar Highlands by feldspar-rich anorthosites, which form when feldspars that crystallise from magmas float because of their lower density, gave rise to the idea that the Moon initially formed as a totally molten mass. That this probably resulted because the early Earth collided with a Mars-sized protoplanet stems from the almost identical chemical composition of the lunar and terrestrial mantles, as worked out from the composition of younger basalts derived from both, together with the vast energy needed to support a large molten planetary body condensing from a plasma cloud orbiting the Earth. Such a giant impact is also implicated in the final stages of core formation within the Earth.

Artist's depiction of the giant impact that is...
Artist’s depiction (after William K. Hartmann) of the giant impact that is hypothesized to have formed the Moon. (credit: Wikipedia)

A core formed from molten iron alloyed with nickel would have acted as a chemical attractor for all other elements that have an affinity for metallic iron: the siderophile elements, such as gold and platinum. Yet the chemistry of post-moon formation basaltic melts derived from the Earth’s mantle contain considerably more of these elements than expected, a feature that has led geochemists to wonder whether a large proportion of the mantle arrived – or was accreted – after the giant impact.

A tool that has proved useful in geochemistry on the scale of entire planets – well, just the Earth and Moon so far – is measuring the isotopic composition of tungsten, a lithophile metal that has great affinity for silicates. One isotope is 182W that forms when a radioactive isotope of hafnium (182Hf) decays. The proportion of 182W relative to other tungsten isotopes has been shown to be about the same in Lunar Highland anorthosites as it is in the Earth’s mantle. This feature is believed to reflect Moon formation and its solidification after the parent 182Hf had all decayed away: the decay has a half-life of about 9 Ma and after 60 Ma since the formation of the Solar System (and a nearby supernova that both triggered it and flung unstable isotopes such as 182Hf into what became the Solar nebula) vanishingly small amounts would remain.

Oddly, two papers on tungsten and Earth-Moon evolution, having much the same aims, using similar, newly refined methods and with similar results appeared in the same recent issue of Nature (Touboul, M. et al. 2015. Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature, v. 520, p. 530-533. Kruijer, T.S. et al. 2015. Lunar tungsten isotopic evidence for the late veneer. Nature, v. 520, p. 534-537). The two of them present analyses of glasses produced by large impacts into the lunar surface and probably the mantle, which flung them all over the place, maintaining the commonality of the ventures that might be explained by there being a limited number of suitable Apollo samples. Both report an excess of 182W in the lunar materials: indeed, almost the same excess given the methodological precisions. And, both conclude that Moon and Earth were identical just after formation, with a disproportional degree of later accretion of Solar nebula material to the Earth and Moon.

So, there we have it: it does look as if Earth continued to grow after it was whacked, and there is confirmation. Both papers conclude, perhaps predictably, that the early Solar System was a violent place about which there is much yet to be learned…

Tectonics of the early Earth

Tectonics on any rocky planet is an expression of the way heat is transferred from its deep interior to the surface to be lost by radiation to outer space. Radiative heat loss is vastly more efficient than either conduction or convection since the power emitted by a body is proportion to the fourth power of its absolute temperature. Unless it is superheated from outside by its star, a planet cannot stay molten at its surface for long because cooling by radiation releases all of the heat that makes its way to the surface.  Any football supporter who has rushed to get a microwaved pie at half time will have learned this quickly: a cool crust can hide a damagingly hot centre.

Thermal power is delivered to a planet’s surface by convection deep down and conduction nearer the surface because rocks, both solid and molten, are almost opaque to radiation. The vigour of the outward flow of heat might seem to be related mainly to the amount of internal heat but it is also governed by limits imposed by temperature on the form of convection. Of the Inner Planets only Earth shows surface signs of deep convection in the form of plate tectonics driven mainly by the pull exerted by steep subduction of cool, dense slabs of old oceanic lithosphere. Only Jupiter’s moon Io shows comparable surface signs of inner dynamics, but in the form of immense volcanoes rather than lateral movements of slabs. Io has about 40 times the surface heat flow of Earth, thanks largely to huge tidal forces imposed by Jupiter. So it seems that a different mode of convection is needed to shift the tidal heat production; similar in many ways to Earth’s relatively puny and isolated hot spots and mantle plumes.

Most of the yellow and orange hues of Io are d...
An analogy for the early Earth, Jupiter’s moon Io is speckled with large active volcanoes; signs of vigorous internal heat transport but not of plate tectonics. Its colour is dominated by various forms of sulfur rather than mafic igneous rocks. (credit: Wikipedia)

Shortly after Earth’s accretion it would have contained far more heat than now: gravitational energy of accretion itself; greater tidal heating from a close Moon and up to five times more from internal radioactive decay. The time at which plate tectonics can be deduced from evidence in ancient rocks has been disputed since the 1970s, but now an approach inspired by Io’s behaviour approaches the issue from the opposite direction: what might have been the mode of Earth’s heat transport shortly after accretion (Moore, W.B. & Webb, A.A.G. 2013. Heat-pipe Earth. Nature, v.  501, p. 501-505). The two American geophysicists modelled Rayleigh-Bénard convection – multicelled convection akin to that of the ‘heat pipes’ inside Io – for a range of possible thermal conditions in the Hadean. The modelled planet, dominated by volcanic centres turned out to have some surprising properties.

The sheer efficiency of heat-pipe dominated heat transfer and radiative heat lost results in development of a thick cold lithosphere between the pipes, that advects surface material downwards. Decreasing the heat sources results in a ‘flip’ to convection very like plate tectonics. In itself, this notion of sudden shift from Rayleigh-Bénard convection to plate tectonics is not new – several Archaean specialists, including me, debated this in the late 1970s – but the convincing modelling is. The authors also assemble a plausible list of evidence for it from the Archaean geological record: the presence in pre- 3.2 Ga greenstone belts of abundant ultramafic lavas marking high fractions of mantle melting; the dome-trough structure of granite-greenstone terrains; granitic magmas formed by melting of wet mafic rocks at around 45 km depth, extending back to second-hand evidence from Hadean zircons preserved in much younger rocks. They dwell on the oldest sizeable terranes in West Greenland (the Itsaq gneiss complex), South Africa and Western Australia (Barberton and the Pilbara) as a plausible and tangible products of ‘heat-pipe’ tectonics. They suggest that the transition to plate-tectonic dominance was around 3.2 Ga, yet ‘heat pipes’ remain to the present in the form of plumes so nicely defined in the preceding item Mantle structures beneath the central Pacific.

Dust: heating or cooling?

In the left image, thin martian clouds are vis...
Mars: with and without dust storms in 2001. Image via Wikipedia

Once every 13 years on average dust blots out most of the surface of Mars turning it into an orange ball. The last such planet-encircling dust storm occurred in 2001, but lesser storms spring up on a seasonal basis. Yet Martian seasons have very different weather from terrestrial ones because of the greater eccentricity of Mars’s orbit, as well as the fact that its ‘weather’ doesn’t involve water. When Mars is closest to the Sun solar heating is 20% greater than the average, for both hemispheres. The approach to that perihelion marks the start of the dust season which last a half the Martian year. Unsurprisingly, the sedimentary process that dominates Mars nowadays is the whipping up and deposition of sand and dust, though in the distant past catastrophic floods – probably when subsurface ice melted – sculpted a volcanic landscape pockmarked with impact craters up to several thousand kilometres across. Waterlain sediments on early Mars filled, at least in part, many of the earlier craters and probably blanketed the bulk of its northern hemisphere that is the lowest part of the planet and now devoid of large craters. Erosion and sedimentation since that eventful first billion years has largely been aeolian. Some areas having spectacular dunes of many shapes and sizes, whereas more rugged surfaces show streamlined linear ridges, or yardangs (http://earth-pages.co.uk/2011/05/08/winds-of-change/), formed by sand blasting. Most of the dust on Mars is raised by high winds in the thin atmosphere sweeping the great plains and basins, and, by virtue of Stokes’s law, the grains are very much smaller than on Earth.

The dustiest times on Earth, which might have blotted out sizeable areas from alien astronomers, in the last million years have been glacial maxima, roughly every 100 ka with the latest 20 ka ago. Layering in the Antarctic ice core records such dust-dominated frigid periods very precisely. Less intricate records formed away from the maximum extent of ice sheets as layers of fine sediment known as loess, whose thickness variations match other proxy records of palaeoclimate nicely. Loess, either in place or redeposited in alluvium by rivers, forms the most fertile soil known – when the climate is warm and moist. The vast cereal production of lowland China and the prairies of North America coincides with loess: it may seem strange but a large proportion of 7 billion living humans survive partly because of dust storms during glacial periods of the past.

Being derived from rock-forming minerals dust carries with it a diverse range of chemical elements, including a critical nutrient common on land but in short supply in ocean water far offshore: iron in the form of oxide and hydroxide coatings on dust particles – the dust coating your car after rain often has a yellow or pinkish hue because of its iron content. Even when the well-known ‘fertilizer’ elements potassium, nitrogen and phosphorus are abundant in surface ocean water, they can not encourage algal phytoplankton to multiply without iron. Today the most remote parts of the oceans have little living in their surface layers because of this iron deficiency. Yet oceanographers and climatologists are pretty sure that this wasn’t always the case. They are confident simply because reducing the amount of atmospheric carbon dioxide and its greenhouse effect to levels that would encourage climate cooling and glacial epochs needed more carbon to be buried on the ocean floors than happens nowadays, and lifeless ocean centres would not help in that.

Dust plume off the Sahara desert over the nort...
Saharan dust carried over the Atlantic Ocean by a tropical cyclone. Image via Wikipedia

At present, the greatest source of atmospheric dust is the Sahara Desert (bartholoet, J. 2012. Swept from Africa to the Sahara. Scientific American, v. 306 (February 2012), p. 34-39). Largely derived from palaeolakes dating from a Holocene pluvial episode, Saharan dust accounts for more than half the two billion metric tonnes of particulate atmospheric aerosols dispersed over the Earth each year. Located in the SE trade-wind belt, the Sahara vents dust clouds across the Atlantic Ocean, most to fall there and contribute dissolved material to the mid-ocean near-surface biome but an estimated 40 million t reaches the Amazon basin, contributing to fertilising the otherwise highly leached tropical rain-forest soils. While over the ocean the high albedo of dust adds a cooling effect to the otherwise absorbent sea surface. Over land the fine particles help nucleate water droplets in clouds and hence encourages rainfall. The climatic functions of clouds and dusts are probably the least known factors in the climatic system, a mere 5% uncertainty in their climatic forcing may mean the difference between unremitting global warming ahead or sufficient cooling by reflection of solar radiation to compensate for the cumulative effects of industrial CO2 emissions.

Recording amounts of dust from marine sediments quantitatively is very difficult and impossible in terrestrial sediments, but superb records tied accurately to time at annual precision exist in ice sheets. Low dust levels in Greenland and Antarctic ice tally well with the so-called ‘Medieval Climate Anomaly’ (a warm period) whereas through the 13th to 19th centuries (the ‘Little Ice Age’) more dust than average circulated in the atmosphere. Crucially, for climate change in the industrial era, there has been a massive spike in dust reaching near-polar latitudes since the close of the 18th century during the period associated with signs of global warming: a counterintuitive relationship, but one that is difficult to interpret. The additional dust may well be a result of massive changes in land use across the planet following industrialised agricultural practices and growing population. There are several  questions: does the additional dust also reflect global warming with which it is correlated, i.e. evaporation of the huge former lakes in the Sahara (e.g. Lake Chad); is the dust preventing additional greenhouse warming that would have taken place had the atmosphere been clearer; is it even the ‘wrong kind of dust’, which may well reflect short-wave solar radiation away but also absorbs the longer wavelength thermal radiation emitted by the Earth’s surface, i.e. an aerosol form of greenhouse warming. Needless to say, neither clouds nor dust can be factored into climate prediction models with much confidence.

Galactic controls

English: Artist's conception of the Milky Way ...
Artists impression of the Milky Way viewed along its axis. Image via Wikipedia

Palaeoclimatologists are quite content that an important element in controlling the vagaries of climate is due to gravitational forces that cyclically perturb Earth’s orbit, it axial tilt and the way the axis of rotation wobbles in a similar manner to that of a gyroscope. The predictions about this by James Croll in the late 19th century, which were quantified by Milutin Milankovich during his incarceration during World War I, triumphed when the predicted periods of change were found in deep-sea floor sediment records in 1972. Authors of ideas that link Earth system changes  to the progress of the Solar system through the Milky Way galaxy haven’t had the same accolades. One of the first to suggest a galactic link was Joe Steiner (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the  Geological Society of  Australia, v.  14, p. 99–132.) but his work is rarely credited.

There has been an upsurge of interest in the last decade or so. In a recent issue of New Scientist Stephen Battersby reviews what galactic ‘forcings’ may have accomplished during the 4.5 billion-year history of our world (Battersby, S. 2011. Earth odyssey. New Scientist, v. 212 (3 December issue), p. 42-45). Having formed probably much closer to the galactic centre than its current position the Solar System has drifted, perhaps even ‘surfed’ gravitationally, outwards to reach its present ‘suburban’ position in one of the spiral arms. There are regularities to the now stabilised orbital movements: once every 200 million years the Solar System completes a full orbit; this orbit wobbles across the hypothetical plane of the galactic disc by as much as 200 light years, moving with and against the Milky Way’s cosmic motion. It has proved impossible so far to detect any sign of the orbital 200 Ma periodicity in events on the Earth, and most attention has centred on the wobble.

Steiner suggested that this motion may have crossed different polarities of the galactic magnetic field, perhaps triggering the periodicity of geomagnetic  changes in polarity, but this now seems unlikely. However, his suggestion that glacial epochs, such as those in the Palaeo- and Neoproterozoic, at the end of the Palaeozoic Era and at present, may have resulted from the Solar System’s passage through dust and gas banding in the Milky Way continues to have its attractions (e.g. Pavlov, A.A. et al. 2005. Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust, Geophysical Research Letters, v. 32, p. L03705). The direction of motion relative to the Milky Way’s cosmic drift governs the exposure to cosmic rays that result from a kind of ‘bow-shock’ ahead of the galaxy

Stellar motion through the Milky Way is semi-independent so that from time to time the Solar System may have been sufficiently close to regions of dense dust and gas that nurture the formation of super-massive stars. These huge objects quickly evolve to end in supernovae, proximity to which would have exposed life to ‘hard’ X- and  γ-rays and would be trigger for mass extinction, for instance by accompanying cosmic rays in destroying the ozone protection from UV radiation from the Sun.

The dynamism of the Earth and the resulting complexity of its surface processes makes it a poor place to look for physical signs of galactic influences. No so the Moon: for almost 4.5 billion years it has been a passive receptor for virtually anything that the cosmos could fling at it, and so geologically inert that its surface layers may well preserve a complete ‘stratigraphic’ record of all kinds of process. Should lunar landings with geological capabilities once more prove economically possible, or politically useful, that hidden history could be read.