How did the planets form?

Animation of the 3-D shape of planetesimal Arrokoth. (Credit: Roman Tkachenko, NASA)

The latest addition to knowledge of the Solar System looks a bit like a couple of potatoes that have lain together and dried over several years. It also has a name – Arrokoth – that might have been found in a novel by H.P. Lovecraft. In fact Arrokoth meant ‘sky’ in the extinct Powhatan language once spoken by the native people of Chesapeake Bay. The planetesimal was visited by the New Horizons spacecraft two years after it had flown by Pluto (see; Most exotic geology on far-off Pluto, Earth-logs 6 April 2016). It is a small member of the Kuiper Belt of icy bodies. Data collected by a battery of imaging instruments on the spacecraft has now revealed that it has a reddish brown coloration that results from a mixture of frozen methanol mixed with a variety of organic compounds including a class known as tholins – the surface contains no water ice. Arrokoth is made of two flattened elliptical bodies (one 20.6 × 19.9 × 9.4 km the smaller 15.4 × 13.8 × 9.8 km) joined at a ‘waist’. Each of them comprises a mixture of discrete ‘terrains’ with subtly different surface textures and colours, which are likely to be earlier bodies that accreted together. On 13 February 2020 a flurry of three papers about the odd-looking planetesimal appeared in Science.

The smooth surface implies a lack of high-energy collisions when a local cluster of initially pebble sized icy bodies in the sparsely populated Kuiper Belt gradually coalesced under extremely low gravity. The lack of any fractures suggests that the accretions involved relative speeds of, at most, 2 m s-1; slow-walking speed or spacecraft docking (McKinnon, W.B. and a great many more 2020. The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt. Science, article eaay6620; DOI: 10.1126/science.aay6620). The authors regard this quiet, protracted, cool accretion to have characterised at least the early stages of planet formation in the Outer Solar System. The extent to which this can be extrapolated to the formation of the giant gas- and ice worlds, and to the rocky planets and asteroids of the Inner Solar System is less certain, to me at least. It implies cold accretion over a long period that would leave large worlds to heat up only through the decay of radioactive isotopes. Once large planetesimals had accreted, however that had happened, the greater their gravitational pull the faster other objects of any size would encounter them. That scenario implies a succession of increasingly high-energy collisions during planet formation.

This hot-accretion model, to which most planetary scientists adhere, was supported by a paper published by Science a day before those about Arrokoth hit the internet (Schiller, M. et al. 2020. Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth. Science Advances, v. 6, article eaay7604; DOI: 10.1126/sciadv.aay7604). This work hinged on the variation in the proportions of iron isotopes among meteorites, imparted to the local gas and dust cloud after their original nucleosynthesis in several supernovas in the Milky Way galaxy during pre-solar times. Iron found in different parts of the Earth consistently shows isotopic proportions that match just one class of meteorites: the CI carbonaceous chondrites. Yet there are many other silicate-rich meteorite classes with =different iron-isotope proportions. Had the Earth accreted from this mixed bag by random ‘collection’ of material over a protracted period prior to 4.54 billion years ago, its overall iron-isotope composition would have been more like the average of all meteorites than that of just one class. The authors conclude that Earth’s accretion, and probably that of the smaller body that crashed with it to form the Moon at about 4.4 Ga, must have taken place quickly (<5 million years) when CI carbonaceous chondrites dominated the inner part of the protoplanetary disc.

See also: Barbuzano, J. 2020. New Horizons Reveals Full Picture of Arrokoth . . . and How Planets Form. Sky & Telescope

Finding Archaean atmospheric composition using micrometeorites

Modern micrometeorites (about 20 μm in diameter) from deep-sea sediments, with shiny magnetite-rich veneers (Credit: D. E. Brownlee)

The gases making up the Earth’s atmosphere and their relative proportions before 2.5 billion years (Ga) ago are known with very little certainty. Carbonate rocks are rare, indicating that the oceans were more acidic, which implies that they had dissolved more CO2 from the atmosphere, which, in turn implies that there was much more of that gas than in present air. There are few signs of widespread glaciogenic sediments of Archaean age, at a time when the Sun’s energy output is estimated to have been at 70 to 75% of its present level. Without an enhanced greenhouse effect oceans would have been frozen over; so that supports high CO2 concentrations too. The fact that water worn grains of minerals such as uraninite (UO2) and pyrite (FeS2), which are stable only in reducing conditions, occur in Archaean conglomerates is a good indicator that there were only vanishingly small amounts of oxygen in the air. That was not to change until marine photosynthesisers produced enough to overcome the general reducing conditions at the Earth’s surface, marked by the Great Oxidation Event at around 2.4 Ga (see: Massive event in the Precambrian carbon cycle; Earth-logs, January 2012. Search for more articles in sidebar at Earth-logs home page). It was then that ancient soils (palaeosols) became the now familiar red colour because of their content of ferric iron oxides and hydroxides The problem is that reliable numbers cannot be attached to these kinds of observation. A common means of estimating CO2 levels comes from the way in which the gas reacts with silicates as soils form at the land surface, estimated from carbon isotopes in soil carbonate nodules. Since the rise of land plants around 400 Ma ago the distribution of pores (stomata) in fossil leaves provides a more precise estimate: the more CO2 in air the less densely packed are leaf stomata. For the Precambrian we are stuck with estimates based on chemical reactions of minerals with the atmosphere. Until recently, one reaction that must always have been extremely common was overlooked.

When meteorite pass through the atmosphere at very high speed friction heats them to incandescence. Their surfaces not only melt but the minerals from which they are composed react very strongly with air. The reaction products should therefore provide chemical clues to the relative proportions of atmospheric gases. Both oxygen and carbon dioxide are reactive at such temperatures, although nitrogen is virtually inert, yet it tends to buffer oxidation reactions. The rest of the atmosphere comprises noble gases – mainly argon – and by definition they are completely unreactive. Pure-iron micrometeorites collected from 2.7 Ga old sediments in the Pilbara Province of Western Australia are veneered with magnetite (Fe3O4) and wüstite (FeO), thus preserving a record of their passage through the Neoarchaean atmosphere. If the oxidant had been oxygen, for these minerals to form from elemental iron suggests oxygen levels around those prevailing today: clearly defying the abundant evidence for its near-absence during the Archaean. Carbon dioxide is the only candidate. Two studies have produced similar results (Lehmer, O. R. et al. 2020. Atmospheric CO2 levels from 2.7 billion years ago inferred from micrometeorite oxidationScience Advances, v. 6, article aay4644;  DOI: 10.1126/sciadv.aay4644 and Payne, R.C. et al. 2020. Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean. Proceedings of the National Academy of Sciences, v. 117 1360 DOI:10.1073/pnas.1910698117). Both use complex modelling of the chemical effects of meteorite entry. Lehmer and colleagues estimated that the Neoarchaean atmosphere contained about 64% CO2, with a surface atmospheric pressure about half that at present. This would be sufficient for a surface temperature of about 30°C achieved by the greenhouse effect, taking into account lower solar heating. The team led by Payne concluded a lower concentration (25 to 50%) and a somewhat cooler planet at that time. Both results suggest ocean water considerably more acid than are today’s. The combined warmth and acidity would have had a fundamental bearing on both the origin, survival and evolution of early life.

See also: Carroll, M. 2020. Meteorites reveal high carbon dioxide levels on early Earth; Yirka, R. Computer model shows ancient Earth with an atmosphere 70 percent carbon dioxide. (both from Phys.org)

Everyone now has their Inner Neanderthal

For 20 years, we have known the full human genome. For 10 years the full content of Neanderthal DNA has been available, courtesy of Svante Paabo’s team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The two were compared and suddenly every living person with a Eurasian ancestry learned that they had significant and functional bits of Neanderthal in their make-up: some beneficial, some not so good (see: Yes, it seems that they did… in Human evolution and migrations, May 2010). Then the Denisovan connection emerged for East Asians and original populations of Australasia. Africans seemed not to share such a privilege. But now it seems that they do, but as a result of a somewhat tortuous route (Lu Chen et al. 2020. Identifying and interpreting apparent Neanderthal ancestry in African individuals. Cell v. 180, p. 1–11; DOI: 10.1016/j.cell.2020.01.012).

Reconstruction of Neanderthal male

Lu and colleagues used a new approach to discover that 2500 people from five widespread subpopulations living in Africa carry in their DNA several million base-pairs of Neanderthal origin (about 0.3% of their genomes). This happened in two steps. The most recent resulted when ancient anatomically modern humans (AMH), who carried Neanderthal DNA as a result of repeated interbreeding, migrated back to Africa from Europe about 20 thousand years ago. But the modern Africans’ DNA also suggests that their ancestral Neanderthals had also interbred with a much earlier group of Africans who had left their home continent between 150 to 100 thousand years ago. The Neanderthals already carried sections of that earlier AMH genome. The relationship between modern humans and Neanderthals seems to have been a great deal more complex that previously thought.

The authors conclude, …  our data show that out-of-Africa and in-to-Africa dispersals must be accounted for when interpreting archaic hominin ancestry in contemporary human populations. It is notable that Neanderthal sequences have been identified in every contemporary modern human genome analyzed to date. Thus, the legacy of gene flow with Neanderthals likely exists in all modern humans, highlighting our shared history’. Palaeo-geneticists have also shown that a similarly complex social relationship may have characterised Neanderthals and Denisovans, where their ranges overlapped (see Neanderthal Mum meets Denisovan Dad in Human evolution and migrations, August 2018). It would come as no surprise to learn, eventually, that wherever different human groups crossed paths in the more distant past they engaged in similar practices, that is, they behaved humanly. Things have changed a bit in recorded history, when only a single human group has existed; perhaps a consequence of the emergence of what today passes for ‘economy’.

Watch Chris Stringer discussing his views on Neanderthal-AMH interactions

See also: Price, M. 2020. Africans, too, carry Neanderthal genetic legacy. Science, v. 367, p. 497; DOI: 10.1126/science.367.6477.497

Note added 14 February 2020

Several studies of DNA from living Africans have suggested introgression (interbreeding) of an even earlier archaic population into ancient AMH in Africa. Because this cannot be related to any known fossils, such as Homo erectus, such a population is known in palaeogenetic circles as a ‘ghost’. A new paper (Durvasula, A. & Sankararaman, S. 2020. Recovering signals of ghost archaic introgression in
African populationsScience Advances, v. 6, article eaax5097; DOI: 10.1126/sciadv.aax5097) suggests that two living groups from West Africa (Yoruba and Mende) derive 2 to 19% of their genetic ancestry from such a ‘ghost’ population. It seems that this archaic group diverged from the descent path of AMH before the split of Neanderthals and AMH. But when the Neanderthal-AMH event took place is uncertain, estimates ranging from 185 to 800 ka. This time uncertainty further obscures the genetic ‘trail’. Curiously, as far as I know non-Africans whose AMH ancestors were of African origin, show no sign of this particular ‘ghost’ among their forebears. That perhaps suggests that few if any West Africans engaged in ‘out-of-Africa’ migrations …

Closure for the K-Pg extinction event?

Anyone who has followed the saga concerning the mass extinction at the end of the Cretaceous Period (~66 Ma ago) , which famously wiped out all dinosaurs except for the birds, will know that its cause has been debated fiercely over four decades. On the one hand is the Chicxulub asteroid impact event, on the other the few million years when the Deccan flood basalts of western India belched out gases that would have induced major environmental change across the planet. Support has swung one way or the other, some authorities reckon the extinction was set in motion by volcanism and then ‘polished-off’ by the impact, and a very few have appealed to entirely different mechanism lumped under ‘multiple causes’. One factor behind the continuing disputes is that at the time of the Chicxulub impact the Deccan Traps were merrily pouring out Disentanglement hangs on issues such as what actual processes directly caused the mass killing. Could it have been starvation as dust or fumes shut down photosynthesis at the base of the food chain? What about toxic gases and acidification of ocean water, or being seared by an expanding impact fireball and re-entering incandescent ejecta? Since various lines of evidence show that the late-Cretaceous atmosphere had more oxygen that today’s the last two may even have set the continents’ vegetation ablaze: there is evidence for soots in the thin sediments that mark the K-Pg boundary. The other unresolved issue is timing: of volcanogenic outgassing; of the impact, and of the extinction itself. A new multi-author, paper may settle the whole issue (Hull, P.M and 35 others 2020. On impact and volcanism across the Cretaceous-Paleogene boundary. Science, v. 367, p. 266-272; DOI: 10.1126/science.aay5055).

K-Pg oxygen
Marine temperature record derived from δ18O and Mg/Ca ratios spanning 1.5 Ma that includes the K-Pg boundary: the bold brown line shows the general trend derived from the data points (Credit: Hull et al. 2020; Fig 1)

The multinational team approached the issue first by using oxygen isotopes and the proportion of magnesium relative to calcium (Mg/Ca ratio) in fossil marine shells (foraminifera and molluscs) in several ocean-floor sediment cores, through a short interval spanning the last 500 thousand years of the Cretaceous and the first  million years of the Palaeocene. The first measures are proxies for seawater temperature. The results show that close to the end of the Cretaceous temperature rose to about 2°C above the average for the youngest Cretaceous (the Maastrichtian Age; 72 to 66 Ma) and then declined. By the time of the mass extinction (66 Ma) sea temperature was back at the average and then rose slightly in the first 200 ka of Palaeocene to fall back to the average at 350 ka and then rose slowly again.

Changes in carbon isotopes (δ13C) of bulk carbonate samples from the sediment cores (points) and in deep-water foraminifera (shaded areas) across the K-Pg boundary. (Credit: Hull et al. 2020; Fig 2A)

The second approach was to look in detail at carbon isotopes (δ13C) – a measure of changes in the marine carbon cycle –  and oxygen isotopes (δ18O) in deep water foraminifera and bulk carbonate from the sediment cores, in comparison to the duration of Deccan volcanism (66.3 to 65.4 Ma). The δ13C measure from bulk carbonate stays roughly constant in the Maastrichtian, then falls sharply at 66 Ma.  The δ13C of the deep water forams rises to a peak at 66 Ma. The δ18O measure of temperature peaks and declines at the same times as it does for the mixed fossils. Also examined was the percentage of coarse sediment grains in the muds from the cores. That measure is low during the Maastrichtian and then rises sharply at the K-Pg boundary.

Since warming seems almost certainly to be a reflection of CO2 from the Deccan (50 % of total Deccan outgassing), the data suggest not only a break in emissions at the time of the mass extinction but also that by then the marine carbon system was drawing-down its level in air. The δ13C data clearly indicate that the ocean was able to absorb massive amounts of CO2 at the very time of the Chicxulub impact and the K-Pg boundary. Flood-basalt eruption may have contributed to the biotic aftermath of the extinction for as much as half a million years. The collapse in the marine fossil record seems most likely to have been due to the effects of the Chicxulub impact. A third study – of the marine fossil record in the cores – undertaken by, presumably, part of the research team found no sign of increased extinction rates in the latest Cretaceous, but considerable changes to the marine ecosystem after the impact. It therefore seems that the K-Pg boundary impact ‘had an outsized effect on the marine carbon cycle’. End of story? As with earlier ‘breaks through’; we shall see.

See also: Morris, A. 2020 Earth was stressed before dinosaur extinction (Northwestern University)

The dilemma of Rwanda’s Lake Kivu

In 1986 the small, roughly circular Lake Nyos in the Cameroon highlands silently released a massive cloud of carbon dioxide. Being a dense gas it hugged the ground and flowed down valleys for up to 25 km. 1700 local people perished by suffocation, together with their livestock (See Geohazards 2000). Having a recent volcanic origin, the lake is fed by springs in its bed that contain dissolved CO2 emitted from the residual magma chamber below. At 200 m deep the bottom water is sufficiently pressurised to retain the dissolved gas so that signs of the potential hazard remain hidden until such a limnic eruption occurs. Far larger, with a surface area of 2700 km2, Lake Kivu bordered by Rwanda and The Democratic Republic of Congo, is even deeper (up to 470 m). It too lies within a volcanically active zone, in this case the western arm of the East African Rift System. Being one of the most nutrient-rich bodies of fresh water on Earth, its biological productivity is extremely high, so as well as bottom water enriched in dissolved CO2 – a staggering 256 km3 – methane (CH4) is also present in very large amounts (~65 km3). This comes partly from anaerobic decay of dead organisms and from microbial reduction of the magmatic CO2 passing through its bottom sediments. Sulfate-reducing bacteria also generate toxic hydrogen sulfide (H2S) in the anoxic bottom waters – Lake Nyos contains less dissolved salts and did not emit H2S.

So Kivu presents a far greater hazard than the volcanic lakes of Cameroon and an emission of a dense gas mixture might fill the rift valley in the area to a depth of about a hundred metres. Being highly fertile the valley around the lake has a high population (2 to 3 million), so the death toll from a limnic eruption could be huge. A further hazard stems from tsunamis generated by such gas bursts. Once bubbles form at depth the bulk density of water drops, so large masses of water surge to the surface rather than the gas itself; a phenomenon known to happen in the periodic eruptions of Lake Nyos. What might trigger such an event in Lake Kivu? The East African Rift System is seismically active, but recent earthquakes did not result in limnic eruptions. Subaqueous volcanic eruption is the most likely to set one off. A surface lava flow from the nearby Mount Nyiragongo entered the lake at the town of Goma in 2002 but, fortunately, did not reach the threatening deeper part of Kivu. Sediment samples from the lake reveal periodic transport of land vegetation to its deeper parts, roughly every thousand years. The sediments with plant fossils also contain abundant remains of aquatic animals, suggesting both tsunamis accompanied by toxic emissions.

KIVUWATT’s methane extraction rig on Lake Kivu. (Credit: Contour Global)

Mitigating the hazard of limnic eruptions at Lake Nyos was made possible in 2002 by linking its bottom waters to the surface by plastic piping. After initial pumping, the release of bubbles at shallower depths and the resulting fall in bulk water density set off something akin to a large soda siphon, slowly relieving the deeper layers of their load of dissolved CO2. This resulted in 50 m high fountains of what was effectively soda ‘pop’. In 2009 this was repeated on a far larger scale on Lake Kivu, the operation being paid for by separation and sale of methane. Yet even this attempt at mitigation has its risks: first of destabilising what may be a fragile equilibrium to trigger a limnic eruption; second by lifting nutrient-rich bottom water that would encourage algal blooms at the lake surface and potential deoxygenation. The current issue of the Journal of African Earth Sciences includes a detailed review of the issues surrounding such dual-purpose hazard mitigation (Hirslund, F. & Morkel, P. 2020. Managing the dangers in Lake Kivu – How and why. Journal of African Earth Sciences, v. 161, Article 103672; DOI: 10.1016/j.jafrearsci.2019.103672). By 2015 the Rwandan KivuWatt Methane Project had a capacity for 25 MW of electrical power generation.

Running at full capacity, degassing the depths of Lake Kivu would provide the economic benefit of low-cost electricity for Rwanda and the DRC, at a maximum generating capacity of 300 mW using the most efficient power plant, as well as removing the risk of a catastrophic gas release. Yet the release of CO2 from the lake and from methane burning would increase atmospheric greenhouse warming significantly, albeit less than if the methane was simply released, for CH4 has 25 times the potential for trapping outgoing heat. Hence the dilemma. Either way, there remains the risk of turning Kivu’s surface water into an anoxic algal ‘broth’ with devastating effects on its fishery potential. Burial of the dead phytoplankton, however, might generate more methane by bacterial decay; a possible source of renewable biofuel that ‘recycles’ the atmospheric CO2 consumed by algal photosynthesis. The geohazards, according to Hirslund and Morkel, are really the ultimate driver for development of Lake Kivu’s fossil fuel potential, now that they are better understood as a real and present danger to millions of people. The authors calculate that a catastrophic gas release may be on the cards in the late 21st century. Yet there are other resource issues bound up with the health of the lake’s surface waters. Preserving the layered structure of the lake water to some extent is also important. Until the rates of natural infiltration of volcanic CO2 and biogenic production of methane are known, a minimum rate of gas extraction to make the lake safe is impossible to calculate. Perhaps matching those rates with gas removal should govern future operation. The total methane content of Lake Kivu is just 1.5 times the annual production from the UK sector of the North Sea. It is sufficient for power generation at 300 MW, at most, for 50 years, which would roughly double Rwanda’s current installed generation capacity – mainly from hydropower. Although Kivu is shared equally between Rwanda and the DRC even half of the short term power potential would be a significant benefit to Rwanda’s ~11 million people, though considerably less to the ~81 million living in the DRC; if access was equitable.

Mineral grains far older than the Solar System

If a geologist with broad interests was asked, ‘what are the oldest materials on Earth?’ she or he would probably say the Acasta Gneiss from Canada’s North West Territories at 4.03 billion years (Ga) (see: At last, 4.0 Ga barrier broken, November 2008. A specialist in the Archaean Eon might say the Nuvvuagittuq Greenstone Belt on the eastern shore of Hudson Bay (see: Archaean continents derived from Hadean oceanic crust, March 2017); arguably 4.28 Ga old. An isotope geochemist would refer to a tiny 4.4 Ga zircon grain that had been washed into the much younger Mount Narryer quartz sandstone in Western Australia (see: Pushing back the “vestige of a beginning”, January 2001). A real smarty pants would cite a 4.5 Ga old sample of feldspar-rich Lunar Highland anorthosite in the Apollo Mission archive in Houston, USA. The last is less than 100 Ma younger than the formation of the Solar System itself at 4.568 Ga. Yet there are meteorites that have fallen to Earth, which contain minute mineral  grains that were incorporated into the initial dust from which the planets formed. Until recently, the best known were white inclusions in a 2 tonne meteorite that fell near Allende in Mexico; the largest carbonaceous chondrite ever found. This class of meteorite represents the most primitive material in orbit around the Sun. Its tiny inclusions contain proportions of isotopes of a variety of elements that are otherwise unknown in any other material from the Solar System and they are older. The conclusion is that these dust-sized, presolar grains originated elsewhere in the galaxy, perhaps from supernovas or red-giant stars.

A presolar grain from the Murchison meteorite made up of silicon carbide crystals (credit: Janaína N. Ávila)

Carbonaceous chondrites, as their name suggests, contain a huge variety of carbon-based compounds and they have been closely examined as possible suppliers of the precursor chemicals for the origin of life. Another large example of this class fell near the town of Murchison in Victoria, Australia in 1969. The first people to locate fragments of the 100 kg body noted a distinct smell of methylated spirits and steam rising from it: when crushed half a century later it still smells like rotting peanut butter. The Murchison meteorite has yielded signs of 14 thousand organic compounds, including 70 amino acids. It has also been a target for extracting possible presolar grains. This entails grinding small fragments and then dissolving out the carbonaceous and silicate material using various reagent to leave the more or less inert silicon carbide grains. The residue contains the most durable grains: despite being described as ‘large’ they are of the order of only 10 micrometres across. Some are made of silicon carbide; the same as the well-known abrasive carborundum. Throughout their lifetime in interstellar space the grains have been bombarded by high-energy protons and helium nuclei which move through space at nearly the speed of light – generally known as ‘cosmic rays’. When interacting with other matter they behave much like the particles in the Large Hadron Collider, being able to transmute natural isotopes into others. Measuring the relative proportions of these isotopes in material that has been bombarded by cosmic rays enables their exposure time to be estimated. In the case of the Murchison presolar grains the isotopes of choice are those of the noble gas neon (Heck, P.R. and 9 others 2020. Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide. Proceedings of the National Academy of Sciences, 201904573; DOI: 10.1073/pnas.1904573117). Analyses of 40 such grains yielded ages from 4.6 to 7.5 Ga, i.e. up to 3 billion years before the Solar System formed. They are, indeed, exotic. The highest age exceeds that of the oldest from such previously measured by 1.5 billion years

Investigations up to now suggest that dusts amount to about 1 % of interstellar matter, the rest being gases, mainly hydrogen and helium. With the formation of the planets and the parent bodies of asteroids a high proportion of presolar grains would have accreted to them to be mixed with other, more common stuff. What Heck and colleagues have discovered puts the Solar System into a broad framework of time and space. The grains must have formed at some stage in the evolution of stars older and larger than the Sun, to be blown out into the interstellar medium of the Milky Way galaxy. One possibility is that about 7 billion years ago there was a burst of star formation in a nearby sector of the galaxy. How the resulting dust made its way to the concentration of interstellar matter that eventually formed the Sun and Solar System is yet to be commented on.

See also: Bennett, J.  2020 Meteorite Grains Are the Oldest Known Solid Material on Earth.  Smithsonian Magazine(online)  13 January 2020.

Active volcanic processes on Venus

Earth’s nearest neighbour, apart from the Moon, is the planet Venus. As regards size and estimated density it could be Earth’s twin. It is a rocky planet, probably with a crust and mantle made of magnesium- and iron-rich silicates, and its bulk density suggests a substantial metallic core. There the resemblance ends. The whole planet is shrouded in highly reflective cloud (possibly of CO2 ‘snow’) at the top of an atmosphere almost a hundred times more massive than ours. It consists of 96% CO2 with 3% nitrogen, the rest being mainly sulfuric acid: the ultimate greenhouse world, and a very corrosive one. Only the four Soviet Venera missions have landed on Venus to provide close-up images of its surface. They functioned only for a couple of hours, after having measured a surface temperature around 500°C – high enough to melt lead. One Venera instrument, an X-ray fluorescence spectrometer – did crudely analyse some surface rock, showing it to be of basaltic composition. The atmosphere is not completely opaque, being transparent to microwave radiation. So both its surface textures and elevation variation have been imaged several times using orbital radar. Unlike the Earth, whose dual-peaked distribution of elevation – high continents and low ocean floors thanks to plate tectonics – Venus has just one and is significantly flatter. No tectonics operate there. There are far fewer impact craters on Venus than on Mars and the Moon, and most are small. This suggests that the present surface of Venus is far younger than are theirs; no more than 500 Ma compared to 3 to 4 billion years.

Volcanic ‘pancake’ domes on the surface of Venus, about 65 km wide and 1 km high, imaged by orbital radar carried by NASA’s Magellan Mission.

Somehow, Venus has been ‘repaved’, most likely by vast volcanic outpourings akin to the Earth’s flood basalt events, but on a global scale. Radar reveals some 1600 circular features that are undoubtedly volcanic in origin and younger than most of the craters. They resemble huge pancakes and are thought to be shield volcanoes similar to those seen on the Ethiopian Plateau but up to 100 times larges. Despite the high surface temperature and a caustic atmosphere, chemical weathering on Venus is likely to be much slower than on Earth because of the dryness of its atmosphere. Also, unlike the hydration reactions that produce terrestrial weathering, on Venus oxidizing processes probably produce iron oxides, sulfides, some anhydrous sulfates and secondary silicates. These would change the reflective properties of originally fresh igneous rocks, a little like the desert varnish that pervades rocky surfaces in arid areas on Earth. A group of US scientists have devised experiments to reproduce the likely conditions at the surface of Venus to see how long it takes for one mineral in basalt to become ‘tarnished’ in this way (Filberto, J. et al. 2020. Present-day volcanism on Venus as evidenced from weathering rates of olivine. Science Advances, v. 6, article eaax7445; DOI: 10.1126/sciadv.aax7445). One might wonder why, seeing as the planet’s atmosphere hides the surface in the visible and short-wavelength infrared part of the spectrum, which underpins most geological remote sensing of other planetary bodies, such as Mars. In fact, that is not strictly true. Carbon dioxide lets radiation pass through in three narrow spectral ‘windows’ (centred on 1.01, 1.10, and 1.18 μm) in which fresh olivine emits more radiation when it is heated than does weathered olivine. So detecting and measuring radiation detected in these ‘windows’ should discriminate between fresh olivine and that which has been weathered Venus-style. Indeed it may help determine the degree of weathering and thus the duration of lava flow’s exposure.

Venus VNIR
Colour-coded image of night-time thermal emissivity over Venus’s southern hemisphere as sensed by VIRTIS on Venus Express (Credit: M. Gilmore 2017, Space Sci. Rev. DOI 10.1007/s11214-017-0370-8; Fig. 3)

The European Space Agency’s Venus Express Mission in 2006 carried a remote sensing instrument (VIRTIS) mainly aimed at the structure of Venus’s clouds and their circulation. But it also covered the three CO2 ‘windows’, so it could detect and image the surface too. The image above shows significant areas of the surface of Venus that strongly emit short-wave infrared at night (yellow to dark red) and may be slightly weathered to fresh. Most of the surface in green to dark blue is probably heavily weathered. So the data may provide a crude map of the age of the surface. However, Filberto et al’s experiments show that olivine weathers extremely quickly under the surface conditions of Venus. In a matter of months signs of the fresh mineral disappeared. So the red areas on the image may well be lavas that have been erupted in the last few years before VIRTIS was collecting data, and perhaps active eruptions. Previous suggestions have been that some lava flows on large volcanoes are younger than 2.5 Ma and possible even younger than 0.25 Ma. Earth’s ‘evil twin’ now seems to be vastly more active, as befits a planet in which mantle-melting temperatures (~1200°C) are far closer to the surface as a result of the blanketing effect of its super-dense atmosphere.

The last known Homo erectus

There are a lot of assumptions made about Homo erectus and, indeed, there is much confusion surrounding the species (see: various items in Human evolution and migrations logs for 2001, 2002, 2003 and several other years). For a start, the name derives from Eugene Dubois’s 1891 discovery of several hominin cranial fragments in sediments deposited by the Solo River in Java. Dubois was the first to recognise in ‘Java Man’ the human-ape ‘missing link’ about which Charles Darwin speculated in his The Descent of Man, and Selection in Relation to Sex (1871). Dubois named the beings Pithecanthropus (now Homo) erectus. Once the “multiregional” versus “out-of-Africa” debate about the origin of anatomically modern humans (AMH) emerged after a variety of H. erectus-like fossils had also turned up in Africa and Europe, as well as in East and SE Asia, ‘Java Man’ was adopted by the multiregionalists as ‘evidence’ for separate evolution of AMH in Asia. Such a view remains adhered to by a tenacious number of Chinese palaeoanthropologists, but by virtually no-one else.

Reconstruction of the Nariokotome Boy from the skeleton found in the Turkana Basin of Kenya (credit: Atelier Daynes/Science Photo Library)

The earliest of the African ‘erects’ were distinguished as H. ergaster, represented by the 1.6 Ma old, almost intact skeleton of Nariokotome Boy from the Turkana area of Kenya. In Africa the specific names ergaster and erectus often seem to be used as synonyms, whereas similar-looking fossils from Asia are almost always referred to as ‘Asian ­H. erectus’. Matters became even more confusing when the earliest human migrants from Africa to Eurasia were discovered at Dmanisi in Georgia (see; Human evolution and migrations logs for 2002, 2003, 2007, 2013). Anatomically they deviate substantially from both H. ergaster and Asian erectus – and from each other! – and at 1.8 Ma they are very old indeed. Perhaps as a palliative in the academic rows that broke out following their discovery, for the moment they are called Homo erectus georgicus; a sub-species. But, then, how can Asian H. erectus be regarded as their descendants. Yet anatomically erectus-like fossils are known in East and SE Asia from 1.5 Ma onwards.

There is another mystery. Homo ergaster/erectus in Africa made distinctive tools, typified by the bifacial Acheulian hand axe. Their tool kit remained substantially the same for more than a million years, and was inherited by all the descendants of H. erectus in Africa and Europe: by H. antecessor, heidelbergensis, Neanderthals and early AMH. Yet in Asia, such a technology has not been discovered at sites older than around 250 thousand years. Either no earlier human migrants into Asia made and carried such artefacts or stone tools were largely abandoned by early Asian humans in favour of those more easily made from woods, for instance bamboo.

In 1996 the youngest Solo River sediments that had yielded H. erectus remains in the 1930s were dated using electron-spin resonance and uranium-series methods. The results suggested occupation by ‘erects’ between 53 and 27 ka, triggering yet more astonishment, because fully modern humans had by then also arrived in Indonesia. Could anatomically modern humans have co-existed with a species whose origin went back to almost two million years beforehand? It has taken another two decades for this perplexing issue to be clarified – to some extent. The previous dates were checked using more precise versions of the original geochronological methods covering a wider range of sediment strata (Rizal, Y. et al. 2019. Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago. Nature, published online; DOI:10.1038/s41586-019-1863-2). No AMH presence in Asia is known before about 80 ka, so can the astonishment be set aside? Possibly, but what is known for sure from modern and ancient DNA comparisons is that early modern human migrants interbred with a more ancient Asian group, the Denisovans. At present that group is only known from a site in Siberia and another in Tibet through a finger bone and a few molar teeth that yielded DNA significantly different from both living humans and ancient Neanderthals. So we have no tangible evidence of what the Denisovans looked like, unlike Asian H. erectus of whom there are many substantial fossils. Yet DNA has not been extracted from any of them. That is hardly surprising for the Indonesian specimens because hot and humid conditions cause DNA to break down quickly and completely. There is a much better chance of extracting genomes from the youngest H. erectus fossils from higher latitudes in China. Once that is achieved, we will know whether they are indeed erects or can be matched genetically with Denisovans.

See also:  Price, M. 2019. Ancient human species made ‘last stand’ 100,000 years ago on Indonesian island (Science)

Chewing gum and the genetics of an ancient human

The sequencing of DNA has advanced to such a degree of precision and accuracy that minute traces of tissue, hair, saliva, sweat, semen and other bodily solids and fluids found at crime scenes are able to point to whomever was present. That is, provided that those persons’ DNA is known either from samples taken from suspects or resides in police records. In the case of individuals unknown to the authorities, archived DNA sequences from members of almost all ethnic groups can be used to ‘profile’ those present at a crime. Likely skin and hair pigmentation, and even eye colour, emerge from segments that contain the genes responsible.

One of the oddest demonstrations of the efficacy of DNA sequencing from minute samples used a wad of chewed birch resin. Such gums are still chewed widely for a number of reasons: to stave off thirst or hunger; to benefit from antiseptic compounds in the resin and to soften a useful gluing material – resin derived by heating birch bark is a particularly good natural adhesive . Today we are most familiar with chicle resin from Central America, the base for most commercial chewing gum, but a whole range of such mastics are chewed on every inhabited continent, birch gum still being used by Native North Americans: it happens to be quite sweet. The chewed wad in this case was from a Neolithic site at Syltholm on the Baltic coast of southern Denmark (Jensen, T.Z.T. and 21 others 2019. A 5700 year-old human genome and oral microbiome from chewed birch pitch. Nature Communications v. 10, Article 5520; DOI: 10.1038/s41467-019-13549-9). The sample contained enough ancient human DNA to reconstruct a full genome, and also yielded fragments from a recent meal – duck with hazelnuts – and from several oral bacteria and viruses, including a herpes variety that is a cause of glandular fever. The sample also shows that the carrier did not have the gene associated with lactase persistence that allows adults to digest milk.

An artist’s impression of the gum chewing young woman from southern Denmark (credit: Tom Bjorklund)

The chewer was female and had both dark skin and hair, together with blue eyes; similar to a Mesolithic male found in a cave in Cheddar Gorge in SW England whose petrous ear bone yielded DNA. By no means all fossil human bones still carry enough DNA for full sequencing, and are in any case rare. Chewed resin is much more commonly found and its potential awaits wider exploitation, particularly as much older wads have been found. Specifically, the Danish woman’s DNA reveals that she did not carry any ancestry from European Neolithic farmers whose DNA is well known from numerous burials. It was previously thought that farmers migrating westward from Anatolia in modern Turkey either replaced or absorbed the earlier Europeans. By 5700 years ago farming communities were widespread in western Europe, having arrived almost two thousand years earlier. The blue-eyed, dark Danish woman was probably a member of a surviving group of earlier hunter gatherers who followed the retreat of glacial conditions at the end of the Younger Dryas ice re-advance about 11,500 years ago. The Syltholm site seems to have been occupied for hundreds of generations. Clearly, the community had not evolved pale skin since its arrival, as suggested by a once popular theory that dark skin at high latitudes is unable to produce sufficient vitamin-D for good health. That notion has been superseded by knowledge that diets rich in meat, nuts and fungi provide sufficient vitamin-D. Pale skins may have evolved more recently as people came to rely on a diet dominated by cereals that are a poor source of vitamin-D.

How marine animal life survived (just) Snowball Earth events

diamict3
A Cryogenian glacial diamictite containing boulders of many different provenances from the Garvellach Islands off the west coast of Scotland. (Credit: Steve Drury)

Glacial conditions during the latter part of the Neoproterozoic Era extended to tropical latitudes, probably as far as the Equator, thereby giving rise to the concept of Snowball Earth events. They left evidence in the form of sedimentary strata known as diamictites, whose large range of particle size from clay to boulders has a range of environmental explanations, the most widely assumed being glacial conditions. Many of those from the Cryogenian Period are littered with dropstones that puncture bedding, which suggest that they were deposited from floating ice similar to that forming present-day Antarctic ice shelves or extensions of onshore glaciers. Oceans on which vast shelves of glacial ice floated would have posed major threats to marine life by cutting off photosynthesis and reducing the oxygen content of seawater. That marine life was severely set back is signalled by a series of perturbations in the carbon-isotope composition of seawater. Its relative proportion of 13C to 12C (δ13C) fell sharply during the two main Snowball events and at other times between 850 to 550 Ma. The Cryogenian was a time of repeated major stress to Precambrian life, which may well have speeded up evolution, sediments of the succeeding Ediacaran Period famously containing the first large, abundant and diverse eukaryote fossils.

For eukaryotes to survive each prolonged cryogenic stress required that oxygen was indeed present in the oceans. But evidence for oxygenated marine habitats during Snowball Earth events has been elusive since these global phenomena were discovered. Geoscientists from Australia, Canada, China and the US have applied novel geochemical approaches to occasional iron-rich strata within Cryogenian diamictite sequences from Namibia, Australia and the south-western US in an attempt to resolve the paradox (Lechte, M.A. and 8 others 2019. Subglacial meltwater supported aerobic marine habitats during Snowball Earth. Proceedings of the National Academy of Sciences, 2019; 201909165 DOI: 10.1073/pnas.1909165116). Iron isotopes in iron-rich minerals, specifically the proportion of 56Fe relative to that of 54Fe (δ56Fe), help to assess the redox conditions when they formed. This is backed up by cerium geochemistry and the manganese to iron ratio in ironstones.

In the geological settings that the researchers chose to study there are sedimentological features that reveal where ice shelves were in direct contact with the sea bed, i.e. where  they were ‘grounded’. Grounding is signified by a much greater proportion of large fragments in diamictites, many of which are striated through being dragged over underlying rock. Far beyond the grounding line diamictites tend to be mainly fine grained with only a few dropstones. The redox indicators show clear changes from the grounding lines through nearby environments to those of deep water beneath the ice. Each of them shows evidence of greater oxidation of seawater at the grounding line and a falling off further into deep water. The explanation given by the authors is fresh meltwater flowing through sub-glacial channels at the base of the grounded ice fed by melting at the glacier surface, as occurs today during summer on the Greenland ice cap and close to the edge of Antarctica. Since cold water is able to dissolve gas efficiently the sub-glacial channels were also transporting atmospheric oxygen to enrich the near shore sub-glacial environment of the sea bed. In iron-rich water this may have sustained bacterial chemo-autotrophic life to set up a fringing food chain that, together with oxygen, sustained eukaryotic heterotrophs. In such a case, photosynthesis would have been impossible, yet unnecessary. Moreover, bacteria that use the oxidation of dissolved iron as an energy source would have caused Fe-3 oxides to precipitate, thereby forming the ironstones on which the study centred. Interestingly, the hypothesis resembles the recently discovered ecosystems beneath Antarctic ice shelves.

Small and probably unconnected ecosystems of this kind would have been conducive to accelerated evolution among isolated eukaryote communities. That is a prerequisite for the sudden appearance of the rich Ediacaran faunas that colonised sea floors globally once the Cryogenian ended. Perhaps these ironstone-bearing diamictite occurrences where the biological action seems to have taken place might, one day, reveal evidence of the precursors to the largely bag-like Ediacaran animals