Centenary of the Milanković Theory

A letter in the latest issue of Nature Geoscience (Cvijanovic, I. et al. 2020. One hundred years of Milanković cycles, v. Nature Geoscience , v.13p. 524–525; DOI: 10.1038/s41561-020-0621-2) reveals the background to Milutin Milanković’s celebrated work on the astronomical  driver of climate cyclicity. Although a citizen of Serbia, he had been born at Dalj, a Serbian enclave, in what was Austro-Hungary. Just before the outbreak of World War I in 2014, he returned to his native village to honeymoon with his new bride. The assassination (29 June 2014) in Sarajevo of Archduke Franz Ferdinand by Bosnian-Serb nationalist Gavrilo Princip prompted Austro-Hungarian authorities to imprison Serbian nationals. Milanković was interned in a PoW camp. Fortunately, his wife and and a former Hungarian colleague managed to negotiate his release, on condition that he served his captivity, with a right to work but under police surveillance, in Budapest. It was under these testing conditions that he wrote his seminal Mathematical Theory of Heat Phenomena Produced by Solar Radiation; finished in 1917 but remaining unpublished until 1920 because of a shortage of paper during the war.

Curiously, Milanković was a graduate in civil engineering — parallels here with Alfred Wegener of Pangaea fame, who was a meteorologist — and practised in Austria. Appointed to a professorship in Belgrade in 1909, he had to choose a field of research. To insulate himself from the rampant scientific competitiveness of that era, he chose a blend of mathematics and astronomy to address climate change. During his period as a political prisoner Milanković became the first to explain how the full set of cyclic variations in Earth’s orbit — eccentricity, obliquity and precession — caused distinct variations in incoming solar radiation at different latitudes and changed on multi-thousand-year timescales. The gist  of what might have lain behind the cyclicity of ice ages had first been proposed by Scottish scientist James Croll almost half a century earlier, but it was Milutin Milanković who, as it were, put the icing on the cake. What is properly known as the Milanković-Croll Theory triumphed in the late 1970s as the equivalent of plate tectonics in palaeoclimatology after Nicholas Shackleton and colleagues teased out the predicted astronomical signals from time series of oxygen isotope variations in marine-sediment cores.

Appropriately, while Milanković’s revoluitionary ideas lacked corroborating geological evidence, one of the first to spring to his support was that other resilient scientific ‘prophet’, Alfred Wegener. Neither of them lived to witness their vindication.

Kicking-off planetary Snowball conditions

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Artist’s impression of the glacial maximum of a Snowball Earth event (Source: NASA)

Twice in the Cryogenian Period of the Neoproterozoic, glacial- and sea ice extended from both poles to the Equator, giving ‘Snowball Earth’ conditions. Notable glacial climates in the Phanerozoic – Ordovician, Carboniferous-Permian and Pleistocene – were long-lived but restricted to areas around the poles, so do not qualify as Snowball Earth conditions. It is possible, but less certain, that Snowball Earth conditions also prevailed during the Palaeoproterozoic at around 2.4 to 2.1 billion years ago. This earlier episode roughly coincided with the ‘Great Oxidation Event’, and one explanation for it is that the rise of atmospheric oxygen removed methane, a more powerful greenhouse gas than carbon dioxide, by oxidizing it to CO2 and water. That may well have been a consequence of the evolution of the cyanobacteria, their photosynthesis releasing oxygen to the atmosphere. The Neoproterozoic ‘big freezes’ are associated with rapid changes in the biosphere, most importantly with the rise of metazoan life in the form of the Ediacaran fauna, the precursor to the explosion in animal diversity during the Cambrian. Indeed all major global coolings, restricted as well as global, find echoes in the course of biological evolution. Another interwoven factor is the rock cycle, particularly volcanism and the varying pace of chemical weathering. The first releases CO2 from the mantle, the second helps draw it down from the atmosphere when weak carbonic acid in rainwater rots silicate minerals (see: Can rock weathering halt global warming, July 2020). All such interplays between major and sometimes minor ‘actors’ in the Earth system influence climate and, in turn, climate inevitably affects all the rest. With such complexity it is hardly surprising that there is a plethora of theories about past climate shifts.

As well as a link with fluctuations in the greenhouse effect, climate is influenced by changes in the amount of solar heating, for which there are yet more options to consider. For instance, the increase in Earth’s albedo (reflectivity) that results from ice cover, may lead through a feedback effect to runaway cooling, particularly once ice extends beyond the poorly illuminated poles. Volcanic dust and sulfate aerosols in the stratosphere also increase albedo and the tendency to cooling, as would interplanetary dust. More complexity to befuddle would-be modellers of ancient climates. Yet it is safe to say that, within the maelstrom of contributory factors, the freeze-overs of Snowball conditions must have resulted from our planet passing through some kind of threshold in the Earth System. Two theoretical scientists from the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology have attempted to cut through the log-jam by modelling the dynamics of the interplay between the ice-albedo feedback and the carbon-silicate cycle of weathering (Arnscheidt, C.W. & Rothman, D.H. 2020. Routes to global glaciation. Proceedings of the Royal Society A, v. 476, article 0303 online; DOI: 10.1098/rspa.2020.0303). Their mathematical approach involves two relatively simple, if long-winded, equations based on parameters that express solar heating, albedo, surface temperature and pressure, and the rate of volcanic outgassing of CO2; a simplification that sets biological processes to one side.

Unlike previous models, theirs can simulate varying rates, particularly of changes in solar energy input. The key conclusion of the paper is that if solar heating decreases faster than a threshold rate the more a planet’s surface water is likely to freeze from pole to pole. The authors suggest that a Snowball Earth event would result from a 2% fall in received solar radiation over about ten thousand years: pretty quick in a geological sense. Such a trigger might stem from a volcanic ‘winter’ scenario, an increase in clouds seeded by spores of primitive marine algae or other factors. The real ‘tipping point’ would probably be the high albedo of ice. There is a warning in this for the present, when a variety of means of decreasing solar input have been proposed as a ‘solution’ to global warming.

Because the Earth orbits the Sun in the ‘Goldilocks Zone’ and is volcanically active even global glaciation would be temporary, albeit of the order of millions of years. The cold would have shut down weathering so that volcanic CO2 could slowly build up in the atmosphere: the greenhouse effect would rescue the planet. Further from the Sun, a planet would not have that escape route, regardless of its atmospheric concentration of greenhouse gases: a neat lead-in to another recent paper about the ancient climate of Mars (Grau Galofre, A. et al. 2020. Valley formation on early Mars by subglacial and fluvial erosion. Nature Geoscience, early online article; DOI: 10.1038/s41561-020-0618-x)

A Martian channel system: note later cratering (credit: European Space Agency)

There is a lot of evidence from both high-resolution orbital images of the Martian surface and surface ‘rovers’ that surface water was abundant over a long period in Mars’s early history. The most convincing are networks of channels, mainly in the southern hemisphere highlands. They are not the vast channelled scablands, such as those associated with Valles Marineris, which probably resulted from stupendous outburst floods connected to catastrophic melting of subsurface ice by some means. There are hundreds of channel networks, that resemble counterparts on Earth. Since rainfall and melting of ice and snow have carved most terrestrial channel networks, traditionally those on Mars have been attributed to similar processes during an early warm and wet phase. The warm-early Mars hypothesis extends even to interpreting the smooth low-lying plains of its northern hemisphere – about a third of Mars’s surface area – as the site of an ocean in those ancient times. Of course, a big question is, ‘Where did all that water go?’ Another relates to the fact that the early Sun emitted considerably less radiation 4.5 billion years ago than it does now: a warm-wet early Mars is counterintuitive.

Anna Grau Galofre of the University of British Columbia and co-authors found that many of the networks on Mars clearly differ in morphology from one another, even in small areas of its surface. Drainage networks on Earth conform to far fewer morphological types. By comparing the variability on Mars with channel-network shapes on Earth, the authors found a close match for many with those that formed beneath the ice sheet that covered high latitudes of North America during the last glaciation. Some match drainage patterns typical of surface-water erosion, but both types are present in low Martian latitudes: a suggestion of ‘Snowball Mars’ conditions? The authors reached their conclusions by analysing six mathematical measures that describe channel morphology for over ten thousand individual valley systems. Previous analyses of individual systems discovered on high-resolution images have qualitative comparisons with terrestrial geomorphology

See also: Chu, J. 2020. “Snowball Earths” May Have Been Triggered by a Plunge in Incoming Sunlight – “Be Wary of Speed” (SciTech Daily 29 July 2020); Early Mars was covered in ice sheets, not flowing rivers, researchers say (Science Daily, 3 August 2020)

Earliest plate tectonics tied down?

Papers that ponder the question of when plate tectonics first powered the engine of internal geological processes are sure to get read: tectonics lies at the heart of Earth science. Opinion has swung back and forth from ‘sometime in the Proterozoic’ to ‘since the very birth of the Earth’, which is no surprise. There are simply no rocks that formed during the Hadean Eon of any greater extent than 20 km2. Those occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay, which have been grossly mangled by later events. But there are grains of the sturdy mineral zircon ZrSiO4)  that occur in much younger sedimentary rocks, famously from the Jack Hills of Western Australia, whose ages range back to 4.4 Ga, based on uranium-lead radiometric dating. You can buy zircons from Jack Hills on eBay as a result of a cottage industry that sprang up following news of their great antiquity: that is, if you do a lot of mineral separation from the dust and rock chips that are on offer, and they are very small. Given a laser-fuelled SHRIMP mass spectrometer and a lot of other preparation kit, you could date them. Having gone to that expense, you might as well analyse them chemically using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to check out their trace-element contents. Geochemist Simon Turner of Macquarie University in Sydney, Australia, and colleagues from Curtin University in Western Australia and Geowissenschaftliches Zentrum Göttingen in Germany, have done all this for 32 newly extracted Jack Hills zircons, whose ages range from 4.3 to 3.3 Ga (Turner, S. et al. 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the HadeanNature Communications, v. 11, article 1241; DOI: 10.1038/s41467-020-14857-1). Then they applied sophisticated geochemical modelling to tease out what kinds of Hadean rock once hosted these grains that were eventually eroded out and transported to come to rest in a much younger sedimentary rock.

Artist’s impression of the old-style hellish Hadean (Credit : Dan Durday, Southwest Research Institute)

Zircons only form duuring the crystallisation of igneous magmas, at around 700°C, the original magma having formed under somewhat hotter conditions – up to 1200°C for mafic compositions. In the course of their crystallising, minerals take in not only the elements of which they are mainly composed, zirconium, silicon and oxygen in the case of zircon , but many other elements that the magma contains in low concentrations. The relative proportions of these trace elements that are partitioned from the magma into the growing mineral grains are more or less constant and unique to that mineral, depending on the particular composition of the magma itself. Using the proportions of these trace elements in the mineral gives a clue to the original bulk composition of the parent magma. The Jack Hills zircons  mainly  reflect an origin in magmas of andesitic composition, intermediate in composition between high-silica granites and basalts that have lower silica contents. Andesitic magmas only form today by partial melting of more mafic rocks under the influence of water-rich fluid driven upwards from subducting oceanic lithosphere. The proportions of trace elements in the zircons could only have formed in this way, according to the authors.

Interestingly, the 4.2 Ga Nuvvuagittuq greenstone belt contains metamorphosed mafic andesites, though any zircons in them have yet to be analysed in the manner used by Turner et al., although they were used to date those late-Hadean rocks. The deep post-Archaean continental crust, broadly speaking, has an andesitic composition, strongly suggesting its generation above subduction zones. Yet that portion of Archaean age is not andesitic on average, but a mixture of three geochemically different rocks. It is referred to as TTG crust from those three rock types (trondhjemite, tonalite and granodiorite). That TTG nature of the most ancient continental crust has encouraged most geochemists to reject the idea of magmatic activity controlled by plate tectonics during the Archaean and, by extension, during the preceding Hadean. What is truly remarkable is that if mafic andesites – such as those implied by the Jack Hills zircons and found in the Nuvvuagittuq greenstone belt – partially melted under high pressures that formed garnet in them, they would have yielded magmas of TTG composition. This, it seems, puts plate tectonics in the frame for the whole of Earth’s evolution since it stabilised several million years after the catastrophic collision that flung off the Moon and completely melted the outer layers of our planet. Up to now, controversy about what kind of planet-wide processes operated then have swung this way and that, often into quite strange scenarios. Turner and colleagues may have opened a new, hopefully more unified, episode of geochemical studies that revisit the early Earth . It could complement the work described in An Early Archaean Waterworld published on Earth-logs earlier in March 2020.

An Early Archaean Waterworld

In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.

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

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.

Should you worry about being killed by a meteorite?

In 1994 Clark Chapman of the Planetary Science Institute in Arizona and David Morrison of NASA’s Ames Research Center in California published a paper that examined the statistical hazard of death by unnatural causes in the United States (Chapman, C. & Morrison, D. 1994. Impacts on the Earth by asteroids and comets: assessing the hazard. Nature, v. 367, p. 33–40; DOI:10.1038/367033a0). Specifically, they tried to place the risk of an individual being killed by a large asteroid (~2 km across) hitting the Earth in the context of more familiar unwelcome causes. Based on the then available data about near-Earth objects – those whose orbits around the Sun cross that of the Earth – they assessed the chances as ranging between 1 in 3,000 and 1 in 250,000; a chance of 1 in 20,000 being the most likely. The results from their complex calculations turned out to be pretty scary, though not as bad as dying in a car wreck, being murdered, burnt to death or accidentally shot. Asteroid-risk is about the same as electrocution, at the higher-risk end, but significantly higher than many other causes with which the American public are, unfortunately, familiar: air crash; flood; tornado and snake bite. The lowest asteroid-risk (1 in 250 thousand) is greater than death from fireworks, botulism or trichloroethylene in drinking water; the last being 1 in 10 million.

Chapman and Morrison cautioned against mass panic on a greater scale than Orson Welles’s 1938 CBS radio production of H.G. Wells’s War of the Worlds allegedly resulted in. Asteroid and comet impacts are events likely to kill between 5,000 and several hundred million people each time they happen but they occur infrequently. Catastrophes at the low end, such as the 1908 Tunguska air burst over an uninhabited area in Siberia, are likely to happen once in a thousand years. At the high end, mass extinction impacts may occur once every hundred million years. As might be said by an Australian, ‘No worries, mate’! But you never know…

Michelle Knapp’s Chevrolet Malibu the morning after a stony-iron mmeteorite struck it. Bought for US$ 300, Michelle sold the car for US$ 25,000 and the meteorite fetched US$ 50,000 (credit: John Bortle)

How about ordinary meteorites that come in their thousands, especially when the Earth’s orbit takes it through the former paths taken by disintegrating comets? When I was a kid rumours spread that a motor cyclist had a narrow escape on the flatlands around Kingston-upon-Hull in East Yorkshire, when a meteorite landed in his sidecar: probably apocryphal. But Michelle Knapp of Peeskill, New York, USA had a job for the body shop when a 12 kg extraterrestrial object hit her Chevrolet Malibu, while it was parked in the driveway. In 1954, Ann Hodges of Sylacauga, Alabama was less fortunate during an afternoon nap on her sofa, when a 4 kg chondritic meteorite crashed through her house roof, hit a radiogram and bounced to smash into her upper thigh, badly bruising her. For an object that probably entered the atmosphere at about 15 km s-1, that was indeed a piece of good luck resulting from air’s viscous drag, the roof impact and energy lost to her radiogram. The offending projectile became a doorstop in the Hodge residence, before the family kindly donated it to the Alabama Museum of Natural History. Another fragment of the same meteorite, found in a field a few kilometres away, fetched US$ 728 per gram at Christie’s auction house in 2017. Perhaps the most unlucky man of the 21st century was an Indian bus driver who was killed by debris ejected when a meteorite struck the dirt track on which he was driving in Tamil Nadu in 2016 – three passengers were also injured. Even that is disputed, some claiming that the cause was an explosive device.

Extraterrestrial sugar

The coding schemes for Earth’s life and evolution (DNA and RNA), its major building blocks and basic metabolic processes have various sugars at their hearts. How they arose boils down to two possibilities: either they were produced right here by the most basic, prebiotic processes or they were supplied from interplanetary or interstellar space. All kinds of simple carbon-based compounds turn up in spectral analysis of regions of star formation, or giant molecular clouds: CN, CO, C­2H, H2CO up to 10 or more atoms that make up recognisable compounds such as benzonitrile (C6H5CN). Even a simple amino acid (glycene –CH2NH2COOH) shows up in a few nearby giant molecular clouds. Brought together in close proximity, instead of dispersed through huge volumes of near-vacuum, a riot of abiotic organic chemical reactions could take place. Indeed, complex products of such reactions are abundant in carbonaceous meteorites whose parent asteroids formed within the solar system early in its formation. Some contain a range of amino acids though not, so far, the five bases on which genetics depends: in DNA adenine, cytosine, guanine and thymine (replaced by uracil in RNA). Yet, surprisingly, even simple sugars have remained elusive in both molecular clouds and meteorites.

Artist’s impression of the asteroid belt from which most meteorites are thougtht to originate (Credit: NASA/JPL)

A recent paper has broken through that particular barrier (Furukawa, Y. et al. 2019. Extraterrestrial ribose and other sugars in primitive meteorites. Proceedings of the National Academy of Sciences. Online; DOI: 10.1073/pnas.1907169116). Yoshihiro Furukawa and colleagues analysed three carbonaceous chondrites and discovered traces of 4 types of sugars. It seems that sugar compounds have remained elusive because those now detected are at concentrations thousands of times lower than those of amino acids. Contamination by terrestrial sugars that may have entered the meteorites when they slammed into soil is ruled out by their carbon isotope ratios, which are very different from those in living organisms. One of the sugars is ribose, a building block of RNA (DNA needs deoxyribose). Though a small discovery, it has great significance as regards the possibility that the components needed for living processes formed in the early Solar System. Moon formation by giant impact shortly after accretion of the proto-Earth would almost certainly have  destroyed such organic precursors. So, if the Earth’s surface was chemically ‘seeded’ in this way it is more likely to have occurred at a later time, perhaps during the Late Heavy Bombardment 4.1 to 3.8 billion years ago (see: Did mantle chemistry change after the late heavy bombardment? In Earth-logs September 2009)

Ordovician ice age: an extraterrestrial trigger

The Ordovician Period is notable for three global events; an explosion in biological diversity; an ice age, and a mass extinction. The first, colloquially known as the Great Ordovician Biodiversification Event, occurred in the Middle Ordovician around 470 Ma ago (see The Great Ordovician Diversification, September 2008) when the number of recorded fossil families tripled. In the case of brachiopods, this seems to have happened in no more than a few hundred thousand years. The glacial episode spanned the period from 460 to 440 Ma and left tillites in South America, Arabia and, most extensively, in Africa. Palaeogeographic reconstructions centre a Gondwanan ice cap in the Western Sahara, close to the Ordovician South Pole. It was not a Snowball Earth event, but covered a far larger area than did the maximum extent the Pleistocene ice sheets in the Northern Hemisphere. It is the only case of severe global cooling bracketing one or the ‘Big Five’ mass extinctions of the Phanerozoic Eon. In fact two mass extinctions during the Late Ordovician rudely interrupted the evolutionary promise of the earlier threefold diversification, by each snuffing-out almost 30% of known genera.

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L-chondrite meteorite in iron-stained Ordovician limestone together with a nautiloid (credit: Birger Schmitz)

A lesser-known feature of the Ordovician Period is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in sedimentary rocks, particularly those exposed around the Baltic Sea (Schmitz, B. and 19 others 2019. An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science Advances, v. 5(9), eaax4184; DOI: 10.1126/sciadv.aax4184). Yet there is not a sign of any major impact of that general age, and the meteoritic anomaly occupies a 5 m thick sequence at the best studied site in Sweden, representing about 2 Ma of deposition, rather than the few centimetres at near-instantaneous impact horizons such as the K-Pg boundary. Intact meteorites are almost exclusively L-chondrites dated at around 466 Ma. Schmitz and colleagues reckon that the debris represents the smashing of a 150 km-wide asteroid in orbit between Mars and Jupiter. Interestingly, L-chondrites are more abundant today and in post-Ordovician sediments than they were in pre-Ordovician records, amounting to about a third of all finds. This suggests that the debris is still settling out in the Inner Solar System hundreds of million years later. Not long after the asteroid was smashed a dense debris cloud would have entered the Inner Solar System, much of it in the form of dust.

The nub of Schmitz et al’s hypothesis is that considerably less solar radiation fell on Earth after the event, resulting in a sort of protracted ‘nuclear winter’ that drove the Earth into much colder conditions. Meteoritic iron falling the ocean would also have caused massive phytoplankton blooms that sequestered CO2 from the Ordovician atmosphere to reduce the greenhouse effect. Yet the cooling seems not to have immediately decimated the ‘booming’ faunas of the Middle Ordovician. Perhaps the disruption cleared out some ecological niches, for new species to occupy, which may explain sudden boosts in diversity among groups such as brachiopods. Two sharp jumps in brachiopod species numbers are preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains in one Middle Ordovician sequence. One possibility, suggested in an earlier paper (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53; DOI: 10.1038/ngeo.2007.37)  is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).

See also: Sokol, J. 2019. Dust from asteroid breakup veiled and cooled Earth. Science, v. 365, pp. 1230: DOI: 10.1126/science.365.6459.1230, How the first metazoan mass extinction happened (Earth-logs, May 2014)