Category Archives: Planetary, extraterrestrial geology, and meteoritics

A major Precambrian impact in Scotland

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol

Simplified geological map of NW Scotland (credit: British Geological Survey)

One such mystery lies close to the base of the vast pile of reddish Torridonian sandstones, the Stac Fada Member of the Stoer Group. Beneath it is a common-or-garden basal breccia full of debris from the underlying Lewisian Complex, then red sandstones and siltstones deposited by a braided river system. The Stac Fada Member is a mere 10 m thick, but stretches more than 50 km along the regional NNE-SSW  strike. It comprises greenish to pink sandstones with abundant green, glassy shards and clasts, previously thought to be volcanic in origin, together with what were initially regarded as volcanic spherules – the results of explosive reaction of magma when entering groundwater or shallow ponds. Until 2002, that was how ideas stood. More detailed sedimentological and geochemical examination found quartz grains with multiple lamellae evidencing intense shock, anomalously high platinum-group metal concentrations and chromium isotopes that were not of this world. Indeed, the clasts and the ensemble as a whole look very similar to the ‘suevites’ around the 15 Ma old Ries Impact crater in Germany. The bed is the product of mass ejection from an impact, a designation that has attracted great attention. In 2015 geophysicists suggested that the impact crater itself may coincide with an isolated gravity low about 50 km to the east. A team of 8 geoscientists from the Universities of Oxford and Exeter, UK, have recently reported their findings and ideas from work over the last decade. (Amor, K, et al. 2019. The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry. Journal of the Geological Society, online; DOI: 10.1144/jgs2018-093).

The age of the Stac Fada member is around 1200 Ma, determined by Ar-Ar dating of K-feldspar formed by sedimentary processes. Geochemistry of Lewisian gneiss clasts compared with in situ basement rocks, magnetic data from the matrix of the deposit, and evidence of compressional forces restricted to it suggest that the debris emanated from a site to the WNW of the midpoint of the member’s outcrop. Rather than being a deposit from a distant source, carried in an ejecta curtain, the Stac Fada material is more akin to that transported by a volcanic pyroclastic flow. That is, a dense, incandescent debris cloud moving near to the surface under gravity from the crater as ejected material collapsed back to the surface. On less definite grounds, the authors suggest that a crater some 13 to 14 km across penetrating about 3 km into the crust may have been involved.

Together with evidence that I described in Impact debris in Britain (Magmatism February 2018) and Britain’s own impact  (Planetary Science November 2002) it seems that Britain has directly witnessed three impact events. But none of them left a tangible crater.

Earth’s water and the Moon

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

Where did all our water come from? The Earth’s large complement of H2O, at the surface, in its crust and even in the mantle, is what sets it apart in many ways from the rest of the rocky Inner Planets. They are largely dry, tectonically torpid and devoid of signs of life. For a long while the standard answer has been that it was delivered by wave after wave of comet impacts during the Hadean, based on the fact that most volatiles were driven to the outermost Solar System, eventually to accrete as the giant planets and the icy worlds and comets of the Kuiper Belt and Oort Cloud, once the Sun sparked its fusion reactions That left its immediate surroundings depleted in them and enriched in more refractory elements and compounds from which the Inner Planets accreted. But that begs another question: how come an early comet ‘storm’ failed to ‘irrigate’ Mercury, Venus and Mars? New geochemical data offer a different scenario, albeit with a link to the early comet-storms paradigm.

earth-moon

Simulated view of the Earth from lunar orbit: the ‘wet’ and the ‘dry’. (credit: Adobe Stock)

Three geochemists from the Institut für Planetologie, University of Münster, Germany, led by Gerrit Budde have been studying the isotopes of the element molybdenum (Mo) in terrestrial rocks and meteorite collections. Molybdenum is a strongly siderophile (‘iron loving’) metal that, along with other transition-group metals, easily dissolves in molten iron. Consequently, when the Earth’s core began to form very early in Earth’s history, available molybdenum was mostly incorporated into it. Yet Mo is not that uncommon in younger rocks that formed by partial melting of the mantle, which implies that there is still plenty of it mantle peridotites. That surprising abundance may be explained by its addition along with other interplanetary material after the core had formed. Using Mo isotopes to investigate pre- and post-core formation events is similar to the use of isotopes of other transition metals, such as tungsten (seePlanetary science, May 2016).

Budde and colleagues showed that the 95Mo and 94Mo abundances in water- and carbon-poor meteorites that come from the Asteroid Belt and formed in the inner Solar System differ consistently from those in volatile-rich carbonaceous chondrites that formed much further away from the Sun. The average abundances of the two molybdenum isotopes in the Earth’s silicate rocks, which ultimately had their origin in the mantle, fall between those of the two classes of meteorites (Budde, G. et al.  2019. Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy, v. 3, online ; DOI: 10.1038/s41550-019-0779-y). They must reflect the materials that accreted after core formation. If the 95Mo and 94Mo abundances resembled those in non-carbonaceous, dry meteorites that would suggest late accretion with much the same composition as expected from Earth’s position in the Inner Solar System. Alternatively, some molybdenum from Earth’s original formative materials failed to unite with iron in the core. The Mo ‘signature’ of volatile-rich carbonaceous meteorites in the mantle’s make-up points to a large amount of accreting material from the Outer Solar System. In contrast, lunar rocks show no carbonaceous meteorite component of Mo isotopes, which helps to explain its overall dryness compared with the Earth. Yet, the Moon is strongly believed to have formed from material blasted away by an impact between the proto-Earth and an errant, Mars-sized body (Theia).

The authors suggest a high probability that Theia was a carbon- and volatile-rich body from the outer Solar System flung inwards by gravitational perturbation associated with the then unstable orbits of the giant planets Jupiter and Saturn. In that case Theia could have delivered not only the anomalous molybdenum, but most of Earth’s water and other volatile compounds.   If the theory is correct, then the cataclysmic event that formed the Moon laid the basis for Earth’s continual tectonic activity and its eventually sparking up life; without the Moon, there would be no life on Earth. That kind of chance event isn’t a factor considered in either the Drake Equation or the Goldilocks Zone. Life, natural selection and sentient beings that might spring from them may be a great deal more elusive than commonly believed by exobiologists.

See also: Formation of the moon brought water to Earth (Science Daily, 21 May 2019)

Chang’E-4 and the Moon’s mantle

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

The spacecraft Chang’E-4 landed on the far side of the Moon in January; something of a triumph for the Peoples’ Republic of China as it was a first. It was more than a power gesture at a time of strained relations between the PRC and the US, for it carried a rover (Yutu2) that deploys a panoramic camera, ground penetrating radar, means of assessing interaction of the solar wind with the lunar surface, and a Visible and Near-infrared Imaging Spectrometer (VNIS). The lander module itself bristles with instrumentation, but Yutu2 (meaning Jade Rabbit) has relayed the first scientific breakthrough.

ChangE

Variation in topography (blue – low to red – high) over the Moon’s South Pole, showing the Aitken Basin and the Chang’E landing site. (Credit: NASA/Goddard)

The landing site is within the largest impact structure on the Moon, the 2500 km-wide Aitken Basin. Unlike the near-side maria, Aitken has only a small masking veneer of flood basalts that formed by internal melting resulting from the mare-forming impacts. Instead it is surrounded by the heavily cratered lunar crust of the Highlands made of calcium-rich plagioclase feldspar, i.e. anorthosite. Within the Aitken Basin lies the 930 km Orientale impact structure. The dark colour of the massive basin contrasts with the highly reflective nature of the Highlands and, in the absence of a basalt veneer, suggests that impacts penetrated the lunar crust to fling mantle material across the surface. The Chang’E landing site therefore offered a chance to examine samples of the Moon’s mantle for the first time – none of the samples returned by the Apollo programme of the 196Os and 70s are of such material.

While Chang’E is not equipped for sample return, the Jade Rabbit’s VNIS is capable of supplying information bearing on the minerals strewn across the basin. The instrument detects reflected radiation from the 450 to 2400 nm wavelength range split into many narrower channels, thereby reconstructing detailed spectra. These can be matched with reference spectra of a large range of minerals. The first results reveal the presence of the minerals olivine ((Mg,Fe)SiO4) and orthopyroxene ((Mg,Fe)Si2O6) in the lunar soil close to the lander, both of which could be from the Moon’s mantle (Li, C. and 16 others 2019. Chang’E-4 initial spectroscopic identification of lunar far-side mantle-derived materials. Nature, v. 569, p. 378–382; DOI: 10.1038/s41586-019-1189-0). Such material may represent the denser, mafic crystalline products of a magma ocean through which they sank, while lower density feldspar floated to the surface to form the Moon’s highly reflective crust.

While the spectral signature of olivine has been detected by similar instruments on satellites in lunar orbit, such results stemmed from broad areas of mixed materials. The Jade Rabbit’s discoveries can be related to actual rock fragments.

Related article: Pinet, P. 2019. The Moon’s mantle unveiled. Nature, v. 569, p. 338-339; DOI: 10.1038/d41586-019-01479-x

A bad day at the end of the Cretaceous

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

The New Yorker magazine normally features journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. So it is odd that this Condé Nast glossy for the chattering classes snaffled online what may be the geological scoop of the 21st century so far (Preston, D. 2019. The day the dinosaurs died. The New Yorker 8 April 2019 issue). The paper that lies at the centre of the story had not been published and nor had the issue of The New Yorker in which Douglas Preston’s story was scheduled for publication. The very day (29 March 2019) that Britain was thwarted of its Brexit moment the world’s media was frothing with news about the end of another era; the Mesozoic. The paper itself was published online on April Fools’ Day with a title that is superficially arcane (DePalma, R.A. and 11 others 2019. A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Science, early online publication;p DOI: 10.1073/pnas.1817407116). But its contents are the stuff of dreams for any aspiring graduate student of palaeontology; the Indiana Jones opportunity.

An ‘onshore surge deposit’ occurs at many Western Hemisphere sites where the K-Pg boundary outcrops in terrestrial or shallow-marine sediments. The closer to the Chicxulub crater north of Mexico’s Yucatan Peninsula the more obvious they are, for they result from the tsunamis that immediately followed the asteroid impact. Lead author Robert DePalma, now of the University of Kansas, became focussed on the dinosaur-rich, Late Cretaceous Hell Creek Formation of North Dakota as an undergraduate. Accepted for graduate studies he was directed to a project on the fauna of lacustrine sediments close to the K-Pg boundary layer, which is well-known in the area, and that’s what he has been engaged with ever since. In 2012 he was guided to a remarkable locality by a rockhound, disappointed because it exposed extremely fossil-rich sediments but was so soft that none could be extracted intact with a hammer and chisel. It turned out to have resulted from a surge along a sinuous river that had washed debris onto a point-bar deposit at the inside of a meander. The debris includes remains of both marine and terrestrial organisms and shows clear signs of having been swept upriver, i.e. from the sea and possibly the result of a tsunami. Being capped by a thin, iridium-rich layer of impactite, the 1.5 metre surge deposit is part of the K-Pg boundary layer, and probably represented only a few hours before being blanketed by ejecta.

This Event Deposit comprises two graded, fining-upwards units and thus two distinct surges, with a thin mat of vegetation fragments immediately below the Ir-rich clay cap that also contains sparse shocked quartz grains. The Event Deposit contains altered glass spherules throughout, which cgradually become smaller higher in the 1.5 m sequence. Some of the larger spherules produced ‘micro-craters’ in the sediments. Fossils include marine ammonite fragments (some still nacreous) and freshwater fish (paddlefish and sturgeon). The fish are so complete as to suggest an absence of scavengers. The paper itself contains little of the information that dominated Preston’s New Yorker article and the global media coverage. This included clear evidence that the fish ingested spherules, found clogging their gills and possible causing their death. There are examples of spherules embedded in amber formed from plant sap, which suggests sub-aerial fall of ejecta, and among the marine faunal samples are teeth of fish and reptiles (see DePalma et al’s Supplemental Data). The most startling finds reported by Preston are nowhere to be found in DePalma et al’s paper or its supplement. These include possible dinosaur feathers; a fragment of ceratopsian dinosaur skin attached to a hip bone; a burrow containing a mammal jaw that penetrates the K-Pg boundary layer; dinosaur remains, including an egg (complete with embryo) and hatchlings of dinosaurian groups found at deeper levels in the Hell Creek Formation. Previously, palaeontologists had found no dinosaur remains less than 3 m below the K-Pg boundary layer anywhere on Earth, prompting the suggestion that they had become extinct before the near-instantaneous effects of Chicxulub, and were perhaps victims of the general effects of the Deccan Trap volcanism. If verified in later peer-reviewed publications, DePalma et al’s work would help resolve the gradual vs sudden hypotheses for the end-Cretaceous mass extinction.

gill spherules

X-ray and CT images of impact spherules in the gills of a fossil sturgeon from the Tanis K-Pg site, North Dakota (credit DePalma et al. 2019; Fig. 6)

Preston reports some academic scepticism about DePalma’s work, and emphasises his showmanship at conferences; for instance, he named the site ‘Tanis’ after the ancient city in Egypt featured in the 1981 film Raiders of the Lost Ark. There are geophysical queries too. If the inundation was by the on-shore effects of a tsunami it doesn’t tally with the abundance of ejecta fallout of glass spherules: tsunamis propagate in shallow seawater at speeds less than 50 km h-1  and more slowly still in channels, whereas impact ejecta travel much faster. This is acknowledged in the paper’s supplement, and the paper refers to a seiche wave activated by seismic waves associated with the Chicxulub impact which could have arrived in North Dakota at about the same time as its ejecta blanket. The paper’s authorship includes the imprimatur of other authorities in different geoscientific fields, including Walter Alvarez, jointly famed with his father Luis for the discovery of the K-Pg boundary horizon and its impact connections in 1981. So it carries considerable weight. No doubt further comment and further papers on the Tanis site will emerge: DePalma has yet to complete his PhD. It may become the lagerstätte of the K-Pg extinction; in DePalma’s words ‘It’s like finding the Holy Grail clutched in the bony fingers of Jimmy Hoffa, sitting on top of the Lost Ark.’ …

Read more on Palaeobiology and Impacts

A unifying idea for the origin of life

The nickel in stainless steel, the platinum in catalytic converters and the gold in jewellery, electronic circuits and Fort Knox should all be much harder to find in the Earth’s crust. Had the early Earth formed only by accretion and then the massive chemical resetting mechanism of the collision that produced the Moon all three would lie far beyond reach. Both formation events would have led to an extremely hot young Earth; indeed the second is believed to have left the outer Earth and Moon completely molten. All three are siderophile metals and have such a strong affinity for metallic iron that they would mostly have been dragged down to each body’s core as it formed in the early few hundred million years of the Earth-Moon system, leaving very much less in the mantle than rock analyses show. This emerged as a central theme at the Origin of Life Conference held in Atlanta GA, USA in October 2018. The idea stemmed from two papers published in 2015 that reported excessive amounts in basaltic material from both Earth and Moon of a tungsten isotope (182W) that forms when a radioactive isotope of hafnium (182Hf), another strongly siderophile metal, decays. Hafnium too must have been strongly depleted in the outer parts of both bodies when their cores formed. The excesses are explained by substantial accretion of material rich in metallic iron to their outer layers shortly after Moon-formation, some being in large metallic asteroids able to penetrate to hundreds of kilometres. Hot iron is capable of removing oxygen from water vapour and other gases containing oxygen, thereby being oxidised. The counterpart would have been the release of massive amounts of hydrogen, carbon and other elements that form gases when combined with oxygen. The Earth’s atmosphere would have become highly reducing.

Had the atmosphere started out as an oxidising environment, as thought for many decades, it would have posed considerable difficulties for the generation at the surface of hydrocarbon compounds that are the sine qua non for the origin of life. That is why theories about abiogenesis (life formed from inorganic matter) hitherto have focussed on highly reducing environments such as deep-sea hydrothermal vents where hydrogen is produced by alteration of mantle minerals. The new idea revitalises Darwin’s original idea of life having originated in ‘a warm little pond’. How it has changed the game as regards the first step in life, the so-called ‘RNA World’ can be found in a detailed summary of the seemingly almost frenzied Origin of Life Conference (Service, R.F. 2019. Seeing the dawn. Science, v. 363, p. 116-119; DOI: 10.1126/science.363.6423.116).

Isotope geochemistry has also entered the mix in other regards, particularly that gleaned from tiny grains of the mineral zircon that survived intact from as little as 70 Ma after the Moon-forming and late-accretion events to end up (3 billion years ago) in the now famous Mount Narryer Quartzite of Western Australia. The oldest of these zircons (4.4 Ga) suggest that granitic rocks had formed the earliest vestiges of continental crust far back in the Hadean Eon: Only silica-rich magmas contain enough zirconium for zircon (ZrSiO4) to crystallise. Oxygen isotope studies of them suggest that at that very early date they had come into contact with liquid water, presumably at the Earth’s surface. That suggests that perhaps there were isolated islands of early continental materials; now vanished from the geological record. A 4.1 Ga zircon population revealed something more surprising: graphite flakes with carbon isotopes enriched in 12C that suggests the zircons may have incorporated carbon from living organisms.

407458aa.2

A possible timeline for the origin of life during the Hadean Eon (Credit: Service, R.F. 2019, Science)

Such a suite of evidence has given organic chemists more environmental leeway to suggest a wealth of complex reactions at the Hadean surface that may have generated the early organic compounds needed as building blocks for RNA, such as aldehydes and sugars (specifically ribose that is part of both RNA and DNA), and the amino acids forming the A-C-G-U ‘letters’ of RNA, some catalysed by the now abundant siderophile metal nickel. One author seems gleefully to have resurrected Darwin’s ‘warm little pond’ by suggesting periodic exposure above sea level of abiogenic precursors to volcanic sulfur dioxide that could hasten some key reactions and create large masses of such precursors which rain would have channelled into ‘puddles and lakes’. The upshot is that the RNA World precursor to the self-replication conferred on subsequent life by DNA is speculated to have been around 4.35 Ga, 50 Ma after the Earth had cooled sufficiently to have surface water dotted with specks of continental material.

There are caveats in Robert Services summary, but the Atlanta conferences seems set to form a turning point in experimental palaeobiology studies.

Read more on Palaeobiology and Planetary science

Impacts increased at the end of the Palaeozoic

Because it is so geologically active the Earth progressively erases signs of asteroid and comet impacts, by erosion, burial or even subduction in the case of the oceanic record. As a result, the number of known craters decreases with age. To judge the influence of violent extraterrestrial events in the past geologists therefore rely on secondary outcomes of such collisions, such as the occasional presence in the sedimentary record of shocked quartz grains, glassy spherules and geochemical anomalies of rare elements. The Moon, on the other hand, is so geologically sluggish that its surface preserves many of the large magnitude impacts during its history, except for those wiped out by later such events. For instance, a sizeable proportion of the lunar surface comprises its dark maria, which are flood basalts generated by gigantic impacts around 4 billion years ago. Older impacts can only be detected in its rugged, pale highland terrains, and they have been partially wiped out by later impact craters. The Moon’s surface therefore preserves the most complete record of the flux and sizes of objects that have crossed its orbit shared with the Earth.

The Earth presents a target thirteen times bigger than the cross sectional area of the Moon so it must have received 13 times more impacts in their joint history.  Being about 81 times as massive as the Moon its stronger gravitational pull will have attracted yet more and all of them would have taken place at higher speeds. The lunar samples returned by the Apollo Missions have yielded varying ages for impact-glass spherules so that crater counts combined with evidence for their relative ages have been calibrated to some extent to give an idea of the bombardment history for the Earth Moon System. Until recently this was supposed to have tailed off exponentially since the Late Heavy Bombardment between 4.0 to 3.8 billion years ago. But the dating of the lunar record using radiometric ages of the small number of returned samples is inevitably extremely fuzzy. A team of planetary scientists from Canada, the US and Britain has developed a new approach to dating individual crater using image data from NASA’s Lunar Reconnaissance Orbiter (LRO) launched in 2009 (Mazrouei, S. et al. 2019. Earth and Moon impact flux increased at the end of the Paleozoic. Science, v. 363, p. 253-257; DOI: 10.1126/science.aar4058).

The method that they devised is, curiously, based on thermal imagery from the LRO’s Diviner instrument which records the Moon’s surface temperature. Comparison of day- and night-time temperatures produces a measure of surface materials’ ability to retain heat known as thermal inertia. A material with high thermal inertia stays warmer for longer at night. When a crater forms it partly fills with rock fragments excavated by the impact. When fresh these are full of large blocks of rock that were too massive to be blasted away. But these blocks are exposed to bombardment by lesser projectiles for the lifetime of the crater, which steadily reduces them to smaller fragments and eventually dust. Blocks of solid rock retain significantly more solar heat than do gravelly to dust-sized materials:  thermal inertia of the crater floor therefore decreases steadily with age.

407458aa.2

Blocky surface of a relatively young lunar crater (Credit: NASA)

As well as day- and night thermal data provided by the Diviner instrument, from which thermal inertia values are calculated, the LRO deploys two cameras that capture black and white images of the surface in the visible range, with a resolution of about a metre. They enable the blockiness of crater floors to be estimated. Sara Mazrouei and colleagues measured blockiness and thermal inertia of the floors of 111 craters more than 10 km across, ages of nine of which had been modelled independently using counts of smaller craters that subsequently accumulated on their floors shown by even finer resolution images from the Japanese Kaguya orbiter. Their results are surprising. About 290 Ma ago the rate of large impacts on the Moon increased by a factor of 2.6. This might explain why the Neoproterozoic and Palaeozoic Eras are deficient in terrestrial craters. Another inference from the results is that the number of objects in Earth-crossing orbits suddenly increased at the end of the Carboniferous. Maybe that resulted from an episode of collisions and break-up of large bodies in the Asteroid Belt or, perhaps, some kind of gravitational perturbation by Jupiter. The age-distribution of large craters on Earth is no help because of their ephemeral nature. Moreover, apart from Chicxulub that is bang on the K-Pg boundary, there is little evidence of an increase in impact-driven mass extinctions in the Mesozoic and Cenozoic. Nor for that matter did igneous activity or sediment deposition undergo any sudden changes. There are sediments that seem to have formed as a result of tsunami devastation, but none greater in magnitude than could have been caused by major earthquakes. Or … maybe geologists should have another look at the stratigraphic record.

Read more on Planetary science

Subglacial impact structure: trigger for Younger Dryas?

Radar microwaves are able to penetrate easily through several kilometres of ice. Using the arrival times of radar pulses reflected by the bedrock at glacial floor allows ice depth to be computed. When deployed along a network of flight lines during aerial surveys the radar returns of large areas can be converted to a grid of cells thereby producing an image of depth: the inverse of a digital elevation model. This is the only means of precisely mapping the thickness variations of an icecap, such as those that blanket Antarctica and Greenland. The topography of the subglacial surface gives an idea of how ice moves, the paths taken by liquid water at its base, and whether or not global warming may result in ice surges in parts of the icecap. The data can also reveal topographic and geological features hidden by the ice (see The Grand Greenland Canyon September 2013).

Untitled-2

Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Such a survey over the Hiawatha Glacier of NW Greenland has showed up something most peculiar (Kjaer, K.H. and 21 others 2018. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances, v. 4, eaar8173; DOI: 10.1126/sciadv.aar8173). Part of the ice margin is an arc, which suggests the local bed topography takes the form of a 31km wide, circular depression. The exposed geology shows no sign of a structural control for such a basin, and is complex metamorphic basement of Palaeoproterozoic age. Measurements of ice-flow speeds are also anomalous, with an array of higher speeds suggesting accelerated flow across the depression. The radar image data confirm the presence of a subglacial basin, but one with an elevated rim and a central series of small peaks. These are characteristic of an impact structure that has only been eroded slightly; i.e. a fairly recent one and one of the twenty-five largest impact craters on Earth.. Detailed analysis of raw radar data in the form of profiles through the ice reveals  that the upper part is finely layered and undisturbed. The layering continues into the ice surrounding the basin and is probably of Holocene age (<11.7 ka), based on dating of ice in cores through the surrounding icecap. The lower third is structurally complex and shows evidence for rocky debris. Sediment deposited by subglacial streams where they emerge along the arcuate rim contain grains of shocked quartz and glass, as well as expected minerals from the crystalline basement rocks. Some of the shocked material contains unusually high concentrations of transition-group metals, platinum-group elements and gold; further evidence for impact of extraterrestrial material – probably an iron asteroid that was originally more than 1 km in diameter. The famous Cape York iron meteorite, which weighs 31 t – worked by local Innuit to forge harpoon blades – fell in NW Greenland about 200 km away.

The central issue is not that Hiawatha Glacier conceals a large impact crater, but its age. It certainly predates the start of the Holocene and is no older than the start of Greenland glaciation about 2.6 Ma ago. That only Holocene ice layers are preserved above the disrupted ice that rests immediately on top of the crater raises once again the much-disputed possibility of an asteroid impact having triggered the Younger Dryas cooling event and associated extinctions of large mammals in North America at about 12.9 ka (see Impact cause for Younger Dryas draws flak May 2008). Only radiometric dating of the glassy material found in the glaciofluvial sediments will be able to resolve that particular controversy.

A hint of life on Mars

We can be certain that life was around on Planet Earth around 3.5 billion years ago, if not before, because unmetamorphosed sedimentary rocks of that age from Western Australia in which stromatolites occur contain a black to brownish, structureless material known as kerogen. The material is a hodgepodge of organic compounds that form during the breakdown of proteins and carbohydrates in living matter. It is the source material for petroleum compounds when kerogen-rich rocks are heated during burial. The vast bulk of organic compounds preserved on Earth are in the form of ancient kerogen, whose mass exceeds that of the living biosphere by about 10 thousand times. A good sign that it does represent ancient life lies in sedimentary kerogen’s depletion in ‘heavy’ 13C compared with 12C (negative values of δ13C), because in metabolising carbon dioxide living cells preferentially use the lighter of these two isotopes. Conceivably, 13C can be removed from inorganic carbon by metamorphic processes, so low values of δ13C in metasediments from West Greenland might be organically derived or, equally, they might not.

At the time of writing, geoscientists specialising in Martian matters had become excited by some results from the geochemical system aboard the surviving functional NASA rover. Curiosity has slowly been making its way up Mount Sharp at the centre of Gale Crater near to Mars’s equator. Analysis of high-resolution images taken from orbit suggest that the rocks forming the mountain are sediments. the lowest and oldest strata are suspected to have been deposited in a crater lake when conditions were warmer and wetter on Mars, about 3 billion years ago. Curiosity was equipped with a drill to penetrate and sample sediment unaffected by ultraviolet radiation that long ago would have destroyed any hydrocarbons exposed at the surface. In late 2016, before the rover had reached the lake sediments, the drill’s controller broke down. Since then, Curiosity had moved on to younger, less promising sediments. More than a year later mission engineers fixed the problem and the rover backtracked to try again. Heating the resulting samples to almost 900°C yields any volatile components as a gas to a mass spectrometer, results from which give clues to the molecules released.

‘Selfie’ of Curiosity rover en route to Mount Sharp. (Credit: NASA)

The Sample Analysis at Mars (SAM) team report a range of thiophenic, aromatic and aliphatic molecules of compounds of carbon, hydrogen and sulfur (Eigenbrode, J.L and 21 others 2018. Organic matter preserved in 3-billion-year-old mudtsones at Gale crater, Mars. Science, v. 360, p. 1096-1101; doi:10.1126/science.aas9185). The blend of pyrolysis products closely resembles those which form from heated terrestrial kerogens and coals, but also from pyrolysis of carbonaceous chondrite meteorites. So, the presence of Martian kerogen is not proven. But the results are so promisingly rich in hydrocarbons that another weapon in SAM’s armoury will be deployed, dissolving organic compounds directly from the drill cuttings. This may provide more convincing evidence of collagen. Yet only when samples are returned to labs on Earth will there be a chance to say one way or the other that there was once life on Mars. The results reported in Science’s 8 June issue will surely add weight to the clamour for the Mars 2020 sample-return mission to be funded. Whether or not there is life on Mars demands a great deal more investment still…

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Impact debris in Britain

These days reports of geological evidence for asteroid impacts are not regarded with a mixture of disbelief, wonder and foreboding: well, not by geologists anyway. But for such a small area as Britain now to have three of widely different ages and in easily accessible places is pretty good for its brand as the place to visit for practically every aspect of Earth history. The first to be discovered lies at the base of Triassic mudstones near Bristol (see Britain’s own impact) and would need some serious grubbing around at a former construction site. The next to emerge was located in one of the best geological districts in the country at several easily accessed coastal exposures in Northwest Scotland. A glass-rich ejecta layer occurs in the basal Torridonian Stoer Group on Stac Fada, Stoer, Sutherland (UK National Grid Reference 203300, 928400). The most recently found (Drake, S.N. and 8 others 2018. Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland. Geology, v. 46, p. 171-174; doi:10.1130/G39452.1) is on the Inner Hebridean island of Skye at the base of its famous Palaeocene flood basalt sequence (UK National Grid Reference 155371,821112).

View to the northwest across Loch Slapin to the Cuillin Hills of Skye (Central Igneous Complex). The flood basalts beneath which the ejecta layer occurs are just above the trees. (Credit: Wikipedia)

The last is perhaps the most spectacular of the three, as it contains the full gamut of provenance, matched only by material from the drill core into the 66 million year-old Chicxulub crater. The 0.9 m thick debris layer rests directly on mid-Jurassic sandstones beneath Palaeocene basalts of the North Atlantic Igneous Province (NAIP). The layer contains a basalt clast dated at 61.54 Ma, but is dominantly reminiscent of a pyroclastic ignimbrite flow as it contains glass shards. But there the resemblance ends for the bulk of small clasts are of quartz and K-feldspar, sandstone and gneiss. Zircons extracted from the debris show shock lamellae and give Archaean and Proterozoic ages commensurate with the local basement, but also with the bulk of the Scandinavian and Canadian Shields. So the impact could have been anywhere in such widespread terrains, although the enclosed basalt narrows this down to areas where basement is overlain by lavas of the NAIP. The Skye impactite contains unmelted meteorite fragments in the form of titanium nitrides alloyed with vanadium and niobium, metallic iron-silicon alloy containing exsolved carbon, and manganese sulfide.

Although it may be coincidental, the situation of the ejecta layer immediately beneath the Skye lavas, its containing a clast of basalt whose age corresponds to the oldest flows anywhere in the NAIP is fascinating. But the actual impact site is, as yet, unknown. Even so, the layer provokes thoughts about whether an impact may have been more than spatially related to the large NAIP flood basalt pile, preserved on either side of the North Atlantic. If the event was large, then surely the ejecta should be preserved near the base of the flood basalts elsewhere in NW Britain and further afield

Hadean potentially fertile for life

The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km2) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.