Tag Archives: Mercury

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

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Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

Planet Mercury and giant collisions

Full-color image of from first MESSENGER flyby

Mercury’s sun-lit side from first MESSENGER flyby (credit: Wikipedia)

Mercury is quite different from the other three Terrestrial Planets, having a significantly higher density. So it must have a considerably larger metallic core than the others – estimated to make up about 70% of Mercury’s mass – and therefore has a far thinner silicate mantle. The other large body in the Inner Solar System, our Moon, is the opposite, having the greatest proportion of silicate mantle and a small core.

The presently favoured explanation for the Moon’s anomalous mass distribution is that it resulted from a giant collision between the proto-Earth and a Mars-sized planetary body. Moreover, planetary theorists have been postulating around 20 planetary ‘embryos’ in the most of which accreted to form Venus and Earth, the final terrestrial event being the Moon-forming collision, with smaller Mars and Mercury having been derived from the two remaining such bodies. For Mercury to have such an anomalously large metallic core has invited mega-collision as a possible cause, but with such a high energy that much of its original complement of silicate mantle failed to fall back after the event. Two planetary scientists from the Universities of Arizona, USA, and Berne, Switzerland, have modelled various scenarios for such an origin of the Sun’s closest companion (Asphaug, E. & Reuffer, A. 2014. Mercury and other iron-rich planetary bodies as relics of inefficient accretion. Nature Geoscience, published online, doi: 10.1038/NGEO2189).

Their favoured mechanism is what they term ‘hit-and-run’ collisions in the early Inner Solar System. In the case of Mercury, that may have been with a larger target planet that survived intact while proto-Mercury was blasted apart to lose much of it mantle on re-accretion. To survive eventual accretion into a larger planet the left-overs had to have ended up in an orbit that avoided further collisions. Maybe Mars had the same kind of lucky escape but one that left it with a greater proportion of silicates.

One possible scenario is that proto-Mercury was indeed the body that started the clock of the Earth-Moon system through a giant impact. Yet no-one will be satisfied with a simulation and some statistics. Only detailed geochemistry of returned samples can take us any further. The supposed Martian meteorites seem not to be compatible with such a model; at least one would expect there to have been a considerable stir in planetary-science circles if they were. For Mercury, it will be a long wait for a resolution by geochemists, probably yet to be conceived.

Mercury: sometimes a moist, organic-rich world

Full-color image of from first MESSENGER flyby

Full-colour image of Mercury from MESSENGER  (credit: NASA via Wikipedia)

Astronomers welcomed in 2013 by suggesting from Kepler spacecraft data that the Milky Way galaxy alone probably hosts at least a hundred billion extrasolar planets and that a potentially habitable world the size of Earth probably lies within 20 light years of ours (go.nature.com/pxgbbt). OK, so there are at least 10-15 planets out there for every person likely to be alive by the mid-21 century when the technology becomes available to judge whether or not any of them hold a shred of interest for a population facing worsening living conditions right here.

Mercury is closer and currently being peered at in considerable detail by NASA’s MESSENGER mission to the Sun’s closest planet. The venture seems to have justified itself – and probably JAXA/ESA’s forthcoming BepiColumbo to be launched in 2015, arriving in 2022 – by showing that the long suspected ‘cold traps’ at Mercury’s poles have indeed trapped something: ice and abundant organic debris (Neuman, G.A . and 10 others 2013. Bright and dark polar deposits on Mercury: evidence for surface volatiles. Science, v. 339, p. 296-300).

The planet is exceeding rough, having been hit by objects of all sizes yet possessing insufficient internal energy to repave itself. Its axis of rotation is at a right angles to Mercury’s orbital plane, much like that of the Moon, so its polar regions are perpetually short of solar radiation. Deeply shadows places have been measured by infrared radiometry to be as cold as 25 degrees above absolute zero. Any volatile materials that might have landed in them or condensed there from earlier atmospheres might seem likely to stay there indefinitely. Not quite so, for the most likely compound, water ice, can sublimate away (shift directly from the solid to vapour state). Nevertheless, remote sensing shows the north pole region to be somewhat mottled dark and light on shadowed poleward-facing surfaces. The properties of backscattered radar beams and detection of emitted neutrons are consistent with the bright areas being water ice (Lawrence, D.J. and 12 others 2013. Evidence for water ice near Mercury’s north pole from MESSENGER neutron spectrometer measurements. Science, v. 339, p. 292-296). First estimates give a total ice volume of around 10 to 1000 km3 compared with almost 3 million km3 in the Greenland ice cap.

It’s the dark stuff that sets Mercury apart from, say, the Martian or lunar poles, the idea being that comets or icy asteroids impacting Mercury would have delivered complex organic compounds as well as water ice. This would temporarily give otherwise airless Mercury an atmosphere of volatiles parts of which might condense in the perpetually shaded parts of the polar region. Sublimation of exposed ice would have left a residue rich in those organic compounds that eventually protected deeper ice from fading away with time.

Now, imagine how supremely excited exo-planet hunters would be if they picked up such signals from a truly far-off world.

Mercury: anything new?

Full color image of from first MESSENGER flyby

Mercury from an earlier MESSENGER fly-by. Image via Wikipedia

The Sun’s nearest planet, Mercury, seems odd in some ways; for instance, it has a proportionately larger metallic core than any other planet. That feature has led some to suggest that somehow most of any original silicate mantle was lost. One possibility is that its proximity to the Sun resulted in Mercury’s surface being ablated. Another looks to a huge collision with another body that tore off much of the mantle; similar to the event that the chemical commonality of the Earth and Moon suggests early in Earth history. Both processes should have left a distinct geochemical signature on the surface of Mercury: some kind of residue of solar ablation or evidence of fractional crystallisation of a magma ocean, such as the feldspar-rich lunar highlands that are probably formed of crystals that floated as such a planetary silicate melt cooled and evolved. The seeming strangeness of Mercury helped underpin a well-equipped un-crewed mission, going by the acronym MESSENGER, that finally settled into Mercury orbit in March 2011 after a planned ‘yo-yoing’ path since launch in August 2004 that took it back and forth between Earth, Mercury and Venus in its early stages. Early analysis of results from the now permanent orbit appeared in the 30 September 2011 issue of Science.

MESSENGER carries several remote sensing instruments: a stereo imaging device to map landforms, and topography; a laser altimeter to back the stereo imager; a visible to short-wave infrared spectrometer to map variations in surface spectra and minerals; gamma-ray spectrometry to map distributions of naturally radioactive isotopes and emissions from other elements triggered by high-energy cosmic ray bombardment; using the Sun as a source of gamma- and X-rays to cause a variety of elements to emit lower energy X-rays – a variant of X-ray fluorescence spectrometry that is a workhorse of lab geochemistry.

The earlier Mercury fly-bys and previous missions clearly showed that its surface is heavily cratered but possesses areas resurfaced by lavas that obliterate older cratering. A little like the lunar maria in age and appearance, these smooth terrains show evidence of accumulations up to a kilometre thick formed by repeated lava flows (Head, J.W. and 25 others, 2011. Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER. Science, v. 333, p. 1853-1855). As regards the age of these major volcanic features, all that can be said is that they post-date the largest impacts, such as the huge Caloris Basin, and are more sparsely peppered with younger craters. Intriguingly, floors of some of the craters show clusters of small depressions and pits surrounded by light-coloured material of some kind, suggested to be solids condensed from gases that emerged from below (Blewett, D.T. and 17 others 2011. Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activity. Science, v. 333, p. 1856-1859). While it is only possible to assign youth of these features relative to the craters in which they occur, they indicate an underlying source of volatiles; a factor weighing against previous accounts of Mercury’s evolution by either solar ablation or giant impact.

Considerably more interesting – at least to me – are the results from the geochemically oriented instruments. Calcium, magnesium, aluminium and silicon estimates by the XRF-like instrument present not the slightest evidence for a feldspar-rich component of the early crust akin to the lunar highlands; another blow for the giant-impact and magma-ocean hypotheses. Mercury’s surface seems to be similar in composition to the most ancient terrestrial lavas: Mg-rich mafic to ultramafic komatiites, compared with the more iron-rich tholeiites of the lunar maria (Nittler, L.R. and 14 others. The major-element composition of Mercury’s surface from Messenger X-ray spectrometry. Science, v. 333, p. 1847-1850). They are ten-times more enriched in sulfur than surface rocks on the Earth or Moon, though iron content seems too low to accommodate it in minerals such as pyrite (FeS2). High sulfur content could point to an origin for Mercury from accretion of highly reduced material in the solar nebula, the Earth-Moon system being broadly more oxidised. Gamma-ray spectrometry to analyse the abundances of potassium, uranium and thorium (Peplowski, P.N. and 16 others. Radioactive elements on Mercury’s surface from MESSENGER: implications for the planet’s formation and evolution. Science, v. 333, p. 1850-1852) doesn’t serve previous ideas about the planet’s history either. Potassium, which is moderately volatile, is too high relative to more refractory uranium and thorium to support any notion of solar ablation of the surface, but the U, Th and K proportions are roughly like those of the Earth’s oceanic crust. One of the plots shows K-Th relationships for supposed meteorites from Mars and the extensive gamma-ray data from Mars itself, in which few of the meteorites fall in the K-Th ‘cloud’ for the Martian surface: now there’s a thing….

It must be emphasised that the geochemical results are but a fraction of what should eventually emerge from these powerful instruments. However, these early data place Mercury in much the same envelope as the other rock worlds of the Inner Solar System (Kerr, R.A. 2011. Mercury looking less exotic, more a member of the family. Science, v. 333, p. 1812).