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.

Origin of the arms race

Global paleogeographic reconstruction of the E...
Global paleogeographic reconstruction of the Earth in the early Cambrian period 540 million years ago. (credit:Ron Blakey, Northern Arizona University)

Palaeontologists generally agree on one broad aspect of animal evolution: the central role of predation versus defence in animal diversification to occupy different ecological niches. Indeed that co-relation has to an extent been responsible for the diversification of potentially habitable niches themselves. Armour and arms form a dialectic within the animal world, but one that only rose to dominate when hard materials became an integral part of animal morphology, allowing some to bite, gnaw or rasp and others to develop shelly or horny skeletons. The Kingdom Animalia within the domain of the eukaryotes – organisms based on cells that bear a nucleus – is united by one life style, that of feeding directly or indirectly on other living things. They are heterotrophs unable to generate energy and tissue through the fundamental harnessing of chemistry and physics to use the inorganic world directly, as do autotrophs.  One of the earliest discoveries about the history of animals was that fossils in the widely accepted meaning of the word appeared suddenly in the geological record, earlier rocks containing virtually no tangible signs of life: fossils explode in numbers from the start of the Cambrian Period at 542 Ma. Subsequently, geologists did discover imprints of clearly quite complicated organisms in rocks a few tens of million years older than the start of the Cambrian. But these were flaccid, bag like creatures that recent research has shown to rely on filtering microorganisms from water or directly absorbing organic matter through their skin.

Cropped and digitally remastered version of an...
An animal from the late Precambrian(Photo credit: Wikipedia)

Another feature of sediments of the oldest Cambrian is that in many parts of the world they rest with or=profound unconformity on deformed older rocks of Precambrian age. Throughout Britain the lowest Cambrian rocks are almost pure quartz sandstones that rest upon older more complex rocks ranging from only a few tens of million years older than 542 Ma to some of the oldest rocks in Europe, the Lewisian complex dating back 3 billion years. Many of the hills of North West Scotland have a gleaming white cap of Lower Cambrian quartzite above what has been termed the Great Unconformity where it occurs in Arizona’s Grand Canyon. Sedimentary sequences that continuously record the Precambrian to Cambrian transition and the biological explosion at the juncture are rare. But they show two curious features in sediments that immediately predate those bearing recognisable fossils: a complete lack of evidence for burrowing and millimetre-scale shell-like bodies made of calcium phosphate and carbonate, which are thought to have adorned the skins of otherwise unprotected animals.

Português: Classe Radiodonta
Creatures from the Cambrian Period (credit: Wikipedia)

Calcium, while a very common element is one of the most dangerous to life. Traces are essential for the signalling that goes on in cell metabolism, and too little snuffs out those vital processes.  Yet too much – still a very low concentration in cell cytoplasm – results in the growth of minute mineral crystals within cells, also spelling cell death. This results from the limited solubility of calcium in water, compared with those of other common metals.  At an early stage in evolution cells developed means of restricting the admission of calcium ions and efficient means of expelling excess amounts of calcium. The ubiquitous occurrence of Ca-rich marine limestones throughout the geological record bears witness to two things: the abundance of calcium ions in seawater; a closer look reveals that a great many limestones, going back some 3.5 billion years show traces of biomineralisation that helped form the limey sediments. In the second case, the calcium carbonate in most Precambrian limestones was secreted by photosynthetic blue-green bacteria in minutely thing layers, probably in the form of a slimy film excreted to avoid calcium toxicity. Palaeontologists have long suspected that the earliest skeletal materials formed by animals evolved from the need to excrete biomineralised films by turning a metabolic necessity into functional and integral parts of their body plans: arms and armour. Yet limestones are not rare signs of the presence of a dissolved calcium threat, so why the sudden adoption of waste products in this way?

A fairly old hypothesis is that calcium in seawater must have risen above a threshold that posed toxic threat to all living things and excretion had to increase to maintain the balance, perhaps matched with increasing sizes of animals in the late Precambrian. Only recently has support been found for this suggested evolutionary trigger, initially from analysis of brines trapped in crystalline materials within sediments, such as salt (NaCl). But the very presence of such halite in a sediment is a universally accepted sign of evaporation increasing ionic concentrations in isolated seawater lagoons, whereas a general increase in marine calcium would be needed to present sufficient chemical stress that the whole of animal evolution would require a step-change for survival.  It turns out that support for the hypothesis stems from two isotopic systems most usually associated with dating the formation and weathering of continental  crust: those of strontium and neodymium. The global record of ratios of 87Sr/86Sr and 143Nd/144Nd show unusually large changes in the run-up to the Cambrian Period, the first rising to the highest level recorded in geological history and the second reaching a historic nadir during the Cambrian. This anti-correlation signifies the greatest chemical weathering of older continental crust in the history of the Earth (Peters, S. & Gaines, R.R. 2012. Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature, v. 484, p. 363-366). Not only would this have poured dissolved ions, including those of calcium, into the oceans and raised their concentrations in seawater, but vast areas of the continents would have been eroded to form huge coastal plains, ripe for marine inundation. The last is exactly what the near-universal unconformity at the base of the Cambrian signifies. Presaging this long drawn-out grinding of continents to their gums had been a protracted bout of continental assembly to form the Rodinia supercontinent around 1000 Ma though collision and mountain building. Then Rodinia broke apart, its fragments being driven by plate tectonics to reassemble, along with vast chains of new crust formed in volcanic island arcs, by yet more orogenesis: tectonically high-energy times matched by the processes of denudation on land.

A nice example of planetary interconnectedness on the largest scale with the greatest conceivable consequences, for we vertebrates anyhow. This comes as a great comfort to me in the twilight of my career, since in 1999 I stuck out my neck with a similar concept in Stepping Stones only to meet a suitably stony silence.