One of the most eagerly followed ocean-floor drilling projects has just released some results. Its target is 46 km radially away from the centre of the geophysical anomaly associated with the Chixculub impact structure just to the north of Mexico’s Yucatan Peninsula. In the case of large lunar impact craters the centre is often surrounded by a ring of peaks. Modelling suggests such features are produced by the deep penetration of immense seismic shock waves. In the first minute these excavate and fling out debris to leave a cavity penetrating deep into the crust. Within three minutes the cavity walls collapse inwards creating a rebound superficially similar to the drop flung upwards after an object is dropped in liquid. This, in turn, collapses outwards to emplace smashed and partially melted deep crustal material on top of what were once surface materials, creating a crustal inversion beneath a mountainous ring of Himalayan dimensions that surrounds a by-now shallow crater. That is the story modelled from what is known about well-studied, big craters on the Moon and Mercury. Chixculub is different because the impact was into the sea and involved debris-charged tsunamis that finally plastered the actual impact scar with sediments. The drilling was funded for several reasons, some palaeontological others relating to the testing of theories of impact processes and their products. Chixculub is probably the only intact impact crater on Earth, and the first reports of findings are in the second category (Morgan, J.V. and 37 others 2016. The formation of peak rings in large impact craters. Science, v. 354, p. 878-882; doi: 10.1126/science.aah6561).
Artist’s depiction of the Chicxulub impact 65 million years ago that many scientists say is the most direct cause of the dinosaurs’ disappearance (credit: Wikipedia)
The drill core, reaching down to about 1.3 km below the sea floor penetrates post-impact Cenozoic sediments into a 100 m thick zone of breccias containing fragments of impact melt rock, probably the infill of the central crater immediately following the first few minutes of impact. Beneath that are coarse grained granites representing the middle continental crust from original depths around 10 km. The granite is intensely fractured and riven by dykes and pods of impact melt, and contains intensely shocked grains that typify impacts that produce a transient pressure of ~60 GPa – around six hundred thousand times atmospheric pressure. From seismic reflection surveys this crustal material overlies as yet un-drilled Mesozoic sedimentary rocks. Its density is significantly less than that of unshocked granite – averaging 2.4 compared with 2.6 g cm3. So it is probably filled with microfractures and sufficiently permeable for water to have penetrated once the impact site had cooled. This poses the question, yet to be addressed in print, of whether or not this near-surface layer became colonised by microorganisms in the aftermath (Barton, P. 2016. Revealing the dynamics of a large impact. Science, v. 354, p. 836-837). That is, was the surrounding ocean sterilised at the time of the K-T (K-Pg) mass extinction?; an issue whose resolution is awaited with bated breath by the palaeobiology audience. OK; so theory about the physical process of cratering has been validated to some extent, but will later results be more interesting, outside the planetary sciences community?
Read more about impacts here and mass extinctions here .
The work done by an asteroid or a comet that hits the Earth is most obviously demonstrated by the size of the crater that it creates on impact, should it have survived erosion and/or burial by sediments. Since some is done in flinging material away from the impact, the furthest point at which ejecta land is also a rough measure of the power of the hit. All this and much more derived from the kinetic energy of the object, which from Newton’s laws of motion amounts to half the product of the body’s mass and the square of its speed (mv2/2). It’s the speed that confers most energy; doubling the speed quadruples the energy. At a minimum, the speed of an object from far-off in space is that due to acceleration by the Earth’s gravitational field; the same as Earth’s escape velocity (about 11.2 km s-1). In March 1989 Earth had a close encounter with Newton’s laws writ large; an asteroid about 500 m across passed us with just half a million kilometres to spare. Moving at 20 km s-1 it carried kinetic energy of around 4 x 1019J. Had it hit, all of this immense amount would have been delivered in about a second giving a power of 4 x 1019 W. That is more than two hundred times greater than the power of solar heating of the day-side of the Earth. A small part of that power would melt quite a lot of rock.
Vredefort Dome impact structure (credit: Wikipedia)
As well as the glass spherules that are one of the hallmarks of impact ejecta on Earth and more so on the Moon’s surface, some of the larger known impact craters are associated with various kinds of glassy rock produced by instantaneous melting. Some of this melt-rock occurs in thin dykes, but sometimes there is an entire layer of once molten ‘country’ rock at the impact site. The most spectacular is in the Manicougan crater in Quebec, Canada. In fact a 1 km thick impact-melt sheet dominates most of the 90 km wide structure and it is reputed to be the most homogeneous large rock mass known, being a chemical average of every rock type involved in the Triassic asteroid strike. Not all craters are so well endowed with an actual sheet of melt-rock. This has puzzled some geologists, especially those who studied the much larger (160 km) Vredfort Dome in South Africa, which formed around 2 billion years ago. As the name suggests this is now a positive circular topographic anomaly, probably due to rebound and erosional unloading, the structure extending down 20 km into the ancient continental lithosphere of the Kaapvaal craton. Vredfort has some cracking dykes of pseudotachylite but apparently no impact melt sheet. It has vanished, probably through erosion, but a relic has been found (Cupelli, C.L. et al. 2014. Discovery of mafic impact melt in the centre of the Vredfort dome: Archetype for continental residua of early Earth cratering? Geology, v. 42, p. 403-406). One reason for it having gone undiscovered until now is that it is mafic in composition, and resembles an igneous gabbro intrusion. Isotope geochemistry refutes that mundane origin. It is far younger than the rocks that were zapped, and may well have formed as huge energy penetrated to the lower crust and even the upper mantle to melt a sizeable percentage of 2.7 to 3.0 Ga old mafic and ultramafic rock.
Oddly, the same issue of Geology contains an article that also bears on the Vredfort Dome structure (Huber, M.S. et al. 2014. Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredfort impact event. Geology, v. 42, p. 375-378). Drill core from a Palaeoproterozoic limestone revealed millimetre-sized glass droplets containing excess iridium – an element at high concentration in a variety of meteorites. The link to Vredfort is the age of the sediments, which are between 1.98 and 2.05 Ga, neatly bracketing the timing of the large South African impact. Using reasonably well-constrained palaeogeographic positions at that time for Karelia and the Kaapvaal craton suggests that the glassy ejecta, if indeed they are from Vredfort, must have been flung over 2500 km.
The South Pole - Aitken basin (blue-magenta) and part of the high lunar far side (yellow-red) on an elevation map. Image via Wikipedia
The most significant discovery from the Apollo lunar landings is that the Earth and Moon shared a fiery early history, when a planetary body around the size of Mars slammed into the Earth to fling off vaporised rock that condensed to create the Moon. Such a catastrophic event reset the geochemistry of the Earth, and both it and the Moon likely had an early phase dominated by a deep ocean of magma. The evidence for a magma ocean comes mainly from the lunar highlands which are dominated by almost pure calcium plagioclase feldspar (the rock anorthosite), suggesting that this high-temperature, low-density silicate mineral crystallised and then floated to the surface of the Moon. Yet there is a great deal of evidence about the Moon that did not depend on people setting foot on its surface. For instance, detailed photographic records of the surface and extremely precise measurements of the surface elevation stem from cheaper orbital missions, including coverage of the unvisited far side of the Moon.
The face of the Moon never seen from Earth has long been known to have one of the largest impact basins in the solar system, the South Pole – Aitken basin. Analysis of the far side’s surface elevation data from the Lunar Orbiter Laser Altimeter (LOLA) also shows that it is significantly higher than the near side. It is also far more heavily cratered than the near side. Now there is a plausible explanation for the dichotomy: the Moon received another stupendous blow (Jutzi, M & Asphaug, E. 2011. Forming the lunar farside highlands by accretion of a companion moon. Nature, v. 476, p. 69-72). But how come that didn’t blast the Moon apart or re-melt it and allow it to re-shape to a near perfect sphere? The modelling study suggests that if the culprit slowly collided – around 2-3 km s-1 – it would have wrapped around the early Moon to plaster the surface with debris, nicely shown by the paper’s graphics. Such a ‘slow’ impact is only possible from a co-orbital companion moon, objects from outside the Earth-Moon system inevitably being accelerated by gravity to at least the equivalent of its escape velocity (about 11-12 km s-1). That exceeds the speed of sound through rock, leading at least to a very large hole, shock metamorphism and, with a massive body, to extensive melting (the energy would be ½ mv2) rather than the observed lunar far-side bulge. Jutzi and Asphaugs’s modelling comes up with a companion moon around 1200 km across, that may have formed from the same massive event that created the Moon itself. It could have accreted from the impact-induced vapour disc at a Trojan point in the lunar orbit, where gravitational forces balance to keep orbital objects apart. The gradual expansion of the lunar orbit in response to tidal forces – large in the early history of the Earth-Moon system – could have destabilised the balance so that the companion moon slowly drifted towards the Moon and eventual collision.
One such modelling becomes closer to known reality, i.e. the far-side bulge, it gets more tempting to look for secondary possibilities. One of these the effect of such a ‘slow’ impact on the remaining magma ocean on the Moon. It may have blurted that by then deep molten layer to the side opposite the impact. That, the authors suggest, may be responsible for the geochemical peculiarities of the flood basalts that filled the much later lunar maria on the near side. There are no signs of these KREEP basalt floors to large later craters on the far side, such as the Aitken basin, formed around 4.0 to 3.8 Ga ago at the same time as the near-side maria. A variety of new instruments orbit the Moon and more are planned, so this model presents a nice hypothesis for them to test: what is the betting that a robotic lander might eventually be sent to return samples from the enigmatic far side?