Extinctions by impacts: smoking artillery

It’s a measure of the resistance to events controlled by processes outside of Earthly ones that evidence in support of an impact cause for mass extinctions has assumed monumental dimensions.  The iridium anomalies at the K-T boundary, found by Alvarez and Son in 1980, were never enough for a great many palaeontologists.  Nor, for that matter, were the co-occurrences of glass microspherules, shocked quartz grains and soot, discovered by later investigators at 30 to 100 sites worldwide.  Even the remains of the 180 km-wide Chixculub impact crater that formed at the same time as the extinction event, off Yucatan in the Mexican Gulf, was insufficient for the most intransigent sceptics.   That the sooty material contained massive carbon molecules in forms akin to Buckminster Fuller’s geodesic dome, and moreover those fullerenes contained trapped noble gases in proportions that could never have been present on Earth, formed the smoking siege gun for most sensible scientists.  The fullerenes contain helium, neon and argon with isotopic proportions comparable with those in carbonaceous chondrites and interplanetary dust, probably created by processes in a supernova that preceded accretion of the solar nebula.  The hypothesis that such odd materials were delivered to the K-T boundary layer by an extraterrestrial object was amply confirmed by Luann Becker’s discovery that carbonaceous chondrites, never affected by extreme events since they formed, also contain fullerenes (Becker, L. 2002.  Repeated blows.  Scientific American, v. 286(3), p. 62-69).  The latest occurrence of such convincing evidence for impact control of mass extinction comes from Permian-Triassic boundary deposits in China, Japan and Antarctica, that coincide with the most severe disruption of eukaryote life – around 90% of marine and continental families failed to survive it (see Land vertebrates snuffed at the end of the Permian in February 2002 Earth Pages).

It now seems that palaeontologists and a great many others, including creationists who envisage some kind of design within the fossil record, will be compelled to face up to an unearthly influence over the shaping of life on our planet.  There are many impact structures that are candidates for having affecting the biosphere from the Mesoproterozoic onwards, yet no pattern to their timing and energy of formation.  Such is the complexity of gravitational fluctuations that fling asteroids and comets into Earth-crossing orbits, that aside from the inevitability that, given time, they will strike with devastating consequences, they are essentially random events.  Our species is a late development from a vast concatenation of events, both from outside and within the Earth system, that spanned the entire 4.5 billion-year physical evolution of our home world.  No-one has yet turned statistics to estimate the likelihood of such chance occurrences being repeated, with one outcome being conscious beings.  If that were possible, then for the seekers of extraterrestrial intelligence, it might well be as welcome as a Semtex suppository on a wide-bodied jet!

Dinosaur digest

Having suffered vivid nightmares about dinosaurs when a kid – I did not even dare watch Jurassic Park alone as an adult – it comes as a huge relief to learn that the scariest of all monsters, T. rex, was about as agile as I am.  Indeed, it seems highly likely that you or I could outrun one.  The reasoning behind this welcome news (Hutchinson, J.R and Garcia, M. 2002.  Tyrannosaurus was not a fast runner.  Nature, v. 415, p. 1018-1021) stems from scaling up the sprinting powers of chickens (a chicken is surprisingly fast!) to the estimated 6 tonne weight of an adult T. rex.  The analysis involves two factors.  First, muscle proteins have the same capacity for powering movement, and the total power of musculature depends on its cross-sectional area, but while body mass and volume grows, this area and potential power falls behind.  Secondly, the bearing capacity of bone decreases with size too, because this also depends on area rather than volume.  Hutchinson and Garcia’s scaling hens to 6 tonnes, and calculating the necessary mass of leg muscle to propel them in their fearsome dashes to grab a tidbit (you or me), resulted in the absurd vision of a creature with 86% of its body mass in its legs.  Tyrannosaur modelling from their skeletons falls a very long way short of that, and they would be hard pressed to clock much more than 5 ms-1, which I think I could manage quite easily, for a short while.  That they would ever break into more than a fast walk is unlikely, for the second factor poses a limit.  One wrong pounce would be curtains, for they would break a leg.  Two possible life styles seem to emerge from the analysis.  They may have subsisted on carrion.  Alternatively, the far bigger herbivorous dinosaurs would have been even more stately, for the same mechanical reasons, which generates the absurd vision of large carnivorous dinosaurs ambling down their prey.

See also:  Hecht, J. 2002.  T. rex was a lumbering old slow coach.  New Scientist, 2 March 2002, p. 6;  Biewener, A.A. 2002.  Walking with tyrannosaurs.  Nature, v. 415, p. 971-973.

That dinosaurs could survive high-latitude winters, in near total darkness, if not glacial conditions, was first suspected in 1960 when their footprints turned up in Spitzbergen.  Since then, palaeontologists have found fossils of a wide variety of dinosaurs in areas that would have been near-polar during the Jurassic and Cretaceous Periods (Rich, T.H. et al. 2002.  Polar dinosaurs.  Science, v.  295, p. 979-980).  Surely, these dinosaurs must have been warm-blooded, as their containing sediments sometimes show signs of the effects of permafrost.  There are signs in some of the fossils for heightened visual powers too.  In the case of Australian faunas, it seems certain that the abundant dinosaurs there did not migrate to high latitudes in summer, because seaways blocked passage to lower latitudes.

Extremophiles and possibilities for extraterrestrial life

Bacteria can survive extremes of temperature (-10 to 110°C) and chemistry, and the biosphere extends to crustal depths in excess of 2 km, as shown by thriving communities in deep wells.  So far as biologists are aware, temperature forms the limit to life’s range, because of the instability of crucial molecules and of course the boiling point of water.  Since temperature increases with depth in the Earth, due to its self-heating by radioactive decay, the biosphere has a depth limit too, depending on the geothermal gradient.  However, recent experiments on two common bacteria show that life can survive at extremely high pressures (Sharma, A. et al. 2002.  Microbial activity at gigapascal pressures.  Science, v. 295, p. 1514-1516).  By compressing bacterial films on ice in diamond anvil cells, a team from the Carnegie Institute in Washington, DC have shown that simple life can survive pressure as high as 1.6 Gpa, that is equivalent to crustal depths of 50 km or an ocean bed160 km below the surface.  Because subduction takes cold lithosphere downwards, and the associated geothermal gradient is low in such environments, the deepest biosphere may be below volcanic arcs.  However, the most significant implication of the experiments is that probing the icy crusts of Europa, Ganymede or Callisto (and liquid water that might be present at great depths there) and the Martian ice caps, conceivably could reveal living organisms, if life ever evolved on these bodies.  Whereas this possibility encourages various plans for such exploration, what the experiments did not show was replication by the bacteria, and that is central to any living organism.


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