Complexities of the deep mantle

The use of seismic signals from many receiving stations to probe physical properties of the Earth tomographically is producing increasingly sharp results from the deep mantle. In a fascinating review of the state of that art, combined with results of high-pressure experiments that throw light on deep mantle changes in mineralogy and density, Edward Garnero and Allen McNamara of Arizona State University present some stunning graphics (Garnero, E.J. & McNamara, A.K. 2008. Structure and dynamics of Earth’s lower mantle. Science, v.  320, p. 626-627). Their scope is global, and dominated by thermochemical upwelling plumes and superplumes, zones towards which whole-mantle convection has swept dense material, and some indication of a connection between the two huge phenomena. It seems there are also pockets of magma close to the core-mantle boundary, which are hinted at by abnormally low shear-wave velocities.

Global wildfires at the K-T boundary debunked

Among the minuscule treasures of the K-T boundary deposits across the world are abundant amounts of what researchers have generally called soot. Interpreted literally, these seem to point to massive combustion of living vegetation at the time of the Chicxulub impact. That presupposes two things: that oxygen levels in the late Cretaceous were sufficiently high (~30%) to support combustion of green vegetation and heating from the entry flash of the Chicxulub projectile. The first is possible, but not the second, for not all the planet would have been bathed in the flash caused by compressive heating of the atmosphere ahead of the inbound planetesimal. Nonetheless, global forest fires were the accepted wisdom. A closer look at the ‘soots’ from eight K-T boundary exposures reveals that they are not made of charcoal, which vegetation burning would produce (Harvey, M.C. et al. 2008. Combustion of fossil organic matter at the Cretaceous-Paleogene (K-P) boundary. Geology, v. 36, p. 355-358). Instead the resemble carbonaceous nanospheres that result from incomplete combustion of pulverised coal or oil aerosols in power stations. By chance, the Chicxulub impact was next to what is now one of the most productive oilfields on Earth; the Canterell field in Mexico.

Astonishing stratigraphy of the north pole of Mars

Since, so far as we know, not a single sentient being has set foot on the Martian surface the title of this item might seem strange; but it is true. One of the features of microwave radiation is that it is capable of penetrating through solid surfaces and imaging the subsurface, given the right conditions. This phenomenon is best exploited by ice, and ground-penetrating radar is routinely used for sounding Earths glaciers and ice caps. To a lesser extent sedimentary layers can be penetrated, provided they are very dry. Radar is also an extremely useful remote-sensing tool with which to examine surfaces, and no planetary mission would be complete without some kind of radar instrument. The US Mars Reconnaissance Orbiter carries a radar system targeted at just such penetration – the Shallow Radar or SHARAD.

SHARAD is operated along traverses and provides cross sections of the subsurface that look very like seismic sections, with structure picked out by reflecting surfaces. Crossing the north polar ice cap of Mars, SHARAD reveals a simple layered sequence (Phillips, R.J. and 26 others 2008. Mars north polar deposits: stratigraphy, age and geodynamical response. Science, v. 320, p. 1182-1185). Nonetheless the layering is interesting as it reveals what appear to be cyclical processes involved in the ice cap’s evolution; perhaps by ~million-year periodicity in Mars’s obliquity or orbital eccentricity. The radar transparency of the north polar region is probably down to almost pure ice, around 1 km thick. Therein lie clues to another Martian feature: its lithosphere is very strong and thick. That conclusion stems from the lack of any significant annular topographic bulge around  the ice cap. Kilometre thick ice on Earth would result in a measurable feature of that kind, due to displacement of the underlying asthenosphere. The post-glacial relaxation of such a bulge that once lay to the south of the British ice cap is responsible for the drowning of valleys in SW England especially, and measurable subsidence of southern Britain today.

See also: Kerr, R. 2008. Layers within layers hint at a wobbly Martian climate. Science, v. 320, p. 867.

Other Martian oddities

A wonderfully written and illustrated summary of some of the strange recent findings about Mars appeared in the 24 May 2008 issue of New Scientist (Clark, S. 2008. Fire & ice. New Scientist, v. 198 24 May 2008 issue, p. 35-39). It emphasises the role of water and the chaotic orbital and spin behaviour of the ‘Red Planet’ in shaping its surface. Clark draws a picture of mystery and weirdness that will surely appeal to all Mars buffs.

How to spot impact sites that others have missed

The Earth’s surface is not peppered with obvious impact craters, as are the surfaces of other planetary bodies, because our planet is active tectonically and in terms of weathering, erosion and sedimentary deposition. Craters here get ‘ironed-out’ or buried quickly. Yet there is no way that the Earth could have escaped the episodic rain of objects large and small that results from gravitational perturbation of asteroids and comets by the complex motions of the giant planets. Finding signs of past impacts adds to knowledge of their effects on life, for example, as well as on the processes that accompany ‘mountains that fall from the sky’: it is a damn sight cheaper than doing the field work on the Moon or Mars. Astonishingly, a large impact site straddling a major highway in New Mexico escaped detection until recently (Fackelman, S.P. et al. 2008. Shatter cone and microscopic shock-alteration evidence for a post-Paleoproterozoic terrestrial impact structure near Santa Fe, New Mexico, USA. Earth and Planetary Science Letters, v. 270, p. 290-299). The clue that something swift and terrible had occurred in New Mexico during the late Precambrian were strange structures in road cuttings that looked like cartoons of Christmas trees. They consist of multiple cone-shaped features nested together in masses up to 2 m long and 0.5 m across. Other processes can form these strange structures, but finds of shocked minerals and signs of melting in the rocks affected by the cones confirmed a suspicion of a nearby impact structure. Shatter cones can easily be overlooked by geologists who have never seen such features before. The fact that those in New Mexico occur in recent road cuttings helped the authors spot them. At known impact sites shatter cones occur exclusively within the zone of uplift at the centre of complex craters. Those in New Mexico occur over an area about 3 km across, suggesting a minimum size for the now vanished crater of 6-13 km across.


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