Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

Planetary Formation Update - September

Perhaps one of the more interesting questions is where did Earth's volatiles come from? The generally accepted theory is that Earth formed by the catastrophic collisions of planetary embryos (Mars-sized bodies), which effectively turned earth into a giant ball of magma, at which time the iron settled to the core though having a greater density, and took various siderophile elements with it. At this stage, the Earth would have been reasonably anhydrous. Subsequently, Earth got bombarded with chondritic material from the asteroid belt that was dislodged by Jupiter's gravitational field (including, in some models, Jupiter migrating inwards then out again), and it is from here that Earth gets its volatiles and its siderophile elements. This bombardment is often called "the late veneer". In my opinion, there are several reasons why this did not happen, which is where these papers become relevant. What are the reasons? First, while there was obviously a bombardment, to get the volatiles through that, only carbonaceous chondrites will suffice, and if there were sufficient mass to give that to Earth, there should also be a huge mass of silicates from the more normal bodies. There is also the problem of atmospheric composition. While Mars is the closest, it is hit relatively infrequently compared with its cross-section, and hit by moderately wet bodies almost totally deficient in nitrogen. Earth is hit by a large number of bodies with everything, but the Moon is seemingly not hit by wet bodies or carbonaceous bodies. Venus, meanwhile, is hit by more bodies that are very rich in nitrogen, but relatively dry. What does the sorting?
 
The first paper (Nature 501: 208 – 210) notes that if we assume the standard model by which core segregation took place, the iron would have removed about 97% of the Earth's sulphur and transferred it to the core. If so, the Earth's mantle should exhibit fractionated 34S/32S ratio according to the relevant metal-silicate partition coefficients, together with fractionated siderophile metal abundances. However, it is usually thought that Earth's mantle is both homogeneous and chondritic for this sulphur ratio,  consistent with the acquisition of sulphur  ( and other siderophile elements) from chondrites (the late veneer). An analysis of mantle material from mid-ocean ridge basalts displayed heterogeneous 34S/32S ratios that are compatible with binary mixing between a low 34S/32S  ambient mantle ratio and a high 34S/32S recycled component. The depleted end-member cannot reach a chondritic value, even if the most optimistic surface sulphur is added. Accordingly, these results imply that the mantle sulphur is at least partially determined by original accretion, and not all sulphur was deposited by the late veneer.
 
In the second (Geochim. Cosmochim. Acta 121: 67-83), samples from Earth, Moon, Mars, eucrites, carbonaceous chondrites and ordinary chondrites show variation in Si isotopes. Earth and Moon show the heaviest isotopes, and have the same composition, while enstatite chondrites have the lightest. A model of Si partitioning based on continuous planetary formation that takes into account T, P and oxygen fugacity variation during Earth's accretion. If the isotopic difference  results solely from Si fractionation during core formation, their model requires at least ~12% by weight Si in the core, which exceeds estimates based  on core density or geochemical mass balance calculations. This suggests one of two explanations: Earth's material started with heavier silicon, or (2) there is a further unknown process that leads to fractionation. They suggest vaporization following the Moon forming event, but would not this lead to lighter or different Moon material?
 
One paper (Earth Planet. Sci. Lett. 2013: 88-97) pleased me. My interpretation of the data related to atmospheric formation is that the gaseous elements originally accreted as solids, and were liberated by water as the planet evolved.  These authors showed that early early degassing of H2 obtained from reactions of water explains the "high oxygen fugacity" of the Earth's mantle. A loss of only 1/3 of an "ocean" of water from Earth would shift the oxidation state of the upper mantle from the very low oxidation state equivalent to the Moon, and if so, no further processes are required. Hydrogen is an important component of basalts at high pressure and, perforce, low oxygen fugacity. Of particular interest, this process may have been rapid. On the early Earth, over 5 times the amount of heat had to be lost as is lost now, and one proposal (501:501 - 504 ) heat pipe volcanism such as found on Io would manage this, in which case, the evolution of water and volatiles may have also been very rapid.
 
Finally, in (Icarus 226: 1489 -1498), near-infrared spectra show the presence of hydrated poorly crystalline silica with a high silica content on the western rim of Hellas. The surfaces are sporadically exposed over a 650 km section within a limited elevation range. The high abundances and lack of associated aqueous phase material indicate high water to rock ratios were present, but the higher temperatures that would lead to quartz were not present. This latter point is of interest because it is often considered that the water flows on Mars in craters were due to internal heating due to impact, such heat being retained for considerable periods of time. To weather basalt to make silica, there would have to be continuous water of a long time, and if the water was hot and on the surface it would rapidly evaporate, while if it was buried, it would stay super-heated, and presumably some quartz would result. This suggests extensive flows of cold water.
Posted by Ian Miller on Sep 30, 2013 3:30 AM Europe/London

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