Friday, March 15, 2013

Did lightning strikes feed early life?

Since the 1953 Miller-Urey experiment which used simulated lightning in a replica of the early atmosphere to create amino acids, speculation on the role of lightning in generating early life has continued. Research by scientists at the University of Arizona now suggests a new mechanism by which lightning may have helped life on its way.

The research involved multi-instrument analysis of rocks called fulgurites, named after the Latin for thunderbolt (and also known as lightning glass). These form when lightning strikes moist silica rich soil or rock, melting and vaporising it as the electrical energy dissipates into the earth. The melt cools swiftly to a glass, similar to that of impact tektites such as Moldavite, and often takes the shape of a hollow tube as the vapour component escapes. Fulgurites resemble roots or branching corals, whose shapes reveal the pattern of electrical dissipation in the earth as the current follows the path of least resistance through the particular material. They are a good illustration of the Chinese concept of Li (literally the markings in jade), which is incorporated into one of their words for science and translates as patterns of organic energy. Their colour depends on the chemical makeup of the rock and their size on the depth of penetration of the strike, which also gives a measure of its energy. They are usually quite small (< 1 metre) but a record 4.9 metre specimen was once found in Florida. They are collectable and can be quite expensive.

The analysis revealed that lightning may have helped enrich the early earth in vital nutrients in a form early life could assimilate. It was already known as one of the few abiotic means of fixing nitrogen (which is why we use agricultural fertilisers), but this research demonstrates its role in providing phosphorous in the reduced form used by early life. Phosphorous is a vital constituent of RNA/DNA and cell membranes, and was once referred to by Isaac Asimov as 'life's bottleneck' as it constitutes 1% of organisms while only being present in 0.1% of the world's minerals. It is usually the limiting nutrient in an ecosystem as its hard to dissolve and moves slowly through the rock cycle.

Nowadays, phosphorous is relatively plentiful, mostly as fully oxidised orthophosphate. In the Achaean and early Proterozoic, before oxygen entered the atmosphere, reduced phosphorous was a larger percentage of the available stock, and early life evolved a different chemical pathway for incorporating it to that of modern life. Modern phosphate comes from the weathering of rock in our current oxidising atmosphere, but on early Earth the geochemical sources must have been different.

Lightning is one of the few natural phenomena powerful enough to reduce phosphorous, electricity providing the energy, while the organic matter in the soil strips off the oxygen, acting as the reducing agent (though other reduced elements more reactive than Phosphorous could have done it on early Earth). Analysis of modern fulgurites showed elevated amounts of reduced phosphorous in proportion to their contents of organic matter. Since the element is rare today in its reduced forms, biologists are puzzled why many micro-organisms retain the enzymes to process it, and the genetic evidence suggests that this is an ancient biochemical pathway. They believe it was more common on the primordial Earth, and had long posited the existence of a geological source of reduced phosphorous, which the Earth sciences have now found for them.

The real joy lies in the feeling that as the work of geoscience progresses each year, each brick builds into an awareness of the whole Earth system. Terra doen't divide itself into pieces in the way that those of us who study it do. As we get to know it as an undivided whole, our love and respect for our wonderful world grows with the profundity of our understanding.

Fulgurite image credit: Stan Celestian and Earth Science Picture of the Day:

Fulgurite and phosphite paper:

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