Thunder and lightening have fascinated and scared mankind since times immemorial. In the earlier days such events were thought to be God's curse and were often described in poetic form. With development of the scientific thought things changed and human beings started to think in terms of causes of lightening and the question, 'were there lightening in the past as well' always haunted the minds of the scientists.
Lightening has yet another extremely important factor. It is considered to be one of the sources for creating life on our planet.
A thunderbolt produces energy around 109 Joules per flash and occurs at a rate of about 65 flashes per second. So powerful is the flash of lightening that it causes sudden flow of electrons through the atmosphere and heats the air rapidly to a peak temperature of nearly 30,000 Kelvin. Such a high temperature produces nitric oxide through a chain reaction. The place where nitric oxide is formed has been a debatable issue. Some say it is formed inside the lightening channel and some have reasons to believe that it is formed within the shock wave. But it is certain that lightening does produce nitric oxide which is a natural source of reactive nitrogen oxides in the atmosphere. These nitrogen oxides are silent workers; they play a major role in the Earth's atmospheric-biosphere systems. So much is the significance of nitrogen oxides that scientists attribute the origin of life to the nitrogen oxides. It is needless to repeat the role of lightening in this process.
Possibly a flash of lightening cause a complex photochemical reaction which caused primitive, unicellular life to be formed on this planet billions of years ago!
It means lightening is not merely a flash reflecting the nature's ire. It is much more and needs a detailed probe.
As already said bolt from the blue produces a tremendous amount of energy in a flash. As it strikes the ground, the sand on the surface melts instantly and outcome is a vaporized hollow tube of glassy froth which is actually a melted mineral called Fulgurites. These tubes vary in size and dimensions from a fraction to several centimeters. As the lightening strikes the ground and melts and partially vaporizes the sand, air around is trapped and preserved as bubbles. These odd shaped tubes are fancied by the rich as pendants and by the scientists as witnesses of the past lightening episodes.
Gas bubbles are trapped during the process of formation of fulgurites. These bubbles are now under the scanner of the scientists, who try to extract maximum information about the atmosphere of the past and the date of lightening event. Navarro-Gonzalez of Laboratorio de Qu'mica de Plasmas y Estudios Planetarios, Instituto de Ciencias Nucleares, Universidad Nacional Aut'noma de M'xico, and Laboratoire Inter-Universitaire des Syst'mes Atmosph'riques, UMR CNRS 7583, Universit's Paris and his team including Dr. A.K. Singhvi of Physical Research Laboratory (PRL), Ahmedabad were able to study the composition of the air bubbles of fulgurites from Libiyan desert. They published the outcome of study in the February 2007 issue of Geological Society of America's magazine- 'Geology'.
Elsewhere in the world, lightening events of the past have been dated with the help of fossil charcoal formed presumably due to burning of existing forest as a consequence of bolt from the blue. In 1996 W.B. Harland and J.L.F. Hackler found the first evidence of ancient lightening strike with the discovery of fulgurites which were considered to be 250 million years on basis of fossils found in the overlying host rocks containing fulgurites. Direct age determination of fulgurites has not been reported so far. The age of these fulgurites was estimated as late Pleistocene to Middle Holocene that is 15000 to 5000 years before present. However the study by Navarro et al is the first record of chemical analyses of gases trapped in the bubbles in fulgurites and also points to the fact that they were formed instantly when the lightening struck the ground in the Libyan Desert.
The leaves of pre-history book opened by Navarro and his co-workers make a fascinating study. Lightening has so much energy that it just shreds air's oxygen and nitrogen molecules in to free atoms which create nitric oxide. Breaking of nitrogen out of its inert molecule is essential for life. In normal routine it is possible only through certain bacterial enzymes. But after the nature had decided that the Earth is going to be a planet with life, lightening was perhaps the precursor, which created the main source of life-the active nitrogen. Details of nitric oxide production still remain an enigma for the scientists, perhaps like the mystery of the creation! The air bubbles within the fulgurites also trap the air in the soil. Thus an analysis of the gases could give some clues about the conditions that existed when the lightening struck. And lastly the glass formed due to lightening could be dated with some amount of precision to find out when the event took place.
Data from satellite based sensors shows that whereas the Saharan region from where these fulgurites have been collected by Louis Carion in 1999 has been totally free from lightening flashes; the regions on the north that is Mediterranean Sea and to the south that is Sahel and further south the humid Savannas have been experiencing frequent lightening flashes currently. The satellite sensors have recorded 826327 flashes per square kilometer in these areas between 1998 and 2005. Obviously it means that the lightening struck the core of the Sahara Desert before this period.
In an event that has taken place in past there is always a curiosity about when it took place and why and how it took place. In this case too the scientific curiosity opened paragraphs of unwritten history recorded in the fulgurites. A Thermoluminescence (TL) dating of the material gave an average approximate age of 15000 years before present when the lightening struck the ground there. It means towards the end of the penultimate chapter of the Earth's history, the Pleistocene the area was experiencing lightening strikes. This is infact the first hard evidence of such an event having taken place. Lightening means thunder and rain and it is exciting to know that the dry Sahara was experiencing rain in the days of the yore!
The material of fulgurites collected from the Sahara Desert was found to be akin to that of the Sahelian sandy soils of today. Unlike the barren sandy Sahara Desert, Sahel is covered with miles of grassland, shrub-land and woodland. The carbon atoms match the isotopic profile of plants which thrive in hot dry places. This confirms that the climate of the Sahara in that period was like the Sahel on the south. Lightening and rain are common in the Sahel region today.
Navarro and his team were lucky to be pioneers in analyzing the gases trapped in the fulgurites bubbles. The composition indicated that nitric oxide was formed within the channel of the lightening. Published records of the past climates of this region point towards a wetter northern African climate around 15000 years ago and state that the Sahara (Desert) was almost covered by annual grasses and low shrubs. The region was under a wet spell till 4,500 to 4,000 years ago.
Well this is just the beginning of unveiling the mysteries of past lightening events with the help of fulgurites, the methodology for which was developed by A.K. Singhvi and another co-worker Shanon Mahan of the U.S. Geological Survey worked with him. 'This study provides a unique and quantitative way to develop paleo-ecological changes in semi arid regions. You can track the movement of vegetation belts in deserts quantitative. This could be the method for future quantitative paleo-ecological studies and hence useful for land use and land cover planning' communicates Singhvi.