Paleolightning

Paleolightning refers to the remnants of ancient lightning activity studied in fields such as historical geology, geoarchaeology, and fulminology. Paleolightning provides tangible evidence for the study of lightning activity in Earth's past and the roles lightning may have played in Earth's history. Some studies have speculated that lightning activity played a crucial role in the development of not only Earth's early atmosphere but also early life. Lightning, a non-biological process, has been found to produce biologically useful material through the oxidation and reduction of inorganic matter. Research on the impact of lightning on Earth's atmosphere continues today, especially with regard to feedback mechanisms of lightning-produced nitrate compounds on atmospheric composition and global average temperatures. Detecting lightning activity in the geologic record can be difficult, given the instantaneous nature of lightning strikes in general. However, fulgurite, a glassy tube-like, crust-like, or irregular mineraloid that forms when lightning fuses soil, quartz sands, clay, rock, biomass, or caliche is prevalent in electrically active regions around the globe and provides evidence of not only past lightning activity, but also patterns of convection. Since lightning channels carry an electric current to the ground, lightning can produce magnetic fields as well. While lightning-magnetic anomalies can provide evidence of lightning activity in a region, these anomalies are often problematic for those examining the magnetic record of rock types because they disguise the natural magnetic fields present.

Lightning and early Earth The atmospheric composition of early Earth (the first billion years) was drastically different from its current state. Initially, hydrogen and helium compounds dominated the atmosphere. However, given the relatively small size of these elements and the warmer temperature of Earth compared to other planets at the time, most of these lighter compounds escaped, leaving behind an atmosphere composed mainly of methane, nitrogen, oxygen and ammonia with small concentrations of hydrogen compounds and other gases. The atmosphere was transitioning from a reduction atmosphere (an atmosphere that inhibits oxidation) to one of oxidation, similar to our current atmosphere. The origin of life on Earth has been a matter of speculation for quite some time. Living things did not spontaneously appear, so some sort of biological or even non-biological process must have been responsible for the generation of life. Lightning is a non-biological process, and many have speculated that lightning was present on early Earth. One of the most famous studies that investigated lightning on the early Earth was the Miller–Urey experiment.

Miller–Urey experiment

The Miller–Urey experiment sought to recreate the early Earth atmosphere within a laboratory setting to determine the chemical processes that ultimately led to life on Earth. The basis of this experiment was leveraged on Oparin's hypothesis, which assumed that some organic matter could be created from inorganic material given a reduction atmosphere. Using a mixture of water, methane, ammonia, and hydrogen in glass tubes, Miller and Urey replicated the effects of lightning on the mixture using electrodes. At the conclusion of the experiment, as much as 15 percent of the carbon from the mixture formed organic compounds, while 2 percent of the carbon formed amino acids, a necessary element for the building blocks of living organisms.

Volcanic lightning on early Earth The actual composition of the atmosphere of the early Earth is an area of great debate. Varying amounts of certain gaseous constituents can greatly impact the overall effect of a particular process, which includes non-biological processes such as the buildup of charge in thunderstorms. It has been argued that volcano-induced lightning in the early stages of Earth's existence, because the volcanic plume was composed of additional "reducing gases", was more effective at stimulating the oxidation of organic material to accelerate the production of life. In the case of volcanic lightning, the lightning discharge almost exclusively occurs directly within the volcanic plume. Since this process occurs fairly close to ground level, it has been suggested that volcanic lightning contributed to the generation of life to a greater extent than lightning produced within clouds that would lower positive or negative charge from a cloud to the ground. Hill (1992) quantified this enhanced contribution by examining estimated hydrogen cyanide (HCN) concentrations from volcanic lightning and "general lightning". Results showed that HCN concentrations for volcanic lightning were an order of magnitude larger than "general lightning". Hydrogen cyanide is yet another compound that has been linked to the generation of life on Earth. However, given that the intensity and amount of volcanic activity during the early stages of Earth's development is not fully understood, hypotheses regarding past volcanic activity (e.g., Hill, 1992) are usually based on present-day observed volcanic activity.

Nitrogen fixation and lightning

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