What is the origin of the building blocks of life on early Earth? Is it necessary that they were delivered by meteorites or interplanetary dust? Or was early Earth "biogenic," and could produce key biomolecules on its own? An atmosphere rich in HCN is a distinguishing feature of what we term biogenic worlds. HCN is a key species produced in Miller-Urey electric discharge experiments simulating lightning-based chemistry in the primordial atmosphere. HCN reacts in water to form nucleobases and ribose, the building blocks of RNA, and amino acids, the building blocks of proteins. To determine whether early Earth was biogenic, we develop a self-consistent chemical kinetic model for the production and rainout of HCN in the early atmosphere, and couple it to a comprehensive model of warm little ponds to compute the in situ production of the building blocks of RNA. We model two epochs of the Hadean eon, at 4.4 Gya (giga-years ago) and 4.0 Gya, which differ in composition, luminosity, UV intensity, and impact bombardment rate. At 4.4 Gya, UV intensity was high due to the active newly formed Sun, and asteroids and comets were bombarding the planet at an overwhelming rate of 1x1015 kg/yr. Impact degassing at this time produced a reducing, H2-dominant atmosphere. At 4.0 Gya, the atmosphere was depleted in hydrogen due to escape from the upper atmosphere, and volcanic outgassing led to an oxidizing CO2-dominant world. The reducing models at 4.4 Ga lead to RNA building block production in ponds that is comparable in concentration to what would result from meteoritic delivery (ppm-range). Unlike the RNA building blocks delivered to ponds by meteorites, which survive for less than a few years, the concentrations produced in situ are maintained indefinitely due to the steady influx of HCN from the troposphere. The oxidizing models at 4.0 Ga lead to substantially lower RNA building block concentrations (ppq-range). These results suggest that early Earth was biogenic at 4.4 Ga, and transitioned out of this phase sometime before 4.0 Ga.
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2021-06-07T15:00:002021-06-07T16:00:00Biogenic Worlds: From atmospheric HCN production to the building blocks of RNA in warm little pondsEvent Information:
What is the origin of the building blocks of life on early Earth? Is it necessary that they were delivered by meteorites or interplanetary dust? Or was early Earth "biogenic," and could produce key biomolecules on its own? An atmosphere rich in HCN is a distinguishing feature of what we term biogenic worlds. HCN is a key species produced in Miller-Urey electric discharge experiments simulating lightning-based chemistry in the primordial atmosphere. HCN reacts in water to form nucleobases and ribose, the building blocks of RNA, and amino acids, the building blocks of proteins. To determine whether early Earth was biogenic, we develop a self-consistent chemical kinetic model for the production and rainout of HCN in the early atmosphere, and couple it to a comprehensive model of warm little ponds to compute the in situ production of the building blocks of RNA. We model two epochs of the Hadean eon, at 4.4 Gya (giga-years ago) and 4.0 Gya, which differ in composition, luminosity, UV intensity, and impact bombardment rate. At 4.4 Gya, UV intensity was high due to the active newly formed Sun, and asteroids and comets were bombarding the planet at an overwhelming rate of 1x1015 kg/yr. Impact degassing at this time produced a reducing, H2-dominant atmosphere. At 4.0 Gya, the atmosphere was depleted in hydrogen due to escape from the upper atmosphere, and volcanic outgassing led to an oxidizing CO2-dominant world. The reducing models at 4.4 Ga lead to RNA building block production in ponds that is comparable in concentration to what would result from meteoritic delivery (ppm-range). Unlike the RNA building blocks delivered to ponds by meteorites, which survive for less than a few years, the concentrations produced in situ are maintained indefinitely due to the steady influx of HCN from the troposphere. The oxidizing models at 4.0 Ga lead to substantially lower RNA building block concentrations (ppq-range). These results suggest that early Earth was biogenic at 4.4 Ga, and transitioned out of this phase sometime before 4.0 Ga.Event Location:
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