Reduction in Haze Formation Rate on Prebiotic Earth in the Presence of Hydrogen

Abstract

Recent attempts to resolve the faint young Sun paradox have focused on an early Earth atmosphere with elevated levels of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) that could have provided adequate warming to Earth's surface. On Titan, the photolysis of CH₄ has been shown to create a thick haze layer that cools its surface. Unlike Titan, however, early Earth's atmosphere likely contained high amounts of CO₂ and hydrogen (H₂). In this work, we examine haze formation in an early Earth atmosphere composed of CO₂, H₂, N₂, and CH₄, with a CO₂/CH₄ ratio of 10 and a H₂/CO₂ ratio of up to 15. To initiate aerosol formation, a broad-spectrum ultraviolet (UV) energy source with emission at Lyman-α was used to simulate the solar spectrum. Aerosol composition and total aerosol mass produced as a function of reagent gas were measured with an aerosol mass spectrometer (AMS). Results show an order of magnitude decrease in haze production with the addition of H₂, with no significant change in the chemical composition of the haze. We calculate that the presence of H₂ on early Earth could thus have favored warmer surface temperatures and yet allowed photochemical haze formation to deliver complex organic species to early Earth's surface. Key Words: Atmosphere—Atmospheric compositions—Carbon dioxide—Early Earth. Astrobiology 9, 447–453.