How has the composition of Earth’s atmosphere changed over time? Briefly outline the evolution of Earth’s atmosphere

Question

What will be an ideal response?

Answer 1

ANSWER: ?Earth's first atmosphere (some 4.6 billion years ago) was most likely hydrogen and helium—the two most abundant gases found in the universe—as well as hydrogen compounds such as methane and ammonia. A second, more dense atmosphere gradually enveloped the Earth as gases from molten rock within its hot interior escaped through volcanoes and steam vents. We assume that volcanoes spewed out the same gases then as they do today: mostly water vapor (about 80 percent), carbon dioxide (about 10 percent), and up to a few percent nitrogen. As millions of years passed, the constant outpouring of gases from the hot interior (outgassing) provided a rich supply of water vapor, which formed clouds. Rain fell upon the earth for many thousands of years. Large amounts of CO2 were dissolved in the oceans. The atmosphere gradually became rich in nitrogen (N2). Oxygen (O2), the second most abundant gas in today's atmosphere, probably began an extremely slow increase in concentration as energetic rays from the sun split water vapor (H2O) into hydrogen and oxygen during a process called photodissociation. Hydrogen, being lighter, likely rose and escaped into space while the oxygen remained in the atmosphere. After plants evolved, the atmospheric oxygen content increased more rapidly, likely reaching its present concentration several hundred million years ago.