<Table> <Tr> <Td> </Td> </Tr> <Tr> <Td> This image shows the Moon at the centre, with the limb of Earth near the bottom transitioning into the orange - colored troposphere . The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange - and blue - colored atmosphere . The silvery - blue noctilucent clouds extend far above Earth's troposphere . </Td> </Tr> </Table> <Tr> <Td> This image shows the Moon at the centre, with the limb of Earth near the bottom transitioning into the orange - colored troposphere . The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange - and blue - colored atmosphere . The silvery - blue noctilucent clouds extend far above Earth's troposphere . </Td> </Tr> <Table> <Tr> <Td> </Td> </Tr> <Tr> <Td> Earth's atmosphere backlit by the Sun in an eclipse observed from deep space onboard Apollo 12 in 1969 . </Td> </Tr> </Table> <Tr> <Td> Earth's atmosphere backlit by the Sun in an eclipse observed from deep space onboard Apollo 12 in 1969 . </Td> </Tr>

When did the chemical composition of earth's atmosphere reach that of today