Nevertheless, water-rich asteroids are still a possibility. The European Space Agency’s Rosetta mission showed that comets like 67P/Churyumov-Gerasimenko are not a good match for Earth’s water. However, it is possible that the chemical precursors to water could have been present inside the rocks that formed Earth. The building blocks of Earth were probably not icy, since the Sun is hot enough in the inner Solar System to sublimate ice - turning it directly from a solid to a gas. It’s also possible that some combination of both theories happened. A second possibility is that water-rich asteroids bombarded Earth, bringing water here. One theory is that Earth was born with the elemental precursors of water locked in its rocks. Life as we know it needs water, so Earth had some when life arose. Our earliest direct evidence of life dates back to about 3.7 billion years ago. It’s possible life came into existence and was wiped out multiple times by giant impacts before taking hold for good. We don’t know exactly when life arose on Earth. This event is known as the Late Heavy Bombardment. These impacts may have spiked between 4.0 and 3.8 billion years ago, when the orbits of the giant planets shifted, flinging objects throughout the inner Solar System. Our planet was pelted by more space rocks throughout its early history. Vaporized rock and silicate clouds circled Earth, making our world shine like a small, fiery Jupiter. Most of Earth’s mantle was melted by the impact, and molten remnants from the collision formed the Moon. The material left over the Sun’s formation coalesced into everything else in our Solar System, including Earth, which was born around 4.55 billion years ago.Įarth was still practically a baby when a Mars-sized world named Theia is believed to have plowed into the planet about 4.51 billion years ago. Our Sun formed about 4.57 billion years ago from a cloud of collapsing dust and gas, in an unremarkable corner of the Milky Way galaxy. As a byproduct, oxygen is released, which has over time given Earth an oxygen-rich atmosphere suitable for complex lifeforms.Įxactly how the Cosmos set the stage for this to happen is not known with complete certainty, but we know the basics. Photosynthesis is the process by which plants and other organisms use sunlight, water, and carbon dioxide to produce energy. So how did Earth’s atmosphere get its oxygen? The simple answer is that early microorganisms produced it using a process you may have learned about in elementary school: photosynthesis.
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