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What evidence suggests that liquid water existed on Earth about 4.4 billion years ago?

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Evidence for the existence of liquid water on Earth approximately 4.4 billion years ago is primarily derived from the analysis of ancient zircon crystals found in the Jack Hills region of Australia.12

While most of Earth's original crust from the Hadean Eon has been recycled through plate tectonics, these resilient mineral grains have survived. Geochemists have dated these specific zircons to at least 4.4 billion years old. Many scientists interpret the chemical signatures within these crystals as compelling evidence that stable continents and liquid water were already present on Earth's surface at that time, with surface temperatures likely remaining below 100 °C (212 °F).21

This discovery suggests that the development of Earth's atmosphere and oceans occurred significantly earlier in the planet's history than previously documented by the intact rock record.2