How do we know there was an ocean on Earth 4.28 billion years ago?

Evidence of Ancient Oceans on Earth

The history of Earth's oceans is a fascinating subject that touches upon the conditions that enabled life to emerge. After the Earth formed around 4.5 to 4.6 billion years ago, the planet was initially molten and lacked an atmosphere and oceans. As the Earth cooled, an atmosphere formed, and subsequently, the oceans began to condense as well. Geological evidence suggests that by around 4.28 billion years ago, there was enough cooling for liquid water to be stable on Earth's surface.

One important piece of evidence for the presence of ancient oceans is the existence of rocks bearing traces of microbial life that date back to 3.5 billion years ago, with debated signs of life appearing as far back as 3.8 billion years. These microbes, particularly stromatolites found in Australia, indicate the presence of an aquatic environment since they are formed in shallow water. Moreover, the idea of a "primordial soup", a hypothesized body of water rich in organic compounds, supports the presence of oceans early in Earth's history as a cradle for the emergence of life.

Geological studies and findings, such as in Greenland where the melting ice revealed ancient rocks, also provide evidence of an early oceanic presence. The solidification of Earth's surface permitted the accumulation of water, eventually forming the oceans. Collectively, these pieces of evidence—chemical signatures in ancient rocks, fossilized microbial formations, and theoretical models of Earth's climate and atmosphere development—help us understand how and when Earth's oceans formed.