Question

What is one scientific explanation for the origin of our oceans, and what is the evidence?

Answer 1

Most of us teach earth science using simple or sophisticated models and imagery that demonstrate to students that the Earth is essentially a blue marble whose surface is dominated by oceans. This vision of a blue Earth has even been espoused by popular science writers such as the non-fiction work 'Pale Blue Dot' by the late Carl Sagan. For many it would be difficult to envision an Earth without its blue blanket of oceans. However this is precisely what the early stages of our planet were like. An ocean-free Earth existed, perhaps for several hundred million years as a consequence of extremely high surface temperatures following planetary accretion. The formation of oceans on Earth represents no less than a global-scale cooling of Earth's surface to temperatures at which water is stable as a liquid phase.

That such a profound transition occurred from the highly energetic conditions of the newly accreted Earth whose surface was dominated by meteorite impacts and transient magma oceans to cooler conditions capable of supporting liquid water and eventually life is not in question. However, the timing of this transition - which has implications for when surface conditions necessary for the development were established - is poorly known. Part of the uncertainty of the timing of this transition is due to the fragmentary nature of the rock record for the first ~500 million years of Earth history. Simply put, not much is preserved in the rock record for this time. It is the 'not much' part of the rock record that hold the clues however. The most promising information has come in two forms: (1) preserved sediments up to ~3800 million years old and (2) oxygen isotope studies of detrital zircons up to 4400 million years old