MYSTERIES OF THE UNIVERSE - WHERE DID EARTH GET ITS WATER FROM?

Two new scientific studies have sailed researchers ever closer to discovering the source of Earth's water

- David Crookes

Water covers 71 per cent of the surface of Earth, and most of it – some 96.5 per cent – is contained within our planet’s oceans. The remaining 3.5 per cent is freshwater and therefore drinkable, but most of that – 68 per cent – is sealed within glaciers and ice. Such statistics are well known and equally well trodden, but they’re intriguing nonetheless. After all, Earth is something of an anomaly, given that it’s the only world in our Solar System with extensive, consistent and stable regions of liquid water at the surface – and no one knows exactly how this came to be.

To understand the mystery of Earth’s water, you need to take into account the Solar System’s snow line – the distance from the Sun beyond which water is present as ice. When the Solar System was formed, this line was between the orbits of Mars and Jupiter. Planets that formed within the line would see water turn to vapour; those beyond could accrete water ice. “It is surprising that Earth has so much water because it formed in the inner Solar System, likely within the snow line,” explains Megan Newcombe, assistant professor of geology at the University of Maryland. “The Earth accreted in a region of the solar nebula that was quite near the Sun, and therefore was likely warm and relatively dry. Water ice accretion to the early Earth was probably limited.”

Yet if the newborn Earth’s proximity to the Sun was likely to have left it dry, how and when did the planet produce its water supply? For many years, scientists have proposed that it came in massive quantities from elsewhere in the Solar System, but proof of the exact source has eluded them.