When astronauts stood on the surface of the Moon and looked back at the Earth, they saw a “blue marble” flecked with snow-white clouds.
If you looked hard through its hazy atmosphere you would see the brownish discolorations marking patches of land. With about 70 per cent of the surface of Earth covered by it, our planet might more realistically be called “Water” rather than “Earth.”
If we smoothed our world into a perfect sphere, without the continents sticking up and the cavities of the ocean basins forming depressions, the whole Earth would be under water almost three kilometres deep. There is a lot of water on Earth. Where did it come from? Without it there would be no life.
Water is a compound made up of two hydrogen atoms and one oxygen atom. When the universe was young there were only hydrogen atoms, no oxygen. Then stars formed from that hydrogen, and started getting the energy needed to shine from nuclear fusion. Hydrogen was converted into other elements, such as helium, carbon and oxygen, which were released into space when those stars ran out of fuel and died. After a few generations of stars had formed and died, there was enough oxygen out there to make ice particles an important ingredient in cosmic dust clouds.
Our Earth formed about 4.5 billion years ago, along with the Sun and other planets making up the Solar System. A big cloud of cosmic dust, gas and ice particles collapsed into a disc, then the central part further collapsed to form the Sun and some of the rest of the cloud coagulated to form the planets, moons and other objects we see in the Solar System today. Thanks to modern telescopes we see in space new stars and planetary systems at all stages of formation. It is still going on.
When a cloud of stuff collapses into small lumps, a tremendous amount of heat is produced. This is how the core of the new Sun reached the 10 to 20 million degrees temperature needed for nuclear fusion to begin and the new star to shine. The planets did not get that hot, but the Earth and other planets were once huge balls of molten rock.
The big question is whether the water in the dust and other stuff coagulating to form the Earth would have been boiled off back into space. However, we don’t really know. We have two theories. Firstly, the steam from boiling water formed a dense atmosphere, and one day, when the surface of the Earth had solidified and cooled enough, it rained, and it rained, and it rained, for thousands or millions of years.
The second possibility is that the Earth cooled to be a dry, rocky object, with all its water boiled off into space. However then it was hit by a series of comets, which are lumps of dirty ice a few kilometres across. All the water on Earth would form a droplet almost 1500 kilometres across. We would have needed lots of comets. This is one reason we are interested in comets.
On one side these ambassadors from the outer Solar System bring with them building material dating back to when the planets formed. We want very much to know the ingredients. The material making up the Earth and planets has been processed and modified over billions of years. And of course we would like to know what sort of water comets carry. There are different forms of oxygen and hydrogen, called isotopes, where the atoms contain different numbers of neutrons. Are the relative proportions of the different isotopes the same as in our oceans? The recent rendezvous of the Rosetta spacecraft with a comet got us a sample. The comet water contained proportionally more deuterium – an isotope of hydrogen – than Earthly water does, although of course we have only looked at one comet so far. Maybe the vision of that incredible deluge falling on the young Earth is our best explanation so far……..maybe. Venus and Jupiter still lie close together in the sunset glow. Saturn lies in the south. The Moon will be New on the 15th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.