Where did all the Earth's water come from?
On the preceding page, we saw that some of the water we find on Earth today may have come from Theia, the planet which impacted with the early Earth to form the moon, and that both the early Earth and Theia must have had 'substantial' amounts of water high in delta-17 oxygen either trapped in minerals or as ice. According to the results of the study referred to [1] much of the water we see on Earth today, which is thought to have been crucial for life forming on our planet, must have come from Theia.
But what about the wider context? Could the Earth have achieved its water via the comets who were hurled into the inner solar system and crashed into the earth’s surface following the migrations of Uranus and Neptune bringing their water with them and leaving it behind, or could it have been the result of asteroid impact during the Late Heavy Bombardment? [2]
Two thirds of the Earth’s surface is covered in water, yet paradoxically this constitutes only about 0.02 percent of the earth’s mass. This rises to 0.04 percent if we include the water present in the Earth’s rocky mantle – the area between the surface and the core. This is not very much, giving rise to the question[3] not why does the earth have so much water, but why does it have so little. The Earth, they say, is 100 times "drier than old bone", but the question of how the water we do have arrived here does demand an answer.
Initially, all the water in our solar system can be traced to the giant primordial cloud of gas and dust that collapsed to form the sun and our planets more than 4 ½ billion years ago. That cloud was rich with hydrogen and oxygen, the two atomic ingredients for water H20, most of which was relegated to the disk’s cold outer regions, past the “snow line” where it existed as ice. So far as Earth was concerned, the pattern of accretion involved collisions with some wet objects from past the snow line, thereby endowing the infant planet with some water. Then large impacts with the Earth’s surface during the later stages of its formation, such as that with the Mars sized object that produced the moon, largely cleansed our planet of nearly all its primordial water[4]. So scientists have long been looking for a source of water that could have been delivered after the formation and cooling of the earth-moon system, and ice-rich comets entering the solar system via the Oort cloud and the Kuiper belt, have been a prime candidate since at least the 1950s.
However, this theory struck trouble in the 1980s and 1990s when researchers made the first measurements of deuterium/hydrogen (D/H) ratios on comets from the Oort cloud. (Deuterium is a heavier isotope of hydrogen with a neutron on its nucleus, and its prevalence compared with that of normal hydrogen serves as a useful fingerprint for tracing an object’s history). If the earth’s ocean was made of melted comets its D/H ratio should closely match those of comets we observe today. But the Oort cloud comets showed D/H ratios twice as high as that of ordinary seawater, leading to the conclusion that most of the earth’s water must have come from somewhere else[5].
These conclusions have recently been corroborated by the Rosetta space probe which arrived on the Kuiper belt-originating comet 67P/Churyumov-Gerasimenko[6] after launching in 2004. Here, samples tested revealed a D/H ration three times greater than that of the ocean, pointing once again to another extra-terrestrial source for the Earth’s water, and asteroids orbiting in the “main belt” between Mars and Jupiter, with a much better chance of hitting Earth than comets, are now the prime candidate. Asteroids from farther out regions in the solar system, more than half-way to Jupiter, are relatively wet and already studies of certain families of meteorites have revealed that their D/H rations align with that of seawater, and tend to produce meteorites called carbonaceous chondrites, conglomerations of hydrated minerals that can make up several percent of the rock’s mass, attesting to a time when water ice melted and flowed through an asteroid’s rocky matrix. In the very late stages of planetary formation, the gravity of the outer giant planets scattered materials throughout the young solar system, flinging these wet asteroids down from beyond the snow line to strike earth and other rocky planets[7].
One major flaw in this theory is the absence of most noble gases on Earth - xenon and argon, relative to those found in meteorites – noble because they are spectacularly inert, reacting with any chemical compounds at all. This may tip the scales back in favour of comets as the Earth’s medium for water. In this regard, scientists are awaiting the first measurements of the comet 67P/Churyumov-Gerasimenko's noble gases from the Rosetta space probe by analysing the comet’s surface chemical composition to test several hypotheses about how the early solar system and the planets were formed and how the Earth came to be covered in water. We already know that comets carry amino acids and other organic chemicals, and scientists are now also keen to test the hypothesis that they may have “seeded” our world with the chemistry needed to help kick-start life.
The findings from 67P/Churyumov-Gerasimenko may shed new light on the problem or simply give rise to other scenarios[8]. Bodies larger than most comets and asteroids have also drawn attention of late. NASA’s Dawn mission targeting the dwarf planet, Ceres, up to one half of which is thought to be composed of water, arrived at its destination in April 2015, and another probe, known as New Horizons, arrived in the vicinity of Pluto in July of the same year. Ceres is believed to be composed of rock and ice and may have an internal liquid ocean. It accounts for about one-third of the total mass of all material in the asteroid belt, and may be an example of the transition between an asteroid and a planet. Building up from other asteroids in the main belt, its mass has accumulated to the point where gravity starts to smooth out its shape and become more spherical or planet-like [9].
[1] Published in the Journal Science and referred to at www.dailymail.co.uk/sciencetech/article-3421350/Earth-TWO-planets-Collision-created-moon-4-5-billion-years-ago-saw-pair-worlds-share-ice-land.html
[2] The substance of the remainder of this commentary is drawn from the article: "Where did all the water on Earth come from?", by David Jewitt and Edward D Young, “Oceans from the skies”, Scientific American, March 2015, 30 at 34-35. The article contains a very useful graphic of the process.
[3] Posed by Jewitt and Young, Ibid at 33, 37.
[4] Ibid at 32-33.
[5] Ibid, 33-34.
[6] As I write, just this morning in fact, 13 November 2014 at 2.30 am Australian Eastern Daylight Saving time.
[7] Jewitt and Young, op cit, 30 at 36.
[8] Explored in Ibid at 37.
[9] Source; Nasa Deep Space Network: https://www.nasa.gov/jpl/rosetta-comet-water-different-than-earth-water/ in ibid, at 37.