From Stardust to Oceans: The Cosmic Journey of Water
I remember when I realised how magical water is, but I certainly didn’t always think so. I was attending my first of many hydraulics modules at University, whilst studying my for my engineering degree. After listening for several hours to the professor derive the Bernoulli principle from first principles, its safe to say my interest was waning. Apart from drinking water and swimming, I would rather stay dry and clear of the rest. Then suddenly to fill the silence whilst sketching on the blackboard, we were told “Oh and don’t forget that water is from outer space!” I was suddenly alert with a dozen questions, but sadly the lecture quickly moved back to pressure differentials. I just had to dig deeper into this.
Fast forward to the current day and I am an environmental engineer working in the water treatment sector. It still surprises me how many people do not know the real magic of water, so I would like to share its story.
Cosmic Origins
Could water really be from out of space? I remember being told at school that a dinosaur had probably previously drunk the same water that I had in my glass, but is this really true? Or has the cup of tea I am currently drinking as I write this article previously been circulated through the kidneys (?) of an extra terrestrial being in a galaxy far, far way? Well unfortunately both of those are extremely unlikely, but it appears I and even NASA are sure that water is indeed from space…..
A water molecule is of course two Hydrogen atoms and a single Oxygen. Hydrogen was created during the big bang and Oxygen in those cosmic ovens known as supernovae, exploding giant stars much larger than our sun. Huge amounts of water in gaseous form exists in the vast steller nurseries of our galaxy. The Hubble space telescope has peered into the Helix Nebula and found water molecules. Nebulas give birth to new stars, but even they are largely vast volumes of empty space, with Hydrogen the most common atom within them. Very small amounts of other gasses exist, but despite this, it appears, given enough time they find each other. The Orion Nebula is thought to create enough water each day to fill all of earth’s oceans 60 times over!
How did the water arrive on Earth?
Around 4.6 billion years ago a nearby supernova explosion collapsed the gasses in a solar nebula, which formed our sun and eventually the planets in our solar system. So goes the Nebula hypothesis. Much material was left over, including oceans worth of water molecules and lots of rock. Water, along with every other molecule created in these stellar nurseries, becomes raw material for the formation of new planetary systems. Its at this point where competing hypothesis are introduced as to how the Earth got its water.
Asteroids and Comets
Around 4.1 - 3.8 billion years ago the new solar system underwent a traumatic event known as the late heavy bombardment (LHB). Huge numbers of water containing asteroids and comets impacted the relatively juvenile Earth and other bodies in the solar system. These bodies were formed far enough away from the sun so that they could attract water, where it could condense and remain. On impact with the Earth the water would have been vaporised only to eventually condense again and cool to form the oceans we see today. There are still many comets and asteroids in our solar system and they are time capsules of clues. Do comets and asteroids have the power of both life and death in our galaxy?
An asteroid is mostly rock and metal, which is material that has not formed into a planetary body. Their orbits tend to be small and circular and they do contain condensed water. There is an asteroid belt between Jupiter and Mars that is material left over form the formation of our solar system. Due to the huge size of Jupiter and its gravitational attraction this material, did not form into a planet. Due to the chaotic nature of orbits at the time, huge numbers of these asteroids were ejected and sent into the solar system and many collided with Earth. Of the asteroids that remain, around 75% are thought to be ‘carbonaceous chondrite’ asteroids and we will shortly see that this is important.
A comet is an object mostly made up on frozen water, ammonia other gases. They do also contain some dust and rocky material. They are much older objects than asteroids and were formed in places much further out in our solar system such as the Kuiper Belt and the mysterious Oort Cloud. Their orbits tend to be long and elliptical from the far edges of the solar system inwards around the sun. They start to lose material as they get close to the sun and eventually break up entirely. As they warm up they leave behind their famous tails, often over a million Km long.
A comet has recently been investigated by the Rosetta mission, where for the first time a probe was successfully landed onto a comet in 2014. The comets name is 67-P. It was found that the water within the comet was of a different deuterium-to-hydrogen (D/H) ratio than that found on Earth. Deuterium is a heavier isotope of Hydrogen and they can both bond with Oxygen to make water (known as heavy or light water).
Earth’s oceans have a specific ratio of D/H and this can be used to compare against other cosmic sources. Earth’s ratio does not match those found on comet 67-P, but the range of ratios analysed from carbonaceous chondrite asteroids is closer aligned, but its still not perfect. This concludes that much of Earth’s water is probably from asteroids, and some from Comets, but to fully explain doesn’t the Ocean’s D/H ratio, there may be another source.
Primordial Water
It may come as a surprise, but there is a lot of water on Earth that we don’t see and I am not referring to the ground water we rely on for agriculture and consumption in many parts of the world. Its much deeper and harder to find than that. It is estimated that the Earth’s mantle holds around 3-10 times the volume of water found in all of the oceans! Unfortunately it does not appear that this is easily accessible in large underground reservoirs or even in liquid form. This water is instead structurally bound within the crystal lattices of hydrous minerals such as Serpentine over 400km from the surface. This water has a lower D/H ratio than that found in the Oceans and it is nearly all the lighter version of water. The question is, where did this come from?
Around 4.6 billion years ago when the Earth was forming many heavy elements such as Iron, Silica and Oxygen were in place and abundant in the area. The solar winds were blowing the lighter elements such as the Deuterium and Hydrogen towards Jupiter and the asteroid belt. They would later combine with Oxygen to produce the water found in the comets and asteroids than would reappear more than half a billion years later. It is hypothesised that the sun would later provide the light Hydrogen that would combine to make water in the mantle. This was achieved by releasing protons in the solar winds that would interact with an steal an electron from the relatively higher densities of dust in the solar system. These dust particles would eventually form part of the initial building blocks of earth, where they would combine with the Oxygen already present in the rocks. Overtime, these materials would continue to build and become locked inside of the mantle as primordial water. The Earth would heat up due to radioactive decay that would cause much of the planet to become molten. Much of the trapped water would be released to form water vapour in the atmosphere. Some of this water would remain trapped in the mantle after the Earths surface cooled and solidified. Volcanic eruption and outgassing would provide further opportunities for the water to be released. The D/H ratio sampled in many eruptions does match the signature found in the Oceans.
Oceans Lost
Do planets lose their oceans over time? Yes they do. Billions of years ago Venus may have been the first in our solar system to develop an ocean, but the planet lacks the strong magnetic field that the Earth has. As a result of this and by being closer to the sun, a runaway greenhouse effect boiled the oceans and they were lost into space. Mars also likely had large bodies of liquid water, lakes and even oceans billions of years ago. A strong magnetic field existed at this time, but this only lasted a few hundred million years as it significantly reduced 4 billion years ago when the planet cooled. Without the field to hold onto its atmosphere and drop in surface temperatures, most of its atmosphere and water was lost to space. Although some remains in frozen ice caps and its likely that there is there is primordial water deep under the surface.
The good news is that the Earth has retained its magnetic field due to the inner core of the planet remaining molten. The radioactive decay within Earth create the heat and the molten core moved and generates a magnetic field. This is likely to continue for some time. The amount of water on Earth has been relatively consistent for around 4 billion years and this is not expected to significantly change. Despite this the water is constantly added and lost in a water cycle. It is estimated that UV light breaking down the bonds of water in the atmosphere allows around 30kg/s of water to escape into space. Although space still supplies us with new water from micro meteoroids and this has been estimated to be 1500 tonnes per year. Metabolic Processes such cellular respiration in living organisms also produce water and its estimated that an average human produces 300ml of additional metabolic water per day! Further to this global fossil fuel combustion produces around 1.5 billion tonnes of water a year.
Conclusions
Once on Earth, water has continuously cycled through the atmosphere, oceans, and land for billions of years. The molecules themselves are ancient, but through evaporation, precipitation, and other processes, they have been constantly recycled, making the water you drink today as old as Earth's earliest water supplies. We should also be thankful that the earth is able to keep hold of this water in liquid form, which is unusual.
In essence, the water we use today has a deep cosmic history, dating back to the early universe and enriched by processes that took place long before Earth even existed.
So it looks like all of our water is indeed from space. It has arrived at different times and through different delivery methods and it may be impossible to understand which processes provided the most water.
