Setting the Stage for Our Biosphere 4.5 Billion Years Ago
The universe hummed along very nicely for nine billion years before events in a far corner of the Milky Way Galaxy conspired to create a moderate-sized planet in the orbit of a relatively new star. Thus far, it is the only planet in the universe known to sustain biological life. Granted, this claim to fame may be more of a statement of ignorance than knowledge since our detailed understanding of the universe is rather meager. What we can be sure of is, Earth formed as a watery planet and water is a sine qua non for life as we know it.
The earliest version of Earth was a hot place because cosmic debris collapsing into the proto-earth gravity well generated heat as it packed together. Earth needed to cool before water in the atmosphere condensed, forming our first oceans. The jury is out concerning the exact date this happened, but the most common view is sometime before 3.8 billion years ago. Most proposed dates for when liquid water first appeared are based on geological evidence. For example, the 3.8 billion year timing rests on the presence of pillow basalts in the Isua Greenstone Belt of Iceland. These rocks are 3.8 billion years old, and pillow basalts only form below water.
Other geological evidence points towards an even younger date. An analysis of detrital zircons from the Jack Hills terrane in Western Australia revealed a chemical composition indicating liquid water was present during their formation. Zircons contain radioactive components and are great for age dating. The formation of the Jack Hills zircons dates to 4.4 billion years, implying a hydrosphere of liquid water existed a mere 100 million years after our planet’s formation. But it takes a lot of water to fill an ocean. Where did it come from?
Comets and Asteroids
Basic science rests on data, measurements, and factual evidence, but at a larger and grander scale, science is about creative visions we call theories or hypotheses. A hypothesis is an untested idea and a theory is speculation based on objective evidence, but the evidence is extrapolated to propose yet unproven ideas. However, our understanding of events occurring billions of years ago relies on theories developed from data collected today. Reliable data is sometimes sparse, so the preference for one theory over another depends upon the weight of evidence supporting each.
One of the facts we observe today is the ratio of deuterium to hydrogen (DH ratio) in Earth’s oceans. That ratio may increase over time as hydrogen, the lighter of the two, leaks into space, but it should not decrease. Based on this observation, scientists can measure the DH ratio of water in asteroids and comets, and they observe that asteroids have water with DH ratios close to today’s oceans, but comets have DH ratios of up to twice that of Earth’s oceans.
This evidence supports a popular theory that the planet’s water originally came from outer space as waves of asteroids inundated early Earth about 700 to 800 million years after the planet formed, during a period called the Late Heavy Bombardment. But theories are simply statements of probability and not definitive facts, so the Late Heavy Bombardment Theory doesn’t necessarily jive with observations from the Jack Hills zircons that liquid water was present 4.4 billion years ago. These types of inconsistencies lead us to search for more data.
Strengthening a Theory
Scientists are realists, and they know the chance a theory is correct increases as more data supports it, bringing us back to the basics of collecting and analyzing data. Enter a new wave of space exploration — the detailed sampling of tiny celestial bodies. Small asteroids are hard to see, much less study, from Earth, and as the saying goes, “If the mountain will not come to Muhammad, then Muhammad must go to the mountain.”
The asteroids will not come to us, at least not in pristine condition, so we are going to them using remote spacecraft to sample an asteroid and return the samples to Earth. JAXA (the Japan Aerospace Exploration Agency) was the first to sample an asteroid in space with their Hayabusa mission in 2010. Later in 2014, the Hayabusa2 mission launched, eventually returning to Earth with samples of primordial material from the asteroid, 162173 Ryugu. Water similar in composition to ocean water was found in the Hayabusa2 samples. Separately, the European Space Agency’s Rosetta mission landed on a comet and found that a quarter of its mass was from organic molecules formed via nonliving processes.
Several data points alone do not prove the theory, but they certainly strengthen it. A young planet rich with water and organic molecules would be more conducive to life than a barren, dry, rocky world, or a gas giant like Jupiter or Saturn. So, perhaps our planet’s generous supply of water is a gift from outer space. When we pollute our oceans, remember we are defiling the precious gift that made life possible.
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Sources:
Why do we have an ocean? (Source: NOAA)
Isua Supracrustal Belt, West Greenland: Geochronology (by Vickie C. Bennett and Allen P. Nutman; Springer Link)
Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago (By Simon A. Wilde, John W. Valley, William H. Peck, & Colin M. Graham)
Most of Earth’s Water Came from Asteroids, Not Comets (by Charles Q. Choi; Space.com)
Small Wonders — How did Earth become an oasis for life? (National Geographic)