What is the origin of water on Earth? (maybe not what some thought)


RELEASE: While everybody agrees that our blue planet is rich in water, this observation is at odd, first, with the exploration of other rocky planets, genuinely lacking surface water, and second, with the idea of a giant impact between the proto-Earth and a planetary embryo the size of Mars that created the Moon. Such a catastrophic event should have vaporized any pre-existing water, leaving behind a dry Earth. After the giant impact catastrophe, we have thus two options to explain the presence of water on Earth : either water was brought back later, after the catastrophe, notably by icy or water-rich asteroids; or the giant impact was not big enough to vaporize all the water on Earth.

Because of the importance of water to sustain life, the question of the origin of water on Earth is primordial. A major challenge in investigating this question is that Earth has lost all the traces of its formation since it is an active planet.

A team of numerical modelers and geochemists led by Cédric Gillmann - Université libre de Bruxelles, ULB, funded by the EoS project ET-HoME - has decided to look far beyond Earth - up to Venus - to investigate the origin of terrestrial water.

While Earth and Venus could be considered as twin sisters, their respective geological and climatic evolutions diverged dramatically in the past, leading to Venus' present-day 92 bar atmosphere heated by an infernal greenhouse up to 470°C, opposed to the mild conditions and only 1 bar pressure at the surface of Earth.

However, Venus' volcanic activity and outgassing are reduced compared to Earth, because it has no plate tectonics, but has a stagnant lid instead. Even better, such a convection mode implies very little recycling of volatile species into the mantle.

As such, despite being an inferno, the evolution of the atmosphere of Venus is much easier to understand and model over geological times. In addition, because of their proximity, the Earth and Venus should have received the same type of material during their history. All these aspects combine to make Venus a perfect place to study the primitive evolution of terrestrial planets.

Using numerical simulations of impacts of different types of asteroids containing various amount of water, the team has discovered that water-rich asteroids colliding with Venus and releasing their water as vapor cannot explain the composition of Venus atmosphere as we measure it today. It means that the asteroidal material that came to Venus, and thus to Earth, after the giant impact must have been dry, therefore preventing the replenishment of the Earth in water.

Because water can obviously be found on our planet today, it means that the water we are now enjoying on Earth has been there since its formation, likely buried deep in the Earth so it could survive the giant impact.

This idea has very deep implications in terms of habitability of ancient Earth, Venus and Mars, as it suggests that planets likely formed with their near-full budget in water, and slowly lost it with time. Because Mars is much smaller, it likely lost all its water while life developed on Earth. For Venus, those results shine a complementary light on recent work advocating that water oceans existed at the surface of the planet, and help constrain the maximum amount of water that can be expected on Venus. They will also help prepare the next generation of space missions to Venus.
I cannot justify it, but I find it difficult to believe that so much water was left after the Theia impact.
With some local relevance I'm going to suggest that the solar system was formed by a series of catastrophes.
One player is hydrogen - the most abundant element in the universe and the one most likely to collect in self-generated gravitational wells.
Another player (of many) is iron - formed in stars as the end product of fission in stars. Sitting on a planet with an iron core it seems a fair amount of iron comes out when a star goes supernova. A feature of iron is that it is magnetic and (maybe) forms clumps independently of gravitational wells.
Yet another player is angular momentum. When a ballet dancer pulls her arms in she spins faster. To spin faster she must have been rotating in the first place. The solar system clearly has angular momentum else we'd all have fallen into the sun  a long time ago. Either I'm missing something (highly likely) or there was an off-centre collision early on in the development of the solar system. So the gas giants being further out than (some of) the rocky planets is (could be) the result of an early highly improbable event. The rocky planets with iron cores don't look they were formed by the same mechanism as either the sun or the gas giants because they weren't.

I don't think any hydrogen has been made since the big bang. Iron, oxygen, carbon and the the rest are new kids on the block. Iron oxidises. Iron forms hydroxides. Iron hydrates. Nickel (also magnetic and sell-clumping (maybe)) catalyses a lot of things that wouldn't otherwise happen. So my guess is that iron collected up the water originally.

Enough speculation.

Going back to:
catastrophe Wrote:I cannot justify it, but I find it difficult to believe that so much water was left after the Theia impact.
I'm just kind'a suggesting you'd have to keep rolling the dice until you got the angle right. With the right angle you get flowers and romance, with the wrong angle you get two barren worlds.

Edit - I've just seen:
catastrophe Wrote:I did say that I am a scientist (B.Sc. Chemical Engineering)
I'm a self-schooled peasant. Your comments on hydration would be appreciated.
Edit2 - Correction to my self-schooled peasant description:
In reality the result of many clever people being kind enough to give up their time to help me. My limitations are those I was born with - no more and no less.

When I said that I was a Chemical Engineer, that was nearly 60 years ago. Chemical Engineering included Chemistry, Mechanical Engineering, Electrical Engineering, more Maths and you name it. As a result I have forgotten most of it and ended up knowing just about nothing about everything anyway Smile

However, you do have a point. Wiki gives: "In molecular formulas water of crystallization is indicated in various ways, but is often vague. The terms hydrated compound and hydrate are generally vaguely defined." What it really means is that water can be included in two ways. One might be a simple association and the water may be driven off just by heating, as simply as drying a wet plate. The other is where the water is more tightly (chemically) bound, and in this case the combined structure may have to be destroyed by much stronger heating to separate the water.

So when I posted "I cannot justify it, but I find it difficult to believe that so much water was left after the Theia impact." I was assuming (rightly or wrongly) that either the impact was sufficient to remove both or that only the first situation applied. You were quite right to make me give more consideration to my statement.  Smile

My impression of the Theia impact, however, was that if it were violent enough to form the Moon, then there would probably have been enough heat to vaporise the water. How much water was then recaptured, I think is unknowable this long after the event.


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