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water on Earth – no problemo

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water with bits of earth sticking out of it


So, as described in my last post, H2O in its various forms is plentiful in our solar system as well as beyond it. But, being more or less scientifically illiterate – despite decades of reading stuff on science – I can’t quite work out how liquid water is so abundant on the Earth’s surface. The story has long been told of water-iced asteroids in the time of the heavy bombardment being responsible, with the major proof being that these carbonaceous chondrite asteroids have, or had, the same signature of heavy (deuterium-rich) water as the water we find on Earth. While this seems a strong argument to me, how did the Earth manage to hold on to that water during those super-heated days? 

I’ve looked at this in a previous post, sort of, but I’m still not clear on the atmospheric conditions that brought about our soggy planet (much more soggy during the Mesozoic though). In any case, I’ve recently read that bonafide researchers on this topic have also been mystified about the sheer volume of water on Earth. 

Enter a new (to me) hypothesis, published in the Journal of Geophysical Research: Planets a little over a year ago. It argues – and other astrophysicists appear to be impressed by the reasoning and the detailed analysis in the paper – that the water came not only from asteroids but also from the solar nebula.

Solar nebula? Never heard of it, but apparently the concept has a long history. The so-called nebular hypothesis for the formation of our solar system was first proposed by Emanuel Swedenborg in the 1730s, and further elaborated by such luminaries as Immanuel Kant and Pierre-Simon Laplace later in the 18th century. Surprisingly for such an early contention, it has stood the test of time and survives today, though the details are still argued, and there are a few competing hypotheses. In any case, without going into too much detail, a nebula of dust and gas began to form around 4.6 billion years ago, and collapsed in on itself due to gravitational forces, spinning around a newly-formed sun. Out of this material, protoplanets gradually formed. 

Water in the Earth’s oceans has approximately the same D/H (deuterium to hydrogen) ratio as that of the above-mentioned asteroidal carbonaceous chondrites, so it has always seemed a safe bet that most if not all water came from those asteroids. Yet the sheer volume of water was still a problem. Jun Wu, the lead author of the recent paper, had this to say about the theoretical situation:

The solar nebula has been given the least attention among existing theories, although it was the predominant reservoir of hydrogen in our early solar system.

What has apparently added credence to the new hypothesis is that samples of hydrogen near the core of the Earth have significantly less deuterium and may fit better with the ratio of hydrogen in the solar nebula. Also the isotopic signatures of the noble gases helium and neon found in the Earth’s mantle fit the signatures of these gases from the time of the solar nebula. The explanation of how the lighter hydrogen found itself drawn to the Earth’s centre, in a process called isotropic fractionation, is provided in the paper, apparently. It’s a very interesting story, if true, and it may have implications for liquid water on habitable-zone exoplanets. That’s to say, there’s no reason for it not to be quite common. Here, to finish, are a couple of thought-provoking comments from members of the research team.

… there’s another way to think about sources of water in the solar system’s formative days. Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have served as the origin of Earth’s water.

Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems.


How did Earth get its water?

Written by stewart henderson

December 27, 2019 at 6:32 pm

reasons to be cheerless, part one…

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Tunguska – caused by an alien

Will anthropogenetic global warming be an unmitigated disaster or will it be a boon to some species, possibly even our own? Will artificial intelligence make slaves of us all or enable us to become masters of the multiverse? Will social media developments turn us all into obese opinionated ignoramuses or will it help to unite the powerless against destructive autocrats? Will virtual reality sex liberate the unattractive or simply diminish real relations? And so on. When considering the future we often make the error of imagining the past as having been more predictable than it was. In particular, we think we know ‘human nature’, and we generally consider it unchanging, so we can predict our response to events, even if we can’t predict the events themselves. IMHO, we’re mistaken even on that count.

But let’s consider an event we definitely can’t control. Maybe there’s an object out there in space, a very big one, that’s on track to smash into our planet, with such speed, power and accuracy, that all concerns about human development and response become superfluous.

Few things can be more chilling than inevitability. I experienced this once in my early twenties, in casual conversation with a friend. Something in our talk struck me, and I realised, in a heart-freezing moment, that I was destined to die. Of course, I’ve had a long time since then to come to terms with it! But in reading of fatal events, what often torments the mind is the gap between the knowing and the happening. As they say, falling off a cliff never hurt anyone, it’s the landing that does it – but that’s probably wrong, you can suffer a lot of hurt in anticipating the end.

So this morning I was reading about Shoemaker-Levy 9, a comet named after those who discovered it, more or less by accident (though it was the ninth comet discovered by the team, hence the name), in 1993, by which time it had been captured by the gravitational field of Jupiter. In fact, studies of its orbital motion showed that it had been orbiting Jupiter for at least twenty years, and had begun to fragment a year or so before its discovery, when it passed close to Jupiter in an eccentric orbit. It was the first comet ever discovered to be orbiting a planet rather than the sun. Its discovery caused a sensation in the astronomical community, especially as further calculations of its behaviour confirmed that it was certain to collide with the planet. Which, in one week in July 1994, it spectacularly did, in bits and pieces, the largest of which had an impact described at the time as ‘500 times more powerful than the detonation of the whole world’s nuclear weaponry’.

Jupiter is, of course, the largest planet in our solar system, and has been described as a ‘cosmic vacuum cleaner’, sucking asteroids and small comets into its orbit at a rate many thousands of times more than Earth does. The assumption being that it’s protecting us little planets from a lot of nasty stuff. And yet…

great pic of Comet Hyakutake, in 1996

That protection isn’t guaranteed, as what is now called the Cretaceous-Palaeogene impact even shows. That was pretty massive, and the Tunguska event of 1908 was relatively tiny, as was the Chelyabinsk event of 2013, and then there are much larger bodies passing by, such as Comet Hyakutake, and so on. But as to the size and placement of these events, we seem not to reliably forewarned. The second largest impact in over 100 years (since Tunguska) occurred in December 2018, but few lay people even know about it. It occurred over the Bering Sea, between Russia and Alaska. The meteor had a diameter of 10 metres – nothing compared to Shoemaker-Levey 9’s five kilometre nucleus – and of course it impacted in a region uninhabited by humans, but the fact that these impacts aren’t picked up until the last minute is a worry. Of course technology is being developed to improve the situation, but it’s not there yet. And then there’s that mighty, faraway object that might be hurtling towards us, beyond our ken…

Written by stewart henderson

November 24, 2019 at 12:00 pm