A couple of interesting scientific stories are making the rounds today, and worth a little physics and general science commentary. The first reminds us just how incredibly limited our sensory perceptions are in telling us about the world, by forcing us to imagine how it may look to animals whose perceptions are slightly different. The second reminds us just how little we know about our own planet.
One story that got my attention is that apparently the flashes of ultraviolet light randomly emitted by insulators on electrical power lines are visible to many mammals with whom we share the planet. And these flashes, up and down a long power line, may be scaring the heck out of them.
Light, or more generally, electromagnetic waves, can occur at any wavelength (the wavelength being the distance from one wavecrest to the next). But we humans see only an extremely narrow range of wavelengths — from about 0.4 millionths of a meter to about 0.70 millionths of a meter. (For scale; an atom is about 0.0003 millionths of a meter across. 0.7 millionths of a meter is also called 700 nanometers.)
It’s been known that many animals see light extending down to somewhat smaller wavelengths, but apparently this was thought not to be true of mammals. But recently reindeer were discovered to have eyes sensitive to light that reaches down to 0.3 millionths of a meter. (We call this “ultra-violet” light simply because it is “beyond-the-violet” — smaller wavelengths than the smallest ones that we can see, which appear violet to our brains. It’s sometimes called near-ultraviolet or UVA, because it’s just beyond the human visible range.)
That slightly broader range of vision is apparently enough to make power line flashes visible to their eyes. It’s been known that reindeer and other animals have been widely avoiding the areas with power lines, with the effect of fragmenting their habitat and populations (which can endanger them), but it hasn’t been understood why. Now it is suggested that these flashes might be the reason. There may be similar effects on other animals in other places, because it turns out that humans and monkeys may be unusual among mammals in not being able to see this slightly ultraviolet light. It may therefore be important, on environmental grounds, for power companies to prevent these flashes completely, which are caused by a buildup of electric charge that is suddenly released into the atmosphere. It’s yet another example of the many unexpected and unintended consequences of modern life… and of the limitations of perceiving the world with only human senses.
Next comes the report that a major step has been taken in the debate as to how much water is stored in part of the Earth’s interior.
Using earthquake waves as a probe over the past century, geologists have assembled a picture of the Earth (which has a radius of 6400 kilometers [4000 miles]) as consisting of a core of radius of nearly 3500 kilometers, and a mantle, extending from the top of the core nearly to the outer edge of radius of about 2900 kilometers, with a thin crust (typically 10 to a few tens of kilometers [6 – tens of miles] thick) on which all the continental land and oceans rest. The dense material in the mantle is in some ways like a solid and in some ways like an extremely viscous liquid that can be squeezed and deformed very slowly. (Caution: that’s a quick over-simplification; the mantle’s more complex than that.) The mantle’s details are probably still incompletely known, but there is an upper region extending from the crust down to about 410 km below the surface, and below that is a “transition zone” (410–660 km below the surface, or from about 5710 km to about 5960 km from the Earth’s center), which separates the upper and lower mantle.
There’s been a long-running debate as to whether the transition zone has a significant amount of water, perhaps dragged down from the ocean floor via the process of subduction. And just recently there’s been evidence (which I’ll describe in a moment) that the watery point of view is correct.
In the reporting on this story, a small number of press reports about this have actually bordered on saying that the Earth is now thought to have an underground ocean! Not so. A somewhat larger fraction of the press reports have said there’s an underground “ocean”, whatever those quotation marks are supposed to mean. Either way, it’s pretty misleading. If the water actually is down there, it’s not stored as liquid water. It would be stored in “hydrous” minerals, which contain water not as droplets or pools or puddles but by incorporating individual water molecules into their molecular structure. An example of a hydrous mineral is opal, which certainly isn’t a puddle.
Also, an “ocean’s-worth” of water may sound like a lot, and in a way it is a lot, but then again, not that much relatively speaking. Let’s do a little calculation to see what we’re talking about.
The volume of a sphere of radius r is (4/3) π r3, so if you dig out your on-line or off-line calculator, you can check:
- the volume of the earth is about 1.1 trillion km3 (where a trillion = a million times a million)
- the volume of the core is about 0.2 trillion km3
- so the volume of the mantle, the difference of the two, is about 0.9 trillion km3 ~ 82% of Earth’s volume
- the transition zone has 4/3 π ([5960 km]3 – [5710 km]3) = 0.1 trillion km3 = 9% of Earth’s volume
- the volume of the world’s oceans = 1.3 billion km3 = 0.0013 trillion km³ = 0.1% of Earth’s volume
Right — the ocean looks big, but the Earth is 1000 times bigger by volume. So that means that even if an amount of water equal to the earth’s oceans is inside the mantle’s transition zone, it is only (by volume) about 1% of the volume of the entire transition zone, and about 0.1% by volume of the Earth as a whole. Since water is less dense than rock (i.e., it has less mass for the same volume, which is why it is easier to lift a cup of water than a cup filled with rocks), the stored water is a lot less than 1% of the mass of the transition zone, etc.
The new story this week is that a small ugly diamond from deep underground has been found to contain a tiny, tiny crystal of blue ringwoodite (a very-high-pressure modification of olivine, containing magnesium, iron, silicon and oxygen). As often happens, the discovery was an accident; the geologists were studying the diamond for another reason. Ringwoodite, a beautiful blue crystal, is expected to form geologically only in the mantle’s transition zone, because of the extreme pressure required to make it. This is the first geologically-produced ringwoodite ever found, and as direct information about the transition zone, it has geologists very excited; maybe, if they look harder, they’ll find many more examples.
Moreover, ringwoodite is a hydrous mineral, and this tiny ringwoodite crystal turns out to have a small amount of water incorporated into its structure. And that connects us to our watery story. From this one tiny piece of evidence, a huge claim has been made that the entire transition zone must be 1% (or so) water, and that therefore there’s as much water down there as all the world’s oceans. Or so says the media based on the claims of at least one of the scientists who discovered this evidence.
“It’s actually the confirmation that there is a very, very large amount of water that’s trapped in a really distinct layer in the deep Earth,” said Graham Pearson, lead study author and a geochemist at the University of Alberta in Canada.”
But a non-expert like me is bound to ask: shouldn’t we have a bit more evidence before drawing such a sweeping conclusion? All we could possibly know so far is that at least one part of the transition zone has water incorporated in it. That’s still interesting, and a strong argument against those who’ve argued that the transition zone has virtually no water in it. But for all we know, the transition zone may have a highly variable water density, and so there’s simply no way to draw a conclusion about its total water content from one crystal from one location. On purely logical grounds, we can’t yet have confidence that there’s an ocean’s-worth of water in the mantle’s transition zone… despite what the media’s been saying and that Pearson is implying. After other scientists have found a few more blue pieces of evidence from completely different locations and found that most or all contain water, then maybe we can start to infer something about the transition region as a whole.
In any case, one shouldn’t go around thinking of the planet as made of water. Even if it turns out to be true that there’s as much water in the mantle as there is above ground, we’re still talking about a few tenths of a percent of the planet’s volume, and even less of its mass. Even if it might be a tiny bit wetter than some scientists thought, it’s still a rocky planet.