I was researching a fun post about the apocalypse today when I suddenly came across a disaster scale I’d never even heard of before. It’s the Volcanic Explosivity Index, which is a way of measuring the magnitude of volcanic eruptions via the amount of ejecta they produce – not a perfect way of ranking volcanic eruptions, if you ask me, but probably the only one that’s really possible given all the different ways a volcano can explode. It then struck me that it might be a good idea to spend a little while talking about the major disaster scales and why they’re set up the way they are, since it’ll be a good setup for whenever I do get around to the apocalypse, as well as ensuring that next time you read a news report about an earthquake you’ll have some idea of what the experts mean when they say it measured 5.8 on the Richter scale.

So let’s start with the Richter scale, which is the obvious one everyone thinks they know about. The Richter scale operates on a logarithmic scale, which is a method of measuring quantities that increase exponentially while keeping them all on the same piece of graph paper – or in this case, on the same ten point measurement device. This means that each point on the Richter scale represents an increase of ten times the power of the previous point on the scale; an earthquake measuring 3.0 on the Richter scale is ten times more powerful than one which measures 2.0, and a hundred times more powerful than one which comes in at 1.0. It’s this measurement quirk more than anything else that leads to most local reported earthquakes coming in at around the 4.0—5.5 mark, since the scale has to cover the entire range of possible earthquakes from tiny microtremors to vast world-ending ruptures in the earth’s crust. 4.0 is the point at which earthquakes become noticeable by the majority of humans, and at 5.0 they start causing minor property damage; these sound like large numbers on a ten point scale but you have to remember that even a 5.0 earthquake is a thousand times less powerful than the 8.0s which cause tsunamis and kill thousands of people in places which are – hopefully – a long way away.

Anyway, what you should take away from this is that the first six points on the Richter scale represent comparatively weak earthquakes that have little chance of killing anyone; the news reports are usually just quoting them to make them seem bigger than they really are. The lethal stuff doesn’t really start until after 6.0, and the good news is that you only tend to experience one of those if you live in close proximity to a tectonic plate boundary. I would also draw your attention to the final point on the conventional Richter scale – 10.0 – which says that an earthquake of this magnitude has never been recorded. That’s not to say it can’t happen, just that it would take something rather extraordinary to prompt that sort of geological upheaval, like the earth being walloped by a bloody great asteroid. We don’t notice earthquakes below 4.0 and the range of known earthquakes only goes up to 9.9, so as far as most people are concerned the Richter scale actually operates between 4.0 and 9.9, and suddenly your local earthquake measuring 5.1 on the scale doesn’t really seem like all that much.

Then there’s the Volcanic Explosivity Index, which works much the same way as the Richter scale (except at lower bounds between 0 and 3 due to the way low-end volcanic eruptions work) except it measures the quantity of volcanic ejecta rather than the amplitude of the shockwaves passing through the earth. The difficult thing to get your head around here is that the VEI measures things in terms of volume, and it can be quite hard for the human mind to comprehend just how large a cubic kilometre of ash really is. It’s one of the questions of scale that often trips up undergraduate physicists, actually¹: how many cubic metres go into a cubic kilometre? The answer isn’t a thousand, as a lot of people think; it’s actually one billion, which is a very large number of cubic metres indeed.

Now, unlike earthquakes volcanoes are usually only locally devastating in the short-term, which is one of the reasons why the VEI is focusing on the amount of ejecta – or in other words, the amount of ash – they release. Not only is this a handy way to rank the violence of a given volcano’s eruption, but it’s the ash and gas output over the long-term that will do the major damage on a global scale.  Eyjafjallajokull shut down all flights over Europe with its ash when it erupted in 2010, and that ranks at a relatively titchy 4 on the Volcanic Explosivity Index with 0.1 cubic kilometres of ash output. Krakatoa in 1883 turned the skies around the world an attractive shade of blood red and dropped global temperatures by a full degree Celsius – and Krakatoa only comes in on the VEI index at six out of eight. Once you get out to eight out of eight – the so-called supervolcano eruptions of Yellowstone and Toba, each spewing out over a thousand cubic kilometres of material – you start to see why volcanic eruptions can be globally threatening events. Both Yellowstone and Toba spread so much ash into the atmosphere and onto the Earth’s surface that not only was there a significant drop in the global temperature for years afterwards, but the human, animal and plant populations of the world started to suffer mass dieoffs as the ash either polluted their food/water sources or made it impossible to get enough light to grow. There’s even a theory that posits Toba was responsible for reducing the human race to just a few hundred living individuals, which is why the seven-billion-odd modern humans are descended from a relatively small number of ancestors. Personally I think this is just a little bit too sensational, but it’s inarguable that supervolcano eruptions on this scale would seriously mess up civilization as we know it.

Finally there’s the Torino scale, which is used to rate asteroid impacts. Given that the vast majority of asteroids tend to miss the Earth completely, the Torino scale differs from the others in that it combines both the potential magnitude of the impact event with the probability that the asteroid will actually hit the Earth to produce a single number that indicates just how much we should be worrying about it. Small asteroids that will definitely hit us but will burn up in the atmosphere rank in low positions on the Torino scale. Large asteroids that could explode a small country but which have low probabilities of impacting are also ranked in low positions on the Torino scale. It’s only a combination of a (reasonably) large asteroid and high probability of impact which results in an asteroid having a Torino number attached to it which is larger than one, and if you look at the chart it is impact probability which is the dominant factor in an asteroid managing to place on the higher regions of the scale.  Which is fair enough, really; an object with a 1% chance of impact is something we should definitely keep our eye on, but until we get improved observations of its trajectory and know for sure it’s not a huge worry unless it’s something large enough to potentially end all life on Earth.

So any object with a Torino scale value of 1 or 0 is basically completely non-threatening, and as far as I’m aware the only asteroid that has made it past 2 was 99942 Apophis back in 2004, which was briefly ranked at 4 due to its size (350 metres diameter, which isn’t world-ending but is still pretty big) and the fact that it had a whopping 1% chance of impacting. That it was eventually downgraded to zero should tell you something about the Torino scale: since the bottom four rungs of the scale are based on impact probabilities calculated before detailed observations, this range of numbers basically exists to tell astronomers what they should be focusing their attention on and not to panic the general public. Not that this stops the media from completely misinterpreting what the scale means, of course;  they actually had to rejig the scale categories at one point because asteroids with a Torino value of 1 kept cropping up in news reports as potential threats when in fact these objects posed no unusual danger and just needed a double-check to make sure there was no chance of them hitting.  (This is why 1 is now in its own category as “NORMAL” instead of “MERITING FURTHER ATTENTIONS OF ASTRONOMERS”.)

But what about the upper regions of the scale? It’s the last three points we should really be interested in, which represent certain impacts with increasing levels of destructiveness. Number 8 is localised destructive potential, which basically equates to the detonation of a large nuclear weapon – annoying if you happen to be standing nearby, but not unduly threatening to everyone else. Number 9 represents regional  destructive potential, which is… hmm. If it was an ocean impact we’d probably be talking about something on the level of the 2004 or 2011 tsunamis, but I’m not sure there’s a good analogue for a land impact. Earthquakes don’t really translate that well into asteroid strikes, but maybe just imagine a similar level of devastation to one which wrecks an entire country (so Haiti 2010, Lisbon 1758 etc.) and you’d have some idea of the amount of damage we’re talking about. And then there’s a Torino number of 10, which has the interesting wording of global climactic catastrophe.   An asteroid that ranks at 10 doesn’t have to be large enough to kill millions of people through direct impact damage (although it probably would just as a byproduct), it just has to be able to throw up enough impact debris to cause mass species dieoffs similar to a supervolcano (or a nuclear winter).

Now, if you ask me there should be a number beyond 10. An impact of 10 on the Torino scale is small enough that it still leaves a fair chance of survival for the human race, albeit with vastly reduced numbers. If there was an impact event powerful enough to crack the earth’s crust open there’d be no survivors whatsoever, so maybe there should be a number 11 in its own category of “WE’RE SO SCREWED”. On the other hand the Torino scale exists to communicate information to the public and I suspect the public would find the existence of this category profoundly depressing, so perhaps it’s for the best.

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If you read just about any major news site with any sort of regularity, you may have noticed a disturbing news item creeping into the most-read sidebar: the conviction of six Italian seismologists for not-really-specified offences over the 2009 earthquake in L’Aquila that killed 309 people. I say not-really-specified because the reporting on this has been uniformly bloody terrible; every single article I have read – bar one – has been based off of the same piece of agency copy, with the same points, same facts, and same quotes in each one. That original agency copy seeks to cast the case as one of the Italian judiciary versus science, and that these scientists have been convicted for failing to predict the unpredictable: an earthquake. But it’s not quite as simple as that.

The one piece I have read that has actually gone into detail on the case is this article in Nature. It was written long before the guilty verdicts came in but is nonetheless chock full of useful information on the rationale behind charging the seismologists in the first place. To hear the prosecution tell it, it wasn’t that they failed to predict the earthquake at all. Rather it was that they failed to adequately inform the population of L’Aquila and the surrounding towns of just what the risks of the earthquake were, and that their public statements were more concerned with calming the populace than they were preparing people for the worst. And this at least I can understand, although I still think it’s a desperately misguided argument on which to base a manslaughter charge. The problem is one that many – if not most – scientists will come a cropper from at some point in their careers, although probably not in such a serious and tragic fashion: miscommunication.

Communicating scientific ideas to the public in a way that they understand can be a very tricky business at the best of times. I should know; not only do I have a sizeable amount of experience in science education (not to mention running this blog for fun) but I’m spending a quite considerable amount of time trying to get a full-time career in science communication, and one of the things I’ve learned is that because of their training there is quite a significant gulf separating a scientist’s perception of the world from the way the average person sees it. This happens to anyone who follows a dedicated career path for a couple of decades, really, but it’s a particular problem with science: you become so invested in it that your sense of perspective will be lost and you’re going to start to take certain things for granted. I do genuinely sometimes have trouble remembering that there are people out there who can’t instantly name every single planet and rank them by size, orbital radius and general surface conditions. That knowledge is just there for me; it’s as natural to know it as it is to breathe air. And it’s so ingrained that it can sometimes be difficult to relate to another person using basic English terms that they’ll understand, kind of like trying to explain the concept of balance (“Don’t fall over.”)

So there’s plenty of space for miscommunication in science. Even scientists can often have trouble understanding one another when scientific papers are not only written in the exclusive language of mathematics, but often contain their own esoteric jargon specific to whatever the particular area of research is. These are complex, hard-to-swallow concepts that are very difficult to reduce to terms a normal person can understand without losing some of the nuance and some of the meaning.  Take probability, for example; most people would interpret a 95% chance of something happening as a guaranteed success because the chances of failure are so low, and they’d be upset if they fell into the 5% that didn’t make it. It’d seem unfair, like it didn’t fit the odds at all. And yet somebody has to. That is what the number means, and to a scientist it seems perfectly logical to say a 5% chance of something happening is unlikely and still maintain that outlook when the event in question actually does happen. It was unlikely, but it still happened. The two aren’t mutually exclusive.

I think that something like this is what happened in the Italian case. Actually I think it’s a number of factors all working together in a rather unfortunate combination that seems crazy to an outsider: grieving families in the area looking for somebody to blame; a media that might not be particularly interested in accurately communicating what the scientists had to say in the first place if it’d get in the way of what people might want to hear; education failing to adequately arm the general population with the critical thinking skills needed to properly parse information from that media; and a judicial culture in which public officials can be and are held responsible for the things they say. Added to that is the fact that the seismologists were facing an impossible situation, as the Nature article states:

[L’Aquila’s] most recent seismic tragedy began in October 2008, when dozens of low-magnitude tremors began to hit the city and surrounding areas along the Aterno River valley. Known as seismic swarms, these tremors continued intermittently over the first three months of 2009; according to Picuti, they numbered 69 in January, 78 in February and 100 in March, with an additional 57 shocks during the first five days of April… Unnerving though these clusters may be, experts agree that seismic swarms rarely precede major earthquakes. In 1988, seismic engineer Giuseppe Grandori, now professor emeritus at the Polytechnic of Milan, and his colleagues published a retrospective analysis of seismic swarms in three other earthquake-prone Italian localities. They concluded that a medium-sized shock in a swarm forecasts a major event within several days about 2% of the time, and Grandori says that the same was probably true for the region around L’Aquila.

Translating these risks is extremely challenging for civil defence officials. In Grandori’s view, there is a 98% probability of a false alarm if officials issue an alert, yet a terrible price to pay in loss of life and property if they fail to issue a warning and a major quake occurs. After a medium-sized shock in a seismic swarm, the risk of a major quake can increase anywhere from 100-fold to nearly 1,000-fold in the short term, according to Jordan, although the overall probability remains extremely low. “What do you tell people in that situation?” he says. “You’re sort of between Scylla and Charybdis on this thing.”

Now, here’s the interesting part of the Nature article: what the seismologists actually said which lead to the manslaughter prosecution. The background for this is an unusual meeting of the risks commission they sat on. Usually the commission meets in Rome, but given the heightened seismic activity in the area this particular meeting was held in L’Aquila. This should have been the seismologists’ first clue that Something Was Up (they didn’t choose to convene it there).

Many people in L’Aquila now view the meeting as essentially a public-relations event held to reassure local residents. Christian Del Pinto, a seismologist with the civil-protection department for the neighbouring region of Molise, sat in on part of the meeting and later told prosecutors in L’Aquila that the commission proceedings struck him as a “grotesque pantomine”. Even Boschi now says that “the point of the meeting was to calm the population. We [scientists] didn’t understand that until later on.”

What happened outside the meeting room may haunt the scientists, and perhaps the world of risk assessment, for many years. Two members of the commission, Barberi and De Bernardinis, along with mayor Cialente and an official from Abruzzo’s civil-protection department, held a press conference to discuss the findings of the meeting. De Bernardinis said that the seismic situation in L’Aquila was “certainly normal” and posed “no danger”, adding that “the scientific community continues to assure me that, to the contrary, it’s a favourable situation because of the continuous discharge of energy”. When prompted by a journalist who said, “So we should have a nice glass of wine,” De Bernardinis replied “Absolutely”, and urged locals to have a glass of Montepulciano.

This links into the commission’s shaky conclusion that increased seismic activity was good because it meant energy was being continually released rather than continually building up, but even discounting that these are some really, really dumb things to be saying to the press without qualification. “No danger” might be a somewhat reasonable pronouncement to make given the 2% odds, but I’d say that while a 2% chance of an earthquake is certainly very small it’s not negligible and cannot be discounted. This should have been communicated to the population of L’Aquila, and the fact that it was not means there is some rather unfortunate merit in the prosecution’s argument that they failed to adequately explain the risk of a quake. Are they criminally liable for what they said? No, as one of the defence lawyers explains later on:

To convey the difficulty of communicating risk assessments, he offers the analogy of being asked the safest way to travel, and recommending flying because it is statistically much safer than car or train. “If the person takes the plane, and the plane is involved in an accident, this doesn’t mean that my advice was wrong,” he said. “I gave the right advice, since scientific advice is based on statistics, and the statistics don’t exclude the possibility of an event that we would like to avoid.”

Saying that there was no danger was pushing it a little too far, but on the whole the commission was perfectly in the right to say that there was little risk. They gave the best prediction they could based on the evidence they had and our prior experience with earthquakes. However, for scientists in public-facing positions of importance the seismologists were incredibly naïve in dealing with the press and absolutely woeful at communicating the reasoning behind their conclusions. They don’t deserve to be sent to prison for it, but they do certainly deserve to never hold that sort of position again because they’ve shown they’re not up to the job.

This incident isn’t going to have any sort of chilling effect on scientific dialogue because a) this is Italy and b) scientists are notoriously publicity-hungry by design. Being perceived as an authority and getting quoted in the press is seen as rather career-enhancing and you’d be extremely hard-pressed to find one who’d actively refuse an interview because he/she thought he might get something wrong and be held responsible. However, it does demonstrate why science education and science communication is important. In an ideal world the press would have said “Wait, what?” and actively demanded that the commission justify themselves rather than taking their word for it, but that’s asking rather a lot of the world. In lieu of that, though, it would have been much better for everyone involved if the commission had been able to express itself more clearly (and less glibly) than it actually did. People need to make an effort to broaden their minds, but at the same time scientists need to make an effort to communicate properly rather than just assuming everyone understands what they’re talking about because they do.

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A hundred years ago plate tectonics wasn’t even a thing. People thought the Earth was basically this boring lump of rock that had been slowly cooling and shrinking ever since it formed, with mountains and other irregular features being explained away as the surface crumpling and wrinkling kind of like the skin of an old apple. This cooling process gave the Earth a predicted age of only a few million years, since it was assumed that the Earth had been losing heat via black body radiation ever since it had formed and there weren’t any other known sources of heat within the Earth that could mitigate this in any significant way. The discovery of radioactivity in the late 19^th century provided the impetus for a slow reassessment of this theory. Radioactive elements produce heat as a byproduct of their decay, meaning that not only could the Earth be much older than previously thought, but that the interior of the Earth might be hot enough to be liquid or semi-liquid allowing new theories about its internal structure and geological processes to be formulated. However, it wasn’t until the 1960s that irrefutable evidence was found for the modern tectonic plate model of the Earth’s crust via a phenomenon called seafloor spreading.

What is seafloor spreading? Well, say you have an area in the middle of the seabed where two tectonic plates are slowly moving apart. Mantle material – in the form of LIQUID HOT MAGMA — will gush up from deeper inside the Earth to plug the gap, which cools and solidifies when it reaches the surface to become part of each tectonic plate. Meanwhile the other side of the tectonic plate is banging up against yet another plate, and is likely being subducted – that is, driven underneath that plate to become part of the mantle. This results in a sort of “conveyor belt” system where the two plates are constantly moving apart, but new material is being added to each plate at the boundary separating the two.

However this still might have been dismissed as an isolated or recent phenomenon had it not been for analysis of the magnetic striping of each side of the plate. As newly formed rock cools – such as the rock created by the upwelling of magma in the centre of the plate boundary – magnetic materials in that rock like magnetite record the current state of the Earth’s magnetic field. The polarity of the Earth’s magnetic field is known to periodically reverse itself every million years or so due to the rotating liquid iron outer core that creates the field not rotating entirely uniformly and introducing gradual irregularities into the field until it “flips” (this is sort of the same process that creates sunspots, except not really). These reversals are therefore encoded into the rock that makes up the surface of the tectonic plate as it forms, creating a “striping” effect as the rock of one polarity is gradually carried away from the plate boundary to be replaced by newly formed rock of a different polarity. When scientists looked at the magnetic patterns of the rock on each side of a plate boundary they discovered that not only did the polarity of each set of magnetic stripes on either side of the boundary match almost perfectly, but that the age of the stripes got younger as they examined rock closer to the boundary area.

This was pretty conclusive proof that:

1)      New rock was being created from mantle material (as shown by the stripes’ age and polarity shifts).

2)      This new rock – and by extension, all the other rock on the plate in question — was then transported away from the plate boundary by continental drift (as shown by the matching magnetic stripes).

That continental drift also neatly explained the discovery of similar fossils on different continents as well as the question of how the same species of animal could exist on either side of an ocean without crossing it was merely the icing on the cake.

Today we know that the surface (or crust) of the Earth is composed of seven primary tectonic plates that very roughly correspond to each of the seven continents along with a whole bunch of subsidiary plates. These plates vary in thickness from 30 – 50 km underneath the continents themselves to just 5-10 km thick underneath the oceans. Oceanic crust is constantly being created and destroyed by seafloor spreading and subduction beneath other plates, and so it is rarely more than 200 million years old. By contrast the rock making up the continental crust is persistent, with ages of up to 3.5 billion years being measured.

So you’ve got these enormous tectonic plates and this continental drift theory. All well and good, but then you run into the tiny problem of having to come up with a mechanism capable of causing the gradual movement of trillions of tons of rock over billion year timescales. Happily such a mechanism does exist: the plates are “floating” (see isostasy for a completely incomprehensible explanation) on top of a vast sea of very hot, highly viscous rock called the mantle. This mantle stretches from the Mohorovičić discontinuity layer¹ separating it from the crust all the way down to the liquid iron outer core of the Earth 2900 km beneath it. Since the mantle isn’t strictly solid it’s possible for internal convection currents to cause it to constantly shift, and the continental plates shift along with it.

The existence of the mantle can be easily inferred both from simple physics — as you go deeper into the earth the pressure and therefore the temperature of the rock making it up is going to rise in a fairly predictable manner, allowing scientists to make decent estimate of what it’d be like down there — and seafloor spreading, since that magma has to come from somewhere. I should make sure to stress at this point that the mantle isn’t liquid hot magma. It’s hot, sure, but because the pressure of all the material above it pressing down on top of it is so immense it cannot liquefy and remains a solid, albeit a highly viscous one. This means that while the mantle does flow it does it over geological time scales of millions of years – much like a glacier, although it’s different in almost every conceivable way. And if you were to provide the mantle with some way of relieving the pressure – say, by punching a vent to the surface through the seabed as at mid-ocean ridges – the mantle rock would undergo what is called decompression melting; now that the pressure of the rock above it is gone there’s nothing keeping it from melting into magma.

Scientists have made a series of amusingly misguided attempts to retrieve material directly from the mantle. Most famous of these was the Mohole, which took the startlingly bone-headed approach of trying to drill down through the thinner ocean crust to get at the mantle. This makes sense in theory, as you’re skipping the tedious job of drilling through forty to fifty additional kilometres of rock you’d have to deal with if you did it on top of continental crust, but you’re merely replacing that little problem with the bigger one of trying to drill a hole at the bottom of 3.5 kilometres of ocean from an untethered surface ship. Needless to say it did not end well, and the project was cancelled after the costs started to spiral. The Soviets made a similar attempt and went about it in a typically Soviet way: they chose to drill their hole through 35 km thick Siberian continental crust but they kept up the drilling program for twenty-two years, eventually reaching the fairly respectable depth of 12.5 km before temperature difficulties (not to mention the collapse of the Soviet Union) stopped the project. This is the deepest hole anyone has ever managed to dig, and it was still barely a third of the way down.

With that in mind, if we haven’t even managed to reach the mantle yet then how on earth do we know how far down it goes and what is beneath it? For that matter, how do we even know how the mantle is structured? (And it is structured, being divided into a more plastic upper mantle which is under relatively little pressure, and a more rigid lower mantle which has less opportunity to flow because it’s compressed so much by all the stuff above it.) The short answer is: earthquakes. By tracking earthquakes and analysing how earthquake shockwaves travel through the Earth (or not), scientists can figure out what exactly is down there.

Earthquakes produce two main types of waves, P-waves and S-waves. P-waves move along the direction of propagation while the motion of S-waves is transverse to it; this is an excellent demonstration of the difference that’ll save me about five hundred words of explanation. The difference between P-waves and S-waves is crucial, because it’s this difference in the direction of motion that dictates how fast they travel and what they can travel through. P-waves can travel through anything, and they do it quickly. S-waves cannot travel through liquids because liquids cannot sustain the shearing up-and-down motion through which S-waves propagate², and they move slowly in comparison to S-waves. Taken together these differences allow us to build a fairly good map of the interior of the Earth if we track them after an earthquake.

Here’s how the two types of waves propagate through the Earth (not stolen from Wikipedia for once, either). They follow curved paths because the material they move through refracts them, and this refraction increases as the density and pressure of that material increases with depth. The P- and S-waves also undergo sharp refractions whenever they encounter a material shift as with the Mohorovičić discontinuity or the boundary between core and mantle. As you can see the liquid outer core of the Earth effectively stops all S-waves, neatly proving that it is a liquid (other proofs include the magnetic field of the Earth being exactly what we’d expect to see if you had a rotating convecting layer of liquid iron somewhere down there). Sharp changes in the trajectories of the P-waves meanwhile provide compelling evidence for a second boundary between the liquid iron core and an interior area which is different — presumably a solid iron core. Finally the speeds of the two types of wave allow inferences about the composition of the material they are moving through to be made, since this is directly dependent on the density and elasticity of that material.

So from earthquake waves we know that the mantle is divided into a more elastic upper layer and a more rigid lower layer, that there is a liquid iron core that stops S-waves, and that there is a solid iron core inside that which alters the path of P-waves. While the other planets are certain to have mantles and iron cores thanks to the way planets form and differentiate over time, we are aware of none which have the liquid iron core that drives most of the active geological processes on Earth – no large-scale radioactive decay releasing heat into the mantle driving convection and plate tectonics, and no magnetic field protecting the surface from harmful cosmic radiation and the solar wind. This is why you’ll hear me throw around the term “geologically dead” quite a lot when describing the other terrestrial planets; they aren’t entirely so thanks to constant low-level radioactive decay and we can see that they weren’t always so from the presence of long-dead volcanoes, but in comparison to the Earth today they’re pretty much geologically inert.

Foof. That was a bit of a random topic, but I felt it was useful to get it out of the way because it’ll be good background should I ever get around to talking planetary physics. Next week I’ll be talking about orbital resonances and how they’ve sculpted the structure of the Solar System. Or more accurately, simply trying to describe what an orbital resonance is without coming across as a gibbering lunatic. Until then, KEEP WATCHING THE SKIS.

Uh, skies.

1. Thanks, Total Annihilation.

2. To use a really bad analogy: this is for the same reason that you cannot walk on – or more accurately through — water; the water will simply deform underneath you and you will sink. Similarly water (or any other low-viscosity fluid) will simply deform under the shear stresses created by S-waves rather than propagating them further, causing them to be lost in a very short space of time. I should also probably note that S-waves are different from the surface waves you might see out on the ocean as they move through the main body of an object rather than along the top.

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