Marsquakes

One of the more interesting aspects of the latest NASA landing on Mars is that the rover has dug into the surface, inserted a seismometer, and is looking for marsquakes. On Earth, earthquakes are fairly common, especially where I live, and they are generated through the fact that our continents are gigantic lumps of rock moving around over the mantle. They can slide past each other or pull themselves down under another plate, to disappear deep into the mantle, while at other places, new rock emerges to take their place, such as at the mid-Atlantic ridge. Apparently the edges of these plates move about 5 – 10 cm each year. You probably do not notice this because the topsoil, by and large, does not move with the underlying crust. However, every now and again these plates lock and stop moving there. The problem is, the rest of the rock is moving, so considerable strain energy is built up, the lock gives way, very large amounts of energy are released, and the rock moves, sometimes be several meters. The energy is given out as waves, similar in many ways as sound waves, through the rock. If you see waves in the sea, you will note that while the water itself stays more or less in the same place on average, in detail something on the surface, like a surfer, goes up and down, and in fact describes what is essentially a circle if far enough out. Earthquake waves do the same thing. The rock moves, and the shaking can be quite violent. Of course, the rock moves where the actual event occurred, and sometimes the waves trigger a further shift somewhere else.

Such waves travel out in all directions through the rock. Now another feature of all waves is that when they strike a medium through which they will travel with a different velocity, they undergo partial reflection and refraction. There is an angle of incidence when only reflection occurs, and of course, on a curved surface, the reflected waves start spreading as the angles of incidence vary. A second point is that the bigger the difference in wave speed between the two media, the more reflection there is. On Earth, this has permitted us to gather information on what is going on inside the Earth. Of course Earth has some big advantages. We can record seismic events from a number of different places, and even then the results are difficult to interpret.

The problem for Mars is there will be one seismometer that will measure wave frequency, amplitude, and the timing. The timing will give a good picture of the route taken by various waves. Thus the wave that is reflected off the core will come back much sooner than the wave that travels light through and is reflected off the other side, but it will have the same frequency pattern on arrival, so from such patterns and timing you can sort out, at least in principle, what route they took and from the reflection/refraction intensities, what different materials they passed through. It is like a CT scan of the planet. There are further complications because wave interference can spoil patterns, but waves are interesting that they only create that effect at the site where they interfere. Otherwise, they pass right through other waves and are unchanged when they emerge, apart from intensity changes if energy was absorbed by the medium. There is an obvious problem in that with only one seismometer it is much harder to work out where the source was but the scientists believe over the lifetime of the rover they will detect at least a couple of dozen quakes.

Which gets to the question, why do we expect quakes? Mars does not have plate tectonics, possibly because its high level of iron oxide means eclogite cannot form, and it is thought that the unusually high density of eclogite leads to pull subduction. Accordingly the absence of plate tectonics means we expect marsquakes to be of rather low amplitude. However, minor amplitude quakes are expected. One reason is that as the planet cools, there is contraction in volume. Accordingly, the crust becomes less well supported and tends to slip. A second cause could be magma moving below the surface. We know that Mars has a hot interior, thanks to nuclear decay going on inside, and while Mars will be cooler than Earth, the centre is thought to be only about 200 Centigrade degrees cooler than Earth’s centre. While Earth generates more heat, it also loses more through geothermal emissions. Finally, when meteors strike, they also generate shockwaves. Of course the amplitude of these waves is tiny compared with that of even modest earthquakes.

It is hard to know what we shall learn. The reliance on only one seismometer means the loss of directional analysis, and the origin of the quake will be unknown, unless it is possible to time reflections from various places. Thus if you get one isolated event, every wave that comes must have originated from that source, so from the various delays, paths can be assigned. The problem with this is that low energy events might not generate enough reflections of sufficient amplitude to be detected. The ideal method, of course, is to set off some very large explosions at known sites, but it is rather difficult to do that from here.

What do we expect? This is a bit of guesswork, but for me we believe the crust is fairly thick, so we would expect about 60 km of solid basalt. If we get significantly different amounts, this would mean we would have to adjust our thoughts on the Martian thermonuclear reactions. I expect a rather tiny (for a planet) iron core, the clue here being the overall density of Mars is 3.8, its surface is made of basalt, and basalt has a density of 3.1 – 3.8. There just is not room for a lot of iron in the form of the metal. It is what is in between that is of interest. Comments from some of the scientists say they think they will get clues on planetary formation, which could come from deep structures. Thus if planets really formed from the combination of planetesimals, which are objects of asteroid, size, then maybe we shall see the remains in the form of large objects of different sonic impedance. On the other hand, the major shocks to the system by events such as the Hellas impactor may mean that asymmetries were introduced by such shock waves melting parts. My guess is the observations will not be unambiguous in terms of their meaning, and it will be interesting to see how many different scenarios are considered.

Was there an Initial Atmosphere from Accretion?

One of the problems with modern science is that once a paradigm has been selected, a layer of “authorities” is set up, and unless the scientist adopts the paradigm, little notice is taken of him or her. This is where conferences become important, because there is an audience that is more or less required to listen. The problem then for the person who has a different view is to show why that view is important enough to be considered. The barrier is rightly high. A new theory MUST do something the old one did not do, and it must not be contradicted by known facts. As I said, a high barrier.

In the previous post, I argued that the chemicals required for life did not come from carbonaceous chondrites or comets, and that is against standard thought. Part of the reason this view is held is that the gases had to come from somewhere, so from where? There are two obvious possible answers. The first is the gases were accreted with the planet as an atmosphere. In this hypothesis, the Earth formed while the disk gases were still there and simple gravity held them. Once the accretion disk was removed by the star, the hydrogen and helium were lost to space because Earth’s gravity was not strong enough, but other gases were retained. This possibility is usually rejected, and in this case the rejection is sound.

The first part of the proposition was almost certainly correct. Gases would have been accreted from the stellar disk, even on rocky planets, and these gases were largely hydrogen and helium. The next part is also correct. Once the disk gases were removed, that hydrogen and helium would be lost to space because Earth’s gravity was not strong enough to hold it. However, the question then is, how was it lost? As it happens, insufficient gravity was not the primary cause, and the loss was much faster than simply seeping off into space. Early in the life of a new star there are vicious solar winds and extreme UV radiation. It is generally accepted that such radiation would boil off the hydrogen and helium, and these would be lost so quickly that the other gases would be removed by hydrodynamic drag, and only some of the very heavier gases, such as krypton and xenon could remain. There is evidence to support this proposal, in that for krypton and xenon higher levels of heavier isotopes are observed. This would happen if most of these gases were removed from the top of the atmosphere, and since the lighter isotopes would preferentially find their way there, they would be removed preferentially. Since this is not observed for neon or argon isotopes, the argument is that all neon and argon in the atmosphere was lost this way, and if so, all nitrogen and carbon oxides, together with all water in the atmosphere would be lost. Basically, apart from the amount of krypton and xenon currently in the atmosphere, there would be no other gases. The standard theory of planetary formation has it that the Earth was a ball of magma, and if so, all water on the surface would be in the gas phase, so for quite some time Earth would be a dry lump of rock with an atmosphere that had a pressure that would be so low only the best vacuum pumps today could match it.

There could be the objection that maybe the star was not that active and we did retain some gases. After all, we weren’t around to check. Can you see why not? I’ll give the reason shortly. However, if we accept that the gases could not have come from the accretion disk, the other alternative is they came from below the ground, i.e. they were emitted by volacanic activity. How does that stand up?

One possibility might be that gases, including water, were adsorbed on the dust, then subsequently emitted by volcanoes. You might protest that if the Earth was a magma ocean, all that water would be immediately ejected from the silicates as a gas, but it turns out that while water is insoluble in silica at surface pressures, at pressures of 5000 atmospheres, granitic magma can dissolve up to 10% water at 1100 degrees C, at least according to Wikipedia. Irrespective of the accuracy of the figures, high temperature silicates under pressure most certainly dissolve water, and it probably hydrolyses the silicate structure and makes it far less viscous. It has been estimated that the water remaining in the mantle is 100 times greater than the current oceans so there is no problem in expecting that the oceans were initially emitted by volcanic activity. As an aside, deep in the mantle the pressures are far greater than 5000 atmospheres. This water is also likely to be very important for another reason, namely reducing the viscosity and lowering the magma density. This assists pull subduction, where the dry, or drier, basalt from the surface is denser than the other material around it and hence descends into the mantle. If the water were not there, we would not have plate tectonics, and if there were no plate tectonics, there would be no recycling of carbon dioxide, so eventually all the carbon dioxide on the surface would be converted to lime and there would be nothing for plants to use. End of life!

However, we know that our atmospheric gases were not primarily adsorbed as dust. How do we know that? In the accretion disk the number of nitrogen atoms is roughly the same as the number of neon atoms, and their heats of adsorption on dust are roughly the same. The only plausible physical means of separating them in the accretion disk is selective sublimation from ice, but ice simply could not survive where Earth formed. So, if our nitrogen came from the disk by simple physical means, then we would have roughly the same amount of neon in our atmosphere as nitrogen. We don’t, and the amount of neon we have is a measure of the amount of gas we have from such adsorption. Neon is present at 0.0018%, which is not very much.

So, in answer to the initial question, for a period there was effectively no atmosphere. To go any further we have to consider how the planets formed, and as some may suspect, I do not accept the standard theory for reasons that will become apparent in the next post.

Meanwhile, may I remind readers that my ebooks on Smashwords are on discount through July. Links to novels:

Puppeteer: http://www.smashwords.com/books/view/69696

‘Bot War: https://www.smashwords.com/books/view/677836

Troubles: https://www.smashwords.com/books/view/174203

Meanwhile, if you want to know scientifically about biofuels:

Biofuels: https://www.smashwords.com/books/view/454344

Wellington shakes

First storms, and now earthquakes! This is not altogether surprising, because there are a mass of fault lines nearby that are caused by the Pacific and Australian tectonic plates grinding by each other. These quakes were centred in Cook Strait, with the small town of Seddon, which is in the northern part of the South Island, being the closest habitation, but it was also not very far from Wellington. The first bigger one was just after 9 am on Friday and was 5.7 on the Richter scale. The news media made quite a fuss about it, with various comments about how scared people were, but I was in the rather odd position of hearing about it first from my wife about two hours later. I had seemingly been driving into Lower Hutt, but it was also interesting that I saw a number of people later, talked to some, and nobody mentioned the quake. Obviously, it was not serious.

 There was an after-shock later in the afternoon, which created discussion, not the least because, since we could barely feel anything, I thought it was nothing more than some nearby road works, which I knew were going on because I had to avoid some seriously heavy machinery a little earlier in the day.

 Then, two days later, while lying in bed and wondering whether to get up, there was another from the same place, this time at 5.8 on the Richter scale. Again, nothing to be particularly bothersome, but of course when you experience these sort of things, the real fear is not what is going on, but what it might develop into. My house is on a hill, and at the bottom of the hill, I suppose about 100 – 200 meters away, there is the main Wellington fault, and if it decides to go, it has seemingly generally produced shakes in the 8s on the Richter scale, and these are really serious.

 Then, at 1709 hrs on the Sunday there was yet another. It started harmlessly enough, but it kept building up, and kept building up. After some time, I could hear the walls in the house straining, and just when I was thinking this could get really serious, it started to subside, but it kept going nevertheless, as was 6.5 on the Richter scale. Over all, it lasted a minute, which is not a very long time, but it feels like it in an earthquake. This time there were more serious outcomes. It was fortunate when it struck because there were relatively few people on the Wellington streets. What happened was that a number of windows on high-rises shattered, and sent sheets of glass downwards, and these might be the most dangerous in a quake. A quake of 6.5 disperses approximately 400 TJ of energy, or about 6 times the energy released by the nuclear bomb at Hiroshima.

 Actually, Wellington is apparently experiencing a continuous “quake” this year. I gather that for several months, a massive land mass around Wellington is drifting north-eastwards, at something like a millimetre a week. Because this is smooth, at this rate the accelerating force is impossible to detect, and we would not be aware of it, but for physical measurements that depend on the GPS. We joke that airlines will probably raise their charges! On the other hand, the bulk of the South Island is not matching this drift, which means either a fault line is slipping, or strain is building somewhere, or both. These quakes could have involved the relief of this strain. The major fault lines that mark the tectonic boundaries are essentially north-south in direction with, if looked from above, a modest clockwise turn.  From our point of view, we regard modest earthquakes as good. They do not do any damage, but they relieve strain, and strain is bad because if it builds up too far, the inevitable bang just gets bigger. We are just hoping there is no follow-up bang, through strain being transferred elsewhere.