In Our Time: The Earth’s Core

Despite being relatively close to us the inside of the Earth, and particularly the core of the Earth, is difficult to investigate. Primarily because we can’t just look at it – and the deepest mines or boreholes are only 10km deep which is tiny compared to the 6,000km that is the Earth’s radius. So everything needs to be logically deduced from the readings that we can take. Discussing what we know about the Earth’s core and how we know it on In Our Time were Stephen Blundell (University of Oxford), Arwen Deuss (Utrecht University) and Simon Redfern (University of Cambridge).

Prior to the 19th Century the assumption was that the Earth was the same all the way through – rocks where we can see, so rocks everywhere. But in the 19th Century scientists realised that the theory of gravity required a denser Earth than is possible if it’s just rocks and so they postulated an iron core. This was also the time when scientists began to be interested in how the Earth was formed. The consensus at the time was that it formed by condensation out of a hot cloud, and it was still cooling. This explained (at a time when radioactivity wasn’t known) why it got warmer the further you went underground. So at the time the best explanation for the structure of the Earth was that it had a hot liquid iron rich core surrounded by a rocky shell.

However even in the 19th Century it was clear that there were problems with this explanation. If you spin an uncooked egg, it wobbles – so why doesn’t the earth? During the 20th Century it began to be postulated that the core was two phase – a solid core with a liquid coating. One of the experts on the programme, Arwen Deuss, used seismological readings to show that this was the case. When there is an earthquake seismographs on the other side of the Earth detect the shockwaves that have travelled through the planet. Before Deuss’s work it was thought that there was a shadowzone where no waves were detected because they had failed to pass directly through the centre of the Earth – so it was thought that the core was a different phase to the rest of the planet and the waves couldn’t travel through it. Deuss showed that there are very faint delayed waves detectable in that shadow zone, and that mathematically the best model to describe how these waves are delayed and how they are diminished is one where the core is solid but it is surrounded by liquid. The seismic waves cannot travel through liquid in the same state as they travel through solid, and each transition between states uses up some of the energy in the wave. A wave that travels directly through the core will transition from solid to liquid to solid to liquid and lastly to solid again. As well as each transition using up energy it takes time (hence the delay) and changes direction (so the waves aren’t in quite the same places you’d expect if they had no transitions).

The current theory is that the inner core is an iron crystal that is forming out of a less pure molten iron fluid around it. This iron crystal is about the size of the Moon, a fact which I find mind-boggling. The crystal is still growing and this is not a consistent process, sometimes it grows more quickly and sometimes more slowly. The experts said there is evidence of some sort of discontinuity that formed 500 million years ago, but no-one knows what caused it. The crystal is also split into two pieces. One of the experts made an analogy with the land/sea divide up here on the crust, but I didn’t really follow that. The crystal is also different in the north/south direction as compared to the east/west direction – seismic waves take longer to travel east & west than they do north & south. It’s not known why this is: perhaps to do with crystal alignment, or perhaps it tells us something about the shape of the core.

This solid iron crystal is rotating within the liquid it sits in, I think at a slightly different (quicker?) speed than the whole of the Earth rotates. It’s this rotation that is the cause of our magnetic field (which is another piece of evidence in favour of the two phase theory). And the magnetic field is what protects us from cosmic radiation so in some sense you can say that the two-phase spinning core of the Earth is why there is life on Earth. The current theory is that Mars and Venus have cores that are too solid or too small to generate enough of a magnetic field to protect against radiation. That’s an untested hypothesis, and so Deuss would like to put seismographs on one (or both) of the other planets to see what she can detect about their internal structure.

Bragg closed up the programme by attempting to encourage them to talk about practical uses that have come out of this blue-skies research – but it seems at the moment this is still in the blue-skies phase.

Pilgrimage with Simon Reeve; Survivors: Nature’s Indestructible Creatures

Pilgrimage with Simon Reeve was a three part series that was partly a travelogue and partly about the history of Christian pilgrimage across Europe and the Holy Land from medieval times through to the modern day. Reeve made it pretty clear several times that he’s not a Christian himself, so this was an outsider’s view on the subject. He did, however, talk to several people who do pilgrimages for religious purposes today, so we got both sides of the subject represented. The first episode started in Lindisfarne and made its way down to Canterbury and mostly talked about medieval experience of pilgrimage. Then the second episode went through France and Northern Spain to Santiago de Compostela, and then across the alps to Rome. The third episode went from Istanbul to Jerusalem (via Bethlehem).

Reeve seemed focussed primarily on the question of what people get out of pilgrimage. His conclusions were that as well as the visiting of spiritually significant sites the journey itself has spiritual benefits for the people who do pilgrimages. They have a time to step outside of their daily lives and reflect on how they’re living and what they’re doing with their lives. Be that in a religious sense or a purely spiritual sense, various of the modern pilgrims he talked to weren’t people who called themselves Christian.

It wouldn’t be a Simon Reeve programme if it didn’t also look at the less uplifting parts of the subject! In the last episode there he was travelling through the West Bank (as one has to, if one’s visiting Bethlehem), and took the opportunity to contrast the modern political situation with the spiritual significance of the region to so many people. Another example was his visit to a very modern cult centre – the town where Padre Pio lived, who died in 1968. This has a massive new cathedral, a TV station of it’s own (run by monks), lots of fancy hotels in the town for all the touristspilgrims. And various rumours of how he kept his stigmata open during life by the judicious application of carbolic acid … He was canonised, but Reeve implied that was more an attempt by the Vatican to keep it in house so’s to speak. The cult was growing up anyway, so he was officially made a saint.

An interesting series πŸ™‚ And in a piece of serendipity the first episode overlapped in subject matter with the end of Neil Oliver’s recent series about Sacred Wonders of Britain that we’d just finished watching (post). The second episode had some overlap with the second episode of Waldemar Januszczak’s series about Baroque art which we’re also watching at the moment. The three presenters have very different styles so it was interesting to get the various perspectives all so quickly together!

The other series we finished watching this week was Survivors: Nature’s Indestructible Creatures – a three part series about species that didn’t die out in mass extinction events presented by Richard Fortey. Each episode covered a different extinction event, and Fortey tracked down 10 species that survived it through to modern times. The first one was about the Great Dying (which occurred 252 million years ago) which is the most significant extinction even with the greatest die-off of species. For this one he looked at things like horseshoe crabs, sea cucumbers and lampreys. The second covered the KT boundary – i.e. death of the dinosaurs, so had an emphasis on birds, mammals and crocodiles. And the third one looked at the Ice Age and at the cold adapted species that made it through to our own times.

This was definitely more my sort of thing than J’s. I thought it managed to combine a bit of geology, a bit of evolutionary biology and a bit of modern day travelogue into an interesting whole (even tho I think I knew of most of it before, it’s nice to actually see things sometimes). Fortey was an engaging presenter, who was also pretty entertaining as he tried to handle live specimens with varying degrees of success and comfort. I had a great deal of sympathy with that as someone who’s significantly worse at handling live animals than he was yet is still a biologist πŸ™‚ Oh, and it also had a running theme of Fortey eating some of these survivors & telling us how they taste, which was a slightly odd (but fun) addition.

Other TV watched this week:

The Coffee Trail with Simon Reeve – one-off programme about coffee growing in Vietnam. Vietnam is the main supplier of coffee for the instant coffee trade, and it’s as exploitative a trade as you’d expect. The regime in Vietnam isn’t particularly nice either.

Episode 2 of Baroque! From St Peter’s to St Paul’s – gloriously over the top series about Baroque art and architecture, presented by Waldemar Januszczak.

Episode 1 of The Stuarts – a series about the Stuart Kings of England & Scotland, presented by Clare Jackson, and about how they shaped the United Kingdom and how they were shaped by it. Broadcast on the Scottish version of BBC2 only.

Nigel Slater’s Great British Biscuit – a similar programme to Slater’s previous one on sweets (post), part nostalgia, part history of biscuits. Lots of “oh I remember those” moments πŸ™‚

Greek Myths: Tales of Travelling Heroes – programme presented by Robin Lane Fox about the early Greek myths about the origins of their gods. Also looking at the links between the mythological stories and the landscape the Greeks knew, and also the links to Hittite mythology. We both had quite a lot of deja vu watching it, and figured out eventually that we’d watched it before about 3 years ago and had just forgotten (brief post on my livejournal). Interesting & worth watching, even for a second time πŸ™‚

Treasures of Ancient Egypt (Ep 2); Strange Days: Cold War Britain; Rise of the Continents

The second episode of Alastair Sooke’s series about the art of Ancient Egypt covered the Middle Kingdom (briefly) and most of the New Kingdom. He only picked a couple of objects from the Middle Kingdom – both from Senusret III’s reign. He gave the impression that this is because the New Kingdom was the Golden Age, which is true in some ways, but the Egyptians themselves looked back at the Middle Kingdom as their “classical age” where art and culture first achieved great heights. I think it’s a shame he didn’t make it more clear the reason it gets short-shrift in programmes like these is because not as much survives for one reason or another. Often because sites were re-used or updated by New Kingdom Egyptians wanting the association with past glories.

The other eight treasures on the programme were from the New Kingdom between the reigns of Hatshepsut and Tutankhamun. As well as looking at some of the iconic art from her reign he spent some time talking about the iconography of Hatshepsut as Pharaoh. Pharaohs are male, so Hatshepsut was represented with all the male accoutrements and a masculine body in her official art works. One thing I hadn’t realised before (or had forgotten) is that it was during Hatshepsut’s time that the term Pharaoh actually started to be used – it translates to “the palace” so it’s the equivalent of talking about the White House doing X or Y in the USA (and surprisingly the example Sooke used was Brits talking about the Crown which I don’t even think is the best of the possible UK equivalents – No. 10 would be better).

There was obviously some considerable discussion of the new art style that Akhenaten brought with him when he changed the state religion. Both in terms of the slightly bizarre body shapes of the earliest stuff, and the new informal poses and domestic scenes on official art works. Which does give a very different impression of the royal family of that time, even as I remind myself it’s propaganda first & foremost. Obviously the bust of Nefertiti featured in this section, you can’t really miss it out. But the item from around that era (just before it) that struck me most was the little glass fish, that’s now in Berlin. I’ve seen it before & it’s a lovely piece, but what made it the highlight of this programme for me was that they showed us how it was made. I’ve read about how these glass objects were made before but it’s different actually watching it happen. And as always I’m somewhat in awe of what people were able to do before the advent of modern technology.

Obviously the programme ended with Tutankhamun’s mask – another iconic piece you can’t miss out, which also illustrates how what we have to admire depends so much on chance. The next episode covers the rest of Egyptian culture up to Cleopatra, quite a wide range. There’ve been a few clips of the temple at Abu Simbel, so presumably that’ll feature πŸ™‚

This week we finished watching Dominic Sandbrook’s series about the Cold War – Strange Days: Cold War Britain. This three part series looked at British history from 1946 through to 1989 through the lens of how the Cold War affected politics and culture. So part of the series was Sandbrook telling us about the major events of the Cold War, and giving some indication what life was like on the other side, to give us context for the effects on Britain. And the other part was looking at events in Britain from a perspective we don’t always think of. Some stuff was obvious when you thought about it – like the popularity of James Bond films tying in to revelations about Russian spies in the UK. And the John le CarrΓ© novel The Spy Who Came in from the Cold as the much less glamorous and more cynical take on the same thing. Other things less so – consumerism being a part of how we differentiated “us” from “them” makes sense when I think about it, but I’d never’ve thought of how capitalism was in some ways kept in check by a desire to prove it was better than the alternatives. Which made more sense when Sandbrook talked about it than I have quite managed to articulate here!

The threat of nuclear war and how that shaped our culture was one of the strands running through the programmes, and the various attempts government made to prepare people for this. Sandbrook highlighted several times the contrast between the almost optimistic government handbooks which aimed not to panic people (even if this backfired at times) and the bleak films and TV serieses which were closer to what the reality might be. He showed us clips from The War Game (a 60s film that wasn’t shown on TV for about 20 years) which was a meticulously researched documentary, and Threads (an 80s film) which was more overtly fictional. Both grim enough even in excerpt that I know I don’t want to watch them in full. In the third programme Sandbrook also mentioned a book he’d read for class when he was 10 (I looked him up on wikipedia, he’s a couple of months younger than me) – as he started to talk about it I knew exactly which book he meant before the reveal. It was “Brother in the Land” by Robert Swindells, which I’ve read. Once. I’ve dipped into it occasionally since, and it’s still on my shelves, because I remember it as a good, well written book. But I’ve never re-read it cover to cover, despite my love of post-apocalypic novels. It’s just an extremely grim and depressing and unrelentingly bleak tale of the first months after a nuclear war. I read it at about 13 or 14, a few years after it was published, and it’s stuck with me since then – it must’ve been pretty traumatising to read at the age of 10 particularly when you had to think about it for school rather than stick your head in the sand (I’ve always adopted the ostrich approach to the idea of The End of the World As We Know It catastrophes).

Anyway, that was a bit of a digression. I liked this series, in particular I thought they did a good job of mixing archival footage with new stuff seamlessly switching between the two in a way that made the old stuff seem more immediately relevant. I even liked the somewhat overblown style, but I think J found the sweeping generalised claims made at times a little irritating.

We also finished another series this week – Rise of the Continents – which I really enjoyed so I wanted to say a few words about it even though this post is already quite long! This was a series about plate tectonics and the geological history of the earth, presented by Iain Stewart. Each week Stewart looked at a different continent (Africa, Australia, the Americas and Eurasia) and followed the geological story of the continent after it split from Pangea (the supercontinent that existed when the dinosaurs roamed the earth). He showed us the evidence that tells us about this geological story, and he also showed the impact that geology has had on both evolution and on human history. He’s a geologist so was strongest on that subject, pretty good on palaeontology but said a couple of dubious historical things we noticed (but otherwise was OK on that). Basically what you’d expect as he got further from his actual area of expertise. He was also a charmingly enthusiastic presenter.

One reason I enjoyed it so much is because I think the idea of plate tectonics is inherently cool. The earth not being static but consisting of vast sheets of crust all moving around and crashing into each other is awesome. It’s also an area I don’t know much about – I think the last time I read a book on it was in the 80s, when the science was still fairly new. So there were all sorts of things I didn’t know, and most of them were in the “neat facts” category. Like did you know that as India travelled on its way to crashing into Eurasia it moved over a magma plume, which turned a big chunk of it into a zone of volcanoes. This thinned the land so India started to move quicker. But also while it sat over this region for a few hundred thousand years the amount of volcanic eruption dumped toxins in the oceans and changed the climate – so this is thought to have contributed to the decline of the dinosaurs (before an asteroid finished them off). Or did you know the silver mines in South America exist because of subduction of the Pacific Ocean floor carries water down under the land. I can’t quite remember how Stewart said this then lead to the silver deposits, but the very idea of water being carried down under the crust is one I’d not thought of before (and it’s kinda cool as a concept).

I think J didn’t like the visual effects on the programme much – there were quite a few transitions where they used a jumble of still shots and mixed up audio before Stewart explained something. It didn’t bother me as much though.

Other TV watched this week:

Episode 2 of Sacred Wonders of Britain – Neil Oliver visits several sacred sites in Britain dating from prehistoric times through to the Reformation.

Episode 2 of The Brain: A Secret History – Michael Mosley series about brains, minds and experimental psychology. We never managed to record episode 1 but we decided to watch the other two anyway.

Episode 6 of Tudor Monastery Farm – part re-enactment, part documentary about what life would be like living on and running a farm in 1500.

In Our Time: Ice Ages

For about 85% of the time that Earth has existed the temperature has been high enough that there have been no polar icecaps – a “greenhouse Earth”. The remaining 15% of the time is referred to as “icehouse Earth”, and during these longer cooler periods are glacial periods (like 20,000 years ago when the ice sheets reached as far as Germany) and inter-glacial periods like the current time where the ice is just at the poles. The experts discussing ice ages on In Our Time were Jane Francis (University of Leeds), Richard Corfield (Oxford University) and Carrie Lear (Cardiff University).

I looked at Bragg’s blog post on the Radio 4 blog for the episode, as I often do before I start writing one up, and was surprised that several people had commented complaining about how the discussion was minimising the impact that climate change and rising temperatures would have on our civilisation. Surprised because J and I came away from the programme with the distinct impression that all three experts thought the planet would be just fine with higher temperatures, and that life would survive as it has done before. But our civilisation? Well, that would be in more trouble.

However that was not the focus of the programme, as In Our Time is not a current affairs programme. Instead the programme was about what an ice age is and how we know about them. My first paragraph is a good summary of what they told us about what an ice age is. Continental drift plays a part in producing the conditions that lead to an icehouse Earth – all 5 that have occurred are correlated with the presence of land at one or both of the poles. When there is open water at both poles then the currents moving the water between the poles and the equator counteract any cooling of polar region – obviously when there’s land there this can’t be true. I’m not sure if every time there is land at the poles then there is an icehouse Earth, or if the correlation is only the other way round (every icehouse has a landlocked pole). I don’t think they said. But this thermal isolation of one of the poles seems to be a requirement to get the process going.

The change from a greenhouse Earth to an icehouse Earth is a slowish process (from a geological point of view) but once it starts there are positive feedback loops that mean the Earth continues to cool. One of these feedback loops is because ice & snow are white and reflect back more of the sun’s energy so the land doesn’t warm up as much as it would if snow were black. Another is that CO2 gets frozen in the ice caps and so the atmospheric CO2 concentration goes down – and low temperatures, and icehouse Earths, are correlated with low CO2 concentrations. They were mostly just saying things were correlated rather than speculating on causes – but I think Lear said that CO2 levels are a driver of temperature change.

Once in an icehouse Earth there are these oscillations between cold-cold-cold and not-quite-so-cold. These are due in part to Milankovitch cycles – cyclical changes in the Earth’s orbit which (effectively) change how cold winter gets compared to summer and how long winter lasts. So when the Milankovitch cycles are in a cold-winter phase then you get a glacial era, and when they’re not you get an inter-glacial such as our current climate. I guess in a greenhouse Earth you get tropical and not-so-tropical eras similarly.

The five icehouse epochs have not been identical. One of them only had ice across the southern pole which was where the continent Gondwana was positioned. This comprised of most of the southern landmasses – India, South American, South Africa, Australia etc. The rest of the land on the planet was situated around the equator and had a tropical climate. Another of the icehouse epochs is what was known as Snowball Earth because the icesheets covered the whole of the planet. Bragg was curious as to how the planet had got out of that, the only answer was that it must’ve involved rising CO2 levels but no clear ideas as to what would’ve kicked off the rise.

The evidence for these climate changes come from a variety of places. Francis told us about the physical evidence you can see in the geological record, for instance particular rock types that’re formed from the bits & pieces that a glacier grinds up and carries with it. There are also distinctive scratches that can be seen where a glacier has been. The problem with this sort of evidence is that it’s incomplete. A far more complete picture has been built up using the sediment in the oceans and the ice sheet on Antarctica.

Corfield told us that the old-fashioned way of using sedimentary cores to look at what the climate used to be was to look at the various species of small fossils and see how many were warm water species & how many cold. Lear told us about the more sophisticated techniques that are used now. The first of these is to look at the ratio of 16O and 18O isotopes in the fossils. This reflects the ratio in the water in which they lived, which is dependent on the temperature of the water and the sea levels. As water evaporates from the sea the molecules containing 16O preferentially evaporate. If there is no ice then once the water rains it ends up back in the sea so the ratio stays the same, but if there are ice caps then some of the rain ends up locked up in the icesheets and the ratio in the water is changed. There is also another way of looking at the temperature using magnesium & calcium, but Lear didn’t explain what that was. Cores from the icesheets can be used to look at the atmospheric conditions during the current icehouse epoch. As the ice forms there are small bubbles in it, and it’s possible to extract these & look at the CO2 levels. For most of the glacial period the CO2 level was around 280ppm, which is pretty low compared to today’s 390ppm. In a greenhouse Earth the CO2 levels might be several times that.

Another indicator of ice levels in the past is fossilised coral. Coral always grows pretty close to the surface of the ocean, so where you find the fossils shows you where the coastline was in the past. At the glacial maximum the sea level was a lot lower than now (by about 70m I think they said), during a greenhouse Earth the sea level is a lot higher. Which is where the problems come in for us humans – think of how many important cities are on the coast … But as I said, the programme didn’t dwell on that or spell it out explicitly.