Genes chart Vikings’ spread across Europe

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, using genomics to map the migration of the Vikings…

Host: Nick Howe

And the world's smallest ultrasound detector. I’m Nick Howe.

Host: Benjamin Thompson

And I'm Benjamin Thompson.

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Host: Benjamin Thompson

First up on the show, we're heading back in time over 1000 years to when the Vikings ruled the waves here in Europe. This week in Nature, a team of researchers have been using modern genomic techniques to work out who these intrepid explorers were and how they left their footprint across Europe and the North Atlantic. Reporter Dan Fox is here to tell us more.

Interviewer: Dan Fox

Vikings hold a particular grip on our popular culture. Films, comic books, operas and pop songs have all been written about these ancient blond-haired barbarian raiders. Now, most of what I just said is based more on Marvel comics rather than historical fact. But what do we actually know about who the Vikings were? Broadly speaking, the Viking Age is defined as the 300 years from about 750 to 1050 AD, and the Vikings themselves as the people living in or moving out of Scandinavia at that time.

Interviewee: Cat Jarman

That is how we usually define it, by these people who move out and migrate. But actually, there's quite a lot more to it. There's a lot more interaction, people going back and forth, a lot of people coming back into Scandinavia as well, and that's what makes the picture a bit more complicated or difficult to say what a Viking really is.

Interviewer: Dan Fox

This is Cat Jarman, an archaeologist specialising in Viking history. Given that it's even hard to define exactly what a Viking is, I asked her what we do know about the Viking Age.

Interviewee: Cat Jarman

So, we know that there's a lot of movement going out of Scandinavia in this time period, and we see this in a lot of the archaeological materials. Part of the problem we have is that we don't really know that much about the actual people involved, in terms of how many are physically moving out of Scandinavia. We don't know how many of these objects, grave goods, towns, settlements are actually founded by people and how many are a result of just more sort of cultural diffusion, I guess. So, the question is really, what are the actual people doing in this time period? What's the population is looking like? Where are they going? What are the sort of finer structures within that?

Interviewer: Dan Fox

So how much did the Vikings move around? Well, a paper published this week in Nature may have some answers. Martin Sikora is one of the authors, and he explained to me how he and his colleagues trying to answer this question using ancient DNA taken from archaeological specimens.

Interviewee: Martin Sikora

In a way, it's quite similar to modern genetic studies that look at population diversity and try to investigate differences and similarities between populations. The only difference is that we do this back in time.

Interviewer: Dan Fox

Martin and his colleagues have sequenced the genomes of 442 humans from archaeological sites across Europe and Greenland in the hopes of gaining a better understanding of the Viking Age and how Vikings themselves moved around.

Interviewee: Martin Sikora

So, we find that the individuals from Norway mostly went to the North Atlantic, so we find Norwegian-like ancestry in Orkney, in the Faroe Islands, in Iceland and in Greenland and, to a large extent, in Ireland. We find Danish ancestry is very present in England, which is also, of course, in line with the historical records. And Swedish-like ancestry is very much present in eastern Europe, so across the Baltic Sea, in the Baltic area and also in Poland and Russia where we do have samples from.

Interviewer: Dan Fox

For Cat, this study was particularly interesting, as it illustrated not only where the Vikings spread to, but they came from.

Interviewee: Cat Jarman

For me, I think the most important thing is actually looking at some of the dynamics of going inwards to Scandinavia. So, they have, for example, pointed out that there are people moving from southern Europe into Scandinavia in this time period, which is something that we've not really had a lot of evidence for before. There's a lot of objects going in the sorts of things being traded. But actually, to have some evidence of people moving north, and from southern Europe especially, is really, really exciting.

Interviewer: Dan Fox

Martin's work is also hinting at the extent of a cultural influence the Vikings had at the time.

Interviewee: Cat Jarman

They've got individuals as well with Pictish ancestry from the Orkney Islands who are buried with what we would typically define as sort of Viking or Scandinavian artefacts, but actually who have got quite a different ancestry and genetic history. And that sort of thing, for me as an archaeologist, is very exciting because it's adding to something that we can't find from the typical archaeological record.

Interviewer: Dan Fox

Despite her excitement, Cat does worry that the paper may over-generalise in some of its conclusions.

Interviewee: Cat Jarman

They are trying to make some generalisations over a very long time period, so several hundred years and a very, very large geographical region. They aren't able to break anything down chronologically, which is a bit of an issue because you can't necessarily assume that what happens right at the start of the Viking Age corresponds to what happens in 1050 or after the turn of the millennium, and I think that is simplifying things a bit too much in a way that isn't very helpful.

Interviewer: Dan Fox

Martin, however, thinks that geographically, the sample is representative. Where he would most like to have more data is from the preceding Iron Age.

Interviewee: Martin Sikora

There is just not that much data yet available across Europe from the Iron Age to be able to say conclusively what changes happened before from the Iron Age to the Viking Age in those different regions. We have some in our dataset, and we already see some interesting patterns but in order to confirm that or more thoroughly test it, we would need some more sampling from the preceding time period.

Interviewer: Dan Fox

While Martin would like to expand the dataset, Cat thinks the next steps should be to drill deeper into some of the examples highlighted in this paper.

Interviewee: Cat Jarman

I think there's an awful lot to unpack from this study, and there's a lot of data there that's really, really exciting. I think what needs to be done now is actually to look at each of these case studies in detail, combine it with all the other evidence, which is something, obviously, they haven't been able to do yet. So, I think once we take this evidence and break it down, look at things like gender, they haven't looked at, and again, also chronological patterns, that's, I think, when we get something really, really important and significant about the Viking Age. So, I feel like this is a really hugely exciting starting point of data, but now there's a real working thing of trying to then put it into more context.

Interviewer: Dan Fox

For now, though, our next steps may be to put some old Viking stereotypes to bed.

Interviewer: Dan Fox

It's not that we find that the Vikings were these very, very tight-knit communities in most places where it's only very, very homogeneous Scandinavian ancestry and like the blond, blue-eyed warriors, but actually in many places there's a large amount of diversity and there is a lot of influx from different regions. So, I think it goes in line with what we have learned about human population history also from other ancient DNA studies. I think that the past was just much more dynamic than we might have appreciated before.

Host: Nick Howe

That was Martin Sikora from the University of Copenhagen in Denmark. You also heard from Cat Jarman from the Museum of Cultural History in Oslo, Norway. You can find a link to Martin's paper in today's show notes.

Host: Benjamin Thompson

Next up on the show, it's time for Coronapod, and I'm joined this week by Noah Baker and Heidi Ledford to discuss the latest coronavirus developments. Hello to you both.

Heidi Ledford

Hello.

Noah Baker

Hi there, Ben.

Host: Benjamin Thompson

Today we're going to be talking about the University of Oxford and AstraZeneca vaccine trial. Now, this vaccine is in phase III testing at the moment, and there is a lot of hope for it. I think it's seen as one of the leading vaccine candidates. But this trial was rather abruptly paused just over a week ago and, well, there's a lot to unpack around what happened and the way it was communicated. But before we get into that, Heidi, could you maybe give us a quick timeline of what's been going on?

Heidi Ledford

Basically, what happened is that the news leaked, right. I mean, there wasn't really a formal announcement about this pause that was put on the trial. There was a bit of information that was coming from here and there but some of it hasn't since been confirmed. But essentially what we believe happened is that someone in the trial experienced some sort of medical event. So, what they did then – I mean, it's a trial, it's going on in several countries –they put a pause on the trial in all the relevant countries so that they could evaluate what was going on and make sure that this event that they saw in this particular participant in the trial wasn’t obviously a result of receiving the vaccine, because if they keep giving the vaccine to more participants but they have this indication that there might be a safety worry, that would be unethical, right. The UK, pretty quickly, actually, jumped back in a few days later and said, ‘Okay, let's crank it back up again.’ So, the indication from that then is that they've evaluated the situation and they've decided that this vaccine does not obviously pose a risk to the participants, and so they're able to carry on with the trial.

Noah Baker

So, I guess there's a lot to talk about here but, at the same time, there's kind of very little to talk about. It's kind of this odd story where, on one hand, a lot of people are very interested in this because so many hopes are resting on a vaccine but, on the other hand, a lot of what has happened – maybe not all because there are some things we can talk about later about how this was communicated or not communicated – but a lot of what has just happened is relatively standard practice for a clinical trial. Events like this happen, pauses happen, they're assessed, and then the trial continues if they're deemed to not be associated strongly enough with the vaccine. You've reported on clinical trials for quite some time. What has your sort of takeaway been as you've seen these events unfold?

Heidi Ledford

I mean, I do think it is fairly normal, but I also kind of think that if you had a mild panic attack when you heard the news that that's kind of a normal response too, right. I mean, these big clinical trials, they've got thousands of people enrolled. During the course of the trial, some of these people are going to experience medical events of some kind, and it's not always going to be the case that what you're testing… in this case, a vaccine caused that medical event but a responsible clinical trial investigator, if there's a chance that it might have caused that event, the investigator is going to put a pause on the trial and take a look just to be sure. So, I think what's different in this case is that the whole world is waiting with baited breath on these vaccines. We are so desperate for them to work and we don't want there to be any kind of hint that there might be something wrong, right? So, it was worrying. I've covered trials for a long time. I knew that it's something that could happen. It's normal for it to happen. My first reaction was, ‘Oh, no’, but then afterwards, you do think, actually, it's kind of comforting because it is a sign of a responsibly conducted clinical trial. And there have been some worries, especially with the political pressure in the United States to get something out quickly and ahead of the US election in particular, there have been some worries that there are going to be corners cut, and to see them pause a trial and evaluate a safety situation like this, that's it's comforting in a way too.

Host: Benjamin Thompson

I mean, it seems like they're doing the right things then from what you are saying, but and yet you've said things like ‘hint’ there, and I think it seems like the way that this information came out was perhaps slightly unusual, to me at least.

Heidi Ledford

Yeah, I think it was less than ideal but I don't know how unusual it really was. The sad truth is that the world didn't find out about it from a big announcement from AstraZeneca or anything. It was basically leaked information that was published, I think, initially by STAT and then some more detail came out in a private call with investors that the CEO of AstraZeneca had. We have seen particularly some academic researchers, I think, complain loudly about this and say there needs to be more transparency. Truthfully, they've been complaining for years, so they're making a particular argument this time because this vaccine is so, so important to so, so many people and also because it's received so much government support. In the US, for example, they're saying, ‘Hey, you owe the taxpayers, you need to let us see the details of what's going on.’ It's a good argument. It's something that might have traction with the public and with politicians maybe but long before that, researchers have been sayingyour primary obligation is not necessarily to the people who are funding the trial. Your primary obligation is to the people who are participating in the trial, right, because these guys are putting their health on the line. Maybe they want some personal benefit, but when they do surveys of people who participate in clinical trials, they will often cite a possible benefit to society as being a key motivating factor there. We sort of owe them to squeeze every bit of information we can from the trial, and we owe them to be transparent about it so that we can be sure that it's well designed, that it's safe and that it's likely to get the answer we're looking for. So, yeah, people have been pushing for it for so long. But truthfully, I don't think it's that uncommon to be a bit secretive. And there are arguments saying that, well, we can't give all the details of this particular event because it could compromise patient confidentiality. And I've seen people argue that well, you could have given us some details without necessarily compromising confidentiality.

Noah Baker

So, Nature has also published a story about this and about this transparency, and there are researchers in that story that are saying there are other arguments for not releasing information. There are rumours that the person that fell sick was with something called transverse myelitis, which is a condition that could be associated with viruses perhaps. It's like an inflammation of the spinal column. But there are scientists that are saying even publishing that information is maybe not good when it comes to the objectivity of the researchers. The point of a clinical trial is that it should be blind as much as possible and so, as much as possible, all the people involved in this trial should know as little as possible about these things so they're not going to start seeing associations where there aren't or missing data because they're focusing too much on one thing or the other thing. So, there is kind of like an argument that, for the sake of the trial, these things shouldn't be released, as well as just the confidentiality of the patients.

Heidi Ledford

Yeah, that's right. It's a really complicated issue. I mean, I know, over the years when I've interviewed investigators or physicians about a trial or a treatment and they'll tell me some interesting anecdote about a patient, and it will seem so safe, it will seem like it wouldn't identify anybody, but then they'll have a think later, and they'll realise, ‘Oh, no, no, actually, that could be identifying because of this thing and this thing and this thing,’ that I had no idea. So, I can't really judge in a particular case, should they have released this information, should they not. I know in some trials, I have gotten more information about adverse events than I have in this particular case, but it may be that the design of this trial is such that it's more damaging to release that information, and it may be there are circumstances around the patient that would make it a breach of confidentiality to release that particular information. There are sort of more broader parameters of the trial that haven't been made public, that I think researchers would really like to know, that aren't specific to this event, but maybe have more to do with how the trial is structured, how it's going to be overseen safety wise and so forth, and that's something that might have a little bit less of a grey area in terms of releasing that information. But that's absolutely right. There are some good reasons in certain cases to hold back.

Noah Baker

I think, as well, one thing that we could think about is, because of the intense sort of scrutiny and media scrutiny on everything that happens, you could almost argue there's a reason not to be so transparent there because of what people might do with that information. This is a very devil's advocate argument and certainly not an argument that I'm making, but you could imagine that people hear ‘study paused’ and then media react with ‘this is a dangerous vaccine,’ and so on and so on. And I think it's probably also worth mentioning that, as far as we know, this is actually not the first time that this trial has been paused as well. This trial has actually been paused before but it wasn't leaked and so therefore wasn't talked about, and I wonder whether or not this pause is any different from the first pause, and whether or not this adds to the confusion that we didn't hear about the first pause at the time back in July. Reportedly, it was because the patient developed multiple sclerosis, which was deemed to be not relevant to the vaccine and so the trial continued. I guess I have a big question about how much weight we put on any of these reports because, depending on when they come and how they come, they can change everyone's perception of them.

Heidi Ledford

That's true, and the problem with vaccine hesitancy, for example, I don't know how best to deal with that. But I do feel like there are ways to sort of provide information, you would hope, without inciting panic, but while at the same time informing the public. But nowadays, I don't know what's going to get picked up on Facebook and distorted anymore. I mean, I guess the counterpoint to that is also we don't want everybody to assume vaccines are safe automatically before they've been fully tested. Again, in the US, where I pay a lot of attention to the news because I'm American, you have Donald Trump pushing for a vaccine to come out before the election to help him win. I mean, in a way, this pause was a good reminder that, no, you've got to make sure these things are safe before you push them out and that you could potentially have an adverse event. So, maybe reminding the world that that's a possibility is kind of a good thing too. I don't know.

Noah Baker

There is this kind of air of underhand that comes across from the way information about this has come across, which I think has really confounded people's reaction to this story. So, a lot of information that has been leaked has come from a kind of closed, private call between the CEO of AstraZeneca and select investors, to give them information about what this means so that the investors would know, and I think that any kind of information that comes through the money channels quite quickly triggers people to smell a rat, I suppose. And you wonder whether or not if there had have just been transparency from the beginning, that wouldn't have been a concern in the same way. People might not have reacted in the same way.

Heidi Ledford

I mean, I think that's possible. I think there's a long-standing scepticism, especially in the United States, that's growing of the pharmaceutical industry because they are charging such high prices for drugs, and it's making it very clear that profit motive is clearly at work, right? So, you've got that. You've also got, in the US, this horrible opioid addiction epidemic, some of which has been sort of pinned on pharma for pushing the over-prescription of these opioids and so on. So, you have a lot of scepticism about the pharmaceutical industry and I think, to some extent, helping us deal with this pandemic as a way to sort of salvage their reputation. But not being transparent doesn't really help that happen. I guess it just sort of feeds into the conspiracy theories and the scepticism that's already there.

Noah Baker

There is also an uncomfortable reality that maybe we're overlooking slightly here, which is that a lot of phase III trials don't work. It's quite common for trials to get to the end of all this, all this effort goes in, and then nothing comes of it because it doesn't work. It's not a question of when it works and how long it's going to be. The outcome of this could be that we don't have a vaccine.

Host: Benjamin Thompson

Yeah, I mean, I've got one of those apocryphal sort of stats that it's sometimes hard to prove, and it's that 10% of drugs make it to phase one, of which only 10% of that make it through to phase two, to which only 10% of that make it through to phase III. So, waiting for success to happen isn't necessarily what is going to happen.

Heidi Ledford

I mean, I think this pause on the trial was a good sort of reality check. I mean, I could find myself kind of slipping into that fantasy as well, that, ‘Oh, these trials are going well, they're going to work.’ And there's reason to be optimistic because the preclinical data looked alright, the early clinical data looked promising. Fauci is out there saying, ‘Hey, maybe we're going to have something by the end of the year.’ But I do think this was a good reality check. Most experimental vaccines fail, and some of these are using platforms to develop vaccines that we don't have a lot of experience with and it's also a virus that we don't have experience with. It's not a flu vaccine, right. So, there's so many issues. Even if these succeed in the clinical trials, how well do they work in the elderly, right? We're going to need to know that. They may not work that well. Or maybe they cut down on symptoms, but do they cut down on infection as well? Do you end up generating a lot of people who are walking around as asymptomatic carriers of the virus and then, of course, the big issue of how do you deploy it in an equitable way? I catch myself looking at these trials as like, ‘Oh, when they're done, we have a vaccine that we’ve just got to get out there and we're set.’ There's still a lot of questions and a lot of details to work through, and I do think the pause was kind of a reminder. That is one reason why it gave us all a panic attack, I think. I think deep down, we know this is a fantasy, right, and so this is a reminder of reality. We really don't want it but it was maybe useful that way.

Host: Benjamin Thompson

It certainly seems like this is a story that is going to run and run. Heidi, I hope you'll join us again to talk more about it when it does. But for the time being, Noah and Heidi, thank you so much for joining me on Coronapod this week.

Noah Baker

Thanks, Ben.

Heidi Ledford

Thanks so much, Ben.

Host: Benjamin Thompson

Listen out for more Coronapod next week. And listeners, if you're involved in a coronavirus clinical trial, either as a participant or as a researcher, Nature would like to hear from you. I'll put up a link to a survey in this week's show notes so, if you fit the bill, please go ahead and fill it in. Coming up later in this week's show, we'll be hearing about an incredibly small ultrasound detector. Before that, though, Shamini Bundell is here with this week's Research Highlights.

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Shamini Bundell

You might think hummingbirds spend their time sipping nectar from tropical plants on warm, sunny days. But hummingbirds can also be found in less hospitable settings, including high in the Andes Mountains, up to 5,000 metres above sea level, when night-time temperatures drop to near-freezing. It's a tough environment for a tiny bird, so these hummingbirds often go into a state of torpor. They become cold, motionless and unresponsive with a slowed heart rate, similar to the state of hibernation, but lasting only a single night. Researchers monitored 26 hummingbirds of 6 different species who spend their nights high in the Andes. They found that the birds’ metabolic rates dropped by up to 95% and, in some cases, their body temperature dropped by over 25 °C. In fact, one metal tail hummingbird set the record by dipping its body temperature to just 3.3 °C, the lowest ever recorded in a bird or non-hibernating mammal. The next morning, the birds warmed themselves up again and off they went. Read more in Biology Letters.

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Shamini Bundell

The burnt remains of an ancient grain silo in Turkey is giving archaeologists new insights into the way of life of the Hittite people who lived in the region over 1,000 years ago. The huge silo was built in the Hittite capital and could store over 5000 tonnes of cereal grain in numerous sealed chambers. But soon after it was built, the silo was partially destroyed by fire. Several of the burnt chambers were abandoned and the charred grains inside were left untouched. To get an idea of where these grains came from, researchers looked at the different species present and the atomic isotopes within the seeds. The wide range of species, which included crops and weeds, and the varied isotope ratios suggest this wasn't just local storage. The food probably came from a large number of different farms in different locations. This suggests the grain may have been collected together as part of a tax paid to the Crown. The fact that the burnt storage chambers within the silo were never cleaned out or reused suggests that after the fire, the Hittites may have thought twice about storing all their grain in one place. Read more in the journal Antiquity.

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Host: Nick Howe

Next up, reporter Ali Jennings is here to tell us about a new type of ultrasound technology that could peer inside people in ultra-fine detail.

Interviewer: Ali Jennings

How do you see through someone's skin? Perhaps you want a scan of your unborn child or to watch a patient's heartbeat through their chest. One solution is to use ultrasound – sound waves at a frequency too high for human hearing. Classical ultrasound, like you might find in a hospital, direct sound waves at an object of interest then detects the waves that bounce back. These reflected waves can be translated into an image. It works a lot like sonar. But Vasilis Ntziachristos from Munich Technical University in Germany does it another way. He uses light.

Interviewee: Vasilis Ntziachristos

When molecules absorb light, essentially, they take this energy and there's a minute temperature increase inside tissue, and because of this temperature increase, there is a volume increase. This is a transient however phenomenon, so essentially what we have is a volumetric expansion and contraction and that generates an ultrasound wave inside the tissue.

Interviewer: Ali Jennings

When tissue absorbs light, it momentarily swells then contracts, fast enough to produce a high-frequency sound that Vasilis can measure. This is called optoacoustics. The technique has been around for a while, but Vasilis wants to use it to image smaller and smaller structures down to a microscopic scale. That's where things get tricky though. The smaller the thing you want to image, the smaller your detector needs to be.

Interviewee: Vasilis Ntziachristos

So, ultrasound detection has been traditionally based on what's called the piezoelectric technology, and this technology essentially converts ultrasound waves, that pressure, to electrical signals. And those piezoelectric detectors, they can be manufactured to be very small. But as you start reducing their area, their sensitivity drops. So, there's a certain limit that is defined essentially by sensitivity to which these detectors can be practical.

Interviewer: Ali Jennings

To solve this detection dilemma, Vasilis and his team have created a new kind of ultrasound sensor, one that can be made extremely small, yet still retain its sensitivity. Here's how the sensor works. Down at a microscopic scale, two tiny mirrors are placed facing each other a very short distance apart. Light is then shone into the gap, where it bounces back and forth between the two mirrors. When an ultrasound wave passes through the sensor, the pressure changes the distance between the mirrors. That changes how the light waves bouncing around inside interact with each other, and the light becomes darker or brighter. By measuring the change in brightness, Vasilis and his team can detect an ultrasound wave passing through. These types of detectors already exist, but the breakthrough was realising you could build these devices 500 nanometres wide on a silicon chip. This is possible because light travels more slowly through silicon than standard optical fibres. That allows the pathways that channel the light to be built only a few micrometres thick.

Interviewee: Vasilis Ntziachristos

Which means we can now have a detector that is so small, that can image way beyond the wavelength of the ultrasound used. So, it is the first time that super resolution imaging at such high subwavelength scale is achieved eve.

Interviewer: Ali Jennings

Vasilis’ detector allows him to make out details less than 1 micrometre in size. Think around the size of a bacterium. But unlike a standard microscope, Vasilis’ 500-nanometre detectors will be able to fit into tight places.

Interviewee: Vasilis Ntziachristos

So, essentially, this is more for biological explorations or for ultra-miniaturised endoscopes, let's say. You can see, now, features that they would be much smaller on a more superficial level, than is the capacity of any classical ultrasound detector. So, we could see very small microvasculature or we can see details of single cells.

Interviewer: Ali Jennings

In the future, these ultrasound detectors could be built into medical probes that would allow us to study a patient's internal systems in unprecedented detail. Vasilis and his team hope that this technology will become widely available because it makes use of existing silicon chip manufacturing techniques. This, they say, means that multiple sensors could be built onto a single chip with relative ease. But Vasilis is keen to point out that the technology is not just for the clinic.

Interviewee: Vasilis Ntziachristos

The detector can not only visualise things in tissue, but it can start the fundamental phenomena better because of this very high resolution. So, imagine that you want to study something relating to all ultrasound from basic ultrasound wave propagation and what happens to waves in a certain material or medium. You can do this fundamental observation of the wave with a discrimination I believe that was never possible before.

Interviewer: Ali Jennings

Because the sensor is much smaller than the size of the ultrasound wave, it can provide a very detailed image of the shape of the wave itself. This opens up a whole new way to study the physics of ultrasound. Although this kind of detail won't be available in classical clinical ultrasound, so don't expect any high-res portraits from the prenatal ward. But we can look forward to taking a better look at sound itself.

Host: Nick Howe

That was Ali Jennings. To learn more about tiny ultrasound detectors, head over to the show notes where you'll find a link to Vasilis’ paper.

Host: Benjamin Thompson

Finally on the show, it's time for the weekly Briefing chat, where we discuss a couple of articles that have been highlighted in the Nature Briefing, and that is, of course, Nature's daily pick of science news and stories. Nick, what's caught your eye this week?

Host: Nick Howe

Well, I'm sure it hasn't escaped your attention, or many of our listeners for that matter, that many parts of the US are experiencing wildfires, and you may have seen that parts of California, for example, are looking sort of orange-tinted. So, I was looking into why that is.

Host: Benjamin Thompson

Yeah, of course. I mean, there's wildfires are up and down the West Coast, and our thoughts go out to anyone affected. But yes, a lot of the pictures that I've seen online do have this kind of orange hue to them, and I assumed that it was just fires in the distance.

Host: Nick Howe

Yeah, so you might think that it's just that sort of light coming through, but actually, many of these places in California that have this orange glow are quite far away from the fires themselves. And so, what the reason is, is because there's something called a marine layer, which is a layer of air, which is a mixture of cool, moist and warmer air that tends to sit near coastlines, and this layer of air can act as a barrier to certain kinds of particles. And so, what that means is the smoke particles, as they come across over California, blown by the wind from where the fires are, they sit on top of this layer and then they block certain wavelengths of light from coming through.

Host: Benjamin Thompson

Ah, and I guess then that the Sun streams down and it's coming through this layer, and that's what's giving this kind of colouration to the sky, right?

Host: Nick Howe

Yeah, I mean, you might be familiar as to why the sky is blue, like certain particles in the atmosphere scatter blue wavelengths of light, but these smoke particles are much larger than particles that normally exist in the atmosphere and so they scatter a much greater array of wavelengths of light, and only really the red and orange ones can get through. So, you actually end up with a very strange sort of orange glow that you might see in Blade Runner or something like that. Ben, what's your story this week? I think you were also looking into something to do with the Sun.

Host: Benjamin Thompson

Yeah, that's right, Nick. This week, I've been looking at a piece which has been exploring kind of the very edges of the Solar System, where it ends and the kind of interstellar medium begins.

Host: Nick Howe

Oh, ‘interstellar medium’ sounds very interesting. Is that just everything not in the Solar System?

Host: Benjamin Thompson

Well, yeah. So, the interstellar medium then, this thing sort of thing between stars, has been studied really for quite a long time with sort of telescopes here on Earth, and it's – I don’t know – ionised hydrogen atoms, cosmic rays, these kind of dust clouds, all these sorts of things, right? But it's where these two things combine – the edge of the Solar System and this interstellar medium – that's kind of been a bit puzzling for researchers.

Host: Nick Howe

Right, okay. So, there's the stuff coming out of the Sun sort of pushing out and showing what the boundary of the Solar System is and everything else from outside pushing against it. So, why has it been hard to work out where that boundary is? Can you not just see?

Host: Benjamin Thompson

Well, there's a great quote in the article that says something like, ‘If you want to know what the outside of your house looks like, you need to go outside and have a look.’ I think that's been part of the problem here. So, the Sun kicks out this kind of solar wind of these charged particles and they sort of radiate out, and it's where these kind of meet up with this interstellar medium that you get sort of a push-pull effect, and it's been quite difficult to know what it looks like. But thankfully for us, the Voyager 1 and 2 probes, which were launched in the 70s to look at the planets that the sort of fringe of the Solar System, have just kept going and they're sort of passed into this medium and can see what's going on.

Host: Nick Howe

And pray, tell, what is going on?

Host: Benjamin Thompson

Great question, Nick. Well, I will say, it's not a kind of defined boundary. It's not kind of the Solar System and then nothing. It's this kind of maelstrom of these kind of clashing magnetic fields and charged particles and it's massive, this area. It's millions of kilometres across, and what's interesting about it is it kind of changes a bit as well. For example, when the Solar System passes through the galaxy into an area with more or less dense interstellar medium, the boundary shifts closer or further away.

Host: Nick Howe

Okay, and now we sort of have an understanding of what the edge is. What does that mean, like what are scientists hoping to do with this information next?

Host: Benjamin Thompson

Well, there's a bunch of questions to answer here, Nick, and this kind of wind coming out of the Sun creates this bubble called the heliosphere. And I think what's of interest a lot of researchers is, how rare is this? Is this quite a common thing? And it's kind of intriguing as well as, is this something that is imperative if life is to exist anywhere else because this heliosphere bubble really protects the Solar System, really, really shields it from being battered by these kind of cosmic rays. And I think it says here it sort of filters out about 90% of them, which otherwise would, well, pretty much scour everything clean.

Host: Nick Howe

Okay, so there's this warring effort that is the edge of the Solar System, but I think we've actually also reached the edge of the Briefing chat. So, listeners, if you'd like more stories like these then make sure you check out the Nature Briefing. We'll put a link of where to sign up and links to stories we've discussed in this week's show notes.

Host: Benjamin Thompson

That’s all for this week, but if you want to get in contact with us, then you can reach us on Twitter – we’re @NaturePodcast – or you can send us an email – we’re podcast@nature.com. I’m Benjamin Thompson.

Host: Nick Howe

And I'm Nick Howe. See you next time.