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Adam Rutherford: This week a glancing planetary blow.

Maria T. Zuber: Elliptical craters only occur with very very low angle impacts, essentially nearly horizontal, so less than 10 degrees from the vertical.

Adam Rutherford: That colossal collision is part of our special impact issue.

Kerri Smith: And that's not the only explosive feature of the show this week. We also tackle a powerful underwater volcano.

Robert A. Sohn: I mean, the amount of energy that was released during this earthquake storm is you know as several nuclear bombs.

Kerri Smith: This is the Nature Podcast. I'm Kerri Smith.

Adam Rutherford: And I'm Adam Rutherford. For starters this week, we are looking deep inside, massive holes in the ground. In a minute, we will be talking to Nature News and Features Editor Oliver Morton about the significance of impacts and craters on Earth and beyond but first Nature publishes three papers this week that describe what could be the largest impact structure in the Solar System. Mars has what's known as the crustal dichotomy, the Northern hemisphere features smooth plains and a significantly thinner crust than in the rugged highlands of the South, making it kind of a bottom heavy planet. Here's Jeffrey Andrews-Hanna from MIT.

Jeffrey C. Andrews-Hanna: So we have known for a long-time that Mars has what we call a hemispheric dichotomy. So the Northern half of the planet is of low elevation and it is fairly smooth whereas the Southern half of the planet is of much higher elevation, of about 5 kilometres higher and is very heavily cratered. Now the origin of this hemispheric dichotomy has been a real mystery in planetary science. It completely dominates the topography of Mars, but we still don't know how it formed.

Adam Rutherford: What's the two competing theories, the new studies are pointing towards a high impact answer.

Margarita M. Marinova: There are two theories of how it formed. One is that through mantle convection, one side of the planet the crust got thinned while in the other it got thickened and another explanation of which we propose was that a very large impact effectively excavated part of the planet leaving this thinner crust in the North.

Oded Aharonson: What we did was set out to test the hypothesis that a single large event could redistribute crust globally from the Northern hemisphere to the Southern hemisphere leaving behind a morphology of a basin that's similar to what we see today.

Adam Rutherford: That was Oded Aharonson and before him Margarita Marinova, both part of the team from Caltech who have modelled how a colossal collision could have caused the geology we see on Mars.

Jeffrey C. Andrews-Hanna: This idea of a very large impact testing it via numerical simulations using a technique called smooth particle hydrodynamic where you represent the planet as collections of particles, was first proposed to me when I was a graduate student at Cornell by professor Steven Squyres. I tried to do this and failed because at that time computers weren't fast enough to properly simulate this event. When Margarita showed up here at Caltech, she took this idea and remained with it and indeed ran simulations on a very large computer downstairs called CITerra dedicated to understanding your physical problems on the Earth and other planets.

Adam Rutherford: Maria Zuber at MIT was initially not convinced by the impact idea.

Maria T. Zuber: I was so sceptic about the dichotomy being due to an impact but for a good reason I think we produced a topographic model for Mars, and if you look at the outline, no matter how you orient it, you don't get a round structure. In the Solar System, most impact craters are round and the dichotomy boundary on Mars was just not round.

Adam Rutherford: The solution to this seemingly insurmountable problem came by understanding how the Martian landscape has been formed over millions of years. Maria Zuber and Jeffery Andrews-Hanna worked together using data from the Mars Global Surveyor Spacecraft and the Mars Reconnaissance Orbiter to go beneath the surface.

Maria T. Zuber: Jeff came along and we started looking at the crustal thickness and there was the issue of this province called Tharsis and Tharsis is a large volcanic region of the planet. It takes up about a quarter of the surface area of the planet and it consists of the very large volcanoes that we're familiar with but it is also just a general topographic rise, which corresponds to volcanic flows.

Jeffrey C. Andrews-Hanna: The crust in the highlands and the lowlands is essentially floating in the mantle, almost like an iceberg floating in the ocean. So in areas where you have high topography it's buoyed up or it's by a thick crustal root underneath, like the 90% of an iceberg that's hidden beneath the water. Tharsis on the other hand form later in Mars' history after the planet had cooled and the outer parts of the planet had become stiff and elastic. So first this is actually supported by the elastic strength of the outer portions of the planet. So if we look at the topography of Mars shown here, and as we unwrap the globe to a map of Mars what we then do is we're going to remove this Tharsis region from the topography and you can now trace the dichotomy boundary right across through the Tharsis province, so we can now for the first time trace the full globally continuous extent of the dichotomy boundary. Now what you are going to see and what surprised us originally is that the dichotomy boundary follows a fairly smooth path around the planet, it doesn't look at all irregular and when we looked in to this in more detail what we found is that this path is actually the projection of an ellipse onto a sphere and if we take this flat map and we collapse it back on to a globe, now we can see how the dichotomy boundary looked before Tharsis on this spherical planet and it still looks irregular but if we now take this globe and then we reorient it, so that we're looking at the centre of this lowlands basin and now just unwrap the planet, we find it out but this time in a polar projection you can see this elliptical shape quite clearly.

Adam Rutherford: And this new shape invokes a persuasive argument.

Maria T. Zuber: Well this changes the ball game, elliptical craters only occur with very very low angle impacts, essentially nearly horizontal, so less than 10 degrees from the vertical and I think that an impact origin for the dichotomy of Mars is now the simplest explanation and when I look for explanations, I like simple explanations when I can find them and that's what I like about this.

Adam Rutherford: Maria Zuber from MIT on the Massive Glancing Blow that shaped Mars. Those Martian papers are available at along with a huge giant holes worth of high impact features. Chief News and Features Editor Oliver Morton masterminded filling this crater and he is with us in the studio. Olli, we have heard about this colossal impact on Mars, but what about a bit closer to home? What have been the biggies here on Earth?

Oliver Morton: Well, the most exciting one in recent history and just about living history if you're very very old is the Tunguska impact in Siberia which was a hundred years ago this week by happy coincidence and Tunguska is a pretty good example of what a not very large rock can do. I mean this is something about 50 meters across people think, that exploded with a force of a fairly large nuclear weapon.

Adam Rutherford: And what about the overall significance of meteors and craters in the whole scheme of things?

Oliver Morton: Well it's very odd, I mean, meteors and craters in the whole scheme of things after the really big ones early on that do things like reshape Mars and give the Earth a Moon. They normally just, sort of you know, they make a big hole, but it is quite remarkable how sensitive the Earth can be to big holes. I mean, when you think about it, the actual amount of energy deposited by the thing that hit the Earth 65 million years ago and killed off the dinosaurs that's like a morning's worth of sunlight or something, but if you concentrated in the right place in the right way, you can have a huge effect and yeah, the history of life on Earth, impacts aren't the only things that have shaped it but they have definitely had a significant role, and any given impact on Mars or the Moon isn't particularly big deal, but if you can look at the surface of those planets they are all circles and that's all there is.

Adam Rutherford: And so when can we expect another impact as big as say Tunguska or Chicxulub which was the one in Mexico that wiped out the dinosaurs?

Oliver Morton: Well Chicxulub, people talk about it, being roughly speaking a 100 million years then, but just to stress, there is no strict periodicity in this. I was going to say it's not like they come along like busses but it's exactly like they come along the busses, in that there is no rational reason why the presence of one should predict or not predict the presence of another, my apologies to anyone listening in an area with better public transport than London. However, the Tunguska one, you can be a little bit more sure about because they're more frequent and estimates for how often we should see a blast that big ranged from about every 1000 years to about every 300 years. There is some debate about how big Tunguska was and hence there is some debate about how often you should expect it. We see ones as large as a, small nuclear blast, pretty much every year, but they happened in the upper atmosphere when they are only seen by spy satellites.

Adam Rutherford: And Tunguska landed in a place where there weren't a great deal of humans, we've all seen some pretty hoaky Hollywood films where we deal with meteors crashing into Earth, what actually is going to happen if a meteor the size of the one that hit Tunguska is aiming for London or New York.

Oliver Morton: Even with the current asteroid surface which are pretty good, we are not going to pick up every Tunguska that is coming in and I think it is something like there is a 50:50 chance that it won't kill anyone, because you know an awful lot of the Earth is empty and so you often see pictures of you know the Tunguska blast site compared to the area of Washington DC and that looks very frightening until you remember that it's not going to hit Washington DC, but more generally if you saw one that was a good bit bigger coming in there are ways that we could imagine for deflecting it and if you see it coming early enough there is a good chance that you know given 20 years forward planning we would be able to move one around, if it wasn't too big.

Adam Rutherford: So you wouldn't have to rely on Bruce Willis. What other impactful features have you got in this issue?

Oliver Morton: Well, this is a little package about Tunguska about looking for potentially dangerous asteroids, about there is a very nice commentary about how the search for asteroids has gone so far, there is a little bit of looking back at Science Fiction about impacts, the famous disaster novel Lucifer's Hammer and there is a thing about the impact crater that a large number of scientists would most want to go to which is the huge basin at the South Pole of the Moon.

Adam Rutherford: Okay, thanks Olli and that whole impact jamboree including videos and features is available at

Kerri Smith: Coming up shortly, the British beetle enthusiast 'have a go' explorer who co-discovered the most impactful - "Oh wait, we've done that bit" - the most central theory in biology. But first, Geoff Brumfiel plunges into the Arctic ocean with the team who've been snapping an underwater volcano in action. Nature 453, 1236–1238 (26 June 2008) ;

Geoff Brumfiel: Earth's tectonic plates are spreading apart deep beneath the ocean. As they do so, lava bubbles upon to the seabed making new crust. Geologists thought this was a smooth process but a new find suggests that sometimes crust is made with a bang instead of a whimper. Deep beneath the Arctic Circle, scientists have found volcanoes that periodically explode with a force of several nuclear bombs. I called Robert Sohn at the Woods Hole Oceanographic Institution to hear about what they saw at a place called the Gakkel ridge.

Robert A. Sohn: So the global mid-ocean ridge system has this leg of it, if you will, that stretches up from, say, Iceland and goes north into the Arctic basin to Siberia on the other side and that's the Gakkel ridge and that is a portion of the ridge that's unique, really in two ways. One obvious way is that it is under the ice and so therefore very hard to observe and then the other thing that's unique about the Gakkel is what we called the spreading rate which is how fast the two tectonic plates are diverging to make this volcanic change and the speed at which the plates are diverging is extremely slow so rather than having this really efficient fast mode of volcanism, we have something that's in fits and spurts so they are much more complicated.

Geoff Brumfiel: What does that mean in terms of how the volcanoes behave?

Robert A. Sohn: So, you know, the kind of volcanoes that we see in mid-ocean ridges is a lot like, a kind of a, burping syrup; the lavas are not very viscous so they're very runny. The lava rises up in the subsurface and then kind of spreads out and runs over the table, so to speak of, the seafloor that is the normal way that we are used to thinking that of the basaltic lavas and that has to do with this fact that they don't have very much carbon dioxide or water dissolved in the lava itself.

Geoff Brumfiel: So I guess that's why ocean ridges aren't thought of as explosive places, but I gather you found evidence that the Gakkel was different, tell me what you saw?

Robert A. Sohn: Yeah! There were two things that really stand out in terms of, you know, what we observed is that typically we found these individual cratered volcanic structures that were literally filling up you know this axial valley where the two tectonic plates, you know, the boundary of them actually lies you know that's pretty different and so that was a kind of a hint that the process was somehow happening differently than most other places where we looked up to now and the other thing that really, you know kind of feel the deal, was when we got up close to the sea floor with our camera systems, we found there was, you know, fine grained black and shiny kind of sediment covering literally the entire axial valley where everywhere that we looked and we found that they were actually angular fragments of lava including bubble-wall fragments, so thin films of you know glass that were formed as a bubble was you know exploding or bursting that's pretty much a 100% conclusive evidence that it was an explosive style of eruption.

Geoff Brumfiel: How big an explosion are we talking about?

Robert A. Sohn: That's a good question and we hope at the moment it's a bit hard to put a number on that but I mean the amount of energy is you know as several nuclear bombs.

Geoff Brumfiel: So what ultimately can the Gakkel tell us about, just I suppose plate tectonics and the way the Earth's functions as a planet.

Robert A. Sohn: You know the conventional wisdom had it that this kind of explosive activity was not possible on a mid-ocean ridge at 4000 meters water depth, because the basalts were not supposed to have enough carbon dioxide or enough volatiles to be able to explode, these really tremendous you know hydrostatic pressures that you experience there you know 2 miles or more under water. So we have this question okay, well, now we've seen that it is possible, it did happen and so what do you do with that information. Well, you have to ask yourself, well what we do we really know about how much carbon-dioxide is being fluxed through these mid-ocean ridges, you know, deep under water where we really have very limited ability to observe and immediately you have to kind of admit that we have based our assessment of how much carbon dioxide is coming out on very limited observations.

Kerri Smith: Robert Sohn of the Woods Hole Oceanographic Institution in Massachusetts.Jingle

Adam Rutherford: You're listening to the Nature Podcast. Now we're feeling generous this month here at Nature and giving away a 160-gigabyte video iPod. All you have to do for a chance of winning is fill in a survey and tell us what you think of the podcast. Just go to and follow the links from there. It will take just a few minutes to complete.

Kerri Smith: Very exciting. Now we turn next to the Victorian naturalist and one of the unsung heroes of biology Alfred Russel Wallace. Here's Sara Abdulla

Sara Abdulla: Thanks to one fateful letter, the theory of evolution by Natural Selection was unveiled at the Linnaean Society in London a 150 years ago this week. That letter was from Alfred Russel Wallace to Charles Darwin. In an essay in Nature, Andrew Berry and Janet Browne celebrate Wallace's life and legacy. Born poor and self-educated, Wallace longed to make his name as a naturalist. At 25 years old inspired by Darwin and Humboldt's books about their seafaring science, Wallace set off to his own adventure with fellow amateur Henry Walter Bates, Andrew Berry takes up the tale. Nature 453, 1188–1190 (26 June 2008)

Andrew Berry: They decided to go to the Amazon, which I think is a bit like getting on a space shuttle to go to the Moon or something, I mean, in terms of today's equivalent, this really was the great unknown. Wallace spent four years in the Amazon extraordinary adventures, nearly died a couple of times, uncontacted tribes, remarkable collections and this was the learning on the job, selling specimens for a living, this is the lowest rung on the professional scientific or biological ladder and it all went completely belly up because his boat caught fire on the way back across the Atlantic; lost everything, literally all his specimens, his living specimens and most of his notes. So he got back to Britain, hoping to have made it to being admitted to the upper rationales of science, no! it is off; all over again this time, 8 years on the road in Southeast Asia; but it was collecting caused him to think about variations and then he is ready, if you like, to deliver his first evolutionary bombshell which was this paper in which he described the phenomenon of Evolution without the mechanism of Natural Selection.

Sara Abdulla: What did that first paper say and how did the scientific community react?

Andrew Berry: So, he published this in fact the bombshell, he send it from Borneo and he basically made the observation that the genealogical process that results in closely related species being in one place in space and also if you look in the fossil record, within related close strata in time, so that employs an evolutionary process when new species are derived from old ones. He sat back and waited excitedly for reaction to this paper; remember he is Indonesia and virtually nothing happened. In fact something had happened, which is that various peoples have seen this paper in the UK and elsewhere and drawn Darwin's attention to it. Darwin had been cautiously developing his theory in the background. Charles Darwin did start writing in 1856 and he also started a lukewarm correspondence with Wallace, which is why Wallace did an extraordinary thing with his next great insight, which came in February 1858. He had a bad malaria and suddenly he glimpses the missing part of his theory. He has this, the existing theory is evolution, but he doesn't have a mechanism whereby adaptation can arise. That mechanism of course we call Natural Selection, he glimpsed it in this malarial fit, scribbled down a manuscript and sent it not to a journal Editor but to Charles Darwin.

Sara Abdulla: So, Darwin receives a paper on the theory of the evolution by Natural Selection, why then a century and a half later do we talk about Darwinism and not Wallaceism?

Andrew Berry: This is a highly debated and contested point. Darwin freaked out. We don't know exactly when he received the letter but he wrote to his colleague, the geologist Charles Lyell on the 18th of June 1858, saying look this is basically my idea written down by someone else and Darwin's was, we can see, was inclined to sort of throw the towel and but it was Darwin's colleagues Lyell and Joseph Dalton Hooker the botanist who came to his rescue and they contrived an extra paper which they presented on behalf of the two "indefatigable naturalists," Charles Darwin and of course Wallace.

Sara Abdulla: And was Wallace delighted or infuriated?

Andrew Berry: He was absolutely thrilled. Remember his whole thing is been about being this nobody and he aspires to become a somebody. There is no better way of becoming a somebody then co-publishing in the Linnaean Society, the most significant and important form of the day, the Nature if you like of its day, with Charles Darwin.

Sara Abdulla: What do you feel is Wallace's lasting scientific legacy?

Andrew Berry: I would say, one is the theory of evolution that he was the co-discoverer. He knew nothing about Darwin's ideas on this subject. Two, as a biogeographer, he wrote a monolithic, two volume tome on the distribution of animals. Truly, he is the father of evolutionary biogeography; probably his most frequent appearance in text books is as, the discoverer of Wallace's line, which divides the Australasian and Asian flora and fauna. Thirdly, his understanding of processes such as speciation and differentiation between populations very sophisticated, still being the so called the Wallace effect which is still discussed to this day.

Kerri Smith: Andrew Berry of Harvard University and for more about why Darwin not Wallace became science's biggest celebrity, you can read the essay Sara mentioned at

Adam Rutherford: Finally this week, Mike Hopkin has been finding out about the chemicals that are hovering up bad ozone from our atmosphere.

Michael Hopkin: We tend to think of ozone loss as a bad thing and when it occurs high up in the atmospheres protective ozone layer, that's true. But closer to ground level in the troposphere, ozone is a dangerous pollutant and green house gas. Now a new study in the sunny Cape Verde islands shows that topical marine ecosystems produce their own ozone-busting chemicals helping to control the pollution. I spoke to the lead researcher Lucy Carpenter of the University of York and began by asking her about this difference between good and bad ozone. Nature 453, 1232–1235 (26 June 2008)

Lucy J. Carpenter: So tropospheric ozone is the ozone in the troposphere which is essentially the lower, sort of, 10 kilometres of the atmosphere, so obviously there is a lot of attention on stratospheric ozone lost due to chlorine atoms from things like chlorofluorocarbons and there is a lot of interest there. In the troposphere though ozone is growing and it has more or less doubled since pre-industrial times and that's of concern for a number of reasons, first of all that ozone in the troposphere is a green house gas itself, so you know it is contributing to global warming and also I mean it is harmful to crops and to wildlife. It is essentially an air pollutant.

Michael Hopkin: And this bad ozone, if we can call it that as opposed to the ozone layer, where does it actually come from, how is it produced and how do its effect go right around the world?

Lucy J. Carpenter: Yeah! It's produced from the combination of nitrogen oxides which are really from fossil fuel combustion and from hydrocarbons which have a, sort of, dual source from both man-made activities, but hydrocarbons are also produced naturally from trees and grasses and so the combination of those with sunlight produces your tropospheric ozone and ozone can be transported relatively long distances, so that your emissions from the US certainly affect ozone's levels in the UK, ozone precursors in Asia effect, levels of ozone in the US etc.

Michael Hopkin: And your research seems to show a process that is hoping to rate the atmosphere of this ozone though, if I understand it correctly?

Lucy J. Carpenter: That's right, so when this effect was predicted several decades ago, then in essence it's analogous, it's exactly analogous to what is going on in the stratosphere. So in stratosphere, you have chlorine radicals and some bromine radicals as well that are causing this chemistry. So they enter into catalytic ozone destruction cycles, in other words, they enter a chemical reaction and they spat out the other end so that they are available to keep destroying ozone over and over and the same thing is happening lower down in the atmosphere in the troposphere, but it's not chlorine, in this case it's bromine and iodine and these are radicals that are formed from species that are broken down in the troposphere rather than in the stratosphere. So this effect was predicted several decades ago, but it is only now that we have the observational evidence that it is happening potentially certainly on a regional basis and potentially on a global basis as well.

Michael Hopkin: And what are the species that are producing these bromine and iodine compounds that are breaking down the ozone?

Lucy J. Carpenter: Well, in the case of bromine in the boundary layer the main source is the sea spray, so sea spray produces what is called sea-salt aerosols, so small, suspended water droplets that happen to contain high levels of salt, so bromine and chlorine and the bromine has released from reactions that happened on the surface of those sea-salt aerosol particles. In the case of iodine it is quite different. The main source we believe comes from ocean biology. So within the ocean you have got very small microscopic plants called phytoplankton and these through their metabolism they produce sort of more stable organic guiding compounds which are quite volatile and once they are in the air above, they can get broken down by action of sunlight and that produces iodine atoms and off they go and enter these catalytic ozone destruction cycles.

Michael Hopkin: And if these are natural processes that happen throughout the ocean, does that mean possibly that we don't have to worry so much about low-level ozone pollution anymore or may be would it offer us ways to try and tackle places with problems with ozone.

Lucy J. Carpenter: It is good news because it's another process that's destroying tropospheric ozone, so that has to be a good thing, but on the other hand, as far as we understand it, these processes had been going on all the time; we have just not really noticed them before. So in a way what it really means, is this current, sort of, global models, get ozone rates, i.e. they predict the concentrations to be more or less what we observe, a kind of means that they must be missing some, sort of, source so that they are not right in other ways, so you can look at it as a good news or you could also say, well actually it means there must be something else going on; there must be other sources going on that we don't know about to kind of make the balance right.

Michael Hopkin: And I mean presumably we do need to find out more and you made these observations by going to the Cape Verde islands just off Africa and flying planes for about a year, which sounds like quite a good work if you can get it. So you're planning any more measurements along the same line.

Lucy J. Carpenter: With the case these measurements in Cape Verde are ongoing, so we, set up an observatory there, and we have started making measurements in October 2006 and the instruments are still there happily chugging away. So, yes there are sort of changes or trends lurking ahead, we are in a good place to try to protect those.

Adam Rutherford: York University's Lucy Carpenter and for all you chemistry nuts out there, the latest instalment of Nature's Chempod is out now, available through iTunes or from That's all from us this week.

Kerri Smith: Remember let us know your thoughts on the Nature Pod by going to and following the links to the survey. We will be back with more research and news next week including what science can tell us about music and vice versa. I'm Kerri Smith.

Adam Rutherford: And I'm Adam Rutherford thanks for listening.


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