Nature Podcast

This is a transcript of the 21st February edition of the weekly Nature Podcast. Audio files for the current show and archive episodes can be accessed from the Nature Podcast index page (http://www.nature.com/nature/podcast), which also contains details on how to subscribe to the Nature Podcast for FREE, and has troubleshooting top-tips. Send us your feedback to mailto:podcast@nature.com.

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Kerri Smith: Coming up, the recipe for a self-healing rubber material.

François Tournilhac: What is very interesting and exciting in this research is to make something very new starting from very simple ingredients like vegetable oil and urea.

Adam Rutherford: Researchers who have recreated a Martian delta on Earth.

Erin R. Kraal: The Eurotank is basically the size of a large swimming pool and it's filled with about a meter of sand and inside of that you can build all different kinds of landscapes or features and experiment with different types of water flow.

Kerri Smith: And Darwin on religion.

John R. Durant: I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well as speculate on the mind of Newton. Let each man hope and believe what he can.

Kerri Smith: Writing in a letter to his American pen pal, Asa Gray.

(Music)

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

Adam Rutherford: And I'm Adam Rutherford. Now Kerri you've just stepped off a plane this morning from Boston. Nice to have you back!

Kerri Smith: Thanks. Good to be here. I have been off at the annual meeting of the American Association for the Advancement of Science or AAAS to its friends and I'll be rounding up the highlights later in the show.

Adam Rutherford: Great! While you were jet-setting around the US, I spoke to Carlos Bustamante and Kirk Lohmueller of Cornell University in New York. They've been comparing small differences called single nucleotide polymorphisms in the DNA of two ethnic groups, Americans of European and of African descent. What they have discovered is that there are more potentially damaging DNA variations in the European group than in the African. First, here's Carlos Bustamante. Nature 451, 994–997 (21 February 2008)

Carlos D. Bustamante: So by comparing population level patterns of variation between the two groups, we saw a shift in the proportion of Single-Nucleotide Polymorphisms or SNPs across the two categories, those that change amino acids and those that don't change amino acids. The amino acid changing SNPs were also then divided into those that are predicted computationally to have a small impact or little impact on protein structures, so called benign changes. Those that may have an impact on protein structure and folding which are termed 'possibly damaging' and then those that are likely to have a change on protein folding instability, which are termed 'probably damaging' and those also showed a shift towards an increase in the 'probably damaging' relative to the other categories in the European-American sample as compared to the African-American sample.

Adam Rutherford: So your study is showing that there's more genetic diversity within Africa than in the rest of the world, is that right?

Carlos D. Bustamante: Well, we didn't look at the rest of the world, as you know, others papers are coming out in your same issue that did and I think it is a long-standing result of human population genetics that African populations show more genetic diversity than populations outside of Africa and this is not a controversial point. I would say, this is what we expected to find and it's a signature of our common human origin in Africa. Right, this is what Francis Collins has called our common genetic humanity, where we all evolved in Africa, so it is not surprising that most of the genetic diversity or there is more genetic diversity in Africa than outside of Africa.

Adam Rutherford: Okay, Kirk if I could turn to you, could you describe the evolutionary history, which has resulted in this difference between Americans of European and African origin?

Kirk E. Lohmueller: So we hypothesized that as humans left Africa greater than 30,000 years ago, there was a small population size that actually left Africa and so during this time of small population size, a lot of mutations were lost as a result of genetic drift, so these mutations that were at low frequency went to frequency zero in the population, then new mutations started occurring and because of the small population size, genetic drift was a more powerful evolutionary force than in a large population. So, some of these weakly deleterious alleles could have drifted up to a slightly higher frequency in the small population that in a larger population would have been effectively eliminated by selection, so that is the first part of the effect. The second part of the effect is, as a result of the bottle neck there was this time of small population size during the migration out of Africa, but then obviously the European population expanded to populate the rest of Europe and during that expansion now all of a sudden because of the large population size, there was a lot more mutations occurring and a lot of these mutations are likely to be weakly deleterious, but since the European population expanded relatively recently within the last, may be 20,000-30,000 years there hasn't been enough time for negative selection to be whittling out these weakly deleterious mutations, so may be in thousands of years from now, the proportion of damaging SNPs will likely go down to what the equilibrium value will be, but we haven't reached that point yet.

Adam Rutherford: Okay, now this is the question that the popular press will really fixate on this week: what does this mean for the health of people of non-African descent?

Carlos D. Bustamante: So, we don't think we know at all what the implications are. I think one really important point that we want to make is that you cannot interpret these results to say that one person's genome is better or healthier or more evolutionary fit than another person's genome. These are population level effects and the strength of selection we are looking at here is incredibly weak. The reason we are able to pick up on it at all is because we are looking at many many many many genes, many more than have been looked up before to try to address this question in some sense and to give you a sort of a sense of how small each of these effects are you would probably need to compare the number of offspring left by thousands of individuals with the mutation to the number of offspring left with thousands of individuals that don't have the mutation in order to detect a difference in the evolutionary fitness, so these are all mutations of, very sort, of small effect and we know that environment plays a really key and such an important role in determining human health that we really want to not have this be seen at an individual level because we don't think that is an appropriate interpretation.

Adam Rutherford: Carlos Bustamante and before him Kirk Lohmueller; theirs and the other paper Carlos mentioned from Noah Rosenberg's group at the University of Michigan are both available on our web site http://www.nature.com/nature.

Kerri Smith: Now here is Mike Hopkin on the sticky end of a new material.

Michael Hopkin: Picture the scene, you're putting a rubber band around a sheet of highly important files when – twang - it breaks. But what if rather than trudging off to the stationery cupboard for a new one you could fix it simply by holding the ends together. That's the possibility offered by material scientists in Paris, who have just created a new form of self-healing rubber. Self-healing materials have fascinated chemists for a long time offering the potential of indestructible medical implants, everlasting paint, and even self-darning socks. These materials often use clever chemical tricks involving complex large molecules, but the new rubber is made from very simple raw materials, many of which you will find on your kitchen shelf or even in your own body. François Tournilhac one of the materials inventors explains. Nature 451, 977–980 (21 February 2008)

François Tournilhac: From chemistry point of view, what is very interesting and exciting in this research is to make something very new with very new property starting from very simple ingredients like vegetable oil for instance and urea which are natural products and available in huge quantities. And it is always frustrating for the chemists to make very lengthy multi-step reactions and to be able just to have any mass spectrum or very little quantity of material. Here we are able to make sample, we can play with the samples. We can show them to non-scientific people and we can communicate our enthusiasm to non-scientists who are understanding what is the new potentiality of this research and imagine themselves an application for this material.

Michael Hopkin: Traditional rubber snaps back into shape because it is made up of long chain-like molecules, each crosslinked their neighbours to form a sort of stretchy lattice. Once broken, these long chains cannot be reformed. In contrast, the new material is made from much smaller molecules that string themselves together to mimic the long chains of conventional rubber, but because these small molecules bond by electrostatic attraction, they can reform their connections even after being ripped apart. All you have to do is, hold the two broken ends together for around 15 minutes the inventors say and the break does not even have to be fresh. The effect works for up to a week. The material although not as stretchy as normal rubber has exciting properties, says another of the researchers Ludwik Leibler.

Ludwik Leibler: What we have is something new. It is something which behaves like a rubber. It can be deformed and then it relaxes back, yes because it's made of these small molecules, it self-heals like a silly putty, yet it doesn't creep in the same way. What is really exciting about it is that usually when you do a new chemistry you do it on a milligram scale and then the scale up will be to have a gram scale of your material. Here we started right away with 100 grams and made molecules that never existed and they are not catalogued in any book of abstract and things like that and so this is very exciting. From industry point of view, it's a very interesting one to start with; two, because it has plenty of advantages including price which is always important. The good thing is it because of the simplicity of this technology these materials will be available very soon and products based on this technology will hopefully be available for people with imagination.

Michael Hopkin: But what kind of products? Well the inventors aren't quite sure yet, but Leibler points to the example of silly putty which was invented in 1943 by engineers at General Electric who were trying to make a synthetic substitute for rubber. It languished unused on the shelves for years before someone realized how much fun it is to play with.

Ludwik Leibler: People didn't find for few years any application and finally there was a brilliant consultant in marketing who had this idea of making a toy and he then made millions of dollars, so my only hope is that this new material I hope at least will be a toy and give fun to many children in the Earth.

Kerri Smith: Ludwik Leibler ending that report by Mike.

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Kerri Smith: This is the Nature Podcast and for your chance to win an iPod Touch, listen right to the very end of the show.

Adam Rutherford: Now could we be looking in the wrong places for the next infectious disease outbreaks? We sent Charlotte Stoddart to find out.

Charlotte Stoddart: Last week I visited the Institute of Cancer Research in Chelsea, this week I am off to Regents Park - not to the zoo, but over the road to the Institute of Zoology where I am meeting Kate Jones. She and her colleagues have been looking at where, when, and why infectious diseases appear, with rather worrying results. Global resources to combat these diseases are concentrated in the richer, developed countries of Europe and North America, but according to this new study future disease hotspots are more likely to be in Africa, Asia, and Latin America. Kate thank you very much for inviting me to the institute today. And to start off with then, what are emerging infectious diseases and could you give us some examples? Nature 451, 990–993 (21 February 2008)

Kate E. Jones: Right, so emerging infectious diseases are diseases which have recently increased in incidence or virulence, so recently evolved into human populations or have jumped from a different host into a human population. A really good example of that is AIDS-HIV, which has come from chimpanzees which has jumped into humans and then evolved rapidly within human populations.

Charlotte Stoddart: And you are a biodiversity expert and I noticed looking around the room that there are lot of bat things, so I am wondering how you got interested in this project that is all about human diseases.

Kate E. Jones: Well it is very interesting actually, so I am interested in trying to look at the processes driving past and present patterns in biodiversity to think about, well, how to model that and then how do we go forward, so how do we understand how mammals are going to decline or birds are going to decline or amphibians and I met Peter Daszak and he is interested in diseases, you know, in predicting emerging diseases and so we got chatting about it would be great if we could predict what diseases are going to threaten wildlife because it is a huge amount of interest you know amphibians' declines with the rickettsia fungus, so we went off and we had a look in the literature for data on diseases and it is so patchily and terribly reported that we thought, well lets just focus on one species, so well, why don't we do us, you know, so we'll do human disease, that'll be great, so this is what we've done and we've just ended up making a predictive model of human emerging diseases which is incredibly interesting.

Charlotte Stoddart: So, could you tell me a little bit more about this modelling that you have done?

Kate E. Jones: Right, so we looked through the literature on emerging infectious diseases and started to build a database on the actual first time it emerged and where about in the world this has emerged and then what kind of pathogen was it, so we found 335 emerging infectious disease EID the events which we deemed as independent over the last 64 years, so its a quite a large span of the data.

Charlotte Stoddart: Talk me through these maps that we have in front of us then.

Kate E. Jones: Using our model we have highlighted places where it's most likely that you would get the emerging infectious disease. Because the drive is different, we split them up into the different categories, so the different types of EIDs predict them occurring in different areas. Now if you look at where we think EIDs evolving drug-resistant microbes will be occurring they are in the kind of more developed North, where a lot more people use antibiotics to treat humans and you got these drug-resistant microbes which are evolving causing problems. Now, that is in contrast, to the ones which we were getting for Zoonotic diseases from wildlife which is much more based on areas which have got high population density and high wildlife biodiversity.

Charlotte Stoddart: So that is where we see on the map the, sort of, red areas around India and I think parts of China, for example.

Kate E. Jones: Yeah, that's exactly right. Now the problem is that if you look at this map over here this is showing you the kind of degree of surveillance that has already happened in the past about where we are looking for these diseases. So areas like Europe and North America are really hot on there surveying for diseases and that's great if you are looking for drug resistance, but it's not so great if you are looking these Zoonotic pathogens which are emerging into the human populations in these different areas in these lower latitude developing countries. About two-thirds of human emerging infectious diseases have a nonhuman animal source. Areas, which are rich in mammalian species richness and have a high impact, so there are lots of humans in that area, have an increased likelihood of the disease emerging. So it does look as if there is a mismatch between where we are looking for things and where actually the likelihood of getting a disease is going to be coming next.

Charlotte Stoddart: Kate, thank you very much. I am now going to go back to the studio and call up your colleague Peter Daszak to find out a little bit more about the implications of this for human health policy.

Peter Daszak: There's been a huge amount of interest in emerging diseases for, you know, the past 30 years. We got organizations like the CDC, World Health Organization, NIH and Wellcome putting billions of dollars into funding vaccines and drugs to deal with emerging diseases. We all know about these diseases; some are HIV-AIDS, Lyme disease and despite all of this money we're still up until recently were no closer to understanding where the next one is going to come from or what it's likely to be, and that's really the significance of what we've done.

Charlotte Stoddart: So what would your advice be to these organizations, what should they be doing?

Peter Daszak: Well they need to look, I think, to these ecological methods and bring them into public health a little bit better and to really say, you know, you have got limited resources to deal with the global emergence of new pathogens – so lets target those resources better, what all the hotspot maps does is, it provides a way to target our resources better.

Charlotte Stoddart: So Peter is there a way that we can then now go out into the field to these disease hotspot areas and actually find the pathogens that are likely to cause the next diseases.

Peter Daszak: Yeah, its almost impossible to say exactly which pathogen is going to be the next new agent, but we can focus it down and I think that we should spend a little bit of our global effort just to be looking in these hotspot regions in wildlife species and in people that interact with wildlife, bushmen hunters, restaurant workers who use wild animals and start to look for these new pathogens. We're going to find not only new pathogens that are of interest in wildlife diseases, but also new pathogens; it could be the next HIV-AIDS.

Adam Rutherford: Peter Daszak from the Consortium for Conservation Medicine in New York ending that report by Charlotte. Now Kerri is fresh, off the plane from Boston, where she has been at the American Association for the Advancement of Science meeting, Kerri.

Kerri Smith: I am indeed; I must admit I have been fresher though. The loosely followed theme for this year's AAAS meeting in Boston was, 'Science and Technology from a Global Perspective'. I say loosely because the sessions I went to took me from human morality through, doing genomics in Africa, the ins and outs of memory and imagination, trafficking nuclear materials, and even analyzing faked works of arts, but given the global theme its fizzing to kick off by finding out of about nothing less than the biggest challenges shaping the world's future. Among the crystal ball gazers were, futurologist Ray Kurzweil and ex-US Defence minister, William Perry, who helped drew up a list of 14 grand engineering challenges facing humanity this century. I spoke to William Perry and he highlighted a couple of the biggest opportunities.

William J. Perry: You heard Ray Kurzweil talking about his view that solar energy is going to play a huge and important role and that's an important judgment because for decades now solar power has been a sideshow, but his judgment which is supported by not only the data provided by their people in the group was that we had a tipping point, there were engineering developments that are going to bring the costs down but that's is not really quite competitive. Similarly with fusion – fusion has been talked about discussed, research had been done on for decades. The judgment of our group was that not now that there would definitely be a winner at this time, but they were close enough for within the decade, we will know and if the answer is yes, then it will be a major move forward in fusion as well.

Kerri Smith: But there are dark sides to some of these opportunities and the panel have stressed that some of the 21st century challengers have to do with containing the ill-effects of 20th century technologies and making sure they are only used for good.

William J. Perry: We celebrate nuclear power and the engineering achievement in developing the nuclear bomb was an unquestioned great achievement. The positive aspect of the nuclear bomb is it for 60 years now, it has kept the world from having another major world war, but then I said, the dark side here is that the engineering which led to the nuclear bomb has further improved so that nuclear bombs can be made rather easily by a small group of terrorists. This great power that was developed by scientists and engineers led to the potential of nuclear power for commercial and civil use has led to the nuclear bomb which is as deadly as it is, has provided the deterrents from world war for now for 60 years, but it also has a potential of falling into the hands of evil people.

Kerri Smith: This job of making sure that nuclear materials don't fall into the wrong hands rests in part with the International Atomic Energy Agency. I caught up with the director of the Office of Nuclear Security, Anita Nilsson, and first asked her just how much trafficking she and her nuclear detectives reckon goes on.

Anita B. Nilsson: The IAEA records on a continuous basis information given officially from States of cases of trafficking and we get around 150 new cases every year and we have now built up in the database more than 1300 cases from the time of its inception in '95.

Kerri Smith: And what kind of things are these cases regarding? How much material's been found, who's doing this?

Anita B. Nilsson: This is a variety of material ranging from very innocent smoke detectors to more serious quantities of nuclear material and more serious radioactive sources and some of them are inadvertent and others are the results of thefts and losses.

Kerri Smith: And so how much of it then you talked in the press conference earlier about some of this being opportunistic – people just stealing it, they don't really know what it is? How much of it is malicious?

Anita B. Nilsson: We say that the criminal context to these cases is somewhere around 40%, but the criminal context may be for different purposes, it doesn't have to be malicious or terrorist uses, on the contrary it could be a financial gain; for example, environmental crime or it can be just the wishes to transform something which is believed to have a value into Dollars or Euros.

Kerri Smith: How does the IAEA then go about preventing these things from happening?

Anita B. Nilsson: The very important thing there is to promote and establish the infrastructure that is necessary to prevent these materials from coming out under uncontrolled circumstances, namely to put accounting in place in facilities, at industrial places in locations, and to put adequate physical protection in place for whatever material there is and while radiation doesn't show, it certainly can be registered if you have a radiation detection instrument and therefore what we do is to promote the use of effective radiation detection instruments at borders or in other places and we also promote the development of user friendly instruments for that purpose so that they can be used by customs officers which normally do not a scientific background for example.

Kerri Smith: Anita Nilsson of the IAEA. A highlight of my AAAS was a different type of detective work, a marathon symposium, delving into the mysteries of the mind. Thinking about thinking for 3 hours, starting at 8:30 on a Sunday morning was almost too much, even for a neuro nerd like me, but even that length of time wasn't enough to more than scratch the surface of this nebulous topic. A surprising prospective came from evolutionary biologist, Terrance Deacon, of UC Berkeley. Here's a snippet of his talk.

Terrance W. Deacon: We are, I would say, degraded apes and by that I mean genetically, neurologically, a whole variety of ways, I think. We seem to have ourselves placed on a pedestal and we are looking for things at it in a way more complicated. We're struggling to figure out even now, but its different about human brains or rather what's the difference that makes the difference. I think there's another way to look at this.

Kerri Smith: What Deacon went onto suggest is that things like language could actually be the products of relaxed selection rather than being positively selected for. Language could one of them and you can't see it in the genes. Incidentally, I think degraded ape describes me fairly well, having just stepped off the red eye this morning. Back to you, Adam.

Adam Rutherford: Thanks Kerri. Sounds like you had a lot of fun. Now Charles Darwin might have likened humans to our primate cousins, but I don't think he ever used the phrase "degraded ape." He did feature at the AAAS though, as one of the evening shindigs was a play about Darwin and his American pen pal, Asa Gray. Director of the MIT museum, John Durant, thinks we should learn from their fond correspondence.

John R. Durant: The Menagerie Theatre Company is currently touring New England with Craig Baxter's play 'Re: Design'. The work was commissioned by the Darwin correspondence project that Cambridge University Library in the UK and is supported by the Templeton Foundation, the philanthropic organization that funds research within and between science and religion. Re: Design is a remarkable and I think remarkably relevant production. It's based entirely on the surviving correspondence between Charles Darwin and Harvard Botanist, Asa Gray. Darwin first wrote to Gray in 1855. He wanted to know more about the distribution of the Alpine flora of North America and Gray was just the person to help. As the two men exchanged letters, so Darwin came to sense a kindred spirit in his American colleague. Summoning all his courage in 1857, he sent Gray a short summary of his ideas about the interrelatedness of the natural world. Gray was sympathetic and when the Origin of Species appeared two years later, he quickly became Darwin's leading supporter in the United States. Now the interesting part is that Darwin and Gray never really saw eye to eye on the significance of Theory of Evolution by Natural Selection. Gray was a convinced Christian, and nothing Darwin wrote could persuade him that the world, especially the living world, was not the handy work of God. "Organic nature abounds," he wrote to Darwin, "with unmistakable and irresistible indications of design" to which Darwin replied, "I cannot see as plainly as other do, and I should wish to do evidence of design and beneficence on all sides of us, there seems to me too much misery in the world." The two men continued to exchange ideas on Evolution and Creation, Natural Selection and Design, Chance and Necessity. As they did so, they gradually drew apart in their philosophical beliefs. Gray tended towards a kind of theistic evolution, and Darwin towards agnosticism. This divergence did nothing though to dim the warmth and closeness of their relationship, which continued through many years and even included a visit by the Gray's to the Darwin's at Down House, just south of London. When Darwin died, in 1882, Gray wrote an openly affectionate memoir of tribute for the American Academy of Arts and Sciences, to the person he called the most celebrated Man of Science of the 19th century. So why is all this remarkable? Well the questions that troubled Darwin and Gray a century and a half ago, continued to trouble many people today. There's been a lot of talk and a great deal of argument about so called 'Intelligent Design', here in the United States in recent years. Scientifically and philosophically speaking, the issues haven't changed that much, but the spirit of the times is totally different. It seems to me that ours is a far less generous minded, far more ill tempered age. Today many Darwinists parade their science under the banner of militant atheism and many theists line up in opposition both to atheism and to any naturalistic attempt to explain the history of life on Earth. This standoff hardly does justice to the complexity of the issues involved. As Darwin put it to Gray, "I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton, let each man hope and believe what he can." A few among Darwin's leading supporters today are inclined to write so humbly. Instead many of them seem to want to force a choice between Evolution and Religion, Science and Faith. Such a choice probably makes a few new friends for science, but I suspect that it makes many more new enemies and what's more it does so quite unnecessarily. For as Darwin and Gray knew there are no easy certainties when it comes to the larger philosophical and religious implications of the Theory of Evolution. If there were more Darwins and Grays in the world of science today, discussions about evolution and creation could continue within science, instead of being relegated to the lunatic fringes of the so-called culture wars. In this way, certainly we would all have a much pleasanter time, and who knows perhaps we might even learn something new.

Kerri Smith: John Durant from the MIT Museum in Cambridge, Massachusetts. Finally in this week's show, Geoff Brumfiel has been in search of water on the red planet.

Geoff Brumfiel: Planetary scientists are fairly certain that Mars once had water on it, but they can't say much more than that. Nobody knows for sure, how much water existed on the red planet or when it was there. Now a paper in this week's Nature, provides at least a few hints about Mars' wet and wild youth by recreating a Martian crater on earth. I called Erin Kraal of Virginia Tech University to learn more. Nature 451, 973–976 (21 February 2008)

Erin R. Kraal: There are probably over a 100 fans on Mars – Now fan is just a depositional feature that looks like what it sounds, it likes a fan, and they can form in different ways, but the stepped fans of which there is only about 10 have a really unusual surface, so if you can imagine looking at a stepped fan, it's the shape of a fan, but the surface is literally like stair steps. Now quite a large, they can be up to 20 kilometres across, and they tend to originate from really defined canyons, so it doesn't look like they are draining from large mountains or big areas, they come from these really really narrow confined canyons.

Geoff Brumfiel: So what particularly were you studying about the step fans? What aspect of them were you looking at in this paper?

Erin R. Kraal: We were specifically looking at the steps, because that is really their distinguishing feature and what sets them apart from all the other fans on Mars and it also sets them apart from fans on the Earth. So we were really curious; how do you form these steps. We couldn't find anything in previous research or in examples from our Earth, where we really felt convinced that, that was how they had formed, so we turned to the laboratory to try and do that.

Geoff Brumfiel: And tell me the experiment you did?

Erin R. Kraal: We made a mock crater in a large sediment flume and a sediment flume- it can take all different shapes, but the Eurotank at Utrecht University is basically the size of a large swimming pool, 5 meters by 12 meters and it is filled with about a meter of sand and inside of that you can build all different kinds of landscapes or features and experiment with different types of water flow. So what we did was to try and recreate part of the Martian surface, we made a crater and then we made a small channel going into it to direct the water and we allowed the crater to fill up with water over time and then because of the scientific equipment in the Eurotank, we were able to take really detailed measurements of the topography of the surface once we were done to make measurements about exactly how the sediment had behaved when it had entered this mock crater and deposited it in the sand.

Geoff Brumfiel: Obviously Mars has less gravity than the earth; substantially less I think. How are you able to take that into account in your laboratory tests?

Erin R. Kraal: Yeah, you're absolutely right. Mars has about one-third the gravity of the Earth and the way that we handled that from a sediment standpoint was we scaled the flow of water and the size of the sand particles that we used and the size of the crater that we used to be appropriate as an analog to Mars. Because sands form through something called bed-load transport which means the particles bounce along the bottom of the channel, that's less affected by the gravity of Mars in terms of scaling than some other processes. So we were able to maintain the similarity of process.

Geoff Brumfiel: And what were you able to determine about how these stepped fans formed?

Erin R. Kraal: What we were able to determine was that in order to form the steps, what you need to do is to have a fan depositing into a basin at the same time, the basin is filling up with water. So the two things have to be happening at the same time. Then to preserve all of these little lips or steps, you have to shut the system off because otherwise it would behave like a delta and you just keep depositing and all those lips would be erased by all of the sediment that would keep washing down through the system. And the second important part is that you can't have the system reactivate over time, you can't turn it off and drain it and then have more water flow through the channel later, because you would incise the steps and you would erase them too and you wouldn't be able to see them.

Geoff Brumfiel: So does this basically mean then that there was water on Mars but it was only there once and for not very long?

Erin R. Kraal: In these places where these stepped fans form, which remember they're really different than all the other fans on Mars, we believe that this does indicate the water was a very rapid event. We think from the sediment scaling that we did, that these fans formed between 10 and 100 years, so this is a really rapid event.

Geoff Brumfiel: And what about the rest in Mars?

Erin R. Kraal: Well the rest in Mars is the mystery. So our hypothesis is that for specifically the stepped fans, they are getting their water from some, sort of, internal source that's causing a rapid outburst of water, may be some hydrothermal activity or some intrusion of volcanic materials and they're not getting their water from an atmospheric source. Now that's contrasting with a lot of the other fans on Mars that do appear to be getting their water from atmospheric sources. So it's a caution that just because we see a fan on the surface of Mars, we cannot always attribute it to an atmospheric source, it may be an internal source and we need to be careful when we separate these populations because when we are trying to merge the climate history of Mars, these stepped fans may be telling us more about the interior process than the atmospheric process.

Kerri Smith: Erin Kraal talking to Geoff.

Adam Rutherford: This week's Sound of Science comes from a new exhibition at the Science Museum in London called the 'Listening Post' by artist, Ben Ruben and Mark Hansen, it takes snippets of text typed in real time by thousands of internet chat room users and transformed them into computer synthesized voices to weave this hypnotic result. This is the Nature Podcast. I'm Adam Rutherford

Kerri Smith: And I'm Kerri Smith, thanks for listening.

[Sound of Science]

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Adam Rutherford: If you're all still listening here's your chance to win an iPod Touch courtesy of the Nature podcast.

Kerri Smith: First you need to listen to the following three sounds of science taken from the podcast archives from autumn last year and then go to our web site that's http://www.nature.com/nature/podcast and follow the link at the very bottom of that page. Here they come.

[Music plays with three different sounds]

Adam Rutherford: So identify those three sounds and follow the link from http://www.nature.com/nature/podcast. This competition closes on the 31st of March 2008. Good luck and thanks for listening.

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