Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.astrosociety.org/abh/pdf/ABH09transcript.pdf Дата изменения: Tue Oct 19 21:16:23 2010 Дата индексирования: Mon Oct 1 23:12:58 2012 Кодировка: Поисковые слова: molecular cloud

A S T R O N O M Y B E H IN D T H E H E A D L IN E S A p o d c a s t fo r In fo r m a l S c ie n c e E d u c a to r s fr o m th e A s tr o n o m ic a l S o c ie ty o f th e P a c ific w w w .a s tr o s o c ie ty .o r g /a b h E p is o d e 9 : C O S M IC C H E M IS T R Y with Dr. David Grinspoon, Denver Museum of Nature and Science

Welcome to Astronomy Behind the Headlines, a production of the Astronomical Society of the Pacific. In this episode, we're on a hunt for the cosmo-chemical origins of life. Astrobiology concerns itself with life in the universe. Astrobiologists and planetary scientists want to know if other planets could support life ­ and what the evidence for such life might be. They are also delving back in cosmic time to understand how the elements for life originated. Dr. David Grinspoon is an astronomer at the Denver Museum of Nature and Science. We talked with him about the chemical fingerprints of life in the universe. Dr. Grinspoon, welcome to Astronomy Behind the Headlines. DR. GRINSPOON: Thanks very much for having me. HOST: The search for life takes astronomers out from our solar system to planets around other stars, and even into the interstellar medium to understand the chemical origins of life. Now, we often hear from astrobiologists about "pre-biotic chemistry." What does that mean? DR. GRINSPOON: Well, life as we know it is made up of essentially giant organic molecules, what we call "macromolecules." But, it couldn't have started out that way ­ those molecules are very complex. So, how did they come to be? We believe, and we have good evidence, that before these giant molecules existed there were smaller organic molecules that were going
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through a phase of what we call "chemical evolution" and gathering complexity and approaching the phase where life could actually exist out of these giant molecules. That previous phase of chemical evolution we call "pre-biotic chemistry." HOST: Okay, so this is sort of the chemical "soup of life." Let's take this a step further back in time... how did the chemical precursors for the "soup of life" get to Earth? DR. GRINSPOON: Well, we used to be very puzzled by that. But, we've learned that the universe seems to like to make organic molecules, that there are multiple ways for a young planet like the Earth to get organic molecules. For one thing, we see them in meteorites and in comet dust. There are organic molecules even now falling to Earth all the time, and when the Earth was young, there was a lot of stuff falling in, a lot of remnants from forming the solar system. And we now know that those materials in the solar system ­ those primitive materials ­ are full of organic molecules. HOST: So, the materials for life were present in our own proto-solar system and we've seen proof of that, for example, in amino acids in samples of material taken from Comet Wild-2. What does that discovery say about how our planet formed and evolved life? DR. GRINSPOON: Well, it's sort of encouraging ­ encouraging for those of us that like life in the universe, which I think most of us probably do. It's encouraging that it doesn't seem that hard to get the organic precursors, that in the young solar system those molecules were forming as a part of the chemical processing of material that led to the formation of the planets. And, furthermore, we now know that in environments like the environments of the young Earth, organic molecules also form naturally. So, it means that it's not hard to get the building blocks for life on a young planet like the young Earth. So, it's encouraging if you're worried about where did those building blocks come from, it turns out they're falling from the sky all the time, not just on the Earth, but on all planets. HOST: So, this term cosmo-chemistry ­ which is really what we're talking about -- also refers to elements being processed in molecular clouds that may or may not have planets in them already -- and long before stars and planets even formed in them. What
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sort of cosmic chemistry is going on "out there" that could influence the formation of life on other worlds? DR. GRINSPOON: Well, it's worth noting that when we talk about organic chemistry ­ organic molecules, which are the stuff of life ­ we're really just talking molecules that have carbon in them. And carbon is a substance that loves to bond with other carbon atoms and with other atoms and form these organic molecules. And, it turns out that the environment of interstellar clouds, which is the sort of feeding ground of which stars and our solar system, and other stars and other solar systems form, is kind of a chemical factory that ­ in interstellar dust you have a lot of carbon. Carbon is one of the most abundant elements in the universe. And, then you have some ultraviolet light coming from other stars, and ultraviolet light tends to process material and process organic chemistry, and breed chemical complexity. So, you have very simple organics ­ like methane ­ it's the simplest organic molecule. And then, if you have any ultraviolet light, methane ­ CH4 ­ is gonna lose some of those hydrogens (H). Ultraviolet light comes in and knocks off a hydrogen, and that makes the carbon say, "Wait a minute! I need to bond to something else!" And, it tends to bond to other carbons, and then starts to form more and more complex organics, and then you know, ultimately, it forms us. HOST: So, you mentioned the methane, and I've heard a number of talks about looking for the chemical signatures of life on other worlds. What would we actually look for at another star or another planet? DR. GRINSPOON: Well, it's a puzzle of course, because you can argue that we don't really know what life is or how to define it until we have other examples. So, how can we possibly claim to know what to look for? But, we have some pretty good ideas, I think, about what life does to its environment, based on Earth life and what life does to its environment here, and based on our sort of evolving general notions about what life is. And, one thing that life does is it chemically

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alters its environment, and in particular, it changes what we call the "equilibrium state" of its environment. If you look at Earth from outer space, and you don't really know anything about life on Earth, you would probably notice ­ in comparison to the other atmospheres of our solar system ­ there's something weird about our atmosphere. There's oxygen. There's methane. Those gases are out of equilibrium, that is, they want to react with each other. Without something perturbing that system, they wouldn't be co-existing. So, that's what we think life, in general, does. So, we can look specifically for oxygen, we can look for methane ­ the kind of gases that life makes. But, in a more general sense, we can look for atmospheres that are out of equilibrium ­ where some process seems to be disturbing those atmospheres. And that process doesn't have to be life, but it MIGHT be life. HOST: How close are we to being able to detect these signs of life? DR. GRINSPOON: Well, we're getting closer. Of course, within our own solar system, we're actively exploring, and if we're lucky and this is happening nearby, we could literally find it any day. And, of course, the big revolution that's happened pretty much in the last decade is that we now know that there are many, many, more planets in the universe than are represented by the planets in our own solar system. And, these exoplanets are, of course, the places we would really like to study for their atmospheric composition. We're not really at the point where we can do that very easily. And, in some cases, we get lucky and we can tell certain things by the sunlight that's shining through those atmospheres and extract some information about the chemistry. But, really the revolution there will happen when we build the next generation of space telescopes ­ really very large space telescopes that are designed to look in the atmosphere of Earth-like planets. And, now that we're right on the edge of finding where all the Earth-like planets are ­ and that announcement's going to come any day now ­ then, I think there will be a push to build this new generation of space telescopes, where we can literally find the chemical fingerprints of these alien atmospheres and look for the signs of life.
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HOST: This is so fascinating and I'm envious of the next generation of scientists and graduate students who'll be able to work on this. It's clear the story's not done yet. We really appreciate you sharing your insights with us, Dr. Grinspoon. DR. GRINSPOON: Well, thanks a lot. This has been fun. HOST: As you've heard, astrobiology is truly a science that looks at the past and future of life on our planet and evidence for it on other worlds. It even encompasses the interstellar medium. If you'd like to learn more about life in the universe and how it forms, point your browser to www.astrosociety.org/abh. And, thanks for listening!

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