News from Online: The Chemistry of Beyond

When my wife and I got married we received as wed- ding gifts several gift ... have done much shopping, but I had a similar feeling when I learned of ...
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News from Online: The Chemistry of Beyond by Mark Michalovic

When my wife and I got married we received as wedding gifts several gift certificates from Bed, Bath, and Beyond. I had no interest in colorful soaps or new pillowcases, but I was drawn to the enigmatic beyond category. That word held out the promise of refreshing mystery amidst the predictability of the suburban mall. I still don’t know what’s in the beyond category, since it’s too soon after the wedding for us to have done much shopping, but I had a similar feeling when I learned of the theme for this year’s National Chemistry Week, namely “Earth’s Atmosphere and Beyond”. My attention was again drawn to the beyond category and the lure of the unknown that went along with it. Since I’d already written the column, Chemistry in a Planet, not a Test Tube, about Web sites dealing with the atmospheres of Earth and other worlds in our solar system (1) in April 2003, I once again saw an opportunity to delve into that mysterious realm of beyond. The Cosmic Atom Factory When we’re talking about Earth’s atmosphere, beyond covers a lot of territory, all of the universe except the little speck within Earth’s gassy blanket. Given that vast domain, it’s hard to known where to start. So I suppose I should start as close to the beginning as possible, not long after the big bang. The use of going that far back in time was brought to my attention by a student’s question. Once when I was teaching a section on nuclear chemistry, I got to the point in the lesson about iron being the most stable nucleus, and how iron could not fuse into larger nuclei. A student then asked, how could these heavy elements ever form in the first place? Good question. To answer it let’s explore how the nuclei of elements come together in the cosmos, a process called “nucleosynthesis”. Astrophysicists think that sometime after the big bang, when quarks finally began to congeal into protons and neutrons, at first the only atomic nuclei that formed were hydrogen, some helium, and a little lithium here and there when nature got daring. The periodic table was small in those days. Eventually hydrogen condensed into stars, and thermonuclear fusion, by which hydrogen is fused into helium, began to light up the previously-dim cosmos. This process is the predominant activity for most of a star’s life. The basics of how fusion works, and the principles of why it releases energy (the difference in binding energy between hydrogen and helium and of course Einstein’s relationship E = mc2) are reviewed thoroughly enough for a general chemistry class at Fusion—Physics of a Fundamental Energy Source produced by the Contemporary Physics Education Project and hosted by Princeton Plasma Physics Laboratory and the Lawrence Livermore National Laboratory at http://fusedweb.pppl.gov/ CPEP/Chart.html. This site provides good content, though

Figure 1. Tycho’s supernova remnant in X-ray. The violence of a supernova provides the energy needed to form nuclei larger than iron. The dust left over from ancient star deaths, rich in heavy elements, eventually coalesced into our sun and its rocky planets.

photo by ROSAT, MPE, NASA

The Great Beyond

the layout and navigation could use some tweaking (For example, the opening page is essentially blank in the main frame, making you think the page hasn’t loaded yet. Don’t worry, just click on the menu items on the left and something will appear in the main frame.) That’s great, but it only gets us to helium. We still haven’t accounted for the existence of most of the elements. In the later stages of a star’s life, larger nuclei are produced by fusing helium nuclei as hydrogen fuel is depleted. Carbon, nitrogen, oxygen, etc. are produced, with iron as the ultimate fusion product. Iron, of course, is the most stable of all nuclei. The fact that nuclei become more stable with increasing mass until we get to iron, and then become less stable as mass continues to increase brings us back to the concept of binding energy, and other topics like radioactive decay. A quick, student-accessible review of how heavy elements are formed in stars during the later stages of their lives is They Came from Outer Space found at http://www.astrocentral.co.uk/ stardust.html, the UK-based astronomy Web site Astrocentral. Fusion expanded the cosmic periodic table greatly, from three elements to 26. But that’s still a far cry from the 100plus we know today. Since iron is so stable, it will not fuse into larger nuclei. It was only when the largest of these first stars had spent their nuclear fuel and became supernovae that there was sufficient energy released to pound together larger nuclei, the theory goes. The violence of exploding stars gave birth to the heavier elements all the way up to uranium. Either way, it is the action of stars that has created the elements of the periodic table. But just when we thought we knew how heavy elements came to be, scientists discovered that some stars in fact contain respectable quantities of lead. An excellent article describing these findings and the production of heavy elements in supernovae is Heavy Metal Stars at http://spaceflightnow.com/ news/n0108/30heavy/ from the private organization Space Flight Now. The article not only provides a review of how the elements up to iron are synthesized in stars in a process gentler than that of a supernova (called the “slow process” as opposed to the “rapid” process that takes place in supernovae), but explores how scientists think some stars can produce the less-stable trans-iron elements, elements as heavy as bismuth and lead. But the slow process can’t make the still-

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Stellar Compositions and Electronic Structure Studying stars can show us a thing or two about the electrons of an atom as well as the nucleus. The element helium was after all discovered by studying spectral lines in sunlight decades before it was discovered on Earth. The spectrum of a star’s light gives us a means of establishing the star’s composition, since each element has a characteristic spectrum. This use of line spectra to determine stellar composition gives us a nice way to teach the origins of line spectra in electronic transitions and the atomic structures that give rise to those transitions. A nice Web site on this topic is Investigating Stellar Spectra: Integrating Chemistry and Astronomy created by Dave Johnson of the U.S. West Oregon Teacher Network Project at http://jersey.uoregon.edu/~djohnson/astro/ prindex.html. Building Simple Molecules in Space So far we’ve only talked about atoms. The creation of heavy elements is just the one step in the path from the big bang to a universe that can do things such as support life. Once there are complicated atoms, there is the possibility of molecules forming. Simple molecules like water, ammonia, and carbon monoxide are common in the universe, often being found in the space dust left over in the nebulae that linger after supernovae blow themselves to smithereens. Our own solar system, headed by a second-generation star formed from the dust of some ancient supernova, is rich in these compounds. A new theory holds that some of the carbon and oxygen nuclei that aren’t hammered into larger nuclei during the giant blasts can be joined chemically to form carbon monoxide. This theory is described in the Dartmouth Col-

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photo by NASA, Chandra X-ray Observatory, Earth orbit

heavier radioactive elements, and this brings up again the topic of radioactivity: radioactive elements decay faster than the slow process can make them. To make uranium, you still need a supernova. I like this article for both of these reasons, but also because it brings up an obvious question that almost never comes up when studying the structure of the atom: how do you get protons to stick together in a nucleus when they should repel each other? Neutrons are the key, of course, as they experience attractions for protons and for each other, due to the strong nuclear force, but this is rarely mentioned in general chemistry books. For a final note on nucleosynthesis, a real gem available online is found at the Web site Singing Science Records at http:/ /www.acme.com/jef/science_songs. This series of albums of the same name was recorded in the late 1950s and has been made available as free mp3s at this site. The song “Why Does the Sun Shine?” is a succinct and catchy earworm that will stick the basics of thermonuclear fusion in students’ heads. A peppier post-punk, new-wave rendition of the song was recorded by They Might Be Giants and is available on their album Severe Tire Damage.

Figure 2. This image, from NASA’s Web site Imagine the Universe (http://imagine.gsfc.nasa.gov/), shows an X-ray image of supernova remnants in the constellation Cassiopeia A. The caption drives home that stellar explosions are one way in which elements are created.

lege news article “Astronomers Find Carbon Monoxide Gas in Supernova Debris” at http://www.dartmouth.edu/~news/releases/1999/jan99/nova.html. The Cosmos Comes to Life It is, of course, these and a few other simple compounds that we think gave rise to life on earth. On Earth prebiotic chemical evolution, as it’s called, mostly involves reactions that we think took place in the oceans, not the atmosphere. But the study of prebiotic evolution does involve a good deal of atmospheric chemistry, because we know that Earth’s atmosphere today is not like the one that blanketed Earth when life first evolved. The nature of our atmosphere at that time is still a matter of debate. This is an important debate because the chemical behavior of the atmosphere would have had a profound impact on the kind of chemistry that could take place in the ocean. For example, today we have what we call an “oxidizing atmosphere”, for obvious reasons. It was thought that an atmosphere of water, hydrogen, ammonia, and methane once prevailed, a so-called reducing atmosphere. (Already we have an excuse to teach a genchem topic: oxidation and reduction.) In the 1950s, Stanley Miller, then an eager 22year-old graduate student, carried out a now-famous experiment. He used salt water solution to simulate the early ocean and placed it in a sealed container, blanketed by a reducing atmosphere of the composition just described. He then fired electrical sparks inside the container to simulate lightning. He discovered that he quickly produced simple amino acids under these conditions. An interview with Miller containing lots of good chemical information and human interest is From Primordial Soup to the Prebiotic Beach at http://www.accessexcellence.org/ RC/miller.html, hosted by Access Excellence.

Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu

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photo by NASA Discovery Mission

photo by NASA, NSSDC

Figure 3. Stars build hydrogen into elements as large as iron through thermonuclear fusion, while heavier elements are sometimes created in healthy stars through the so-called “slow process” of nucleosynthesis.

Figure 4. Neptune, like the other gas giants of the outer solar system, is made of hydrogen, methane, ammonia, and water. This was once thought to be the composition of Earth’s original atmosphere, and this view shaped early experiments in exobiology.

While the conditions he used are not universally thought to be those of early Earth, his experiment heralded the beginning of the study of prebiotic evolution. While this was strictly an Earth-bound investigation at the time, today scientists hope that by understanding the conditions under which life evolved on Earth, we will better know what type of places in the universe are likely to give rise to life similar to life on Earth. A good site to begin exploration of prebiotic evolution theories and their implications on the search for extraterrestrial life is ThinkQuest’s Astrobiolog y: The Living Universe at http://library.thinkquest.org/ C003763/index.php?page=index&tqskip1= 1&tqtime=0713. You might also want to explore NASA’s Astrobiology: Exploring the Living Universe found at http:// astrobiology.arc.nasa.gov or go to http://astrobiology.com produced by SpaceRef Interactive Inc. Though an amazing chemical cycle got underway on primitive Earth, when we last left the realm of deep space we had only discussed the presence of water, methane, and ammonia in the interstellar medium. It was once thought that such simple first-month-of-class chemistry was as exciting as chemistry got in deep space. But today scientists are learning more about the interstellar medium, and finding that some very interesting organic chemistry is going on in the night sky. The presence of complex organic compounds has led scientists to wonder whether space organics could have had any role in prebiotic evolution. As an aside, you’ll remember that the possibility that complex organic compounds could form before the evolution of life was at the heart of the controversy surrounding the Mars meteorite, which contained polycyclic aromatic hydrocarbons. Scientists still disagree whether this is a sign that life once existed on Mars. A site with lots of information on interstellar organic chemis-

try is NASA’s “Astrochemistry Lab” at http://ww.astrochem.org. Following the link to Scientific Studies will lead you to lots of information on the subject and its implications for prebiotic evolution and the emergence of life. Buckyballs, Spaceflight, and a Universe of Chemistry In addition, it was in trying to simulate deep space conditions to study the organic chemistry of stellar atmospheres that buckminsterfullerenes were discovered in the 1980s by Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard E. Smalley. The three were awarded the Nobel Prize for Chemistry in 1996 and their story can be found at The Nobel Prize for Chemistry 1996 from the Nobel Foundation’s e-Museum (http://www.nobel.se/chemistry/laureates/1996/). I would have liked to discuss Web sites about the chemistry of human activity in space, but these were wanting. This is too bad, because space flight brings up polymer and materials science since robust materials are needed to withstand the heat of re-entry and punishing cosmic radiation. Rocket propulsion can be used to study basic chemical reactions such as the burning of liquid hydrogen and liquid oxygen in the space shuttle’s main engines. Personally, I prefer telling my students about the use of powdered aluminum as the propellant in the shuttle’s solid rocker boosters. This not only demonstrates simple chemical reactions, but also the fact that metals do indeed burn. Nuclear chemistry is definitely the field most easily taught using the chemistry of deep space, what with the various processes of thermonuclear fusion, the slow process, and the rapid process of violent supernovae all contributing to a cosmic element factory that illustrates so many basic principles. Meanwhile, stellar spectra can teach us a lot about the electrons surrounding the nuclei that stars create. Once those

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atoms start joining to form molecules, the possibility of teaching biochemistry emerges, with the study of prebiotic chemical evolution and the search for extraterrestrial life illustrating the basic concepts of amino acids, proteins, and nucleic acids. While I didn’t find Web resources on the chemistry of deep space as easy to find as I did for material on the atmospheres of Earth and our neighbors in the solar system for that earlier column (1), much information is there. There is an amazing amount of chemistry in the cosmos that can help illuminate classroom lessons. I’ve listed a few ideas, but let your own knowledge and creativity be your guide when you

boldly go where no chemistry teacher has gone before. Literature Cited 1. Michalovic, Mark J. Chem. Educ. 2003, 80, 362.

Mark Michalovic is an adjunct professor of chemistry at Temple University and an education specialist at the Beckman Center for the History of Chemistry, Chemical Heritage Foundation, 315 Chestnut Street, Philadelphia, PA 19106; [email protected].

Fusion—Physics of a Fundamental Energy Source http://fusedweb.pppl.gov/CPEP/Chart.html

From Primordial Soup to the Prebiotic Beach http://www.accessexcellence.org/RC/miller.html

They Came from Outer Space http://www.astrocentral.co.uk/stardust.html

Astrobiology: The Living Universe http://library.thinkquest.org/C003763/ index.php?page=index&tqskip1=1&tqtime=0713

Heavy Metal Stars http://spaceflightnow.com/news/n0108/30heavy/ Singing Science Records http://www.acme.com/jef/science_songs/ Investigating Stellar Spectra: Integrating Chemistry and Astronomy http://jersey.uoregon.edu/~djohnson/astro/prindex.html Astronomers Find Carbon Monoxide Gas in Supernova Debris http://www.dartmouth.edu/~news/releases/1999/jan99/ nova.html

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Astrobiology: Exploring the Living Universe http://astrobiology.arc.nasa.gov, or http://astrobiology.com Astrochemistry Lab http://www.astrochem.org Nobel Foundation’s e-Museum http://www.nobel.se/chemistry/laureates/1996/ Access date for all sites is Aug 2003.

Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu