August 8, 2012 | 1
Light and crystals
In 1802 the English physicist William Wollaston took a prism and squinted at the spectrum of sunlight produced by it which his fellow Englishman Isaac Newton had observed in an iconic experiment more than a hundred years before. Wollaston saw black lines interposed between the familiar set of colors corresponding to the rainbow and wondered at their origin. A few years later Josef Fraunhofer observed the same lines but went one step further and invented an instrument, the spectroscope, which collected together and then refracted the light to produce a clear band of colors with separating dark and bright lines. Wollaston and Fraunhofer had invented the discipline of spectroscopy, a science which would have an incalculable impact on the development of physics and chemistry over the next one hundred years.
But the most important development in spectroscopy came about forty years later when the Germans Gustav Kirchhoff and Robert Bunsen made the striking observation that the lines observed by Wollaston and Fraunhofer in the solar spectrum could be mapped on to similar lines appearing in the light produced by certain elements when heated. The implications were clear; there must be elements in the sun and these elements could be identified by looking at their spectra. Bunsen had recognized the significance of the technique for element identification in chemistry and Kirchhoff had the key insight of applying it to astronomy. Experiments with spectroscopy led among other things to the discovery of the helium, the first element identified outside earth. Since then spectroscopy has become the dominant tool for discovering molecules in interstellar space. Of all the wondrous facts unearthed by science, the fact that one can deduce the composition of a heavenly body that is 93 million miles away by simply looking at the nature of its light when passed through a cheap prism has got to rank at the very top. Very few comparable discoveries convey the sheer power and reach of the scientific method.
Kirchhoff and Bunsen had set the stage. Every element when heated or excited produces light of a characteristic color, and this fact is exploited by high school students identifying specific elements by heating them (incidentally on a Bunsen burner) and looking at the color of the flame; calcium glows brick red, copper emits a tranquil green. Technically the spectra can arise either from absorption (in which the atoms absorb certain frequencies) or, in the case of the flame test, by emission. It was emission spectroscopy that cast a pall of mystery over physics during the first half of the twentieth century. Out of the morass of the spectral lines of hydrogen came Niels Bohr’s seminal idea about atomic orbits and transitions and the birth of the quantum theory of matter. Bohr also explained the nature of emission spectra which arise when atoms whose electrons are excited to specific energy levels emit well-defined frequencies when reverting to their more stable “ground states”.
A concomitant development around the same time that Bohr was developing the quantum theory was also key to the future of science. At Manchester and Leeds, William Henry Bragg aimed a beam of x-rays at a crystal of sodium chloride. When the resulting scattered radiation was analyzed, it again showed the presence of lines, but this time lines which indicated the spacing of different atoms within the crystal. Bragg had invented the science of crystallography, and he and his son Lawrence (who still holds the record as youngest Nobel laureate at age 25) were pivotal in establishing its use in determining the structure of unknown materials. Since then crystallography has been responsible for a scientific revolution that has given us the structures of both DNA and proteins.
Why am I recounting the story of spectroscopy and crystallography? Because the Braggs, Bunsen and Kirchoff were essentially chemists. In the intervening century or so, both spectroscopy and crystallography – light and crystals – have been foremost in deciphering the composition of all kinds of mundane and exotic substances, from semiconductors to snake venoms. These tools are used on a routine basis by graduate students in chemistry departments around the world. And on August 5, a 2000-pound, nuclear-powered chemist which employed these techniques descended on the surface of a world 154 million miles away, having had its speed reduced with pinpoint precision from catastrophic to a leisurely 1 meter per second. The name of this chemist was Curiosity, and it crucially relies on light and crystals to explore the new world it has been asked to survey.
Curiosity: A marvel of engineering with a mandate of chemistry
The launch, manipulation and descent of Curiosity were rightly hailed as marvels of engineering. The accuracy with which the rover went through all its stages of operation, culminating in a spectacular, perfectly timed parachute-guided touchdown on the surface of the Red Planet is breathtaking. No less important was to understand the forces of gravity that would tug and kick Curiosity as it strained to escape Earth’s gravity and gracefully enter Mars’s. And as for Mars itself, the planet has been an astronomical object of fascination for decades. There’s no doubt; Curiosity is a triumph of engineering and astronomy. But what is equally certain is that Curiosity’s mandate is chemistry. Engineering and astronomy got Curiosity to its destination, but now that it has reached Mars, Curiosity is a chemist. And it’s a chemist that continues the legacy of Bunsen, Bragg and others in using spectroscopy and crystallography to chart the face and innards of the martian landscape.
Curiosity’s main purpose now is to tell us what its Martian surroundings are made out of, essentially what’s called “structure determination”. Structure determination is a goal that falls squarely within the realm of chemistry, one that chemists have ceaselessly pursued in their labs for as long as there was a science of chemistry. No molecule, natural or synthetic, can be analyzed further or synthesized until its structure is known. The moment a material substance is glimpsed, both chemists and Curiosity follow Marcus Aurelius in asking the key question: “What is it, in and of itself?”. And just like they are for chemists, crystallography and spectroscopy are going to be Curiosity’s weapons of choice in tackling this question. On earth chemists will continue to use these methods to determine the structures of new drugs, plastics and catalysts. On Mars Curiosity will use these methods to ask questions about the planet’s present and past habitability.
To accomplish its goals Curiosity hinges on three instruments that are really the jewels in its multifaceted crown of electronics and instrumentation. Let’s look at these in some detail:
1. ChemCam (Chemistry and Camera): The ChemCam module located on the top of the rover does what can be summed up as “blast and analyze”. The centerpiece of the ChemCam is a spectrometer that does Laser Induced Breakdown Spectroscopy (LIBS). LIBS uses a laser to heat a piece of matter to an intensely high temperature so that it briefly ends up as a hot, ionized plasma. ChemCam accomplishes this using an infrared laser that shines like a scalpel on an area just 0.6 mm across. The laser excites the atoms of the elements in the plasma and while reverting to their ground states, they emit a panoply of characteristic frequencies, like notes from a dramatic organ overture. This spectral light enters a spectrometer, a sophisticated prismatic descendent of Fraunhofer’s original contraption. The spectrometer dissects the emitted frequencies and classifies them by element. In one shot you can get an idea of the elemental composition of a complex Martian rock. The instrument can even quantity the elements within error.
The LIBS technique is very versatile and one of the most valuable things it can do is to detect hydrated minerals, that is minerals which have water bound to their constituent molecules. It can also detect frozen water. When testing for life on a remote planet, the “follow the water” adage is an imperfect albeit enormously useful starting point. Water dissolves many specific ions and when it evaporates these ions stay behind with their exact percentages trapped in time for posterity to examine; in addition the distribution of elements can also indicate whether the environment was ever conducive to life. This is what ChemCam is doing, assessing the elemental makeup of Mars for possible clues to the presence of water and its possible suitability for life. As an added benefit it can also find out based on the chemical composition whether a particular piece of the landscape may be hazardous to life. ChemCam also performs other important functions like imaging and microscopy. The imager on ChemCam is remarkably accurate; it can detect a human hair from two meters. The rover’s leisurely top speed of 1.5 inches per second gives the imager plenty of time to do its job. The combination of the laser spectrometer and imager turns ChemCam into a sophisticated instrument for quickly dissecting and analyzing fine features of its environment.
2. ChemMin (Chemistry and Mineralogy): The ChemMin unit on Curiosity does something which is in some ways even more sophisticated than the ChemCam: it analyzes the structure and abundance of minerals, crucially including elemental isotopes, and it can do this for as many as 74 samples. ChemMin follows William Bragg in shooting a razor thin beam of x-rays at a powdered and heated sample of rock or soil to quickly create a diffraction pattern. On the other side of the sample is a charged coupled device (CCD) detector that records the scattered x-rays with exquisite sensitivity so that there is a minimum loss of x-ray photons. Every mineral has a specific x-ray pattern that can be used as a fingerprint to nail down its identity and this is exactly what ChemMin does. Some elements when they are hit by x-ray also give off fluorescence, and ChemMin is designed to process these fluorescence signals as well. The resulting diffraction (XRD) or fluorescence (XRF) patterns can be plotted as graphs of intensity vs scattering angle. Using a relationship first derived by Lawrence Bragg, these plots can provide data on the atomic spacing and the identity of a particular mineral. Certain minerals can be extremely suggestive of the circumstances under which they formed; for instance the compound jarosite generally precipitates out of acidic water. ChemMin is sensitive enough to detect individual minerals that are as low as 3% in abundance . For higher percentages ChemMin can also quantify the amounts.
Perhaps the most pivotal thing that ChemMin can do however is to identify isotopic abundances. Over the last fifty years isotopic abundances have generated invaluable insights into planetary events and the accompanying rise and fall of life on earth. Staggering clues to geological and biological upheavals have come from examining the percentages of isotopes. Just a few examples; ratios of oxygen isotopes from ice cores have provided crucial information on ice ages, sulfur isotopes have shed light on the momentous Great Oxidation Event that saw a dramatic rise in the levels of oxygen and the ensuing explosion in biodiversity, and of course, carbon and nitrogen isotopes can reveal the ages and identity of creatures that lived in a particular environment. ChemMin is equipped to analyze isotopic ratios of all the major elements and there is no doubt that these analyses will be paramount in interrogating the Red Planet’s surface and interior for current and past signs of life. The rover will store all the gathered x-ray data as CCD images which will be faithfully transmitted to earth for further analysis.
3. SAM (Sample Analysis at Mars): From a biological standpoint, another major component of the Curiosity payload is SAM, and this is the component that will perhaps be most recognizable to a modern graduate student in chemistry. It contains three main instruments; a gas chromatograph, a mass spectrometer and a laser spectrometer. Of these the first two can be found (usually as a combined “GC-MS” package) in any academic or industrial chemistry lab. The mass spectrometer is essentially an atomic-level precision balance that weighs and quantifies atoms, isotopes and molecular fragments. The gas chromatograph is required to separate complex mixtures into individual chemical components. Isotopic abundances of essential elements are analyzed by the laser spectrometer.
Together the MS and the GC will be able to breakdown and analyze both organic and inorganic molecules, although the focus will be on the building blocks of life. They will search for compounds like methane which is partly produced by bacterial decomposition and fermentation. A few simple compounds like hopanes have been recognized as fossilized molecular signatures of life on earth and Curiosity could look for these using its GC-MS probe. SAM can also try to find fragments of these compounds which could help us understand how they might have built up; often, the intermediate chemical species in the formation of a complex biomolecule are as indicative of its biogenic origins as the end product. And we can be sure that the detection of any of life’s well-known building blocks like amino acids, carbohydrates, lipids or ATP will lead to justified front page news and a renewed understanding of life in the solar system. Of all the equipment on board Curiosity, SAM has the greatest potential to provide direct evidence of life’s chemical origins.
That’s it then, a brief tour of the chemistry that Curiosity plans to carry out on Mars. It involves novel implementations of known chemical methods and instruments, and chemists should be ecstatic that the science that they have been practicing on earth for so long is finally aloft in the skies. Curiosity also contains other kinds of chemistry. For instance the alpha-particle spectrometer again examines elemental abundances, including those of trace elements. Then there is the description of the rover as “nuclear-powered”; this refers not to nuclear fission but to a thermoelectric battery that generates electricity through heat created by the alpha-particle mediated decay of plutonium-238 oxide. Unlike its notorious cousin Pu-239 which was used in the Nagasaki bomb almost exactly sixty-seven years ago, Pu-238 has been used in life-giving cardiac pacemakers. And it’s been a mainstay of the batteries used in space exploration for decades. One form of plutonium brings death, the other allows us to search for life; such is the nature of the double-edged sword that is science.
We can be certain that the chemist called Curiosity will continue to use its chemistry to explore the Martian world for as long as electrons run through its circuits. And since spectroscopy and crystallography are techniques rooted in deep physical principles, explorers beyond Curiosity will continue to use these tools to map the material nature of the universe even as they transcend our solar system and glide onwards into the great star-studded void. If he knew about Curiosity and LIBS, William Wollaston would be, literally, starstruck.
1. JPL’s MSL Science Corner website has technical information on Curiosity.
2. There’s a great account of the development of spectroscopy and its use in astronomy in Marcus Chown’s “The Magic Furnace“