Reading a Meteorite with Laser Light
Text and photos by Taylor Ha
Jordan Young, a PhD student at Stony Brook University, prepares to search a slice of meteorite, looking for chemicals composed of carbon and hydrogen. He will use a Raman spectrometer, a device that uses lasers to determine the chemical makeup of rocks and minerals, molecule by molecule. Young is looking specifically for polycyclic aromatic hydrocarbons (PAH’s), which are common on Earth and are thought to be common throughout the solar system. He already knows that PAH’s are present, but he wants to distinguish kinds of PAH’s in different meteorites. Comparing PAH’s will help him understand how the whole rock has changed over time.
The Raman spectrometer sits in the lab of Timothy Glotch, RIS4E primary investigator. Young will shoot light from the laser to molecules in the sample, exciting their electrons. Then he will measure the resulting energy shift. The nature of the shift is like a signature that can help researchers identify the molecule that interacted with the laser light.
Young uses a remote to navigate the microscope’s lens around the tiny meteorite sample. He searches for fine-grained matrix – a “hodge-podge of materials” that includes tiny mineral fragments finer than sand, he said. It appears to be dark material in the form of pores, veins or rims. These black deposits have the carbon-containing material he’s looking for.
Now that he’s found the fine-grained matrix, Young shines the laser on a very tiny spot to see if the organic material he’s looking for is present. Then he takes a 20 x 20 micron scan of the image. Typically, the PAHs have two peaks at approximately 1,375 cm-1 and 1,600 cm-1. That’s the shift of the energy from a ground state to an excited vibrational state, and also a sign of a carbon-carbon vibration.
Young applies a filter, or color value, to an image of the organic molecules. Yellow areas, which look like sunspots in the sun’s corona, represent areas of high intensity – high peaks. The filter can help him see where certain molecules are clustered and pick out areas to analyze more closely as he distinguishes kinds of PAH’s.
Young subtracted background, extraneous noise in the graph that didn’t tell him anything new. After extracting and averaging spectra, Young is left with cleaner, clearer spectra of the whole image. This allows him to distinguish PAH’s in one meteorite from those of another meteorite. When he makes these comparisons, Young is able to recognize what types of alteration the whole rock has experienced.