Lesson Plan: ProcedureSo let's imagine you're a project scientist on the Mars mission, specializing in mineralogy and Mössbauer spectroscopy. You've studied the list of minerals likely to be found on Mars very carefully. Your job is: 1. Pick three minerals from the table of Mars minerals, and research the environments in which they form. A good web site for this search is http://www.mindat.org, but you should also search out other books and web sites for information regarding mineral formation, or paragenesis. Pay close attention to whether or not your three minerals might contain the element hydrogen (chemical symbol: H), which might indicate that they form in the presence of water. 2. Using the Mössbauer Mineral Spectroscopy website, examine the listings for your spectra of interest. Note that for some minerals, there is only a spectrum acquired at room temperature; for others, spectra have been acquired at many different temperatures. a. Examine a set of spectra acquired at different temperatures, and see if you can determine how the spectrum changes with temperature. (This process is exactly what scientists are doing right now: we can't predict, based on theory, how the spectra change with temperature, so we have to make all these measurements. We don't even know the temperature at which Mars surface measurements will be made -- we won't know until we get there!). b. Decide whether or not you think the temperature change is important to consider... c. It will take approximately 12 hours for the MER and Beagle 2 spectrometers to acquire a single spectrum on the surface of Mars. What is the surface temperature on Mars, and how fast does it change? How will this affect the interpretation of Mössbauer spectra? 3. Go back to your three chosen mineral spectra, and find three spectra acquired at the same temperature (if the spectra don't exist on the web site, you may have to choose another mineral -- but we're acquiring them as fast as we can, and new spectra are posted daily!). Copy the raw data into a spreadsheet and plot them on the same graph, if you have that capability. Alternatively, print them out or trace them onto clear transparencies, and line them up on top of each other. Are the spectra different enough to be distinguished? If the rocks on Mars are made up of a mixture of your three minerals, would the MER or Beagle 2 spectrometer be able to tell them apart? 4. Finally, note that the spectra on our database were acquired under ideal laboratory conditions here on Earth, which allowed us to use an experimental apparatus that gives wonderful spectra. In particular, our set-up uses what is called "transmission" mode geometry, so that the gamma rays pass through the sample to a detector on the other side. On Mars, it's impossible to put the detector behind the rock we're analyzing, so the remote landers use what is called "backscattering geometry." At this time, we are assuming that transmission mode experiments give exactly the same results as backscattered ones (theoretically, they should!), BUT this assumption has yet to be rigorously tested.
This
Web site is supported by NASA grant NAG5-12687 and Mount Holyoke College. |
