Yesterday we talked about ChemCam, and how Curiosity will be able to sample rocks up to 7m away using a laser. Today we’ll look at CheMin, short for Chemistry and Mineralogy. This is the first of the instruments we’ve looked at that is served by the sample acquisition system (I’ll talk about this more tomorrow when we look at the Sample Analysis at Mars (SAM) system), and is contained within the body of the rover.
While LIBS can tell you what elements are present in a rock, the vapourization process destroys the crystal structure created by the bonds between the atoms. So LIBS can’t tell you precisely what minerals (like hematite or magnetite) are present in a rock. That’s where CheMin comes in. Using X-rays it will be able to determine what mineral crystal structures are present, as well as being able to give a rough breakdown of the elements in a sample.
The technique used to determine the mineral structure is known as X-ray diffraction. When X-ray photons are fired at a material with a precise crystal lattice structure the X-rays will be scattered when they pass through the lattice in a very specific way (the basics of the theory were worked out by the Braggs around 1913 or so). For a given incoming energy (or wavelength) the spacing between atoms will determine the angle that the X-ray photons are scattered through. So from the angles you can then determine the type of lattice (actually it’s good to have two sets of angles from different incoming energies, and then a bit of sleuthing is required to figure out the precise minerals present on the basis of the lattice size and the elements present).
CheMin will perform a very specific type of x-ray diffraction commonly known as powder XRD (or often just XRD), where the sample is provided in a powder form. XRD produces what you might call an average signal that combines all the different lattice scatterings super-imposed on one another because the powder contains the minerals in random orientations. The result is just a series of rings that specify the lattice spacings (one for each dimension).
As well as using X-ray diffraction, CheMin will also use X-ray fluorescence (XRF). The physics behind this process is actually quite a bit simpler than the X-ray diffraction. In fluorescence, X-rays that strike atoms will often knock electrons entirely out of the atom, leaving behind an ionized atom (i.e. one missing an electron) and an unoccupied electron energy level. Importantly, the unoccupied energy level is usually one of the lowest ones and many other electrons will “want” to be in this state. Thus an electron with a higher energy will drop down to this lower energy and emit an X-ray photon as it does so – the source of the fluorescence.
The key part here is that the energy levels of atoms are very specific to each element. So the energies of the X-rays produced in the fluorescence precisely describe which elements are present. This is exactly the same idea as used in LIBS, just using X-rays.
CheMin thus gives you the atoms and the crystal lattice structure you need to figure out what minerals are present in the sample. From this information we’ll build up a picture of the role of water in the formation of minerals, how they were deposited, and how they have been altered through geological processes. It’s also possible to use this information to look for mineral signatures of life, whether there were environments in the past that were, at least at one time, habitable.
CheMin is thus really important part of Curiosity’s main mission goal, to determine the current and past habitability of Mars.
Tomorrow: Sample Analysis on Mars (SAM)