The color of Mars tells us which minerals are present, and these minerals provide information about water and environmental factors on Mars. The red color comes from iron oxides and varies from orange to red to violet depending on the mineral structure. In the visible region a spectrometer acts like our eyes do and recognizes colors such as green, blue and red. The Pancam on Spirit and Opportunity records these colors in spectral images. The Mini-Thermal Emission Spectrometer (Mini-TES) is another spectrometer on the Mars Exploration Rovers (MER) and measures infrared radiation. Our eyes cannot "see" the infrared radiation, but the spectrometer can. Rocks are composed of minerals and each mineral has a certain spectrum that can be measured by the spectrometer.
Spectroscopy involves measuring the energy absorbed or reflected at certain wavelengths. Infrared spectroscopy primarily measures vibrational energies of the atomic bonds in the mineral structure. Bonds such as Si-O, Fe-O, H2O (water), SO4, CO3 each have different vibrational energies that are measured by the spectrometer. These clusters of atoms are the building blocks of minerals and each mineral has several infrared absorptions in its spectrum. The Mini-TES instruments on Spirit and Opportunity and the TES instrument on the Mars Odyssey orbiter are measuring these mineral components and the scientists must try to recreate which minerals are present in the martian rocks and soil by comparing the infrared energies detected with the known spectral properties of minerals from lab measurements. The Mars Express orbiter also has a spectrometer called Omega that is measuring the near-infrared region. This works in a similar way to the Mini-TES, but collects data from a complimentary wavelength region. A third spectrometer called CRISM will cover visible and near-infrared wavelengths (some that we can see, plus some that are similar to those measured by Omega) and is scheduled to fly to Mars on the Mars Reconnaissance Orbiter in 2005. Combining visible, near-infrared and mid-infrared spectra provides the most clues to scientists trying to figure out the mineralogy of Mars.
The rocks and soils on Mars are composed of a variety of minerals such as silicates (pyroxene, feldspar and olivine), iron oxides, sulfates, and carbonates. The minerals tell a story about how each rock or soil unit formed and what has happened to it since it formed. We know a lot about the minerals present on Mars from detailed studies of the martian meteorites and from chemistry and spectroscopy of the surface. Still, we only have some pieces to the puzzle and many that are needed to assemble the full picture are missing.
In order for scientists to be able to interpret the spectral data of Mars, it is necessary to measure the spectral patterns of rocks and minerals in the lab and in the field on Earth. Mars scientists are studying rocks from a number of field sites including volcanoes, deserts, hydrothermal areas, impact craters, the Arctic and the Antarctic. My field sites have focused on alteration of volcanic material (e.g. Hawaii, Iceland), sedimentation of volcanic material in Antarctic lakes, and rocks forming in hydrothermal regions associated with volcanoes. These samples include a number of minerals such as iron oxides/oxyhydroxides, clays, carbonates and sulfates that provide information about aqueous processes, temperature, pH, etc. Pure samples of these minerals are obtained as well in order to characterize their spectral properties. In many cases, small differences in chemistry or grain size can influence the spectral properties.
I am a Co-Investigator for the UC Berkeley NAI team called BioMars and we are particularly interested in finding ways to characterize and identify Fe and S-bearing minerals that may be associated with life. My team is in the process of measuring spectra of rocks collected at new field sites. Our goal is to learn how to remotely characterize rocks that can provide information about whether or not conditions were supportive of life on Mars. Because each instrument on the many Mars orbital and landed missions measures spectra covering a different wavelength range and resolution, the lab and field data measured for this project will also be modified to match the various spectrometers collecting data of Mars. This will enable us to know what the spectrum of key rocks and minerals associated with life would like on Mars if they are observed by any of the martian spacecraft.