by Vicky Hamilton, Arizona State University
The main purpose of the Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument is to provide information about the geology of Mars. One of my primary interests as a geologist is volcanology.
On Earth, volcanoes are classified into different types depending on where they are located, what types of eruptions they generate, and what types lavas/rocks they produce. For example, explosive volcanoes that generate ash and light-colored, silica-rich, rocks are commonly found near the edges of continents where the oceanic crust is being subducted under the continental crust. An example of this type of volcano is Mt. St. Helens which erupted in 1980. Less explosive, but still dynamic, eruptions are common at shield volcanoes such as found in Hawaii. These volcanoes produce rocks of basaltic, or mafic, composition. Mafic rocks are usually black and have abundant iron and magnesium. I have chosen to study basalt because it is likely to be found on Mars.
Based primarily on observations from the 1970's Viking missions, a great deal of the surface of Mars is believed to be covered by basaltic rocks. If this is the case, then the more we know about the thermal infrared properties of these rocks and their minerals, the better we will be prepared to interpret the data sent back by the Mars Global Surveyor TES.
Hubble Space Telescope view of Syrtis Major region of Mars, in February 1995. Syrtis Major is a large dark-hued area (SM) that 100 years ago was suspected to be covered with vegetation. We now know that Syrtis Major is a broad shield volcano covered by dark soils and windblown sand. Observations from the 1989 Phobos 2 mission have led Brown University (Providence, RI) scientist John F. Mustard and his colleagues to conclude that the region has sand or rocks containing pyroxene minerals- perhaps the MGS TES will confirm this view and provide more detailed information. Photo courtesy NASA and P. James (U. Toledo).
"Pyroxene" is a general term used to refer to a whole class of minerals that have the same crystal structure, but different chemical compositions. Pyroxenes found in volcanic rocks are made of silicon and oxygen plus magnesium, iron, calcium (and magnesium or iron), or some combination of magnesium and iron (with or without calcium). Each of the different pyroxene minerals form under different pressure and temperature conditions (underground) in the basaltic magma. Therefore, the presence of certain pyroxenes in a basaltic rock can tell a geologist a lot about the conditions under which the original magma was generated.
In the thermal infrared, pyroxene spectra are different from the spectra of other silicon-oxygen-based minerals (CLICK HERE for spectra of some key minerals). However, each of the pyroxene minerals produces a slightly different spectrum due to the differences in composition. My research is focused on establishing exactly how the pyroxene spectra change as composition changes, and how accurately we can measure those compositional changes with thermal infrared spectroscopy. When this is done, we will know how accurately we will be able to identify these minerals when we get data from the TES at Mars.
Once we are able to identify and study the minerals of martian volcanic rocks, we will be able to make interpretations about the way those rocks formed, and how their processes of formation may have changed over the course of Mars' geologic history. These changes provide information on the evolution of Mars' interior processes, and may give us clues as to why Mars is so unlike our own planet today.
Edited by K.S. Edgett, April 1995