Dr. Sprague and her collaborators will make spectroscopic measurements of Mercury's surface materials through major .spectral windows in the Earth's atmosphere, between 7.5 and 25 micrometers for the purpose of gaining knowledge about Mercury's composition. The spectroscopic measurements will be made with a state-of-the-art mid-infrared spectral imager (Boston University's MidInfraRed Spectral Imager--MIRSI) on the Infrared Telescope Facility (IRTF) on Mauna Kea, HI. The telescope is at high altitude and permits making measurements from the ground through a minimum of atmospheric water vapor, a substance that attenuates the thermal flux coming from other planets to detectors at the telescope. Mercury is the planet with the greatest density and has a significant magnetic field. These two things together indicate it must be an iron-rich planet. However, the iron at Mercury must be sequestered in the core, or at least beneath the surface layers, or be completely transformed to native iron metal, because no ground-based observations have found convincing evidence for ferrous iron in the surface materials. One goal of the new observations is to determine if there has been volcanism on Mercury that has vented materials from the deep interior. If so there should be measurable signatures of materials that have very high melting temperatures. Such signatures would indicate that heat from Mercury's interior has vented to the surface. In turn this would help constrain the thermal history of the planet and whether there may still be a liquid outer core. If there is a liquid outer core, perhaps dynamo action is still operating on Mercury and generating the magnetic field measured by instruments on Mariner 10 spacecraft. It is thought that Mercury may have formed in a section of the planetary nebula that was characterized by a reducing environment (one with a relatively low partial pressure of oxygen). If this was so, then Mercury's surface should contain silicates favoring the presence of Mg over Fe. A good place to look for such material is at locations which appear to have undergone volcanic resurfacing. A search for spectral features associated with magnesium in silicates, especially pyroxene, will be made at optically smooth and low albedo regions and at locations where freshly excavated materials are exposed by relatively recent impact gardening.
These measurements will occur at an important time in the study of the solar system because the first spacecraft mission to Mercury since the mid 1970's is due to launch in 2004 and go into orbit about Mercury in 2009. The spectral measurements made by Dr. Sprague and collaborators will help to place the results of the spacecraft mission MESSENGER (Mercury Surface, Space ENvironment, GEochemistry, and Ranging) in a better context. This project includes active participation of undergraduate physics majors from Susquehanna University, a small four-year liberal arts college in central Pennsylvania. By sharing the resources of a large University and NSF with students who would otherwise be unable to participate in research with new, highly capable instrumentation at premiere telescope facilities, Dr. Sprague and collaborators greatly extend the importance of their work. A graduate student will also be trained in the techniques of infrared remote sensing analysis. Such techniques are important to astronomy in general and to the analysis of data from new space missions like Space Infrared Telescope Facility (SIRTF).
