In a transmission electron microscope (TEM), a beam of electrons is transmitted through an ultra-thin specimen, interacting with the material as it passes through. For crystalline materials such as the minerals found in extraterrestrial samples, the interaction between the electrons and the specimen allows direct imaging of the atomic arrangement in mineral structures at scales ranging from a single column of atoms up to crystal imperfections and defects several nanometers (nm) in size. At the same time, the use of various X-ray and electron spectroscopy techniques also available in the TEM provides information on the chemical composition and chemical bonding on the same scale as the images. The combination of both capabilities provides unique nanoscale information for studying the formation conditions and history of astromaterials from the Moon, Mars, comets, and asteroids.
The ARES TEM lab houses two 200 kV instruments, both with combined imaging and chemical analysis capabilities. The JEOL 2500SE field-emission scanning transmission electron microscope (STEM) uses a field-emission electron source designed to allow quantitative chemical composition and chemical bonding information to be obtained from localized regions as small as 0.5 nm, and sample atomic arrangements to be imaged with 0.2 nm resolution. This includes the ability to produce digital "maps" of the quantitative distribution of chemical elements, including C and O on nm-scale regions. The second instrument, a JEOL 2000FX analytical TEM, uses a thermionic electron source and can measure quantitative chemical composition on 10 nm regions and larger, and image crystal structures on the 0.25 nm scale. Both instruments are fully digital in their image acquisition and processing abilities.