THEMIS is a 13 kg 14 W pushbroom imager being built for NASA's Mars 2001 orbiter to acquire 9 IR bands (6.5-15.5 /spl mu/m) of 100 m resolution images to map mineralogy and search for hydrothermal activity. A 20 m resolution multispectral visible sensor, from Malin Space Science Systems, is also integrated into THEMIS. THEMIS data will support selection of landing sites for sample collection and return missions. Optics and detector designs are keys to meeting the low-cost mission requirements. THEMIS' telescope is a compact 3-mirror f/1.6 diamond-turned design. Spectral selection is via stripe filters fixed above the focal planes. The IR focal plane incorporates a 2D array of Si microbolometer detectors, based on a Honeywell patent and further developed by Raytheon. These uncooled detectors relieve the instrument design from accommodating the bulk/weight/power of an active/passive cryocooler. The detector high responsivity range is being extended down from 8 to 6 /spl mu/m to image carbonate absorption bands.