Thermal Emission Imaging System

The Thermal Emission Imaging System (THEMIS) is a camera on board the 2001 Mars Odyssey orbiter. It images Mars in the visible and infrared parts of the electromagnetic spectrum in order to determine the thermal properties of the surface and to refine the distribution of minerals on the surface of Mars as determined by the Thermal Emission Spectrometer (TES). Additionally, it helps scientists to understand how the mineralogy of Mars relates to its landforms, and it can be used to search for thermal hotspots in the Martian subsurface.

THEMIS is managed from the Mars Space Flight Facility at Arizona State University and was built by the Santa Barbara Remote Sensing division of Raytheon. The instrument was named after the Greek goddess of justice.

THEMIS detects thermal infrared energy emitted by the Martian surface at nine different wavelengths. Eight of these have wavelengths between 6 and 13 micrometers, an ideal region of the infrared spectrum to determine thermal energy patterns characteristic of silicate minerals. The ninth band is at 14.9 micrometers and is used to monitor the Martian atmosphere. The shortest infrared wavelength, at 6.78 micrometers, is measured twice in two bands to improve the signal-to-noise ratio. THEMIS is therefore a 10-band instrument that detects nine different wavelengths [1].

The absorption spectrum measured by THEMIS contains two kinds of information: temperature and emissivity. The temperature contribution to the measurement dominates the spectrum unless the data is corrected. In effect, a THEMIS infrared image taken during the day will look much like a shaded relief map, with slopes facing the sun being bright (hot) and shaded areas being dark (cold). In a THEMIS image taken at night however, thermophysical properties of the surface can be inferred, such as temperature differences due to the materials grain size (thermal inertia).

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