TheThermal Emission Imaging System (THEMIS) is a camera on board the2001 Mars Odyssey orbiter. It imagesMars in the visible andinfrared parts of theelectromagnetic spectrum in order to determine the thermal properties of the surface and to refine the distribution ofminerals on the surface of Mars as determined by theThermal Emission Spectrometer (TES). Additionally, it helps scientists to understand how themineralogy of Mars relates to its landforms, and it can be used to search for thermal hotspots in the Martian subsurface.
THEMIS is managed from theMars Space Flight Facility atArizona State University and was built by the Santa Barbara Remote Sensing division ofRaytheon Technologies Corporation, an Americanmultinationalconglomerate headquartered inWaltham, Massachusetts. The instrument is named afterThemis, the goddess of justice in ancientGreek mythology.
THEMIS detects thermal infrared energy emitted by the Martian surface at nine differentwavelengths. Eight of these have wavelengths between 6 and 13 micrometers, an ideal region of theinfrared spectrum to determinethermal energy patterns characteristic ofsilicate minerals. The ninth band is at 14.9 micrometers and is used to monitor theMartian atmosphere. The shortest infrared wavelength, at 6.78 micrometers, is measured twice in two bands to improve thesignal-to-noise ratio. THEMIS is therefore a 10-band instrument that detects nine different wavelengths[1].
Theabsorption spectrum measured by THEMIS contains two kinds of information:temperature andemissivity. 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 ashaded 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).
The effect of temperature can be removed from THEMIS infrared data by dividing the image by ablack body curve. The resulting energy pattern is anemissivity spectrum characteristic of the specific minerals (or other things) found on the surface. The presence of minerals such ascarbonates,silicates,hydroxides,sulfates, amorphoussilica,oxides, andphosphates can be determined from THEMIS measurements.
In particular, this multi-spectral method allows researchers to detect the presence of minerals that form in water and to understand those minerals in their geological context.
The THEMIS infrared camera was designed to be used in conjunction with data from theThermal Emission Spectrometer (TES), a similar instrument onMars Global Surveyor. While THEMIS has a very highspatial resolution (100 m) with a lowspectral resolution of only 10 bands between 6 and 15 micrometers, TES has a low spatial resolution (3×6 km) with very high spectral resolution of 143 bands between 5 and 50 micrometers.
The instrument's approach provides data on localized deposits associated with volcanoes, hydrothermal processes, and the alteration of minerals by surface and/or subsurface water.
TheAdvanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), an Earth orbiting instrument on the Terra spacecraft, has used a similar approach to map the distribution of minerals on Earth. Variations in the thermal infraredfalse-color image are due to differences in the minerals that make up rocks and soil.
THEMIS found a wide range of igneous rocks and minerals. Some of the rocks were low-silicabasalts, high silicadacite,olivine basalts, ultramafic (picritic) basalts, and quartz-bearing granitoid rocks. The olivine basalts were present in a variety of locations, such as on crater floors and in some canyon wall layers. The mineral olivine is important because it is common in more primitivemagmas from themantle and it weathers quickly when moisture is present. So, if olivine is present, the climate must have been dry since the time that olivine was exposed.Quartz-bearing rocks were found in the central uplifts in craters. Rocks in the central uplifts[clarification needed] were probably once buried several kilometers beneath the surface, but raised by the impact process. Rocks of dacite composition show that, within magma chambers, fractional crystallization occurred.[1] In this process, some minerals form crystals, then settle to the bottom of the chamber. Having a variety of rocks increases the chances that some useful/valuable minerals may be found on Mars.
THEMIS has a visible imaging camera that acquires data in five spectral bands, takes images with a spatial resolution of 18 m (59'), and can resolve objects about the size of asemi-trailer. This resolution is intermediate between large-scale images from theViking Orbiters (150 to 300 meters per pixel) and the high-resolution images from the Mars Orbiter Camera (MOC) on boardMars Global Surveyor (1.5 to 3 meters per pixel). Visible images from THEMIS are usually close to 20 km wide (12 miles).[2]
The THEMIS visible camera's stated purpose is to determine the geological record of past liquid and volcanic environments on Mars. Additionally, this dataset can be used in conjunction with the infrared data to identify potential landing sites for future Mars missions.
The Thermal Emission Imaging System weighs 11.2 kilograms (24.7 lb), is 54.5 x 37 x 28.6 cm (21.5 x 14.6 x 11.3 in) and runs on 14 watts of electrical power.