Tuesday, September 4, 2007


Credit: NASA/JPL/Arizona State University

The Thermal Emission Imaging System (THEMIS) is a camera on 2001 Mars Odyssey. THEMIS takes images of 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), which is on board Mars Global Surveyor. Additionally, THEMIS 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 detects thermal infrared energy emitted by the Martian surface at 10 different wavelengths. Nine 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 tenth band is at 14.9 micrometers, and is used to monitor the Martian atmosphere.

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 material's grain size.

The effect of temperature can be removed from THEMIS infrared data by dividing the image by a black body curve. The resulting energy pattern is an emissivity spectrum characteristic of the specific minerals (or other things) found on the surface. The presence of minerals such as carbonates, silicates, hydroxides, sulfates, amorphous silica, oxides, and phosphates 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 the Thermal Emission Spectrometer (TES), a similar instrument on Mars Global Surveyor. While THEMIS has a very high spatial resolution (100 m) with a low spectral resolution of only 10 bands between 6 and 15 micrometers, TES has a low spatial resolution (3x6 km), but with very high spectral resolution of 143 bands between 5 and 50 micrometers.

THEMIS also has a visible imaging camera that acquires data in five spectral bands, takes images with a spatial resolution of 18 meters (59 feet). This resolution is intermediate between large-scale images from the Viking Orbiters (150 to 300 meters per pixel) and the high-resolution images from the Mars Orbiter Camera (MOC) onboard Mars Global Surveyor (1.5 to 3 meters per pixel).

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 above picture shows both a visible and a thermal infrared image taken by THEMIS on November 2, 2001. The images were taken as part of the ongoing calibration and testing of the camera system as the spacecraft orbited Mars on its 13th revolution of the planet.

The visible wavelength image, shown on the right in black and white, was obtained using one of the instrument's five visible filters. The spacecraft was approximately 22,000 km (about 13,600 miles) above Mars looking down toward the south pole when this image was taken. The season is late spring in the Martian southern hemisphere.

The thermal infrared image, center, shows the temperature of the surface in color. The circular feature seen in blue is the extremely cold Martian south polar carbon dioxide ice cap. The instrument has measured a temperature of minus 120° Celsius (minus 184° Fahrenheit) on the south polar ice cap. The polar cap is more than 900 km (540 miles) in diameter at this time.

The visible image shows additional details along the edge of the ice cap, as well as atmospheric hazes near the cap. The view of the surface appears hazy due to dust that still remained in the Martian atmosphere from the massive dust storms that had occurred over the previous several months.

The infrared image covers a length of over 6,500 km (3,900 miles) spanning the planet from limb to limb, with a resolution of approximately 5.5 km (3.4 miles) per pixel at the point directly beneath the spacecraft. The visible image has a resolution of approximately 1 km (0.6 miles) per pixel, and covers an area roughly the size of the states of Arizona and New Mexico combined.

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