Thermal Emission Spectroscopy: the Technique
By S.W. Ruff
How Is Emissivity Used to Map Mars?

The View from Above
The measurement of emissivity can be done remotely. No part of the measurement requires contact with the material being measured. Thermal infrared energy travelling at the speed of light reaches the measuring instrument no matter how far away it is. This means that an instrument placed in orbit around Mars can view the entire surface over time. With the benefit of a telescope, an orbiting spectrometer can see relatively small areas of the planet and determine the emissivity from the materials within the field of view. Because Mars has an atmosphere, this view includes molecules of gas as well as molecules of rock, dust, and ice on the surface. As described in the last section, each of these molecules contributes to the emissivity spectrum. The mixed spectra from the orbiting instrument have to be deconvolved (unmixed) using a spectral library, in order to identify the different materials in the scene.

A Firehose Stream of Data
An orbiting spectrometer needs to be able to send the emissivity spectra it has collected back to Earth. Thermal IR energy that leaves the surface of Mars is converted to electrical impulses by the detector. An onboard computer changes these to digital values that can be broadcast as radio waves. The radio waves are picked up by dish antennas on Earth and ultimately converted back to digital data to be read on computers. In the case of the Thermal Emission Spectrometer, this process occurred 100,000 times per day for the Mars Global Surveyor mission. Even with the help of computers, the wealth of spectral data returning from Mars will keep many scientists busy for years (decades?) to come.