TES

The number of spectra collected in any given month varied according to the rate at which data could be returned, the number of spacecraft maneuvers that precluded “typical” data acquisition patterns, and other factors. The final count of TES spectra collected during the aerobraking, mapping, and extended phases of the mission was 206,219,475. See the TES site for more detailed information.

Light and Heat

Thermal IR energy is more commonly known as “heat”. Everyone is familiar with heat because of our sense of touch. But what exactly is heat? Heat is a form of light invisible to our eyes, but detectable with our skin. Visible light is part of a large spectrum of energy that includes other familiar electromagnetic energy regions: microwaves, radio waves, ultraviolet, and X-rays all are forms of light that we can not see. The colors of a rainbow form a continuous spectrum of light in the visible wavelength region as does the “light” in the other regions. Infrared light occurs at wavelengths just below red light, hence the name, infra- (below) red. Near-infrared is the “color” of the heating coil on an electric stove just before it glows red. The thermal (or mid-) infrared colors are found at even longer wavelengths.

Glowing vs. Reflecting

The light that we see with our eyes originates from a glowing source, a light bulb or the sun, for example. But that light also can reflect off of surfaces and reach our eyes. This allows us to see things that don’t emit their own light. Infrared light also is emitted and reflected. Unlike visible light though, infrared light is emitted by any object that has a temperature above absolute zero. With hotter temperatures comes brighter infrared light until the object emits visible light.

Temperature and Color

We can determine the temperature of a glowing object from the color of the emitted light. This is true of the sun, a light bulb, or a rock heated by the sun. The color that we see results from the wavelength region where the maximum emitted energy occurs. In the first two cases, this radiance maximum occurs in the visible wavelengths that we call yellow. In the case of the rock, its peak radiance is in the thermal IR wavelengths. Determining temperature in this manner is a useful feature of infrared measurements. As described in the next section, much more information is available if we can see a full spectrum of infrared colors.

A Super-Technicolor World

The colors that we see in the world around us provide more information than would be available in a black-and-white world. An orange is easy to distinguish from a grapefruit because of color. Red and yellow leaves on trees signal the transition from summer to fall. It is because of the interaction of light and matter that we see objects in different colors. If we could see the world in infrared colors, we would discover even more details.

The Interaction of Light and Matter

When matter is exposed to the electrical and magnetic components of light, the atoms and molecules within it are affected. The chlorophyll molecules in plants absorb UV and most visible wavelengths, but reflect green and near-IR light. The minerals in rocks absorb and reflect thermal IR light of different wavelengths (colors) depending on the composition of each mineral. Because every mineral absorbs and reflects thermal IR differently, each has its own spectral signature in these wavelengths. If our eyes could see thermal IR light, every mineral would have a unique color. This is also true of the gas molecules in the atmosphere and frozen molecules in ice. The TES instrument views Mars with thermal IR detecting “eyes”. This means that we can identify what minerals are on its surface, what gases are in its atmosphere, and what ices are at the poles. To learn how we measure thermal IR energy, read on.

Seeing the Heat

The rods and cones at the back of our eyes are designed to see visible light. When we hold up our hand to a flame, we are sensing the infrared light. But how can we “see” this light? It is possible to design detectors that sense infrared energy and convert it to electrical energy. These are similar to solar photovoltaic cells that convert visible light to electricity. Once infrared light is converted to electricity, we can easily measure the variations in electrical voltage that represent the different wavelengths. In this way, we can measure the colors that our eyes can’t see.

Making a Thermal IR Rainbow

Our eyes are not able to see the full spectrum of colors in sunlight. So much of the sun’s energy peaks in the yellow wavelengths that we need a prism or a rainbow to see the rest of the colors. Similarly, in order to see the full spectrum of thermal IR colors, we have to change how we view them. One of the ways to do this is with filters. Just like a red filter held up to the sun only passes red light, filters that pass different thermal IR colors can be created. Another way is to use a device called an interferometer. Somewhat like a prism, an interferometer can create a full spectrum of infrared light all at once. The TES instrument uses an interferometer to create a spectrum and an infrared detector to measure it. This combination is called an infrared spectrometer.

In response to a NASA request for instrument proposals, Dr. Phil Christensendeveloped the idea of TES. His experience as a graduate student at UCLA exposed him to Mars exploration. It was there that he worked with data from the Infrared Thermal Mapper. The IRTM was a more primitive version of TES that flew on the two Viking orbiters in 1976. In 1983, when NASA called for instruments to fly on board the planned Mars Observer spacecraft, Phil answered with the TES concept. He assembled many of the same IRTM engineers from the Hughes Santa Barbara Research Center (SBRC), to help design and build TES. A team of scientists also was recruited by Phil to help guide the process. One of these scientists, Dr. Hugh Kieffer, was the principal investigator for the IRTM instrument.

A Blackbody: the Perfect Emitter

Imagine looking into a small opening of a deep cave. In the visible wavelengths, the opening looks black because the light that enters the cave is not easily reflected back out. However, the cave glows with emitted thermal IR energy. This energy emerges as a complete spectrum of all wavelengths of IR light. The radiance at each infrared wavelength is the maximum amount possible for a given temperature. If we used an infrared spectrometer to measure this emitted energy and plotted the result, it would follow a Planck distribution.Anything that emits energy with a Planck distribution can be called a blackbody. A blackbody emitter is useful for comparison with materials that do not emit perfectly at all wavelengths, which is the case for most of the matter in the universe.

A Mixed-Up Universe

Using the emissivity spectrum from a pure material, it is easy to identify its composition. The emissivity of quartz for example, is different from that of every other substance in the universe. But materials in the universe rarely are found in their pure state. What happens to emissivity when it is measured from a mixture of materials? Fortunately, the emissivity spectrum from the individual components of a mixture add together in a simple linear fashion. A rock for example, is a mixture of different minerals. If it is composed of quartz, feldspar, and hornblende, its emissivity spectrum will appear as a mixture of all of the absorption features of each mineral. A mixed spectrum at first may appear to be hopelessly convoluted, but the fact that the component spectra add together linearly allows it to be deconvolved.

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.