The 2001 Mars Odyssey mission makes use of many innovative technologies, but the most important among them are the three instrument packages. All three involve the use of spectrometers.
We call the part of the electromagnetic spectrum that we can see "visible" or "optical" light. To fully appreciate the complexity of the world around us, however, we need to rely on human-made devices to provide views of the "invisible" world -- that is, the parts of the electromagnetic spectrum we cannot see without the aid of technology: gamma rays, X-rays, ultraviolet waves, infrared waves, microwaves, and radio waves.
All of these different types of energy are "light," even if our human eyes can only "see" part of it. The only difference between them is wavelength (the distance between the peaks of each). Wavelengths get larger as we move across the electromagnetic spectrum from gamma rays to radio waves. The wavelength of visible light is about 1/10th of a micrometer, but the full electromagnetic spectrum includes both shorter and longer wavelengths.Familiar ways of studying "invisible light" (with wavelengths greater or less than what our eye can see) include x-rays for medical diagnosis and radar for guiding airplanes. By using spectrometers at Mars, scientists can learn a great deal about the planet's composition (what it is made of) and its radiation environment. None of this knowledge would be possible if we only relied on our eyes.