Regular cameras record light intensity at 3 wavelengths (red, green, and blue), which is sufficient to produce an image the looks accurate to the human eye. In imaging spectroscopy, a camera measures the intensity of light at more than just 3 different wavelengths. For every pixel in the photograph, the camera records data that can be viewed as a graph showing how the intensity of light varies across the visible region of the electromagnetic spectrum (and sometimes beyond the visible region). That graph can reveal details about the chemical makeup and physical properties of the photographed object.
Imaging spectroscopy has been used by telescopes and spacecraft to study Mars.
Reflectance spectroscopy and thermal emission spectroscopy are two types of imaging spectroscopy.
Reflectance spectroscopy measures the visible and infrared light spectrum of the sunlight reflected from an object. After the spectrum of the light emitted by the sun is taken into account, a spectrum that is specific to the reflecting material is calculated. This spectrum can be compared to a library of known spectra. The Mars Express Orbiter uses an imaging spectrometer named Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) to study the elements and minerals present on the surface of Mars. OMEGA's detection of hydrated minerals in 2005 was, at the time, the strongest evidence that surface water was once present in large amounts and for a long duration. The Compact Reconnaissance Imaging Spectrometer for Mars, an instrument on the Mars Reconnaissance Orbiter, uses reflectance spectroscopy to examine the surface and dust in the atmosphere. Among other findings, it was used to better understand how dust warms the Martian atmosphere by absorbing sunlight.
Thermal Emission Spectroscopy
Thermal emission spectroscopy, also known as infrared imaging, measures the infrared light that is released by any object as a result of normal molecular vibrations. The spectrum of this light provides information on the composition of the object that emitted it, and that object's temperature. The Thermal Emission Spectrometer on Mars Global Surveyor used thermal emission spectroscopy to learn about dust in the Martian atmosphere and the surface temperature on Mars. The Thermal Infrared Imaging Spectrometer on the Mars Orbiter Mission spacecraft also uses this technique.
It is also possible for a single instrument to combine both of the above methods to perform multispectral imaging. The Thermal Emission Imaging System on Mars Odyssey is capable of multispectral imaging.
Compact Reconnaissance Imaging Spectrometer for Mars web site: http://crism.jhuapl.edu/index.php
Library of thermal infrared spectra maintained by Arizona State University's Mars Space Flight Facility: https://speclib.asu.edu/
- Shaw GA & Burke HK. 2003. Spectral Imaging for Remote Sensing. Lincoln Laboratory Journal, 14(1), 3-28. https://courses.cs.washington.edu/courses/cse591n/07sp/papers/Shaw2003.pdf
- The European Space Agency. Mars Express orbiter instruments. http://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Mars_Express_instruments
- The European Space Agency. September 1 2019. Mars Express science highlights: #1. Hydrated minerals – evidence of liquid water on Mars. https://sci.esa.int/web/mars-express/-/51821-1-hydrated-minerals-ndash-evidence-of-liquid-water-on-mars
- Johns Hopkins Applied Physics Laboratory. CRISM's Investigations and New Discoveries (2006-present). http://crism.jhuapl.edu/science/themes/index.php
- Arizona State University. Mars Global Surveyor Thermal Emission Spectrometer. http://tes.asu.edu/index.html
- Indian Space Research Organization. Payloads. In PSLV-C25/Mars Orbiter Mission. https://www.isro.gov.in/pslv-c25-mars-orbiter-mission/payloads
- Arizona State University School of Earth & Space Exploration. Frequently Asked Questions. In Mars Odyssey THEMIS. http://themis.asu.edu/faq