The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor orbiter has been tracking the weather on Mars for six years of mapping. This movie shows the abundance of dust in the martian atmosphere each day as determined by the Thermal Emission Spectrometer. This movie covers three full martian years, beginning in April of 1999 and continuing through February of 2005. The dust abundance was determined using the infrared spectrum of Mars, which measures the spectral properties of the dust and the atmospheric temperature. These two properties can then be used to derive how much dust is present in the atmosphere.
Of particular interest are the large regional and global dust storms that occurred during summer in the southern hemisphere during each Mars year. The 2001 storm was by far the largest, lasting over six months (June to October, 2001) and covering the entire planet. The storms in the other two Mars years observed by the Thermal Emission Spectrometer were much smaller and never covered the planet. The most recent storm season (June 2003 through January 2005) actually had two separate storms, one in June and a second storm in December. Unlike most large martian dust storms that start in the southern hemisphere, the second of these storms began in the north and swept down toward the equator. In between these storms the atmosphere becomes quite clear, with much smaller dust storms occurring scattering throughout the year and over the planet.
Seasons on Mars are measured by the position of Mars in its orbit around the Sun. This system is measured in degrees of aerocentric longitude (Ls) around the orbit, beginning at 0° Ls at the northern spring equinox, progressing to 90° Ls at the start of northern summer, 180° Ls at the fall equinox, 270° Ls at the start of northern winter, and finally back to 360°, or 0°, Ls at the spring equinox. Dust abundance is measured as opacity (tau), with values of 0 tau being a completely clear atmosphere, and values of 2 being nearly impossible to see through to the surface.