What is a Mars Daily Global Map (MDGM)?

Each Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) Mars Daily Global Map (MDGM) is composed of a polar stereographic north polar map (from 45°N to the north pole), a cylindrical equatorial map (from 60°S to 60°N) and a polar stereographic south polar map (from 45°S to the south pole). Each MDGM is made from 13 consecutive pairs of red and green MGS MOC global map strips or waths (7.5 km/pixel or 3.75 km/pixel) [Wang and Ingersoll, 2002]. Each swath was captured as the MGS spacecraft orbited in a near polar orbit, passing from the pole, over the equator, and then over the opposite pole on each daylight side of each orbit. Data were not collected during the night side passes.

Examples of What Can be Seen in MDGMs

The MDGM images above show the two Martian poles captured at two different seasons. On the left, dust and polar hazes - probably water ice 'clouds' - can be seen over the northern pole in northern summer. On the right, the southern pole can be seen in the processes of being slowly uncovered from under a massive seasonal carbon dioxide ice cap in southern spring. Each image is a "daily average" - it is made of 13 individual strips collected over a full Martian day - so the images do not represent a single "snap shot" of time. In the images, dust hazes and dust clouds appear yellowish to reddish, while water ice clouds appear white to blue. The carbon dioxide ice on the surface also appears white. In the right hand image, the penetration of air containing dust can clearly be seen over portions of the seasonal carbon dioxide ice cap. In the left image, some of the hazes appear yellowish and blue/white. These are probably mixed dust/water ice hazes. The MDGM images have been used to study the evolution of dust and water ice over the course of the seasonal cycle and over several years. In addition, they have been used to study curvilinear frontal and flushing events (interesting travelling dust storms with very well defined "fronts" - just like rain storms on Earth) and global dust storms - huge storms that can greatly increase the haziness of the atmosphere on a global scale (see below) [Hinson and Wang, 2009; McConnochie et al., 2010; Strausberg et al., 2005; Toigo et al., 2002; Wang and Ingersoll, 2002; Wang et al., 2003, 2005; Wang, 2007; Wang and Fisher, 2009].

The two MDGM's shown above provide a glimps of Mars before and during a major global dust storm in 2001. The first image (top) was taken a few days before a small scale storm began in the Hellas basin - Hellas is a deep depression in the southern hemisphere that appears as the largest bright white feature in the southern hemisphere in this image. That small storm grew over a few weeks, engaging other areas in dust lifting, until the whole atmosphere was filled with dust to the point where surface features could no longer be seen (bottom image). Note that in the first image you can see craters, bright and dark surface regions, and cloud-capped volcanoes at the left of the image. By late in the storm, as shown in the second image, Mars is "hazed out" - like a bad smog day in Los Angeles. The very tips of the massive 20km+ high volcanoes can barely be seen above the deep haze. The evolution of this 2001 storm has been traced in great detail thanks to the MOC MDGM images [Strausberg et al., 2005].

More Questions about the Data and Processing?

For questions about the MDGM products, please contact Huiqun Wang at hwang@cfa.harvard.edu.