Mars Global Surveyor
Mars Orbiter Camera (MOC)

Mars: An Active Planet

MGS MOC Releases MOC2-166 to MOC2-172, 10 August 1999

Among the goals of the Mars Surveyor program are to characterize the planet's climate and the interaction of the atmosphere with the planet's surface. Both the Mars Global Surveyor (MGS, presently in Mars orbit) and the Mars Climate Orbiter (MCO, which reaches Mars in September) address these goals in part by using cameras to observe martian weather and changes on the surface that occur from season to season.

Mars Global Surveyor has been orbiting the Red Planet for just over 1 martian year (687 days). Although the spacecraft only recently attained its final design orbit after 18 months of aerobraking and other orbit phasing activities, "snapshots" of Mars acquired during this period by the MGS Mars Orbiter Camera (MOC)--when it has been on--and more recent views from the present mapping orbit have captured a unique record of seasonal and meteorological events that demonstrate that the planet is quite active and dynamic today.

Evidence for present-day activity comes in two forms--weather, and surface changes. Detailed weather observations include the tracking of dust devils and the daily mapping of cloud and storm patterns. Other changes on the planet have been seen among frost-covered sand dunes. These changes are connected to the passage of martian seasons and the retreat of polar ice as winter draws to a close and spring begins. As the winter ice begins to sublime, dunes develop small dark spots that grow and eventually coalesce until the frost disappears. Some dunes show evidence that wind and gravity are actively moving the dune sands, as well.

The images shown below were presented at a Space Science Update briefing at NASA Headquarters, Washington, DC, on 10 August 1999. For the corresponding JPL Press Release, see:

1. The Mystery of the Spotted Dunes

"Bushes" On Dunes
August 1999

Snow Leopard Dunes
August 1999

Changes In 26 Days
August 1999

Wind Streak Dunes
August 1999

Every year, Mars grows two large, seasonal frost caps (one during each hemisphere's winter) out of a combination of atmospheric carbon dioxide (the major component of the cap) and water vapor. At the end of each winter, these caps recede in latitude as the sun moves across the equator and into the spring-time hemisphere. This annual cycle of frost deposition and sublimation (i.e.,the process by which ice transforms when heated directly from solid to gas without first "melting" to form a liquid) is one of the definining elements of the martian climate. There is much that is unknown about this process, but recent observations of sand dunes within the polar regions are providing new information about the seasonal retreat of the polar ice caps.

Observations made in 1998 at the end of the north polar winter showed some evidence that dark spots develop on sand dunes as the winter frost begins to dissappear. This year, 1999, similar features have been observed in the southern hemisphere as winter has proceeded into spring (southern spring began August 2, 1999). The evidence from the dunes suggests that defrosting begins more or less simultaneously at many small, localized sites. Each site then grows radially from the initial spot, enlarging and eventually merging until all of the frost is gone. The rate of growth of the defrosted surfaces, and the details of their appearance, indicate that the frost is probably repeatedly sublimed and locally re-deposited, with this local cycle acting to "refridgerate" the ground and moderate or slow the rate of polar cap retreat. Each picture shown above (MOC2-166 to MOC2-169) provides examples of the dark spots that develop on dunes as they "thaw out," including comparisons over a 26-day period and an example of local wind transport of sand exposed in the spots.

2. Recent Dune Activity

Proctor Dunes
August 1999

Since first seen in Mariner 9 images of Mars, the isolated dune fields within large impact craters have been of great interest, as their dark color indicates that the light dust that covers much of the planet does not accumulate on the sandy surfaces. This indicates that the dunes must be active--moving--and that we might, with time, eventually see evidence of changes that allow us to measure the effectiveness of wind erosion on Mars. The dune field in the picture above shows evidence of recent activity, as dark sand has been mobilized and transported across surfaces covered by the late-winter remains of seasonal frost.

3. Dust Devils

Dust Devils
August 1999

Dust devils result from spinning vortices of air that lift dust from a planet's surface. They occur on Earth, where they are relatively small features and are commonly seen on hot, dry summer days in desert and farmland settings. On Mars, dust devils are thought by some to be a major source of the fine, pinkish dust that gives the sky its unearthly brownish color (as seen from the Mars Pathfinder and Viking landers). The MGS MOC wide angle camera has been used to look for dust devils--a clear demonstration that the planet is active today.

4. North Polar Clouds

North Polar Weather
August 1999

The MGS MOC wide angle cameras were designed specifically to monitor and document the red planet's weather over the course of a martian year. At the north pole, storm clouds have been brewing all through July and August 1999, as northern summer transitioned into fall. The picture shown here above is one of a series that shows how the weather patterns on Mars evolve over a couple of days.

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