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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:
www.jpl.nasa.gov/news/.
MOC2-166 "Bushes" On Dunes August 1999 |
MOC2-167 Snow Leopard Dunes August 1999 |
MOC2-168 Changes In 26 Days August 1999 |
MOC2-169 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.
MOC2-170 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.
MOC2-171 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.
MOC2-172 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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