Edge of the Polar Cap
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Edge of the Polar Cap

August 16, 2006

The two polar caps of Mars pile up like gigantic pancake stacks of ice intermingled with countless layers of dust and fine sand. Scientists believe both caps hold a priceless record of the Red Planet's climatic history.

The northern polar cap has a sawtooth outline, produced as sunlight and wind eroded the icecap's edge. In many places the transition from the icecap to the surrounding ground is dramatic. In Chasma Boreale, for example, the edge is steep.

In this scene, about 350 kilometers (220 miles) west of Chasma Boreale, the edge of the polar cap drops off less precipitously, although the elevation difference is about the same. The gradual descent, however, helps scientists better examine details in the layers. The wide slope also brings out variations in erosion that are likely due to physical differences in the dust and sand layers or how they were deposited.

This mosaic is made from images taken at visible wavelengths by the Thermal Emission Imaging System (THEMIS), a multi-color camera orbiting Mars on NASA's Mars Odyssey spacecraft. Broadly speaking, the view approximates what you would see looking down from orbit with a telescope on a clear day.

Fanning the Pages

The polar cap's edge exposes layers of ice alternating with sheets of dust and sand. These accumulate like pages in a climate history book as the Martian years roll past.

Scientists know that Mars' axis has changed its tilt relative to the Sun and the planet's orbit over timescales of hundreds of thousands of years. At present, the axis tilts some 25° - by coincidence, nearly the same as Earth's.

At times, however, the tilt has been almost 0°, largely cancelling the planet's seasons and heating the equator while letting the polar regions become very cold. At other times, the tilt has been as high as 80° - turning Mars all but pole-on to the Sun. This makes the poles the warmest part of the planet, while the tropics become cold.

Calculations show these climatic variations have a semi-rhythmical character, and scientists are closely studying exposures like that seen in the small image to unravel the sequence of events. Given a bit of luck, they may be able to relate the layers to the most recent changes in the Martian climate cycles.

Stepped Edge

A broad, dark shelf extends from the edge of the icecap in this area, its dark color indicating it contains much dust and fine sand. In places, however, bright patches show where ice lies at the surface.

The complex pattern indicates the dusty layer contains depressions that trap ice and partly protect it. Similarly, stepping down the edge, we can see - almost like stairs - layers with bright, west-facing edges. Because these face toward the Sun (and warmth), they may indicate that the layers are rich in ice and only thinly coated with dust.


As the icecap erodes in the present climate, the edge retreats northward. But due to variations in the dusty and sandy layers, the retreat doesn't go smoothly.

Here we see a butte like many found in desert regions on Earth. Erosion may one day complete the illusion by removing the connection with the main icecap, leaving this feature to stand alone. Already, several tiny versions of it stand free, along the polar cap edge to the east.

The fate of the dust and sand that emerge from the layered icecap is, literally, blowing in the wind. The lightest and smallest dust particles will be picked up and mixed with others in the atmosphere to make a homogenous mixture that may be deposited planet-wide.

The larger particles can't move as far or as fast - most will end up in the giant sand sea that surrounds the northern icecap.

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