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Mercury: The Key to Terrestrial Planet Evolution

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Question 5: What are the unusual materials at Mercury's poles?

A radar image of the north polar region of Mercury

A radar image of the north polar region of Mercury shows radar-bright regions concentrated in circular floors of craters with permanently shadowed interiors. The radar-bright material might be water ice, but alternative suggestions have also been proposed. The image is 450 km across. Image courtesy of J. K. Harmon, Arecibo Observatory.

Mercury's axis of rotation is oriented nearly perpendicular to the planet's orbit, so that in polar regions sunlight strikes the surface at a near-constant grazing angle. Some of the interiors of large craters at the poles are thus permanently shadowed and perpetually very cold. Earth-based radar images of the polar regions show that the floors of large craters are highly reflective at radar wavelengths, unlike the surrounding terrain. Furthermore, the radar-bright regions are consistent in their radar properties with the polar cap of Mars or the icy moons of Jupiter, suggesting that the material concentrated in the shadowed craters is water ice. The idea of water ice being stable on the surface of the planet closest to the Sun is intriguing.

The temperature inside these permanently shadowed craters is believed to be low enough to allow water ice to be stable for the majority of the observed deposits. Ice from infalling comets and meteoroids could be cold-trapped in Mercury's polar deposits over millions to billions of years, or water vapor might outgas from the planet's interior and freeze at the poles. A few craters at latitudes as low as 72° N have also been observed to contain radar-bright material in their interiors, and at these warmer latitudes maintaining stable water ice for longer periods of time may be more difficult; a recent comet impact, in the last few million years, may be required to satisfy all radar observations. Alternatively, it has been suggested that the radar-bright deposits are not water ice but rather consist of a different material, such as sulfur. Sulfur would be stable in the cold traps of the permanently shadowed crater interiors, and the source of sulfur could be either be infalling meteoritic material or the surface of Mercury itself. It has also been proposed that the naturally occurring silicates that make up the surface of Mercury could produce the observed radar reflections when maintained at the extremely low temperatures present in the permanently shadowed craters.

MESSENGER’s three flybys of Mercury passed nearly over the equator and did not allow for viewing of the planet’s poles. Once in orbit around Mercury, however, MESSENGER's neutron spectrometer will search for hydrogen in any polar deposits, the detection of which would suggest that the polar deposits are water-rich. The ultraviolet spectrometer and energetic particle and plasma spectrometer will search for the signatures of hydroxide or sulfur in the tenuous vapor over the deposits. The laser altimeter will provide information about the topography of the permanently shadowed craters. Understanding the composition of Mercury's polar deposits will clarify the inventory and availability of volatile materials in the inner Solar System.


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