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This color composite image produced from Mariner 10 data reveals evidence that different terrains on Mercury have different compositions. Image courtesy of M. S. Robinson (Robinson and Lucey, Science, 1997). |
It is more than 30 years after Mariner 10 visited Mercury and still only 45% of Mercury's surface has been imaged by a spacecraft. The part that has been seen appears cratered and ancient, with a resemblance to the surface of Earth's Moon. Slightly younger, less cratered plains sit between the largest old craters. A volcanic origin has been suggested to explain the formation of these plains, and slight color differences between the plains and ancient cratered terrains are consistent with the plains being volcanic. However, the typical resolution of images from Mariner 10 is not high enough to search for diagnostic volcanic surface features to support fully that idea, and thus the origin of the plains remains uncertain.
Mercury's tectonic history is unlike that of any other terrestrial planet. On the surface of Mercury, the most prominent features due to tectonic forces are long, rounded, lobate scarps and cliffs, some over a kilometer in height and hundreds of kilometers in length. These giant scarps are believed to have formed as Mercury cooled and the entire planet contracted on a global scale. Understanding the formation of these scarps thus provides the potential to gain unique insight into the thermal history and interior structure of Mercury.
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The white arrows in this Mariner 10 image point to the longest known lobate scarp on Mercury, named Discovery Rupes. The scarp is about 550 kilometers long and over a kilometer in height in some regions. Image courtesy of M. S. Robinson. |
MESSENGER will bring a variety of investigations to bear on Mercury's geology, in order to determine the sequence of processes that have shaped the surface. The X-ray, gamma-ray, and visible-infrared spectrometers will determine the elemental and mineralogical makeup of rock units composing the surface. The camera will image the previously unseen portion of the planet, and the typical imaging resolution for MESSENGER will far surpass that of most Mariner 10 pictures. Nearly all of the surface will be imaged in stereo to determine the planet's global topographic variations and landforms; the laser altimeter will measure the topography of surface features even more precisely in the northern hemisphere. Comparing the topography with the planet's gravity field, measured by tracking the MESSENGER spacecraft, will allow determinations of local variations in the thickness of Mercury's crust. This large breadth and depth of data returned by MESSENGER will enable the reconstruction of the geologic history of Mercury.
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