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Mercury: The Key to Terrestrial Planet Evolution

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Question 2: What is the geologic history of Mercury?

picture of Beagle Rupes

A portion of the long, lobate scarp named Beagle Rupes (right side of this image) deforms an impact crater seen in the upper right. This image was taken during MESSENGER's first flyby of Mercury, and the width of the image is about 110 km.

Prior to MESSENGER, only 45% of Mercury's surface had been seen by spacecraft during the Mariner 10 mission. Combining the Mariner 10 photos with the images from MESSENGER’s three Mercury flybys, about 98% of the surface of Mercury has been seen in detail. It is possible for the first time to begin to investigate Mercury's geologic history on a global basis.

Much of Mercury's surface appears cratered and ancient, with a resemblance to the surface of Earth's Moon. Slightly younger, less cratered plains sit within and between the largest old craters. Many of these plains are volcanic, on the basis of their age relative to nearby large impact features and other indicators of volcanic activity. Data from MESSENGER’s flybys indicate that volcanism on Mercury persisted for at least the first half of the planet’s history, and that the style of volcanism included both effusive and explosive eruptions.

Mercury's tectonic history is unlike that of any other terrestrial planet. On the surface of Mercury, the most prominent features produced by tectonic forces are long, rounded, lobate scarps or 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 insight into the thermal history and interior structure of Mercury.

Caloris basin

Caloris basin is the best-preserved large impact basin known on Mercury. The full extent of Caloris basin was imaged for the first time during MESSENGER's first flyby of Mercury, revealing that the basin is even larger than previously thought (yellow line = Mariner 10 diameter of 1300 km; blue line = MESSENGER diameter of 1550 km).

Once in orbit, 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 cameras will image Mercury's surface in color and at a typical imaging resolution that surpasses 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 diversity of high-resolution data returned by MESSENGER will enable the reconstruction of the geologic history of Mercury.


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