MESSENGER did its best to map shortening structures across the entirety of the planet, but two things made that job a challenge. For starters, the more powerful of its two imaging systems, the laser altimeter, could not operate effectively if the spacecraft was more than 930 miles away from the planet—a proximity it reached only when it was over Mercury’s north polar region during its sweeping elliptical orbits; at other points it was as much as 9,500 miles distant. The dual-imaging system could survey the surface during those more remote approaches, but it did not produce the detailed three-dimensional pictures the laser altimeter did.
Then too, there is the condition of Mercury’s surface itself. The shortening structures formed early in the planet’s history, and in the eons since, Mercury has been steadily pounded by incoming asteroids, gouging out craters and scattering debris, obscuring the telltale scars of planetary contraction. It’s possible to read through that rubble—tracing shortening structures until they are covered up by craters or rocks and estimating what their course and reach was—but that takes planet-wide three-dimensional maps, something that MESSENGER did not provide. There was, however, a way to create them.
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