Looks to me like sections of the planet that weren't photographed when the craft observing it took pictures of it, I couldn't find anything about the massive stripe of lighter-colored surface on mercury so that's my guess. (pictures of planets (at least the earth) to my knowledge are made by cobbling together several photos taken by a craft orbiting the planet, so maybe the craft had a vertical orbit and just didn't get there)
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Further research required
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Further research required
http://planetary-mechanics.com/2017/08/08/how-rough-is-mercury/