The old joke goes that the only thing worse than finding a worm in an apple is finding half a worm. Planetary scientists had a similar feeling on March 29, 1974, when the Mariner 10 space probe flew by Mercury and gave humanity its first good look at this tiny inferno of a world. It discovered, among other features, one of the largest impact basins in the solar system, later named Caloris. Yet its pictures captured only half the basin; the other half remained cloaked in darkness. In fact, between this visit and the second and third flybys later in 1974 and in 1975, Mariner 10 imaged less than half the planet’s surface.
It was not until 34 years later that we finally saw the entire basin illuminated, and it was even more impressive than the early images suggested. On January 14, 2008, the MESSENGER spacecraft swung by Mercury, and the first image it transmitted to Earth was very nearly centered on Caloris. When our colleague Nancy Chabot showed the image to the team, everyone cheered—but only briefly, because then we launched into an intense discussion of what exactly we were seeing. It looked like a negative image of the moon. Although Mercury’s cratered surface was reminiscent of the moon’s, lunar basins have dark, lava-filled interiors, whereas Caloris was filled with light-colored plains—a difference we have yet to fully understand.