Given its proximity to the sun, Mercury is one of the more difficult planets to image. Before today, maps of the planet Mercury were mostly blank. Views by telescope revealed little detail. Mariner 10 flybys in the 1970s had photographed only a portion of the planet.
When Mariner 10 flew past Mercury three times in 1974 and 1975, the same hemisphere was in sunlight during each encounter. As a consequence, Mariner 10 was able to image less than half the planet. Planetary scientists have wondered for more than 30 years about what spacecraft images might reveal about the hemisphere of Mercury that Mariner 10 never viewed.
But now the unseen side has been revealed: NASA’s Messenger spacecraft took this picture from a range of 17,000 miles on Jan. 14, 2008. At first glance, it seems to show little more than a repetitive expanse of craters, much like our own moon. But researchers are excited. One of the craters is the giant Caloris Basin never before seen in its entirety. Formed by the impact of a large asteroid or comet, Caloris is one of the largest and perhaps youngest basins in the Solar System.
Close-up photos of the 800-mile-wide crater (still being downloaded from the spacecraft) may reveal new things about the history of Mercury and the physics of catastrophic impacts. BTW, Mercury’s nearness to the sun causes a daytime temperature of more than 400 °C (752°F). Mmmmm TOASTY!
More photos will be coming in the next few days, I expect there will be some surprises. (h/t Spaceweather)