Mercury was the “dead” planet when NASA’s
MESSENGER mission launched in 2004. But the
spacecraft’s 2011 orbital arrival, and more than 250,000
subsequent images, documented an active planet. Running
on fumes this year, MESSENGER (short for Mercury
Surface, Space Environment, Geochemistry and Ranging)
saved the best for last, swooping just miles above the virtually
atmosphereless planet. It gathered fresh evidence for frozen
water in crater corners, strange rock features called hollows
and the evolution of a magnetic field almost 4 billion years old.
The craft finally smacked into Mercury at 8,800 mph on April
30. This view of 1,000-mile-wide Caloris basin — among the
largest known asteroid impacts in the solar system — shows
how lava (orange) filled the blast site before new craters
excavated the original basin (purple).