In March 2011 MESSENGER became the first spacecraft to orbit Mercury. NASA's satellite has been taking measurements since then, and has collected nearly 100,000 images of the solar system's smallest planet, and the one closest to the sun.

The image above was created using laser radar to measure surface elevation. It depicts a 1,200-kilometer-long swath of the planet’s northern plains, falsely colored so that darker colors represent lower regions and lighter areas represent higher elevations. The sinuous ridges, cracked craters and an odd bulge (the large whitish area shown on the left) indicate that the four-billion-year-old plains underwent significant distortion after they formed.

Until now many scientists thought Mercury became a geologically dead world soon after its formation. Instead, MESSENGER found several topographic features that appear to have tilted over time, suggesting that Mercury was geodynamically active for a longer period than expected. It may even be active today.

Scientists do not understand the forces that guided the topographic tilting and bulge formation. On other planets, a viscous molten mantle is responsible. But because Mercury has an extremely thin mantle, it is difficult to imagine how it could have transported such vast geologic expanses, researchers say.

Understanding Mercury's unusual dynamics could help scientists learn more about the formation and evolution of rocky planets.

By Sarah Fecht