NASA's Curiosity Mars rover has imaged the planet Mercury passing in front of the sun, visible as a faint darkening that moves across the face of the sun. This is the first transit of the sun by a planet observed from any planet other than Earth, and also the first imaging of Mercury from Mars. Mercury fills only about one-sixth of one pixel as seen from such great distance, so the darkening does not have a distinct shape, but its position follows Mercury's expected path based on orbital calculations. The observations were made on June 3, 2014, from Curiosity's position inside Gale Crater on Mars. In addition to showing the Mercury transit, the same Mastcam frames show two sunspots approximately the size of Earth. The sunspots move only at the pace of the sun's rotation, much slower than the movement of Mercury. Many viewers on Earth observed a Venus transit in June 2012, the last visible from Earth this century. The next Mercury transit visible from Earth will be May 9, 2016. Mercu...