NASA's Curiosity Mars rover clicks pic of Mercury passing in front of Sun

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.

Mark Lemmon of Texas A and M University, College Station, a member of the Mastcam science team, said this is a nod to the relevance of planetary transits to the history of astronomy on Earth.

He said observations of Venus transits were used to measure the size of the solar system, and Mercury transits were used to measure the size of the Sun.

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.