Jupiter's moon Ganymede is the Solar System's biggest moon, though Saturn's Titan comes a very close second. It's larger than the planet Mercury. The image was taken a couple of decades ago by NASA's Galileo mission.
The moon is described as having
an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The large circular feature on the upper right, called Galileo Regio, is an ancient region of unknown origin. Ganymede is thought to have an ocean layer that contains more water than Earth and might contain life.
Image Credit: NASA, JPL, Galileo Probe