The question of life on Venus depends largely upon the length of the planet's rotation period. This is still uncertain since no definite surface markings can be found on the planet by which the period of its rotation can be determined. So dense is the atmosphere of Venus that its surface is, apparently, always hidden from view beneath a canopy of clouds. It is the more general belief that Venus, as well as Mercury, rotates on its axis in the same time that it takes to make a revolution around the sun. In this case the same side of the planet is always turned toward the sun and, as a result, the surface is divided into two hemispheres--one of perpetual day, the other of perpetual night.