The length of the day on Mars is known very accurately, for the rareness of its atmosphere permits us to see readily many of its surface markings. The length of the day is about twenty-four and one-half hours, and the seasonal changes on Mars strongly resemble our own, though the seasons on Mars are twice as long as they are on our own planet since the Martian year is twice as long as the terrestrial year.
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.
This peculiar form of rotation in which the period of rotation and revolution are equal is by no means unknown in the solar system. Our own moon always keeps the same face turned toward the earth and there are reasons for believing that some of the satellites of Jupiter and Saturn rotate in the same manner.
Life on any one of the outer planets is impossible. The density of these planets averages about the same as the density of the sun, which is a little higher than the density of water. The density of Saturn is even less than water. In other words, Saturn would float in water and it is the lightest of all the planets. It is assumed from these facts that the four outer planets are largely in a gaseous condition. They all possess dense atmospheres and, in spite of their huge size, rotate on their axis with great rapidity. The two whose rotation periods are known, Jupiter and Saturn, turn on their axis in about ten hours. On account of this rapid rotation and their gaseous condition both Jupiter and Saturn are noticeably flattened at the poles.
The terrestrial planets are separated from the outer group by a wide gap. Within this space are to be found the asteroid or planetoid group. There are known to be over nine hundred and fifty of these minor bodies whose diameters range from five hundred miles for the largest to three or four miles for the smallest. There are only four asteroids whose diameters exceed one hundred miles and the majority have diameters of less than twenty miles. The total mass of the asteroids is much less than that of the smallest of the planets. It was believed at one time that these small bodies were fragments of a shattered planet, but this view is no longer held. The asteroids as well as the comets and meteors probably represent the material of the primitive solar nebula that was not swept up when the larger planets were formed.
