No. 87: May-Jun 1993
In SF#85, we learned that the evolution of advanced life forms on earth may have depended upon the protective influences of Jupiter and Saturn. These two giant planets can gravitationally deflect potentially devastating asteroids and comets away from the earth.
It seems now that we are doubly lucky! Computer runs demonstrate that the presence of our large moon has stabilized the earth's spin axis down the eons. Presently, the earth's spin axis makes an angle of 23.5° with the plane of the earth's orbit (its "obliquity"). The well-known result is our yearly procession of seasons. Without the steadying effect of the moon, however, the earth's obliquity would probably have swung chaotically over much larger values. Such extreme changes would have been inimical to the development of life, particularly advanced life.
As a case in point, the polar axis of Mars, with only two tiny moons to dampen its spin excursions, seems to have gone through many wild swings, as indicated in the figure. What deadly climatic changes must have wracked our sister planet!
(Touma, Jihad, and Wisdom, Jack; "The Chaotic Obliquity of Mars," Science, 259: 1294, 1993. Also: Laskar, J., and Robutel, P.; "The Chaotic Obliquity of the Planets," Nature, 361:608, 1993.)
Comment. The successful evolution of higher life on earth (a presumption!) therefore seems to have depended upon the gravitational shields of Jupiter and Saturn as well as the presence of our unusually large satellite. How likely is this combination of planets and satellites in the rest of the universe? Of course, life-as-we-know-it also requires just the right kind of central star and a planet with good air and water. Perhaps lifeas-we-do-not-know-it is more likely!