Читать книгу Exploring the Solar System - Peter Bond - Страница 27

In the outer reaches of the solar nebula, temperatures were low enough for ices to form. Indeed, it seems that ice particles were much more abundant than silicate dust particles. This being the case, any planetesimals born in the frigid outer zone would have resembled icy dirt balls, much like the comets we see today. However, the main constituents of Jupiter and Saturn are hydrogen and helium, rather than water. Since temperatures in the nebula would have been too warm for these gases to condense, accretion of hydrogen and helium snowflakes cannot have occurred. Another explanation must be found.

There seem to be two possibilities. Studies of gas giant interiors suggest that Jupiter and Saturn may possess rocky cores at least as large as the Earth. It may be, therefore, that the early stages of growth of these planets resembled the accretion taking place in the inner Solar System, with the growth of massive nuclei of ice and dust. Once these became sufficiently large, about five to 15 times the mass of Earth, they were able to attract and hold onto even the lightest gases in the surrounding solar nebula. As their mass and gravitational grasp grew, their spheres became ever more bloated.

Alternatively, they could simply have developed as the result of large‐scale gravitational instabilities in the solar nebula. Since the disk in the outer reaches contained both dust and condensed ices, there was plenty of raw material for large planets to develop and grow.