Computer models of the formation of the Solar System show that the giant planets may not have formed in their current location, and could have migrated substantially in the early Solar System. A key point seems to be the gravitational influence of Saturn on Jupiter, especially the ratio of their orbital periods. In one set of models, when Saturn’s orbital period became twice that of Jupiter’s, their respective orbits became more elongated. The result was an outward migration of Uranus and Neptune, whose orbits grew larger, and they actually may have switched places. This shuffling cleared away nearby planetesimals, sending some to the inner Solar System, creating the late heavy bombardment (Figure 10.24). In another set of models, Jupiter migrated inward to 1.5 AU—the current orbital distance of Mars. When Saturn migrated inward even faster to a point at which its orbital period was 1.5 times that of Jupiter’s, they then both migrated outward, pushing Uranus and Neptune into larger orbits. Some of these computer models could explain the lower mass of Mars (at 1.5 AU) and the distribution of material in the small bodies of the Solar System.