National Geographic : 1985 Jan
Each time this happened, Earth reassem bled, a bit bigger than before. Our moon may be a relic of the time when Earth grew big enough to hold its own against the planetesimal assault. Increas ingly many scientists suspect that when Earth was only a few tens of millions of years old, an object perhaps as large as Mars struck our accreting planet at more than 35,000 kilometers an hour. The planet sur vived, although much vapor and molten rock were ejected into space. Some of this ejecta would have coalesced into the moon. FROM MAUNA KEA my unaided eyes cannot see Uranus, Neptune, or Pluto this morning. They are so far away that even with large telescopes on other mountaintops I have found them unremark able spots of light. NASA's Infrared Tele scope Facility (IRTF) on Mauna Kea has become our primary source of knowledge about these outer bodies, which we are find ing riddled with primordial mysteries. Uranus and Neptune, for instance, have rocky cores about the same size as those in the two gas giants, Jupiter and Saturn. Yet the outer planets are much smaller, only about four times the diameter of Earth. Why did they end up with so much less gas? Uranus, moreover, is lying on its side, ro tating around horizontal rather than verti cal poles. Many astronomers suspect that a body the size of Earth once struck the big planet and knocked it askew. Neptune also shows signs of catastrophe.