On the previous page the comparison between the Earth and Pluto (2,274 km) was addressed by the planets both having large companion moons. It is believed the Charon (1,212 km) itself was created by a larger body striking Pluto, similar to the formation of our own moon (fig. 1). The theory of Earth having a second moon could also be drawn from the recent discovery of two additional moons around Pluto. What this tell us is that a planetary collision, followed by an accretion of debris into a single moon does not totally discount the possibility of the planet having additional moons. Although these two moons around Pluto are most likely captured bodies from the Kuiper Belt - perhaps Earth itself captured either a rogue moon of one from an orbit the crossed Earth's orbit several hundred million years ago. We have seen earlier how Pluto's orbit takes it inside that of Neptune's, and additional, where did the Mars size object that struck the Earth four billion years ago come from. If it can happen once, then it could easily be considered possible to happen twice.

  I have selected Uranus's moon Miranda (470 km) as a captured moon because of its size, mass and uncompleted composition due to failed accretion (which is covered in another page).

  The test of any theory is its ability to stand up to the challenge by making sense physically. A friend, Mitchell Porter has raised the following issue. "It's over 300 million years from the first to the last of the mass extinctions you list [in an email], which sounds like a long time for fragments of the lost moon to hang around in orbit."  That is absolutely a challenge indeed to explain.

  This could depend on the bodies actual density; being the quantity of matter, 'its mass divided by its volume'. Density is related to how compressed the material is and the weight of its atoms or molecules per unit volume. As an example, water has a greater density than steam because the atoms in a liquid are closer together than those in a gas. Iron is denser than ice, because the iron atoms are heavier than the water molecules, even though they both are solids.

  Looking at the graph below, one can see the density of Miranda, the Moon and the Earth. It is interesting to see that Miranda is quite less then the Moon in density. If will took both moons and orbited them around a planet with the mass, density and bulk size of Earth, a moon like Miranda would be quickly pulled apart and fall progressively into a decaying orbit. This would mean that there could simply be no 500 hundred million year, periodic impacts from this debris to explain the extinction cycles.

  If we look at Mars (3,397.2 km) , we can see that the small moon Phobos (13.4x9.2 km), now orbiting the planet at only 6,050 km - will eventually crash into the planets surface within 70 million years. Phobos's currently decaying orbit is well within the distance we find between that of the Earth and our Moon. In fact, it would be awfully close. Allowing a 1.3 density of our second moon, it would off had to been outside the current orbit of our moon (even allowing for the moons closer orbit back then).

  This second moon would have to have been orbiting in an elliptical orbit similar to that of Pluto and Neptune and to that of the object that struck the Earth, the debris forming the moon. On periodic passes of the Earth, the moon may have been pulled apart over millions of years by passing tidal forces between the Moon and the larger Earth. If the second moon had been broken up, this could account for larger sections of this body not only impacting on the Earth, but also the moon. The smaller debris could have also formed a minor ring system around the Earth, additional accounts for cratering on the darkside of the moon. (Next: The Impact Extinction's)