As of this date, there are 171 known moons orbiting planets in the solar system, and over 200 additional moons orbiting asteroids or dwarf planets. Among the planets, Mercury and Venus have no moons, Earth has a single Moon, Mars has two very small moons. The vast majority of moons, therefore, orbit the outer gas giant planets: Jupiter with 66, Saturn with 62 (not counting the trillions of particles of ice forming its rings), Uranus 27, Neptune with 13. All of these numbers are certainly going to be increased in the years ahead as our ability to discover smaller and fainter objects improves. For example, the moons of Pluto have been increased from 3 to 5 over the past year, with the latest discovery announced only a few weeks ago.
In orbit around Jupiter are the Gallilean moons Io, Europa, Ganymede and Callisto — each large enough to be easily visible by eye from Earth, though they cannot be seen because of the intense glare coming from Jupiter itself. About Saturn we find the extremely fascinating moon Titan (discussed in an earlier post), larger than the planet Mercury, and 5 other moons of significant size. Neptune has the moon Triton, about ¾ the size of our moon. Uranus has 4 somewhat large moons. The remainder of the set of planetary moons — with the exception of Earth’s moon — are small objects, a few hundred miles to a few miles in diameter.
Most of the planetary moons in the solar system have been observed from spacecraft we have sent to visit their host planets. Based upon these direct observations, which allow us to understand the chemical makeup of most of the moons, as well as a measurement of their shapes and sizes with great accuracy, we find that the vast majority of them bear very little resemblance to the planets they orbit. This strongly suggests that these objects formed independently in the early solar system, and were later captured by the gravity of the giant planets as they passed near those planets in their original orbits.
In fact, a large number of the smaller moons of Jupiter and Saturn very closely resemble comets. In modern history we have seen the effects of the gravitational pull of Jupiter alter the course of comets visiting the inner solar system. In July of 1994, we witnessed the comet Shoemaker-Levy 9 break up into 21 separate pieces and crash into the atmosphere of Jupiter over a period of 6 days. The evidence overwhelmingly suggests that most of the outer planet moons are captured comets.
Among the smaller moons, those that are not captured comets are likely captured asteroids. While a comet consists almost entirely of frozen gas and water, an asteroid is basically a rock. The two moons of Mars — both less than 15 miles in diameter — are identical in form and composition to common asteroids. As Mars lies just inside the orbital band where asteroids are commonly found, it is strongly believable that these small moons are captured asteroids.
The larger moons in the solar system tend to lie in orbits very close to the equator of their planets, and orbit in the same direction as the planet turns. These facts suggest that these large moons formed from the same dust and gas clouds that collapsed to form the planet they orbit.
There is one moon in the solar system that stands out as quite unique from the others. While the other moons orbiting planets are very small in size compared to their host planets (Titan is huge, but Saturn is a mammoth planet), the Moon is about 27% the diameter of the Earth, or holds a volume about 2% that of Earth. In comparison, the ratio of Titan to Saturn’s volume is 0.008%. The Moon is also highly unusual as moons go, in that it is composed of material very similar to Earth — it has a solid iron core surrounded by a molten layer of nickel-iron, within a vast mantle and crust of compounds commonly found on Earth.
Put together, these observations indicate that it is nearly impossible for Earth to have captured the Moon as a stray body wandering through the solar system. The Moon therefore could be assumed to have formed along with the Earth, just as we suspect the other large moons in the solar system were formed. There is, however, at least one major problem with this explanation.
The side of the Moon facing Earth contains large grey areas, known as Maria, or “Seas." The far side of the Moon is almost entirely lacking in these features. The Seas formed nearly 4 billion years ago as liquid magma burst through the thin crust of the Moon when it was hit by large meteors. The far side of the Moon was similarly bombarded, but the crust was much thicker on that side, and no Seas were formed. The very lopsided thickness of the crust of the Moon in its early age defies explanation if the Moon and Earth were formed through the simple collapse of a cloud of rock and gas.
Instead, the leading current hypothesis for the formation of our Moon is a much more violent birth story. It is proposed that a proto-Earth, of slightly less mass than our current planet, was struck by a planet the size and mass of Mars about 4.52 billion years ago, approximately 20-40 million years after the solar system initially formed.
The collision force was immense, enough energy was released to completely melt and vaporize most of the Earth, and nearly the entire impacting planet. In the aftermath, two bodies formed, orbiting very close to one another. Stray material from the collision rained down on both objects, creating deep craters which eventually filled with water on Earth, and permanently scarring the waterless Moon.
As the molten center of the Moon gradually cooled, the much stronger gravitational pull of the Earth created a strong “tide” in the molten mass continually bulging toward the Earth, resulting in a thinner crust on the Earth-facing side of the Moon. Models of the collision further explain the relatively small iron core of the Moon compared to Earth’s core, as well as giving very good agreement with the relative abundance of various minerals found on the lunar surface.
If you have children who are fascinated by the science behind our knowledge of the stars and planets, I welcome you to join our upcoming astronomy class sessions. Our next session starts in August and runs through late December, and we still have a limited number of spaces open. Please visit www.turnerclasses.com for more information.