Moons are the constant companions to the planets of our solar system, and it’s likely they also exist elsewhere in the universe.
They can be large or small and are often oddly shaped. They are covered with ice or dotted with volcanoes, and they circle all of the large planets in our solar system. For scientists, moons are historical records that provide a wealth of information about the origins of the universe—and all of the life in it.
Have you ever wondered why the Moon keeps a fairly stable distance from the Earth (average distance: 238,855 miles), has been regularly cycling through its phases for some 4.5 billion years, and directs the rhythm of the rising and falling tides as if it were following the dictates of a tide chart (instead of the other way around)? Could it be that the Moon— with a diameter of 2,159 miles, which makes it the fifth-largest satellite in the solar system—will someday seek to wander farther afield? To move on? To decide to ally itself with a bigger and perhaps better planet than Earth? It’s not unthinkable. Considering that our Moon is the planetary satellite closest to the Sun, why haven’t the immense gravitational forces of our huge star not drawn in and devoured the gray chunk of rock in our night sky? Or forced it into a solar orbit the way it has the planets? As is so often the case, the answer is physics. Earth possesses its own range of influence in which terrestrial attraction prevails. That is the Earth’s Hill sphere, named for the American astronomer and mathematician George William Hill. The principle: Every heavenly body creates its own sphere of attraction. If one heavenly body lies within the Hill sphere of a larger one, then it will revolve around the larger orb.
The range of a planet’s Hill sphere depends largely on its distance from the Sun: The greater the distance, the weaker the Sun’s influence and thus the larger the planet’s Hill sphere. But it also depends on the mass of the object: A huge planet like Jupiter, for example, has a much bigger Hill sphere than one the size of Mars. These are the reasons Jupiter has so many moons (79 are known so far). The Earth’s Hill sphere has a reach of about 930,000 miles—putting the Moon well within it—and things are expected to remain that way for the foreseeable future. The same applies to the 180+ other moons and their mother planets. The number is vague because new moons continue to be discovered. And the various moons of our solar system couldn’t be more different from one another.
MERCURY AND VENUS ARE THE ONLY SOLO ACTS
Naked rubble, volcanic hellfire, or a potentially life-bearing environment: The moons of our solar system offer a little bit of something for everyone. If people someday settle on Mars, they will see two moons in the sky, Phobos and Deimos, which were probably asteroids before the Red Planet captured them. The larger of the two small moons, Phobos, orbits dangerously close to Mars and gets closer to its mother planet at a rate of 6 feet every 100 years. In 50 million years it will either crash into Mars or break up into a ring around it.
Beyond Mars, the moons become extremely varied: The largest planet, Jupiter, has 79 confirmed moons, the ringed planet Saturn has 62, Uranus has 27, and Neptune has 14. This uneven distribution is due in part to the gigantic gravitational fields of the gas giants Jupiter (Hill sphere: 33.55 million miles) and Saturn (41 million).
In addition, there is also the matter of having enough elbow room: The solar orbits of the smaller planets— Mercury, Venus, Earth, and Mars—are too close together to provide stable orbits for additional moons. The gas giants have a lot more space between them. The smallest of the gas giants, Neptune, has the added benefit of its great distance from the Sun (average: 2.8 billion miles), giving it a Hill sphere of 72.7 million miles—a record among all the planets.
CHUNK OF DEBRIS OR CAPTURED ASTEROID?
Before NASA’S Apollo missions had brought Moon rocks back to Earth, there were a number of hypotheses as to how our Moon was formed. The capture theory suggested that it was a wandering body that had formed elsewhere and was captured by the Earth’s gravity when it had passed into our Hill sphere. According to the accretion theory, both the Earth and the Moon were created at the same time billions of years ago. The fission theory suggests that the Earth was spinning so fast that the centrifugal force hurled a substantial amount of material away from it, and the Earth’s gravity had pulled it into orbit around itself. The giant-impact hypothesis is currently favored: In the early days of our solar system, an object the size of Mars crashed into the young Earth. The debris then collected in an orbit around the planet to form the Moon. Dwarf planet Pluto and its satellite Charon are believed to have been created in the same way.
The moons in orbit around the gas giants include Ganymede, Europa, and Callisto (Jupiter), Rhea, Dione, and Enceladus (Saturn), and Titania (Uranus). Most of them have their own atmospheres, composed primarily of the gases oxygen, methane, nitrogen, or carbon monoxide. Saturn’s largest moon, Titan, even has clouds and a dense atmosphere that’s similar to that of a planet. Scientists think the conditions there may resemble those that had prevailed on the Earth during the very early years of our planet. The plumes of the geysers on Enceladus rise more than 100 miles high. When they escape the moon’s gravitational pull, they feed the E-ring of Saturn, adding about 200 pounds of material each second. Even more importantly, Enceladus could actually support life: The Cassini space research mission has discovered that the vapor plume that vents from Enceladus’s south pole contains hydrocarbons, and the moon also has a salty ocean with hydrothermal vents on its seafloor— further indications of the possibility of life. Jupiter’s moon Europa has a vast deep ocean beneath an ice shell up to 15 miles thick and is considered to be the most promising place of all to search for extraterrestrial life in our solar system.
So if some moons are the size of small planets, could a moon have a moon of its own? In theory, yes. The question harks back to the concept of a Hill sphere. Earth’s Moon has its own sphere of gravitational influence that extends about 37,000 miles in all directions. That’s around one-sixth of the Moon’s distance from the Earth. Considering how often fragments of space debris whiz through this zone, one might think the Moon would have captured satellites of its own by now. However Earth’s Hill sphere extends some 930,000 miles into space, thus totally overlapping and dominating the Moon’s Hill sphere and exerting far greater attraction. So the Moon has remained moonless, until fairly recently. Not content to leave well enough alone, NASA has artificially created a moon to circle our satellite: The Lunar Reconnaissance Orbiter was launched in 2009 and has been orbiting the Moon ever since. Having already returned more data than all other planetary missions combined, it is still going strong.