What If the Earth Had Two Moons? (Excerpt)

Excited about the possibilities of an Earth that might have had two moons? Enormous tides, a hell hanging in the sky, and eventual collision!

Keep reading! We’ve got excerpted selections from recent non-fiction release What If the Earth Had Two Moons? by Neil F. Comins, which explains how it could happen and what the planet would be like.

The Earthlike planet in this chapter, which we’ll call Dimaan, begins its life identical to the early Earth in size, composition, and distance from the Sun. Based on geological and fossil evidence, the Earth was initially spinning much faster than it is today. Although that rate is not yet known, I give Dimaan a plausible eight-hour day when it first formed. Neither Earth nor Dimaan had a moon at first. Ours came into existence within about 200 million years of the Earth’s forming.

Moons can form in four ways: from impacts, in which the planet is struck and thereby ejects debris that becomes one or more moons; simultaneously with a planet, in which the moons and planet condense together; by fission, wherein the moons are literally thrown off a rapidly rotating planet; and by capture of the moons after the planet has formed.

Most astronomers believe that our Moon formed as the result of a collision between Earth and a Mars-sized body. The intruder hit Earth at an angle that ejected debris into orbit in the same general direction in which our planet was spinning. This rubble formed a short-lived ring that was much smaller but, interestingly, much more massive than all of Saturn’s rings combined. As this material orbited, it began colliding with itself and bunching together under the influence of its own gravitational attraction until it coalesced into the Moon. This is how I posit Dimaan’s first moon, which we’ll call Kuu, formed.

Although it is entirely possible for an impact of a small planet onto a larger one to splash enough debris into orbit to form two moons similar to ours, such moons would drift together and collide billions of years before advanced evolved on Dimaan. Because I want that second moon around for people to enjoy, I posit that Dimaan captures its second moon long after the first one formed.


The Capture

Four effects contribute to the capture of the second moon, which we’ll call Lluna: most important is the face that Lluna’s companon feels a slightly different gravitational attraction from Dimaan and Kuu than does Lluna itself. This occurs bceause as Lluna and the companion approach Dimaan, these two intruders are at slightly different distances from the planet and its original moon. Therefore they feel different amounts of gravitational force from them. This difference can be enough to pull the companion free of Lluna and fling it away, taking with it a substantial amount of energy, which has the effect of slowing Lluna down, making it possible for the final three effects to complete its capture.

Upon approaching the Dimaan-Kuu system, Lluna’s gravitational attraction pulls on the moon Kuu, causing its orbit to become more elongated (more elliptical). Moving Kuu causes Lluna to lose energy. At the same time, Lluna creates tides on the planet Dimaan that pull back on Lluna, slowing it down further. Finally, the gravitational pull of Dimaan on Lluna coupled with the planet’s orbit around the Sun cause Lluna to lose even more energy. In this final process, energy is taken from Lluna and given to Dimaan. The combination of all these effects can remove enough energy from Lluna for it to become locked in orbit around Dimaan.

I set Lluna’s intial orbit around Dimaan to be half Kuu’s distance from the planet, with both moons orbiting in the same direction and in about the same plane that our Moon orbits Earth. This leads to eclipses related to both Kuu and Lluna. Virtually all objects in the solar system have elliptical orbits (egg-shaped), however, most of these are very close to circular. Lluna and Kuu will initially have more elliptical orbits than any other moons because the capture of Lluna was so messy.

It will take roughly two weeks from the time that Lluna is first close enough to generate noticeable tides on Dimaan until this moon is securely in orbit. During that transient period, all hell breaks loose on the planet. Lluna’s gravitational pull creates tides on Dimaan eight times higher than those from Kuu. While Lluna is settling into orbit, it will also create monster tidal waves on Dimaan that will make any tsunamis that we have on on Earth seem like tiny ripples in comparison. The water will slosh like the waves created in a large pan filled with water as you carry it from the sink to the stove.

These tidal waves and the tidal bulges generated by Lluna will create Dimaanquakes and increased volcanic activity that will persist for years. The dust released by the volcanic emissions will darken the skies and cool the atmosphere dramatically. The volcanies active during this time will also release vast volumes of water vapor, carbon dioxide, sulfur dioxide, carbon monoxide, stinky hydrogen sulfide, and hydrochloric acid, among other gases. All of this activity will cause a mass extinction in the ocean life of Dimaan.


Life With Lluna

Lluna capture and the damage to Dimaan and life on it in the process don’t mean that the planet will thereafter be lifeless. Life on Earth has experienced over half a dozen similarly catastrophic mass extinctions, episodes caused by geological and astronomical events during which large fractions of all life on our planet were eradicated. Perhaps the most dramatic of these events, the Permian-Triassic extinction, occurred 251 million years ago. It wiped out over ninety-five percent of all species of life. Nevertheless, the remaining life-forms grew, diversified, and became the progenitors of the life on Earth today. What Lluna’s presence does mean is that the sequence of evolutionary events on Dimaan would be profoundly different from what occurred here on Earth or that would occur on Dimaan had Lluna not appeared on the scene. Let’s explore some of the differences that would result.



At half the distance, Lluna will have twice the diameter as does Kuu in Dimaan’s sky (or does our Moon in our sky). Twice the diameter means that the area Lluna covers in Dimaan’s sky will be four times greater than that of Kuu. Because moonlight is sunlight scattered from the surface of a moon, Lluna will be four times as bright on Dimaan as is Kuu. (This is assuming their surfaces are made of the same materials as the Earth and the Moon.) Combining the light from both moons, nighttime on Dimaan when both moons are full will be five times brighter than the nighttime surface of the Earth under a full Moon. It would be quite easy to read a book under those conditions.

Lluna and Kuu orbit Dimaan at different speeds, therefore it is more likely that at least one of the moons is up at night than it is for us with our single Moon. When a moon is high in the sky at nght it is at least half full (technically the moon is in either a gibbous or full phase). Therefore, Dimaan will have more nights brightly lit with moonlight than does the Earth.

In what follows, let’s assume that the sensory equipment available to life on Dimaan is the same as on Earth. That means people there will evolve seven senses: touch, taste, smell, sound, sight, heat, and gravity. The last two of these are often left off lists of senses taught to children, but we have them nevertheless. Sensitivity to heat is straightforward: put your hand near a fire and you know that it is hotter than its surroundings. Sensitivity to gravity is our ability to know our posture and to sense when we are falling.

Because it will be easier for predators to see their prey at night on Dimaan, camouflage will be more highly refined than it is on Earth. This, in turn, will require more acute hunting skills using sight,  sound, smell, and heat detection for animals that are active at night. The cycle of protection and detection driven by the brighter nights on Dimaan could well lead to creatures that are more aware of their surroundings than early land animals were on Earth. This, in turn, is likely to increase various aspects of intelligence in these creatures compared to what was necessary for survival here. Perhaps the first sentient creatures on Dimaan will evolve from nocturnal hunters rather than from arboreal creatures, as occurred on Earth.


Tides and Shorelines

With a few notable exceptions, such as Swansea, Wales, and Mont Saint Michel, France, where the ocean bottom slopes downward especially slowly, beaches on Earth tend to vary in width by a few hundred feet or less between high and low tide. Although tidal erosion is significant, it usually takes place over periods of decades or longer, giving people who live or work on the shore time to develop defenses against the changing landscape. The presence of Lluna will make both the range of tides and the speed at which they erode the shoreline of Dimaan much greater.

As noted earlier, Lluna creates tides 8 times higher than Kuu. Combining their tidal effects with that of the Sun leads to tides on Dimaan that are as much as 6 and one third times greater than the range of tides on Earth today. This occurs when Dimaan, Lluna, Kuu, and the sun are in a straight line. Therefore, the typical intertidal region (the area that is exposed at low tide and hidden at high tide) on Dimaan will be much more extensive than it is on Earth. Because more tidal water is flowing on Dimaan each day, the amount of erosion of the shoreline there will be much greater than it is here. The shore would therefore wear away and expose coastal buildings to damage more rapidly than occurs on Earth.

Cities built near the mouths of rivers, such as New York, San Francisco, New Orleans, or even London (as far upriver on the Thames as it is), would experience unnacceptable erosion problems due to the tides and tidal bores generated on Dimaan. Shorelines would erode so quickly that without heroic effort, such as thick concrete walls lining the rivers, cities could not be built on most ocean shores or on especially active rivers.


Volcanoes on Lluna

By far the most spectacular thing about Lluna’s presence at the time people exist on Dimaan will be the moon’s active volcanoes. Their existence is analogous to the volcanoes that occur on Jupiter’s moon Io today. Recall that because of the way it was captured, Lluna’s orbit around Dimaan is not especially circular. Eventually, its orbit becomes more so, but Kuu prevents it from ever being perfectly circular: when Lluna is between the planet and the outer moon, Dimaan pulls it in one direction, and Kuu pulls it in the opposite direction. As a result, Lluna is pulled into an orbit slightly farther away from Dimaan than when Lluna is on the opposite side of the planet. In the latter position, both the planet and the other moon are pulling it towards Dimaan and so Lluna is then closer to the planet than it would be if Kuu were not there.

The result of the noncircular orbit is that when Lluna is closer to Dimaan, the land tides on the moon are higher than when Lluna is farther away. Seen from afar, it would look as if Lluna were breathing as it orbits Dimaan. This change in height of the land creates the same friction that kept it molten before it was in synchronous rotation. As a result, the inside of Lluna will be molten throughout its existence in orbit around Dimaan and that magma will continually be leaking out through volcanoes and cracks in its surface.

Lluna is going to present a spectacular sight from Dimaan. Peppered with huge volcanoes, Lluna will be a world alive with red-hot lava being ejected in several places at once. Some of these events will be seen along the edge of the moon, like mammoth mushroom-shaped fountains leaping miles into the air and then crashing silently back down. These are analogous to stratovolcanoes on Earth. Other features on Lluna will include volcanoes that ooze lava, and rivers of lava that flow and, upon cooling, freeze into place. Although that would be very romantic today, I can imagine that prescientific civilizations on Dimaan would create a wide variety of mythical scenarious based on it. Hell, yes.


The Collision Between Lluna and Kuu

Lluna and Kuu are destined to collide. Our Moon spirals away from the Earth. Kuu spirals away from Dimaan. After Lluna forms, the tides it creates on Dimaan will act back on it, forcing it, too, to spiral outward. Calculations reveal that after Lluna is captured, it recedes from Dimaan faster than does Kuu. Just as the recession of our Moon slows the Earth’s rotation and Kuu’s recession slows Dimaan’s rotation, the recession of Lluna will make Dimaan’s day even longer. As Lluna closes in on Kuu, the day on Dimaan will exceed twenty-eight hours.

The collision will take place tens of thousands of miles farther from Dimaan than our Moon is from the Earth. As seen from Dimaan, Lluna will approach Kuu from behind. In their final days, the gravitational forces that each moon exerts on the other will cause the two to become more and more egg-shaped as land tides miles high form on them. This will cause Lluna to crack open, allowing its molten interior to pour out and cover its surface in glowing lava. Kuu, heated by friction as it distorts, will also have an outpouring of molten rock. Then the worlds will kiss.

Inexorably Kuu and Lluna will come together. The collision, however, will not be even remotely as horrendous as the impact on Domaan that created Kuu or the one on Earth that created our Moon. The intruders in these latter impacts were moving much more rapidly, compared to the planets they struck, than Kuu and Lluna will be moving when they collide. People on the side of Dimaan facing their moons can prepare a jug of their favorite libation, set up a comfy chair, and watch the first phases of the event over a period of hours, as they might watch a science fiction movie at home. Only this event will be real and its consequences deadly.

After the kiss, rings of crushed rock will fly off the moons from the regions where they come into contact. At the same time, the sides of the moons opposite the impact site will erupt with lava shooting out as their liquid interiors collide and bounce away from each other. By the time a quarter of their mass has come into contact, both moons will begin to break apart. Seen from Dimaan, they will appear to explode in slow motion. For many hours the sky will be filled with bright red lights from the impact region and wherever else molten rock is emerging.

Debris from the impact will fly in all directions, most notably perpendicular to the direction that the two bodies were moving when they struck. Put in blunt terms, a lot of stuff is going to fly toward Dimaan. The devil of this impact is in the details (such as the relative speed between the moons, their internal temperatures, and their chemical compositions), but some of their debris, including chunks big enough to create craters miles across, will drift toward the planet for several days and then fall into Dimaan’s atmosphere.

Fortunately, this event is going to occur long after advanced civilizations have been established on Dimaan. They are likely by then to have technology and techniques (Hollywood, take note) to prevent impacts of collision debris that would be able to save the civilizations they had established on the two moons before the collision is another question altogether.

The two moons will eventually become one. Following the impact, debris that wasn’t blown completely out of orbit would form a significant ring around Dimaan. Within a few years, the densest part of the ring would clump together due to its own gravitational attraction and due to relatively slow collisions between its pieces. A single body would form, growing as it collected more and more of what used to be Lluna and Kuu. Eventually this new moon would absorb the remaining ring debris and a new era in the life of Dimaan would begin.


What If the Earth Had Two Moons? Copyright © 2010 Neil F. Comins


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