What would happen to Earth if the moon were only half as massive?

Neil F. Comins, author of What If the Moon Didn't Exist? and professor of physics and astronomy at the University of Maine, cuts this mystery down to size:

A less massive moon would be closer to Earth, for starters: the tidal forces that slowly widen the moon's orbit around Earth would be curtailed. There would be profound effects on our planet and its denizens as well.

The real moon orbits at an average distance of around 238,600 miles, but every year it drifts about 1.6 inches farther away. The cause? Ocean tides. The moon's gravity, combined with the waltz of Earth and the moon around a common center of mass, forces the oceans into an oval shape, with two simultaneous high tides. One high tide is on the side of Earth facing the moon; the other is on the opposite side of our world. Because Earth spins so rapidly compared with the moon's orbit, our planet drags the high tide closest to the moon a little bit ahead of it. The gravitational pull of the water imparts energy to the trailing moon, spiraling it a little farther outward with every orbit.

If the moon were half its mass, the tides would be correspondingly smaller and would impart less orbit-expanding energy to the moon. Even though a half-mass moon would require less energy to repel, the tides would be so weakened as to nonetheless bring about a closer moon.

The energy given to the moon comes from Earth's rotation, and our planet is slowing down as a result—in other words, days are getting longer. Geologists believe that an Earth day was originally five or six hours long. If the moon were less massive, thereby creating less drag on Earth, our planet would not have slowed as much. The day would be perhaps 15 hours long.

Weaker tides also would not have eroded Earth's landmasses so drastically over the past few billion years. Less soil leaching into the ocean might have had profound effects on the origin of life: some organic (carbon-based) compounds thought to have seeded life may not have reached the primordial soup of the early oceans.

Assuming that life had still arisen, it would have had to contend with more frequent ice ages and more extreme warm snaps, because large moons act to stabilize planets. Mars, whose two tiny moons combined are millions of times less massive than our full-size moon, wobbles dramatically on its axis and so endures bigger climatic swings and seasonal temperature changes than Earth does.

The outlook for life would have been dim—literally. A smaller moon means less moonlight, which means darker nighttimes. Whatever life-forms did evolve on this altered Earth would have had to develop bigger or more sensitive eyes to help them navigate, forage and spawn at night under the diminished glow.

Why don't we get more drinking water from desalinating the ocean?

Peter H. Gleick, president of the Pacific Institute, a nonprofit environmental and water policy think tank based in Oakland, Calif., distills an answer:

The desalination of water requires a lot of energy and, hence, money. The price varies widely from place to place, ranging from just under a dollar up to several dollars to produce a cubic meter (264 gallons) of desalted water, and efforts to reduce the energy requirements have not kept pace with rising energy costs. The cost of drawing freshwater from a river or aquifer is much lower—about 10 to 20 cents per cubic meter—and farmers often pay even less. As a result, desalination currently satisfies less than half a percent of human water needs.

Desalination carries environmental costs as well: seawater intakes can suck up small ocean creatures, upsetting the food chain, and the process's leftover brine is so strong that its return to the ocean can prove harmful to coastal ecosystems.

Nevertheless, desalination's appeal is growing as other sources of water disappear and the price gap closes. Finding a new source of freshwater or building a dam in a place such as California, for instance, can drive costs up to 60 cents per cubic meter. Far more must be done to use water more efficiently, but with the world's population swelling and the water supply dwindling, the economic tide may soon turn in favor of desalination.

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