Our Moonstruck Planet

Next time the conversation gets boring, try this ploy: "Alaska just wouldn't be Alaska without the moon. Imagine--no earthquakes, no volcanoes, no auroras. Probably no moose, bears, or even people either." Your listeners may speculate on the lunar root of the word "lunatic," but you will have their attention. Best of all, you'll be absolutely right.

At least, that is the opinion of Jerome Pearson, who presented many theories of how Earth came to be the kind of planet it is in an article for the British journal New Scientist. When Pearson puts them all together, the theories give the moon a lot of the credit--or the blame.

Take earthquakes. It's the heat of our planet's core that provides energy to drive volcanoes and shift continental plates, grow mountains and widen oceans. All that activity is accompanied by earthquakes. Given its fairly small size, as planets go, and relatively sedate pace of rotation, it is odd that Earth has so warm a heart.

The moon may have caused that. Earth and its moon are more like a double planet than a planet-satellite combination, going by the size of the moons of the other planets in the solar system. Exactly how and when this pairing arose is still the subject of lively dispute among scientists, but Pearson favors the view that the earth captured its moon by a grazing collision--a kind of cosmic fender-bender instead of a smashup--when both were barely formed. Thus he can go on to speculate that for a billion years after the solar system began, the moon and the earth were very close together.

With all that mass in the immediate neighborhood, the gravitational forces were ferocious. The young planets raised enormous tides on one another, and the friction generated by these rising and falling bulges of material raised their temperatures far higher than normal for their sizes and rates of rotation. This heating went on until the violent tidal action pushed the moon out to near its present location and slowed the earth's rotation period from close to 4 hours to 24 hours.

The moon's relationship to the aurora is trickier to explain, but also relates to the high heat levels of the earth's core. The explanation requires a few steps.

Auroras begin with the sun. Our star spits out vast quantities of electrons and protons, which stream away as the solar wind. When these particles are captured by Earth's magnetic field, they are pulled into the atmosphere along the magnetic field lines. The molecules of atmospheric gases glow with auroral light when they are struck by the cascading particles.

So, to have auroras, a planet needs a strong magnetic field. The planets in our solar system possess magnetic fields that relate nicely to their rotation and their distance from the sun--except for Earth. Our magnetic field is 100 times stronger than it should be, because the earth's core is so hot. The earth does not follow the magnetic pattern of the other planets because it was heated in the process of slowing down, and that we might blame on the moon.

As for moose, bears, and people--Pearson reports that for life to exist on its surface, a planet may need to have a hot core. The ensuing strong magnetic field offers protection from potentially deadly cosmic radiation; the shifting, moving crust releases gases from the interior, recycles surface material, and provides changing habitats to encourage evolution. Score another point for the moon.

Furthermore, the tides raised by the moon on Earth's oceans may have been important for letting life come ashore. Tidewater zones, alternating between flooding and drying out, perhaps made it easy for early plants and animals to emerge on land.

If you convey Pearson's points firmly, you should convince all your listeners of the moon's importance--unless, of course, there's an expert in your crowd. I ran into that problem quickly here at the Geophysical Institute.

"He's treading on thin ice in a number of his arguments," said David Stone, a professor of geology who has given lectures on lunar subjects. "One involves the time at which the moon was in a low orbit. There is no evidence in the geologic record of the huge ocean tides that would be expected. In fact, there is circumstantial evidence for very small tides." He then went on to tell me quite a bit more than I wanted to know about the contradictory theories of how the moon formed.

Scientists may not be certain of how the moon formed, but they do know that without it, Earth would be a very different plant indeed.