The universe is a dangerous place. Extremely energetic radiation can come from unexpected directions at almost any time.
In 1957 we knew that the earth had a magnetic field — that is the reason why compasses point to the north — but scarcely understood the implications. When the United States decided to launch Explorer 1 (after Sputnik and the Vanguard failure) a radiation detector was one of the principal instruments. Its goal was to measure cosmic rays to see if they were as prevalent above the atmosphere as predictions indicated.
After launch it appeared that the instrument periodically failed. After Explorer 3 was launched and much analysis, the investigators at Iowa State University discovered that the reason the instrument failed was that the earth’s magnetic field trapped high energy radiation in bands which came to be known as the Van Allen Belts after the leader of the team. These belts of trapped charged particles were basically unexpected. A serendipitous find.
If the Earth and Mars and Venus all formed about the same location in the solar system, they should have similar basic constituents. And so they do, except for oxygen on Earth (largely produced by plants) and water — abundant on Earth, lacking on Mars and Venus. What caused this? Both Mars and Venus have weak magnetic fields. Solar radiation strikes the top of their atmosphere basically unimpeded while the Earth’s magnetic fields deflect most of it away; only a fraction comes into contact with the atmosphere and mostly at the magnetic poles — we can see that interaction as the aurora. This radiation has the interesting property that it tends to split up molecules high in the atmosphere; H2O is broken down into hydrogen and oxygen and each constituent atom is energized to basically escape velocity. The water on Mars and Venus likely evaporated away.
We knew none of this in 1957. Some of it was suspected. How it played together has been puzzled out ever since.
The Earth has a planetary protection system. Water is the necessary ingredient for life. No strong magnetic field, no bands of trapped radiation, and your water leaves the planet. No water, no life.
It really isn’t all that simple, but that is close. Close enough for the blogosphere anyway.
Why does the Earth have such a strong magnetic field and Mars and Venus weak ones? Yet another unlooked for event. It has to do with the moon. Moon rocks are unlike almost any on earth, but in a family resemblance sort of way are like the rocks on the earth’s crust, quite different from what we think the composition of the earth’s mantle or core may be. And the crust of the Earth is considerably thinner than the crust of Mars or Venus. And the moon basically doesn’t have a mantle or molten core (or magnetic field for that matter). What is going on?
The best hypothesis — the scientific guess that best matches the available data — is that early in Earth’s history, a Mars sized planet collided with Earth and the two planets blurped together (that’s not a scientific term). Most of the heavy mantle and core type stuff stayed in what would become the Earth and most of the lighter crust type stuff got thrown off into space by the violence of the collision and subsequently coalesced into the Moon. All of this is backed up by computations, mathematical models, and lots of long scientific papers that you probably don’t want to read. And it took going to the moon and bringing back rocks and studying them in detail to make this clear.
The Earth’s large core and mantle generate the magnetic field. Venus, although almost the twin of Earth in total size and mass, does not have nearly as large a core and mantle. Poor little Mars, about half the size of Earth, with a thick crust and small core, is basically frozen. No magnetic field to speak of at either Mars or Earth.
So when you look at Earth, it really is unique among the rocky planets of the solar system. Makes you feel special doesn’t it?
We would never have known half this stuff if we hadn’t ventured off the planet, gone to the Moon, sent probes to Mars and Venus, and then thought really hard about what it means to have Van Allen Belts.
You may well ask, “so what?” This planetary protection is there and its natural and we can’t do anything about it.
Well, not exactly. But when mankind releases chemicals which punch out the ozone layer, we are affecting life on Earth in very significant ways. We stopped producing and releasing CFCs because of that damage. In the early 1960’s, the United States exploded an atomic bomb in the middle of the Van Allen Belts. Totally disrupted them for weeks. Now we have a treaty not to do that sort of thing again. Its good to know how your planetary protection system works so you don’t mess it up.
Seems like there should be a moral here . . . .
Oh yeah. Going into space has allowed us to find out things that we didn’t have any idea about — and which are really important to our continued survival.