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The Late Heavy Bombardment
A Perfect Balance > One of the awe-inspiring things we wannabe astronomers look at through our telescopes is the Moon. When our satellite is around its first quarter phase, the time when the sun's light strikes it at an oblique angle, its craters seem so real and well-defined that you'd believe they could reach through your scope and poke you in the eye.
Of course those brighter, crater-filled areas have obviously taken a beating. They are the "highlands" of the Moon. But what about those darker areas we see? What about the "maria"?
Turns out they got hit even harder. Through a telescope they appear relatively smooth and featureless, but it is precisely those characteristics that lead us to believe that those parts of the Moon suffered severely.
And that we must have suffered, too.
Those dark maria are the lava-filled basins from monstrous impacts that occurred about 3.9 billion years ago. These were no cute little rocks that struck our little buddy. They were enormous asteroid types that pummeled the Moon, all in the same blink of geologic time.
And when we look at battered Mercury we notice that there, too, and at the same time, it rained down destruction.
The plot thickens when we see that Mars' whole southern hemisphere shows the same widescale blasting, and it dates – surprise, surprise - from that same time of terror now called the Late Heavy Bombardment (LHB).
What is tarnation is going on? Why all this ruination, and during such a short spurt of time? And why don't we have the scarred face that the rest of our neighbors do?
Well, the latest story goes like this, and its outcome is another one of those "great coincidences" we are becoming more accustomed to in astronomy.
Early on in the history of the solar system this was one dirty place, filled with space debris from nearby supernovae that littered the spacescape with the whole periodic table.
Much of the stuff collected into a new star, our sun, and a lot of floaties, rocky and icy flotsam, eventually became planets and asteroids and comets.
This congestion meant a lot of crashing and slamming and kabooming, like driving down the 15. But this demolition derby all died down considerably just over 4 billion years ago. The solar system was relatively calm and clean by then.
Then why the sudden and frightful pulse of projectiles that wrecked havoc in the inner solar system? What exactly happened in this Late Heavy Bombardment?
Well, to give an honest scientific assessment, nobody knows what exactly happened. But we do have a good idea based on a truckload of evidence and some fine computer work. And it involves Jupiter and Saturn.
As these two great giants were forming their orbits were not fixed, but were influenced by each other, and by the asteroid belt, other planets, and a gigantic dumpsite in the outer solar system called the Kuiper Belt.
Through a complex Newtonian dance Jupiter gradually moved in, Saturn slowly moved out until they reached what we call in astrospeak a "resonance."
Briefly, they apparently agreed on orbital distances which allowed Jupiter to go around the sun approximately two times for every one time Saturn did. In the process Jupiter nudged closer to the asteroid belt, Saturn closer to the outer Kuiper Belt and this perturbed the tiny rocky bodies there.
Many asteroids and comets got gravitationally knocked out of their orbits and plunged down, down, down toward the sun. Well, this was not good news for Mars, Earth, the Moon, Venus, and Mercury. They were all in the way.
And they got hit to a fare-thee-well. It is estimated that this brief bombardment doused our planet alone with more than 20,000 impact craters as big as San Diego, about 40 impact basins bigger than Texas, and several impact basins larger than Australia! Fair dinkum!
Of course we can't see these craters anymore because plate tectonics have covered their traces.
Good news: This whole shower gave us an extra supply of innards and heat to run the plate tectonics of our planet even more efficiently.
Amazing news: Life appeared not billions of years later, but in the same geologic breath that the LHB dissipated and the Earth cooled, at about 3.8 billion years ago.
More amazing news: If it weren't for this precisely timed dance of the outer planets, and their distances from the junkyards, and the density of those debris fields, the LHB could have happened later and completely sterilized our life-filled planet.
Next time you see the Moon in all its glory, spot the great dark basins, basins formed by those colossal collisions, and be thankful those days came -- and went.
Posted by Mark Ritter at 2006.05.28 04:38 PM | Comments (0)
