FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

The Late Heavy Bombardment

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.

Moon Myths Revealed!

There are several common misunderstandings about our little satellite above, also know as The Moon. I hear them all the time. Let's look at some, and try and clean up its image a bit.

1. One myth is that the Moon doesn't spin around like earth does. All we see is one side of the moon, ever. If the Moon really does spin about, so the thinking goes, we should see the other side of it, right?

Well, it's precisely that it does spin around that allows us to see just one side. Imagine being up a couple stories looking down on two children, one walking a big circle around the other.

In order for the circling child to continue to face the stationary child, that little one must continually turn while walking. If she didn't "spin" slowly while orbiting around, she would appear to be staring at some distant object the entire time. The child in the center would only then see all around the orbiting child's head.

Like the child, the Moon spins around one time for every orbit.

2. The Moon is bigger when it is full and near the horizon.

Sorry, this is a myth, as well. A rising Full Moon is the same size as it is hours later when it is high in the sky. Some believe that this optical illusion flies so well because at the horizon we compare the Moon with points of reference - distance buildings or trees or lights - which make the Moon appear bigger.

We even expect this illusion in artwork. If the Moon is drawn to real scale in the sky of a painting, rather than as a big bright ball, we are not amused; it just looks like it's too small.

Next time there is a full moon, go out when it is on the horizon, when it appears so big, and you'll see that you can cover it with a finger held at arm's length. Wait for a couple hours when it is higher in the sky and try it again. You can cover it then exactly as you covered it when it appeared so big earlier.

3. There is what we can see of the Moon, and then there is the "dark side of the moon."

Not really. There is always a dark side of the Moon, but sometimes it faces us. This is New Moon, when the Moon is between the sun and us. And when we see the so-called "half moon," more accurately called quarter moon, we see some of the lit side and some of the dark side. More accurately that part of the moon we cannot see should be called the "far side" of the moon, not the "dark side."

4. The sun is up during the daytime, the moon is up during the night.

Of course we can define daytime as when the sun is above horizon. But we do not define the night as when the Moon is up. "Nighttime" is when the sun is below horizon. The Moon is part of the definition of neither daytime nor nighttime.

As a matter of fact, during many "daytimes" you can see the Moon - if you know where to look. During this coming week you can see the Moon in the early morning in the western skies. You can then follow it day after day as it moves over towards the sun.

5. We never went to the Moon.

There are some people, few but vocal, who believe all the Apollo missions to the Moon in the late 60's and 70's were a giant hoax perpetrated by our government. I wouldn't even mention this except for the fact that there are a good number of good people out there who are actually sitting on the fence with this one.

The best way I can address this in such a small space is to point you to a website called "Bad Astronomy." Specifically one of the pages there rebuts the Fox TV show "Conspiracy Theory: Did We Land on the Moon?" It is here. Go and read and be enlightened.

Actually the next myth is easier to believe than the last...

6. The Moon is made of green cheese.

This one may actually be true. No, I jest! As far as I can tell, using the Oxford English Dictionary as a source, this phrase goes way back, hundreds of years. It was probably used with at least a pinch of sarcasm. Judge for yourself:

John Cotgrave in 1611 gives us, "(Wee say of such an Idiot) hee thinkes the Moone is made of greene cheese." Sir Charles Wilkins in 1638 wrote, "You may as soon perswade some Country Peasants, that the Moon is made of Green-Cheese (as we say) as that 'tis bigger than his Cart-Wheel." (Green in both cases means "new" or "fresh.")

It's like someone today saying, "Soon you'll have him believing the Apollo missions were faked!"

Until next time, clear skies!

The Young Faint Sun Paradox

There are coincidences, and then there are amazing coincidences. If you've been reading this column for a while you know already that there are some pretty "fortunate circumstances" that have occurred over the last billions of years, circumstances without which you would not be alive to read this.

Our just-right distance from the sun, the preternatural creation of our Moon, at least two incredibly well-timed nearby supernovae before our sun was born, our extremely well-placed planetary neighbors, etc., etc. - the list is endless.

But when several sets of separate, seemingly unrelated events occur over a period of billions of years - a number of simultaneous and diverse phenomena which must work and change in flawless unison or it's curtains for life - then it's time now to drop the jaw in utter awe.

When life was first introduced here, nearly four billion years ago, the sun wasn't the bright and shiny star it is today. It was probably about 30% less luminous than now, meaning it poured out 30% less energy.

If we know that a drop in energy of only about a percent or two from what we have now would turn us into a giant snowball, then how could life survive back then at a time the sun was so much dimmer?

Well, back then, so the records show, there was a lot more carbon dioxide in the air. Carbon dioxide is a greenhouse gas, efficiently trapping energy like a huge blanket around our planet.

So although the sun poured out a lot less energy eons ago, the greenhouse effect was in full kick back then, offsetting the wimpy sun, and keeping the planet nice and warm.

The big questions now? Where did all the carbon dioxide go as the sun got brighter and brighter? Why didn't life on this planet bake away a long, long time ago under our heavy blanket? Put on the proverbial thinking cap and take a seat.

Carbon dioxide can be cleaned from the sky in several ways, but it can't happen too fast, nor too slowly - it has to happen at just the perfect rate to match the brightening sun. One way involves water.

Early oceans were important imbibers of CO2 from the air. Carbonated beverages are perfect examples of water filled with dissolved carbon dioxide. But even the oceans have a limit. Could land help out here perhaps?

Yes, land can absorb CO2, as well. There is a class of a very abundant rock called silicates that can undergo a chemical reaction with carbon dioxide. (Fear not! I won't give the equation!) When silicates become exposed to the air during erosion, they get together with CO2 to form carbonates and sand. Yes, sand.

But erosion rates and plate tectonics and the uplift of mountains to expose new rock, etc., all have to be perfect so that the decrease in CO2 matches the increased energy output of the sun.

Surprise! It was! And still is.

A long time ago, when there was much more intense lifting of the land and exposure of new silicates, more CO2 could be sucked up at just the right rate. But the plates and uplift have slowed considerably in the billions of years that have gone by. Why aren't we baking right now in all the carbon dioxide that couldn't be absorbed?

Amazingly, plant life showed up at just the right time to work as a team with the land. Remember from school that CO2 is something plants require. Well, early plants didn't require much CO2, later plants required more. These requirements helped offset the fact that the silicates were playing a smaller and smaller role in the removal of CO2.

And then shell critters showed up at just the right time to continue helping! These tiny beasts turn CO2 into special carbonates, the building blocks for their shells.

And when the plants and shell critters went and got themselves dead and buried they effectively took the CO2 with them.

So as the sun got brighter and brighter, and bathed us in more and more energy, our warm blanket got thinner and thinner.

It is even more amazing than I can describe in this short space. It is such an astonishing set of events it has its own name - the Faint Sun Paradox. But the Big Picture here, that several entirely disparate events took place at just the right time to maintain perfect temperatures for billions of years goes beyond cute coincidences and enters into the realm of the miraculous.

What a world!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me