I am fascinated and enthralled by things that fall from space and the marks they leave behind. It’s not just my love of space, it’s is something far more profound, it is in part what those things signify.
Go to a museum, one that has meteorites. Often there will be at least one display of a metallic body that you can touch. Lay your hands on it, press your palms against it, feel the soft curves, the slightly nubby surface, the coolness of the blackened metal. You are touching the core of an extinct planet. That should give you pause and send a small shiver up your spine.
On Earth there less than 200 known, confirmed, impact structures. Just looking at the map it is clear that the distribution is skewed to areas where there are many people (North America & Europe), exposed bedrock (Canada & Scandinavia), or regions where weathering is slow (Australia & North Africa).
Every other rocky body in the solar system is liberally coated in the scars left by impacts. The Earth bears the history of its impacts in a different way. Weathering, plate tectonics, and the oceans have served to hide the marks of the numerous past impacts. Except…
The global ocean, that covers 70% of the surface of the planet to a depth of 7 miles in some places, this, the single largest surface feature of the planet, is impact derived. It is believed that ALL the water on the planet arrived by cometary impacts soon after the planet formed. The Moon is another large impact structure, a relict left over from the collision of the proto-Earth and another roughly Mars sized body.
The frequency of large impacts has, thankfully, fallen over time, but they still happen. Some of you, I hope all of you, may remember the comet Shoemaker-Levy 9, the comet that crashed into Jupiter in 1994 after being torn apart by Jupiter’s immense gravitational field. The fireballs in Jupiter’s atmosphere were larger than the entire Earth, and there were multiple fireballs.
The energy released by each of the Shoemaker-Levy impacts was on a par with the Chicxulub impact in the northern Yucatan 65 million years ago that is implicated in the demist of all terrestrial animals larger than a piece of carry-on luggage.
On Earth impacts are still frequent, but most are small and do not survive passage through the atmosphere. Think shooting stars, grains of sand and dust traveling at orbital speeds, around 20km/second. Several months ago, on the last day of February, I was treated to a something more dramatic than one of these little grains of dust. A little after 10pm on the 28th I was driving under a clear sky and the snow covered landscape lit-up with a bright blue flash. I later found out that the flash of light had been seen from New Jersey to Quebec. This was just one of the many fireballs that flash in the sky each year, probably something small only a few meters in diameter, an explosion not more than a few kilotons.
In a few places the scars left on the ground from large impacts are still visible. One of my favorite ones is in NE Canada. Canada is an excellent place for finding impact structures as much of the Canadian Shield is ancient, exposed bedrock.
The Manicoaguan impact is about 215 million years old and approximately 60 miles across. It has been dammed and the island in the middle is now one of the largest fresh-water islands in the world. Big impacts like this are rare, but they leave dramatic remains behind.
Small impacts are surprisingly common, the frequency rapidly trailing off the larger the impact. This is good news, but the picture is very incomplete as we have only been able to watch carefully for a short period of time.
We are struggling to understand how the universe fits together and have tremendous difficulty comprehending the scales and energy involved. We are too used to thinking on our small scales, our bodies, our houses, maybe our planet, for a few our solar system or galaxy. Our solar system is huge, our galaxy immense, yet in the lager context of our body of knowledge and what we can see even the Milky Way galaxy is barely a microscopic speck.
Look at the ocean, lay back and watch the trails left by falling meteors, look at the background of stars, go to a museum and touch the heart of a planet, if you live near an impact crater go visit.
We often say, “We are all connected,” and this is true, and that web of connection is far greater, wider, and deeper than most of us realize.