I was looking at selection of close up views of craters on the moon and it struck me that the larger diameter crater were not (apparently) proportionally deep. Googling, I see I’m not the first one to make this observation.
Some of the answers as to why this is so, seem to indicate a low gravity combined with a sturdy regolith surface.
I’m curious, if this is true, why we wouldn’t see part of the asteroid or comet in the center of the crater.
The crater is created by a combination of the seismic effects (propagation of the shock wave through the regolith, which is anything but solid, and the more solid granite-like substrate) and the superheated gas produced by the rapid conversion of the kinetic energy of the meteorite into thermal energy. Since Earth’s Moon has much lower gravity than the Earth, material can be seismically disturbed over a broader area and kicked up by significantly less energy, translating into a much larger impact crater. The thermal energy and movement of ejecta is also not retarded by an atmosphere, so it can travel further, and of course there is effectively no erosion from wind or hydrological action on the surface of the Moon. (Technically, the Moon does have an atmosphere of highly charged dust but it is extremely diffuse and can’t really be treated like a normal fluid.) The core of an iron-nickel meteorite would probably remain somewhat intact but buried deeply under the regolith, while a watery or hydrocarbon comet would likely completely evaporate due to the energy released in the impact and any remaining exposed fragments would sublimate or vaporize due to exposure to solar radiation.
A geophysicist could provide a more detailed answer (and would probably correct some of my terminology because it has been a while since I’ve read on planetary geoscience) but I think that gives the highlights of why the Moon has such extraordinarily large craters. Of course, the Earth has some pretty massive impact craters, too, but most of them are hidden by subsequent eons of erosion and tectonic activity.
Stranger
Impact craters are much larger than the diameter of the asteroid/comet that created it (called “asteroid” for simplicity) . On an airless body (called “rock” for simplicity) like the moon, everything strikes with it’s full relative speed to the rock that it is hitting, which can vary considerably depending on whether it is a “head on” collision between the two or if the rock is hit from behind by a “tailgater”. But either way, the collision is at tens of thousands of miles per hour. At that speed, the asteroid (and a big chunk of the rock) are literally vaporized. The crater is what is created when the energy from the impact continues to travel downwards but then is reflected back upwards, exploding a much larger hole than the original “entrance wound”. An impact isn’t analogous to a pebble being dropped into mud, it is like a hand grenade being dropped into mud. (Bomb craters on Earth aren’t as deep as they are wide, either.)
https://craterexplorer.ca/crater-formation/
On airless bodies, every impact is a cratering event, even from dustmote-sized particles. On a body with an atmosphere as thick as Earth’s, the size of asteroids that hit most commonly are areobreaked to a standstill (or complete destruction) miles before they can reach the ground. Impact craters form on the moon every day. Impact craters on Earth are thousands of years apart. The most recent cratering event on Earth was an extremely weird one with an asteroid fragment small enough that it “shouldn’t” have reached the ground with enough remnant orbital velocity to make a crater, but through freak accident, did. The crater (in soil) was about 3 times wider than deep (45 feet by 15 feet).
Another recent crater is the Kamil crater, which is also around 3:1, around 150x50 feet
(The delicate structures of Kamil crater have been destroyed by heavy mining for meteorites, but Wikipedia refuses to update the page because that information is “original research”.)
And probably the most famous crater, “Meteor Crater” in Arizona is currently around 7 times wider than deep (partially infilled since impact, originallymore like 6 times wider).
Ah, OK. Thanks for the info and the links. Very interesting reading.
Yes, the owners of that Arizona crater were incredibly lucky. Looking on Google Earth I see the meteor just missed the visitors’ center.
Yes, but too bad about the petting zoo.
I’ve mentioned it here before, but Barringer bought the crater back in 1902 when it was still assumed that meteorite craters were like a pebble dropped in mud, and there was a gigantic lump of iron roughly the diameter of the crater buried at the bottom. He started a mining company and spent more than 20 years (and more than 600,000 early-20th century dollars, around $15-20 million today) attempting to drill down to the mountain of iron. Eventually in 1929 Barringer asked for the advice of an astronomer named Forest Moulton. Moulton mathed the math, and realized that an object traveling at extremely high speeds would make a crater much larger than itself and be shattered/vaporized in the process.
Here is an article on it:
Since the OP’s question has been addressed, Barringer / Meteor Crater is very interesting to visit. The Visitor’s Center is modern and clean and nicely appointed with good exhibits, and one particularly interesting thing was knocking on the meteorite with my wedding ring or with a coin and hearing the metallic ping! that resulted. I plan to return to visit again.
That visit made quite an impression on you: while searching for an earlier post about the crater last night I saw you mention the ring thing. Somebody other than md-2000 made the visitor’s center joke for that thread, though.
Some of Barringer’s mining equipment is still lying abandoned down in the crater. At least it was the last time I visited about 20 years ago.
You might enjoy this short episode of StarTalk! In it, Neil deGrasse Tyson explains why high-speed moon craters are always perfect circles, regardless of the angle at which the impact occurs and why they were once thought to be volcanic calderas.
About the vaporization of large impactors, for miles around Meteor Crater there are tiny blebs of iron in the soil. Named Nininger Spherules after the meteoritics pioneer Harvey H. Spherule–um, Harvey H. Nininger, there are an estimated thousands of tons of them, resulting from droplets of liquid iron that solidified. Nininger used to collect them with magnets and sell them.
I have a few spherules that came with a micromount-sized Canyon Diablo (the name for the meteorites from the crater). Hard to make them out through a plastic dome and a plastic baggie, but they are there at the bottom.
As for the Canyon Diablo fragments larger than spherules, it is probably a combination of bits that spalled off the top of the main impactor as it hit (the shrapnel) and separate pieces of iron that were originally sitting on the main chunk and fell off along the way.
My biggest piece of Canyon Diablo shrapnel is 72 grams
A bit smaller than the 4.8 kg piece available here for 13.5 kb (kilobucks).
A couple of questions:
Could he have reasonably expected to recoup the equivalent of 7 million dollars ( and make a profit to boot)? (Yes, I read that he said that there might be “hundreds of billions of dollars” worth of iron, but that sounds more like a tale for investors than something he actually believed)
And, what does “vaporized” in this context mean? Wouldn’t the pieces still be available with a strong magnet?
He was probably trapped within the sunk cost fallacy.
And yeah, the spherules are recoverable, but Barringer died in 1929 and the spherules were discovered in 1946. As for recovery now, the estimate was
Dr. Nininger estimated that there were between 4,000 and 8,000 tons of the tiny metallic spheroids still in the top four inches of the soil within five miles of the crater.
Plugging in all the numbers I’m getting something on the magnitude of 100 grams per cubic foot. If processing several hundred million cubic feet of soil for that amount of nickel and iron would be profitable I don’t know, but the Nininger family isn’t interested in trying it. (They long ago stopped allowing people to dig for meteorite fragments in the surrounding land because assholes would leave open cavities that their cattle could trip and break a leg in.)
ETA: I see that just iron (ignoring the nickel) sells for around $100 a ton. So 4,000 to 8,000 tons of iron is worth less than a million dollars.
He assumed that the meteorite would be close the diameter of the crater, so on the order of a 3,500 foot sphere of iron and nickel. (What actually hit was more in the order of 150 feet, so on the order of 200 times smaller than his expectation.)
I believe the central peak that some craters have is an artifact of the impact physics and may not contain much if any of the impactor.
Correction: 8 million times smaller. It’s the volume that matters, not the diameter.
Yeah, I don’t know where my math went wrong there. But a correction to your correction: he thought it was around 20 times wider, so 20x20x20 or 8 thousand times smaller.
Interesting article. I wonder if this applies to gambling? ‘I’ve already put $X in the slot, might as well see it through until it pays off’.
Do they sell online or at the gift shop these nodules? The price of generic iron-nickel is one thing. Stuff from space is another. (Yeah, all our stuff was once from space but …)
