Are all the craters on the moon made by round objects?
Posted 31 August 2020 - 11:04 PM
Or would any shape object still produce a round crater?
Posted 31 August 2020 - 11:28 PM
Most craters are formed not by a physical splash, like throwing a rock into soft mud, but by the explosion of hot gas from the vaporized impactor + vaporized target material (rock, on the moon, as well as ocean water, plants, animals, etc. if on Earth)--more akin to a nuclear blast. As long as the impactor is traveling fast enough to vaporize when it hits, then it can be almost any shape and enter at almost any angle and still produce a circular crater. You can get oblong craters, but they are usually from very low impact angles, I think something like 10* or less (I'm sure someone who knows the physics better will correct me).
That's not to say that impactors can't make lower-energy craters that are also round, like throwing a rock into soft mud. I think some of the pits gouged out by the debris from the Sikhote-Alin metorite airburst were like that. But essentially all of the craters on the moon that are big enough to be seen with amateur instruments are of the vaporization/explosion type. The latter are also MUCH bigger than the original impactor. Meteor Crater is 3900 feet in diameter, but the impactor is thought to have been only 160 feet in diameter. But again, it's not like throwing a 160-foot rock into soft mud at low speed to blast out a pit a few hundred feet across. It was more akin to converting a 160-feet sphere of nickel and iron into gas at immense pressure--essentially a gigantic bomb, 10 megatons in the case of Meteor Crater. Similarly, the K-Pg dinosaur killer that hit at Chicxulub in the Yucatan is thought to have been about 6 miles in diameter, but the crater is somewhere between 120 miles and 180 miles in diameter, depending on what we think the outermost ring of the original crater was--even wider circular structures can be formed when the temporary crater slumps inward.
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Posted 31 August 2020 - 11:29 PM
The craters you see in any object be it the moon or planets are generally made by tiny objects relatively speaking. the crater itself is more like what happens when a rock falls into a pond and the water ripples in a round shape. The speeds these objects travel at cause even small ones to create such a rippling effect even on solid ground, and the material that was impacted settles into a more or less perfectly circular shape. This "droplet" sort of effect is why some craters have a weird nipple in the center. Much like when an object falls in the water and for a brief moment a "nipple" at its spot results from the water collapsing inwards.
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Posted 02 September 2020 - 09:18 AM
Posted 03 September 2020 - 01:47 AM
As others have noted, the high velocity of the impacts is the reason that the shape of the impactor and other factors like impact angle (except for very shallow angles) don't affect the shape of the crater. A mass moving at 1 km/sec has a kinetic energy comparable to its mass in TNT, and most impacts on the moon occur at 10 km/sec or more, so you get a huge explosion at the point of impact. That creates a hemispherical shockwave that expands outwards from the impact point and excavates a roughly hemispherical cavity in the surface. Small craters don't experience much further collapse, and retain a classic bowl shape, but larger craters can collapse to form central peaks and terraces around the perimeter.
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