Creating Round Craters
Most or nearly all impact craters in the solar system are round.
The accepted explanation is not logical.
This observation of round craters includes the Moon (both sides),Mars, Mercury, and Callisto (orbits Jupiter). Even Pluto has 'halo' craters.
It is very suspicious they are always round given anyone would naturally assume random impact angles would result in a variety of shapes. There is consistency, not randomness, which must be explained.
Here are two references for the standard explanation:
1) from Scientific American
"When geologists and astronomers first recognized that lunar and terrestrial craters were produced by impacts, they surmised that much of the impacting body might be found still buried beneath the surface of the crater floor. (Much wasted effort was expended to locate a huge, buried nickel-iron meteorite believed to rest under the famous Barringer meteor crater near Winslow, Ariz.) Much later, however, scientists realized that at typical solar system velocities--several to tens of kilometers per second--any impacting body must be completely vaporized when it hits.
"At the moment an asteroid collides with a planet, there is an explosive release of the asteroid's huge kinetic energy. The energy is very abruptly deposited at what amounts to a single point in the planet's crust. This sudden, focused release resembles more than anything else the detonation of an extremely powerful bomb. As in the case of a bomb explosion, the shape of the resulting crater is round: ejecta is thrown equally in all directions regardless of the direction from which the bomb may have arrived.
"This behavior may seem at odds with our daily experience of throwing rocks into a sandbox or mud, because in those cases the shape and size of the 'crater' is dominated by the physical dimensions of the rigid impactor. In the case of astronomical impacts, though, the physical shape and direction of approach of the meteorite is insignificant compared with the tremendous kinetic energy that it carries.
"An exception to this rule occurs only if the impact occurs at an extremely shallow, grazing angle. If the angle of impact is quite close to horizontal, the bottom, middle and top parts of the impacting asteroid will strike the surface at separate points spread out along a line. In this case, instead of the energy being deposited at a point, it will be released in an elongated zone--as if our 'bomb' had the shape of a long rod.
"Hence, a crater will end up having an elongated or elliptical appearance only if the angle of impact is so shallow that different parts of the impactor strike the surface over a range of distances that is appreciable in comparison with the final size of the crater as a whole. Because the final crater may be as much as 100 times greater than the diameter of the impactor, this requires an impact at an angle of no more than a few degrees from horizontal. For this reason, the vast majority of impacts produce round or nearly round craters, just as is observed.
2) from scienceabc:
When a meteor hits the lunar surface, a shockwave spreads out in all directions and the impact creates a dent in the surface that is much bigger than the size of the impacting object itself. Since the impact sprays ejecta in all directions in equal proportions (just like the shockwave of an explosion), the direction and incident angle of the impact become irrelevant in determining the shape of the crater.
I suggest more than just 'the vast majority of craters' are round. When looking at those with a very small diameter they are nearly 100% and these are the most frequent.
I totally agree with ' this may seem at odds with our daily experience of throwing rocks.'
When I throw a flat rock as fast as I can (for maximum kinetic energy) to get as many skips at possible on a lake, I will never see a big splash at the moment of each touch, on each skip. The kinetic energy must be transferred from the moving body when the velocity stops because the energy in the momentum must be conserved.
It makes no sense to propose an explosion to cause the velocity to stop the motion by vaporizing the object and a hole in the surface at the moment of a touch, regardless of angle.
This proposal, though widely accepted (as shown by the same story repeated in different sources), is illogical.
The asteroid must stop for it to explode. It will not stop at a shallow angle so there can be no explosion.
Also an asteroid cannot create the energy for its explosion as that violates thermodynamics.
Here is the NASA interpretation of Tycho, a well known crater (posted in 2019).
Tycho Crater is an one of the most prominent craters on the moon. It appears as a bright spot in the southern highlands with rays of bright material that stretch across much of the nearside. Its prominence is not due to its size: at 85 km in diameter, it's just one among thousands of this size or larger. What really makes Tycho stand out is its relative youth. It formed recently enough that its beautiful rays, material ejected during the impact event, are still visible as bright streaks. All craters start out looking like this after they form, but their rays gradually fade away as they sit on the surface, exposed to the space environment which over time darkens them until they fade into the background.
From a different NASA article (also 2019):
Tycho's central peak is thought to be material that has rebounded back up after being compressed in the impact, and though it's a peak now, it originated at greater depth than any other portion of the crater. The floor of the crater is covered in impact melt, rocks that were heated to such high temperatures during the impact event that they turned to liquid, and flowed across the floor. Impact melt flowed downhill and pooled, where it cooled.
Those bright rays are not ejected material (substantial rubble is not visible anyway) but are surface electrical scarring.
Tycho is a near perfect example of a Lichtenberg figure.
Lichtenberg figures are branching electric discharges that sometimes appear on the surface or in the interior of insulating materials. Lichtenberg figures are often associated with the progressive deterioration of high voltage components and equipment. The study of planar Lichtenberg figures along insulating surfaces and 3D electrical trees within insulating materials often provides engineers with valuable insights for improving the long-term reliability of high voltage equipment. Lichtenberg figures are now known to occur on or within solids, liquids, and gases during electrical breakdown.
Rayed craters are created by a very high voltage discharge to the surface, resulting in the Lichtenberg figure.
This is like electric discharge machining (EDM).
When the current is high enough a birkelund current pair of filaments can form a helix. As the helix rotates it machines the bottom surface of the crater floor but sometimes leaves a peak at the center between the two filaments. The outside of the filament rotation machines the round crater rim while maintaining the diameter. This machining process will result in a flat floor
This perpendicular electrical process explains the convex floor of Mimas.
For the multitude of small round craters observed in so many places the electrical discharge was brief, like a lightning bolt but always perpendicular. No ejected material is ever observed beyond the sharp circular rim around these craters. Material was vaporized not ejected.
EDM process can sometimes cause sputtering which are observed as many small shallow craters. These are also seen on the Moon.
Linear crater chains (consecutive duplicate craters in a line with slight overlap) are also observed like the Davy crater chain on the Moon and Enki Catena on Ganymede.
The Moon has several 'walled plains' or very wide round craters with a flat floor.
When a substantial electric discharge stops, sometimes a follow up discharge occurs almost immediately. Like with lightning it targets the highest point. This explains an observed combination of one larger crater that has a much smaller very round crater located precisely on the rim's highest point. For these crater pairs to arise from random impacts is an incredible coincidence. Examples of this rim shot on the Moon are the Taruntius and Schikard craters.
Proposing so many impacts everywhere to always form a round crater from random angles is rather unbelievable.
However there are some craters observed in our solar system, round or not, that might not be from an impact.
Mimas, orbiting Saturn, has the Herschel crater whose diameter is 1/3 of the moon. On the scale of Earth that would be Canada.
Rather than being deep for such a diameter, the floor appears to be somewhat convex. There is a central peak though not tall. The rim appears almost like a hexagon (6 rough segments).
This crater with its combination of floor, rim, and size is an unlikely impact.
The minor planet Vesta has Rheasilvia crater with a diameter at 90% of Vesta.
Phobos, orbiting Mars, has the Stickney crater, whose diameter is about 9km in diameter vs the moon's diameter of about 20km. For a crater to be half the size of the moon and not break the moon on impact seems unlikely. There are no visible fractures around this crater, from an impact.
However the LLNL found a way 'in a 3D simulation' the moon Phobos would not break.
From a 2016 research paper in the Geophysical Review Letters:
"We've demonstrated that you can create this crater without destroying the moon if you use the proper porosity and resolution in a 3D simulation," said Megan Bruck Syal, an author on the paper and member of the LLNL planetary defense team. "There aren't many places with the computational resources to accomplish the resolution study we conducted."
Alternately the crater is not from an impact.
Tycho on the Moon presents the alternate explanation for round craters.
Langrenus on the Moon has a larger diameter than Tycho at 132km
Grattieri, a rayed crater on Mars looks the same as Tycho, but just 32 km vs 85 km.
Debussy, a rayed crater on Mercury also looks the same as Tycho and both are 85 km.
The non-impact explanation for round craters (rayed or not) was first proposed by Ralph Juergens in an academic journal in 1974. An excerpt can be found on line. This post includes observations after 1974 . This topic followed a review of the photographs and data available for several planets and moons (so any mistakes are mine).
Obviously the nebular hypothesis for the formation of the solar system must deal with craters. This is how:
The Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm) is an event thought to have occurred approximately 4.1 to 3.8 billion years ago (Ga), at a time corresponding to the Neohadean and Eoarchean eras on Earth. During this interval, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.
The Late Heavy Bombardment happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth's history.
Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors (i.e. asteroids and comets) in the inner Solar System, but no consensus yet exists. The Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration and in doing so, scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, and into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact-melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They also note that the rate of impact cratering could differ significantly between the outer and inner zones of the Solar System.
Round craters are a challenge for cosmologists especially if rayed craters are Lichtenberg figures.
The nebular hypothesis is a separate topic.
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