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Craters

Are all craters in the solar system from an impact?
Those atop volcanoes certainly are not.
According to the Earth Impact Database in Wikipedia  there are more than 190 confirmed impact sites on Earth.

However there are some craters observed in our solar system that might not be from an impact.

Some examples:
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.

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.

However the LLNL found a way the moon 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."
'



Our moon has many, many perfectly round craters. These indicate a perfectly perpendicular impact.

There is a site with a topographic map of the far side of the moon (search 'topographic map moon far side')
Most or nearly all craters there are round as well.


In comparison an oblong crater would indicate an impact at an angle. A visual scan of a moon map will find non-circular craters are extremely rare.
This observation of round craters implies most or nearly all impact craters on the Moon occur with a perpendicular trajectory around the entire sphere. That is rather quite an incredible coincidence if they are truly meteor impacts such consistent  angles of trajectory.
Our moon has many round craters with a flat (could be smooth or rough) floor and often a central peak that is nearly always right at the very center. From a moon map I can find many like this, including a few that are well known like Archimedes, etc. A crater with a flat floor is awkward to explain for a violent impact.
Among these is Plato which is noticeable with a very smooth floor. That feature is explained this way:

'
Compared to other craters of similar size, Plato should have a 2.2-km-high mountain rising from its floor. However, since Plato is filled with a 2.6-km layer of lava, the peak is buried.
'

This remark means an analysis has been done on the expected peaks for ranges of crater diameters, and I assume it used a large statistical sample of "similar" craters to define this expectation.

Wikipedia has an article titled 'List of geological features on Callisto'  This is a moon of Jupiter.
This wikipedia page has a 'USGS map of Callisto' - a graphic that can be zoomed. All or nearly all craters are round - though it is not a high resolution image.

NASA has an image of the planet Mercury surface. All or nearly all of its craters are round. A number of these round craters have a similar rayed appearance as Tycho on the Moon. Tycho will be mentioned below.

This is too incredible to assume nearly all meteors in the solar system hit the surface of its impact so close to perpendicular, to be so round.

Tycho on the Moon presents the alternate interpretation for round craters.

This non-impact explanation for round craters was first proposed by Ralph Juergens in an academic journal in 1974. An excerpt can be found with links below. This post includes observations documented after 1974. This topic summarizes my review of the photographs available for a number of planets and moons.

A link below has the NASA interpretation of Tycho.

It says:
'
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.'

Because there are so many smaller craters than Tycho and many much larger craters I am very suprised by the comment 'All craters start out looking like this after they form'.
That is an overgeneralization given Tycho is rather exceptional on the moon.

Langrenus is larger 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 (according to wikipedia).

Here is an alternate explanation for craters:
The rays are electrical searing; they are lighter than the terrain and are only at the surface.
If the rays were ejected material it is quite unlikely they could have fallen into such a symmetrical pattern.

These rays are Lichtenberg figures.  From wikipedia:
'
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. 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 of the crater floor but sometimes leaving a peak between the two filaments. The outside of the filament rotation  creates the round crater rim.
This perpendicular process explains the convex floor of Mimas.

For the many,many small round craters observed in so many places  the electrical discharge was brief but always perpendicular. No ejected material is ever observed beyond the sharp circular rim.

When the electric discharge stops, sometimes a follow up discharge occurs almost immediately. Like with lightning it targets the highest point. This explains a frequent 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 appear from random impacts is an incredible coincidence.

Proposing so many impacts, approaching at angles that  should be random around the body surface (ranging from 90 degrees to nearly tangential) always form a round crater is rather unbelievable.

NASA description of Tycho
link


references for the 1974 view of craters.

link1



link2


opposing view of craters from scientific american:

link

I question their explanation  an impact from an angle releases all its kinetic energy precisely at the point of impact meaning the angle is irrelevant. I am skeptical of that explanation for round craters.

date updated (03/05/2019)

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