Most astronomers and planetary scientist seem to assume that any crater that cannot be attributed to a volcanic origin is automatically classed as an impact crater or a crater produced by fall out of impact ejected material. There are no large scale terrestrial craters that we can examine, so our findings must be based on astronomical observations and the study of physics and geology. The following are the facts and assumptions employed in developing the alternative methods for the origin for many of the craters observed on moons and planets
FACTS AND ASSUMPTIONS
1. The orbits of our planets were swept fairly clean of debris during their period of planet accretion and that the observable craters on the planets and moons originated after the time of accretion.
2. The findings of Dr. A.C. Gifford of New Zealand are correct in that the force of an impact at astronomical speeds would result in an explosion of the meteorite, and whatever the original angle of impact, the result would be a circular crater.
3. The size of an impact crater would be dependent upon the size of the meteor and it's velocity at time of impact. The larger the meteor or the greater it's velocity, the larger the crater.
4. The range in size of meteors striking one planet would not be significantly different than the range in size of meteors striking another planet.
5. The friction of an atmosphere on an incoming meteor can generate so much heat that the meteor can explode before impact. On planets and moons that are void of an atmosphere, the explosion would not occur until actual impact.
6. Meteors like the leonids, move in an orbit around the Sun in a direction opposite to that of Earth, and slam into our atmosphere almost head-on and at a very fast velocity. Meteors approaching Earth from the direction of our orbit would have a slower impact velocity. The relative velocity of meteors beyond that created by the gravity of the receiving planet or moon, would be dependent upon the orbital velocity of the planet or moon and the direction and velocity of the meteor approach.
7. The further a planet is from the Sun, the slower it's orbital velocity. Mercury's orbital velocity is about 48 km/sec, the Earth, about 30 km/sec and Mars about 24 km/sec.
8. Degassing of planets and moons occurred very early in their existance.
9. On Earth, upward movement of heat from a deep magma source, can produce superheated water in confined ground-water aquifers. When the built up pressure of the superheated water ruptures the the overlying strata, the water immediately turns into steam and the resulting steam explosion can create craters (maars). Such craters are indistinguishable in topographic expression from an impact crater (Hole in the Ground, Oregon vs Barringer Meteorite Crater, Arizona).

ORIGIN BY STEAM EXPLOSION
Mars undoubtedly had surface water in it's past, and where there was surface water, there was ground water. Basaltic lava flows, which are believed to be common on Mars, are often interbedded with scoraceous or other permeable interflow strata. Thes strata serve as confined aquifers. Underground sources of heat, which dissipate by moving up to the surface, could heat the ground water in the confined aquifers to a superheated state. The temperature of water in a high pressure steam boiler is about 500 degrees C. The pressure on the overlying or confining lava flow continues to increase until rupture occurs. The sudden drop in pressure caused by the rupture results in the superheated water turning into steam and erupting with great force and explosion, creating a crater at the surface. These are like the black powder explosion that created the crater at Petersburg, Virginia during the Civil War, the craters created by nuclear tests in Nevada, and the natural steam explosion that created Hole in the Ground, Oregon. In the case of a steam explosion, no lava is produced, and no conduit is required to transmit heat up to the aquifer. The craters created by steam explosions are indistinguishable from craters created by meteor impact explosions. The identification of the Barringer Meteorite Crater in Arizona took many years and was not bvy it's topographic expression., but by the the occurrence of pulverized silica, and laterr by the presence of the mineral "coesite", which is created by high pressure and heat.
Consequently, many of the craters on Mars could have been created by subsurface explosions of steam. By looking at such craters from afar, and not being able to to make on-the-ground investigations, it would be impossible to state for cetain which is a meteor impact crater and which is a crater created by a subsurface explosion.

ORIGIN BY OUTGASING AND COLLAPSE
As mentioned above, the size of an impact crater is determined by the mass of the meteor and it's impact velocity. Our Moon and Mercury are both void of an atmosphere and are both heavily cratered. Mercury has about twice the mass of our Moon, so it has a greater gravity than our Moon. Being close to the Sun, Mercury has about a 60% greater orbital velocity than our Moon. If the size of the meteors impacting our Moon were about the same as those impacting Mercury, the crater size on Mercury should be related to it's greater gravity and a much higher orbital velocity. Consequently the size of most meteor impact craters on Mercury should be substantially larger than those on the Moon. On appearance, there seems to be little difference in size between the craters on the Moon and those on Mercury. There are some very large craters on both that were undoubtedly created by meteor impact.
Early in the history of our planets and moons, there was an outgassing as the planets and moon reacted to gravity induced compaction. On the smaller planets and moons, the gas spewed into space as there was insufficient gravity to create an atmosphere. The period of degassing was a very violent time. Gases, mainly water vapor and carbon dioxide often became trapped beneath the surficial mantle. As the trapped gases continued to accumulate, they produced large surface bulges in the surface mantle. When the pressure of the trapped gases finally ruptiured the confining surficial mantle, the gases escaped and the bulges collapsed into large circular flat bottomed calderas or craters. These craters were ringed by steep sided fault scarps. After collapse, gases continued to escape through fumaroles. In some craters, the mantle resealed itself and a secondary bulge occurred that resulted in the formation of a caldera within a caldera. In many craters, the continued gas emmisssions created a central cone of ejected material.
When examining pictures of craters on our neighbors, be aware that there are other possible methods for their origin other than meteor impact.
Jack

I liked most of your proposition. The one thing I disagreed with is something that doesn't really undermine your claim very much:

I think the fact that Mercury's craters are not substantially different in scale from the lunar ones perhaps means that there was a differentiation in the size of the planetessimals that were allowed to form in at the different distances from the sun. This would have been influenced by:
- heat and evaporation of the icy bodies at an earlier epoch for inner orbits
- velocity of collision, which affects what size bodies can colide and stay gravitationally bound, vs simply exploding into high surface area and easily evaporated debris
- time at which the solar nebula had been blown outside of that particular orbit.

Also, related to Mercury, I suspect that Mercury was fairly anhydrous very early in its history, so I wouldn't expect too many of its visible craters to be from out-gasing of water.

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Your last comment about Mercury lacking sufficient water got me to thinking. Do we know when the planets and moons came into existance in relation to the timing of the ignition of fusion on the Sun. If the planets had completed their accretion prior to ignition, the heat of the initial fusion may have triggered the beginning of degassing on all the planets and Moon. Thanks--Jack

Theory of formation of planets is a subject I've only read about in popular articles, so don't assume too much credibility in this speculation.

My thought on Mercury was two-fold: first I think Mercury had a higher heat of formation, as the objects hitting it would all have had considerably more energy on impact. I imagine that this would speed the differentiation process; driving out things with low boiling points [including perhaps Sodium] before a crust formed. Second even before ignition, there was tremendous heat from the collapse of the protostellar cloud/disk and I imagine that this would drive lighter gases such as hydrogen, methane, ammonia, and water out from the center of the solar system. Note, there must be some boundry where hydrogen is infalling too rapidly to escape this way. I'm not sure where that is , but it may explain why there are no planets inside the orbit of Mercury.

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:unsure: I have read such statements in several sources and remain puzzled. It seems the faster the object passes through the atmosphere, the more heat is generated (no puzzle) and the less time for the heat to be conducted into the interior of the object most of which I presume are silicates of some sort, setting aside comets, with low thermal conductivity. It seems that ablative-like erosion from the outside of the object should dissipate the heat faster than it can be conducted to the interior to create an explosion in the atmosphere. This should be more likely in the case of icy objects especially the loosely aggregated ones. I don't have a good feel for the iron ones. So you can imagine how puzzled I am about the Tunguska event. Can enough heat be applied to the atmosphere to produce an explosion of just the atmospheric constituents?

This brings to mind another puzzle. How and where are the "solid rocks" and "almost pure iron" objects of equal to or less than a few thousand cubic kilometers (below the critical size to produce differentiation) formed? Does one necessarily have to assume they were produced in or on a large body (planet or moon sized object) which subsequently exploded or is there another explanation? Since in its last few hours before going supernova, a star produces the elements up to iron in sequential order thereby generating concenric shells of differentiated elements, could solid chunks of this material possibly survive the explosion of the supernova event? It seems unlikely. :unsure:

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

The heat generated by a falling meteor is not caused by friction with the air but it's ram pressure. The air in front of the meteor gets compressed by the falling object and the high pressure heats up the air to incandescence.
Mercury's craters are no different from lunar craters which creates a problem with the energy of incoming missiles. A possible explanation may be that mercury's surface has been altered since it's inception and the current surface has seen few cratering events and most of them after solar ignition when most planitesimal were already dispatched by the explosion.

Hi JESMKS, The news story about the surface of Comet Wild 2 supports your idea [at least on comet-scale objects].

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[quote] Hi JESMKS, The news story about the surface of Comet Wild 2 supports your idea [at least on comet-scale objects].
Do you mean the shape of the craters? It seems they are not believed to be impact craters by the Stardust team. Are flat floors and mesas to be expected by outgassing and steam explosions?

Cheers.

Yes, that's his theory.

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I've been thinking about Gourdhead's posting about meteors moving through an atmosphere. I had never questioned the "Why" in "Why should a meteor explode in the atmosphere prior to impact". I now believe that such meteor explosions would be very rare events, and that the Tunguska explosion was an anomaly.
I've come up with a scenario for the Tunguska explosion that I'll call the "Half-Baked Theory". in this theory, a fairly large meteor composed of ice and frozen mud entered the atmosphere at a very low angle. The initial heating caused a melting and evaporation of the surficial ice and a baking of the residual mud. As the meteor moved into lower and lower orbits. the mud baked into a brick-like shell that encased and confined the core of ice. When heat losses from the meteor failed to keep up with heat gains, the ice core melted and turned into super heated water. When about five miles above Tunguska, the internal pressure became so great, it ruptured the shell resulting in a tremendous explosion and the pulverizing of the shell. The downward blast from this explosion devestated the underlying forest creating a blast pattern that is somewhat similar to that created by the blast of Mt St Helens. Something to think about.
Jack