Origin of the perfect double ringed crater on Mercury

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silylene

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APOD today had this interesting article about a curious double ringed crater on Mercury. Large double ring craters are not uncommon on planets and moons, and are thought to be caused by rebound effects, slumping and volcanism. However, this one is curious...

The blurb below dismisses the idea that a second impact could occur right in the center of the first impact. But I think it is possible to have a double impact in one spot, near simultaneous, and I have been waiting for evidence of it. I think we should've discovered about one such event by now somewhere in the solar system.

Here is why: we know that quite a few asteroids are doubles or have close-in moonlets. It's rather common in fact (~2-5% total, IIRC). My hypothesis is this: if a double asteroid hit Mercury and the alignment happened to be just right so that the companion asteroid was in a trailing position at the moment of impact, then what would happen is the second strike would be in the center of the first strike. Statistically, a close asteroid pair would be in an aligned formation for a double strike in one spot (aligned within +- 10 degrees of straight in) about 2*2*10/360 * 100% = 11% of the time [assuming that the double asteroid co-rotation plane is congruent the ecliptic plane....if no tendency for the asteroid doublet to be congruent with the ecliptic plane, then double strike in the same spot would be about 1.2% of collisions]. So chances are I think that we should see at least one example of a near simultaneous double strike in one spot somewhere within the solar system. Maybe this is one? If so, I'd expect a rougher surface in the region between the inner and outer ring, as also observed.

http://antwrp.gsfc.nasa.gov/apod/astropix.html
Explanation: What created the internal second ring of this double ringed basin on Mercury? No one is sure. The unusual feature spans 160 kilometers and was imaged during the robotic MESSENGER spacecraft's swing past our Solar System's innermost planet last week. Double and multiple ringed basins, although rare, have also been imaged in years past on Mars, Venus, Earth, and Earth's Moon. Mercury itself has several doubles, including huge Caloris basin, Rembrandt basin, and enigmatic Raditladi basin. Most large circular features on planets and moons are caused initially by a forceful impact by a single asteroid or comet fragment. Since it is unlikely that a second impact would occur right in the center of the first, large double rings are usually attributed to a subsequent volcanic lava flow inside the impact crater. Possibly, though, a second ring could be caused by the melting and flowing of material upon impact. One clue to the origin of the above-imaged double ring is that the basin center appears much smoother than the region between the rings. MESSENGER has now completed its last flyby of Mercury but will return and attempt to enter orbit in 2011 March.

doublebasin_messenger.jpg
 
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MeteorWayne

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Hi silyelene. Interesting theory, buty I see a flaw. Near simultaneous impacts leaving two aligned craters would be hard to justify with ones that big. A 160 km crater impactor deposits a huge amount of energy, more than would be dissapated in a sort amount of time between two coorbiting asteroids. In fact, it might even create molton magma..I'll have to work on that.

Just a first thought, though.
MW
 
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silylene

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MW, I rather agree too. If the second impactor landed in a pool of magma, that would create perhaps more of a splash as opposed to a second concentric ring. Whether this would create a second ring, i don't know. Would require some nice simulation studies. I think in this Mercury case, based on the ring size the hypothetical second impactor would have had to have been much smaller than the first, perhaps 1/4 the mass.

I do think odds are we should've found an example by now of an aligned double asteroid impact, somewhere in the solar system. I have been looking for several years for such an example. The Mercury crater might be one (?)
 
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xXTheOneRavenXx

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I do have a question for both of you because purhaps you may have different opinions. If the inner-most ring was caused by a magma flow from the initial impact, then would we not see an outward-flow pattern of sorts even with the settling of sediments after the impact? The detail in the image to me does not represent that of a magma flow (though quite a smooth area); but also hides details of that of a normal crater. I think a second impact and how it's material would have been ejected from the area could have been affected by the already present impact area depending on how deep the initial crater was and how high the surrounding ring wall rises and angle of incline from that point. What do you think?
 
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MeteorWayne

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While I'm reading up on crater formation, etc, here's a comparably sized crater on Venus called Meitner.

Note this is Magellen radar data, not a visible image, so the color differences represent radar reflectivity, not visual brightness.

Meitner_crater_on_Venus.jpg
 
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kg

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silylene":3leoudl1 said:
I do think odds are we should've found an example by now of an aligned double asteroid impact, somewhere in the solar system. I have been looking for several years for such an example. The Mercury crater might be one (?)

"Bullseye" craters do seem fairly common. Why do you suspect this one might be a double asteroid impact?
 
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