comet hitting Mercury?
MAT
meteor?
http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/CN0131774306M.png
Chris L Peterson
>Is the ray system the result of a comet impact rather than an asteroidal
>meteor?
It is very difficult to say. Impact theory doesn't have much to say
about distinguishing the two, especially as it becomes increasingly
clear that some comets are rather asteroidal, and some asteroids are
rather cometary.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
Sam Wormley
And at the speed and energy involved, it hardly matters whether
the impactor is most rock or more of a mixture of rock and ice.
MAT
Dennis Woos
>
>>
>> And at the speed and energy involved, it hardly matters whether
>> the impactor is most rock or more of a mixture of rock and ice.
>>
How about this for a try: Force = Mass x Acceleration, so with a large
acceleration (or in this case a large deceleration as the comet/asteroid
strikes the planet) even the mass difference between rock and ice doesn't
make much of a difference, and the force will still be tremendous.
Dennis
Chris L Peterson
>And why is that?
Kinetic energy is proportional to mass, but to velocity squared. And
mass is proportional to the diameter cubed. The total density variation
between comets and asteroids isn't more than a factor of five or so. In
other words a comet only needs to be a bit larger than an asteroid to
have the same mass.
The effects of velocity are dominant (and there is something like a ten
to one range there), and only a small difference in size makes a big
difference in mass.
As far as material strength is concerned, any possible impact velocity
will be greater than the speed of sound in the impactor. That means that
it will behave the same whether it's liquid or solid, powdery,
fragmented, or rigid. The impactor itself will be largely vaporized
regardless of composition, so the ray structures are made up of the
underlying material, not the impactor.
Dennis Woos
>
> Kinetic energy is proportional to mass, but to velocity squared. And
> mass is proportional to the diameter cubed. The total density variation
> between comets and asteroids isn't more than a factor of five or so. In
> other words a comet only needs to be a bit larger than an asteroid to
> have the same mass.
>
> The effects of velocity are dominant (and there is something like a ten
> to one range there), and only a small difference in size makes a big
> difference in mass.
>
> As far as material strength is concerned, any possible impact velocity
> will be greater than the speed of sound in the impactor. That means that
> it will behave the same whether it's liquid or solid, powdery,
> fragmented, or rigid. The impactor itself will be largely vaporized
> regardless of composition, so the ray structures are made up of the
> underlying material, not the impactor.
> _________________________________________________
Wow, this is a much better answer than my post. Seems like I remember
posting in a physics context some years ago with similar results, and
deciding then to stick to topics that I know more about. I forgot. Last
spring during a phone conversation with my older son at college about a
physics class he was taking, he said "Even dad knows F=MA." Even me.
Dennis
Dennis
MAT
"Dennis Woos" <dpw...@gmavt.net> wrote in message
news:2sOdnZwXSePVP3PV...@posted.greenmountainaccess...
>> And why is that?
>>
>>>
>>> And at the speed and energy involved, it hardly matters whether
>>> the impactor is most rock or more of a mixture of rock and ice.
I asked Wormley for the answer.
MAT
composed of.
Only conjecture. Very fine grained material seems to be a good bet. I have
seen a similar pattern
emerge dropping a marble or small stone into flour.
Chris L Peterson
>Yes...but no one yet knows exactly how ray structures form or what they are
>composed of.
>Only conjecture.
No, that's not the case. There is good theory describing hypersonic
impacts, which has been tested experimentally. There is certainly more
work to be done, but what is known is far from "conjecture".
>Very fine grained material seems to be a good bet. I have
>seen a similar pattern
>emerge dropping a marble or small stone into flour.
A marble into flour is a very poor model of a high speed impact. The
situation when you have enough energy to completely vaporize both the
impactor and a good deal of the impact area is very different.
Sam Wormley
How about E_k = 1/2 mv^2 ?
MAT
>
> A marble into flour is a very poor model of a high speed impact. The
> situation when you have enough energy to completely vaporize both the
> impactor and a good deal of the impact area is very different.
Ok then, no earthly model can recreate that scenario?
Also, rays such as the Tycho phenom have not been fully explained...still
conjecture.
What is your personal opinom as to why rays are seen in certain situations
but not most?
Chris L Peterson
>Ok then, no earthly model can recreate that scenario?
Not sure what you are asking. Hypersonic impacts can be created
experimentally with special projectile guns. Experiments are used to
test impact theories.
>Also, rays such as the Tycho phenom have not been fully explained...still
>conjecture.
Again, there is a difference between "conjecture" and "not fully
explained".
>What is your personal opinom as to why rays are seen in certain situations
>but not most?
I don't think there is much doubt that rays are simply ejecta. An impact
event is highly variable: the kinetic energy of the impact, the momentum
of the impactor, the velocity of the impactor, the impact angle, the
nature of the area that is hit, possibly down to a much greater depth
than is actually excavated. I can't tell you how these factors affect
the formation of ejecta patterns, but I don't find it hard to believe
that they do.
Martin Brown
>> A marble into flour is a very poor model of a high speed impact. The
>> situation when you have enough energy to completely vaporize both the
>> impactor and a good deal of the impact area is very different.
>
> Ok then, no earthly model can recreate that scenario?
Laser ablation can simulate it pretty well. Although the people using
the technique these days tend to design their optics to produce clean
flat bottomed steep sided craters to get more even depth profiling.
Early kit with Gaussian beamshapes and IR wavelengths tended to produce
things that look a lot like lunar craters. More so if the target is
something with a low melting point like lead. These days they are mostly
working in the UV. The objective is to create a uniform plasma aerosol
of the material being ablated for ultra trace analysis.
I expect some of the devices for testing tank armour can simulate the
required conditions more or less perfectly. Most shaped charge
penetrating weapons rely on turning a part of the projectile into a hot
plasma ahead of a stream of molten metal. The main research now is into
reactive armour to counteract penetrating rounds . I doubt if you will
find much about this in the open literature.
>
> Also, rays such as the Tycho phenom have not been fully explained...still
> conjecture.
There is a big difference between "not fully explained" and conjecture.
>
> What is your personal opinom as to why rays are seen in certain situations
> but not most?
If the material jets out in specific directions then you get rays. If
the material spreads more uniformly you don't. The shape of the original
impactor and the surface topology of the impacted location probably
plays a part in determining the ray pattern. I expect once the impact is
well underway it will become pretty symmetrical. There are high speed
photos of how a nuclear groundburst proceeds on the net whch are broadly
comparable with the sorts of KE yeild of a serious impactor.
Regards,
Martin Brown
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