[meteorite-list] Impact Question

From: John Hendry <pict_at_meteoritecentral.com>
Date: Tue, 26 Apr 2011 21:41:51 -0700
Message-ID: <C9DCDE15.BBE8%pict_at_pict.co.uk>

Hard to pick typical melting points for mafics as composition seems to
have quite an influence. Just looking at the olivine end members you have
forsterite melting at 1890 degC and fayalite at around 1200 degC.

In the 1g melting energy calcs I ignored the nickel content so the numbers
I used for iron alone would only approximate meteoritic matter, but from
the above fact I was far less confident of what the typical melting point
of a stoney would be so plumped for Fe.

I was wondering if in the course of scientific study if samples of
different types of meteorites have been taken to their melting point, or
if the melting points can be reliably inferred from the mineralogy? I
would think this information might be related to predicting atmospheric
ablation rates.

Anyway with regard to the second sentence of your post Sterling; although
iron has a higher melting point than most rocks it may not be easier to
melt than basalt in terms of energy. The specific heat capacity of basalt
at 0.84 (kJ/kg/K) is substantially higher than iron at 0.45. Heating a
kilo of these two materials from 0K to their respective melting points
takes...

Basalt 1210K * 0.84 = 1016 kJ
Iron 1803K * 0.46 = 811 kJ

Not sure if the respective latent heat at the solid liquid phase change
influences this result much. I'm presuming not.

Do you have any spud gun designs?

John




On 25/04/2011 16:32, "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
wrote:

>John, List,
>
>Well, I was calculating for rock, not iron. Rock
>melts more easily, of course. My source (John S.
>Lewis, Physics and Chemistry of the Solar System)
>says it takes about 1.2 x 10^10 ergs to melt one
>gram of "average" rock. Since a joule is 10^7 ergs,
>it would take 1200 joules to melt a gram of rock,
>or the kinetic energy per gram achieved at 1550 m/s.
>
>OK, I used my calculator for that one, but when I was
>clicking away at the Post, I didn't even use "back
>of the envelope" figures, just the back of my head.
>The figure was worse than approximate, more like
>order-of-magnitude rough, just to be sure that
>melt velocities were a fraction of impact velocities.
>
>"Rock" covers a lot of ground, with a high variability
>of melting points, different for each. Likewise, the
>"boiling" or vapor point of rock tends to be niot greatly
>about the melting point. Basalts melt at 1210 K.
>and boil at 1450 K., for example.
>
>The point is that the velocities associated with the
>energy necessary to vaporize a rock or even an iron
>are small compared to the velocities of the encounters.
>Further, the bigger (and more massive) the incoming
>object, the less effect the atmosphere will have on it.
>If it's big enough, there is little or no retardation.
>
>Partly, it's a matter of time. The time for atmospheric
>heating of a 10 meter rock to commence and reach
>high temperatures is very short, a few 100 milliseconds,
>so retardation starts at high altitudes. The retardation
>is called compressibility drag, "pushing" asgainst the
>atmosphere. The ability of an object to overcome this
>drag is proportional to its "loading," the mass per unit
>cross section.
>
>Whatever the loading of a 10 meter rock, the loading
>of a 100 meter rock is ten times greater (1000 volumes
>divided by 100 cross-sections); the loading of a 1000
>meter rock, 100 times greater, a 10,000 meter rock,
>1000 times greater.
>
>To a Chicxuluber, the Earth's atmosphere is effectively
>a vacuum that offers so little resistance as to not even
>be mentioned in the footnotes. Not that there's time
>to write footnotes.
>
>A Chicxuluber transits the atmosphere in 2 to 4
>seconds, and when the surface of impactor strikes
>the surface of the Earth, its backside is still in the
>stratosphere, moving at Full Tilt Boogie while the
>frontside is stopping dead in its tracks.
>
>A great deal of the converted kinetic energy goes
>into the shock wave that starts at the contact and
>flashes back into the still-oncoming rock. Shock waves
>have no inherent limitations.
>
>The stellar explosion we call a supernova is the result,
>not of nuclear reactions, but a gravity-driven shock
>wave (when the radiation pressure fails and the material
>of the expanded star falls back in on the center).
>
>(Answering other objections here...)
>
>Unlike nuclear reactions which convert a small fraction
>of a mass into energy, the conversion of kinetic energy
>is purely mechanical. The term "conversion" may not
>be the best, but that's the term they use in physics,
>with the context of "transforming" energy from one
>form to another: motion into heat which vaporizes
>rock which in turn imparts kinetic energy to the rock
>vapor molecules which expand, and so on.
>
>We tend to think of nuclear reaction devices as the
>ultimate weapons, of course, but imagine if you had
>one of those railgun thingees and could speed up a
>10 kilogram steel slug to 920 kilometers per second.
>OK, I know that's really fast, but in a vacuum with lots
>of energy on hand (about 1200 gigawatt-hours) and
>a long enough railgun, all possible in orbit somewhere,
>why not?
>
>Unless my calculator is confused, that's a kinetic
>energy very close to 4,183,999,999,994,176 joules,
>or one megaton, stored in a 10 kg slug, a mass which
>if it were plutonium (and standing still) could "only"
>produce an explosion of 0.00002 megatons.
>
>Potent stuff, kinetic energy.
>
>
>Sterling K. Webb
>--------------------------------------------------------------------------
>--
>----- Original Message -----
>From: "John Hendry" <pict at pict.co.uk>
>To: "'meteorite list'" <meteorite-list at meteoritecentral.com>
>Cc: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
>Sent: Monday, April 25, 2011 12:56 PM
>Subject: Re: [meteorite-list] Impact Question
>
>
>Sterling,
>
>On 24/04/2011 23:28, "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
>wrote:
>
><snip>
>
>>It takes a little over a joule to melt a gram of rock; that's
>>the kinetic energy of that gram traveling at the sedate
>>velocity of a mere 2100 m/s. A good-sized, high-speed
>>impactor would turn to plasma with close to 100%
>>efficiency.
>
><snip>
>
>I followed all but the aboveS
>
>Assuming physical properties for say pure ironS
>
>Specific Heat Capacity for iron = 460 J/kg/degK
>(http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html)
>Melting point of iron = 1530 deg Celcius = 1803 Kelvin
>(http://www.muggyweld.com/melting.html)
>Assuming incoming temperature of impactor is 200 Kelvin
>(http://en.wikipedia.org/wiki/Asteroid_belt)
>
>Then to raise the 1 gram impactor to its melting point requires a
>temperature increase of 1603 K and the energy required to do this should
>be roughly this
>1603 x 0.001 x 460 = 737 Joules.
>
>So a typical value would be more like one *Kilojoule* to melt a gram of
>meteorite if I have my sums right (stone would be higher, maybe around
>twice as much as iron)
>
>Considered as kinetic energy, 1000 Joules would represent a velocity of
>sqrt[1000/(0.5*0.001)] = 1414 m/s which is ballpark consistent with your
>velocity estimate, but the energy you quote is a tad on the light side
>is
>it not?
>
>Regards,
>John
>
>
>
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Received on Wed 27 Apr 2011 12:41:51 AM PDT