[meteorite-list] Re: Meteorites on Mars

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:00 2004
Message-ID: <12b.38f248a6.2d2e9932_at_aol.com>

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Hi Francis,

It's nice and passes the smell test, and I now realize we are looking for a
threshold size limit for survival. I would support your ballpark figure and
combining some of the ideas I mentioned in the prior comment on the subject, add
that the slowing frictional force taking the initial velocity down to the
impact velocity must be integrated over distance...it is really a work question
in an exponentially increasing pressure situation, so I wouldn't put much faith
in the 0.004 multiple linear assumption you did, already shaky based on
limited Earth limiting size data points...and I see you acknowledge this by varied
it by 1000% (to 10 cm), so I guess that's directionally a good guess as any
can be. Also add that a 45 degree angle of incidence vs. vertical will add
about 41% (sqrt(2)) more frictional damping, so angle of incidence is quite
important to consider important, will help reach the thresholds you discuss. And I
think the potential terminal velocity will be limited proportional to the
sqrt(mass), as mentioned previously, so stone will be 2-3 times iron upper size
limit for similar shapes.

Now does that sound right to you? It's quite late here...
Saludos,
Doug Dawn
Mexico


En un mensaje con fecha 01/08/2004 4:10:31 AM Mexico Standard Time,
francisgraham_at_rocketmail.com escribe:

>
> Dear List,
> I was pondering what Ron had to say about hypersonic
> impacts and other comments.
> From the Wilemette, Alnighito and Hoba meteorites,
> it's safe to say the largest non-hypervelocity
> impactors on Earth are about ~10 meters, as an order
> of magnitude.
> To avoid hypervelocity impact, the object must be
> slowed by some dynamic pressure, which is proportional
> to the density of the atmosphere x velocity squared.
> The entry velocity is escape velocity for the planet;
> For no hypervelocity impact the final velocity must be
> less than the speed of sound in rock, which, in
> comparison to the planet escape velocity, and for the
> purposes of this crude proportionality calculation, is
> close to zero.
> The escape velocity squared is proportional to g
> for the planet. This is 0.4 for Mars, approximately,
> compared to Earth.
> The desnity of the Mars atmosphere is about a
> hundredth of Earth's, i.e., .01.
> So the max dynamic pressure available on Mars is
> about 0.4 x 0.01 = ` .004 compared to Earth. So a
> MASS only .004 of the largest meteorites on Earth
> could be brought below hypervelocity on Mars.
> Since mass is proportional to radius cubed, the
> largest meteorites on Mars to survive hypervelocity
> impacts are therefore in the order of about 1 meter in
> size. Since that is an approximate upper limit, we
> would expect to find centimeter-size to 10 cm. size
> meteorites in the Gusev strewnfield.
> Is my thinking right on this? I admit I made a
> great many handwaving assumptions and used a very tiny
> envelope to write on the back of. Am I in the ball
> park?
>
> Francis Graham
>


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<HTML><FONT FACE=3Darial,helvetica><HTML><FONT SIZE=3D2 PTSIZE=3D10 FAMILY=
=3D"SANSSERIF" FACE=3D"Arial" LANG=3D"0">Hi Francis,<BR>
<BR>
It's nice and passes the smell test, and I now realize we are looking for a=20=
threshold size limit for survival.&nbsp; I would support your ballpark figur=
e and combining some of the ideas I mentioned in the prior comment on the su=
bject, add that the slowing frictional force taking the initial velocity dow=
n to the impact velocity must be integrated over distance...it is really a w=
ork question in an exponentially increasing pressure situation, so I wouldn'=
t put much faith in the 0.004&nbsp; multiple linear assumption you did, alre=
ady shaky based on limited Earth limiting size data points...and I see you a=
cknowledge this by varied it by 1000% (to 10 cm), so I guess that's directio=
nally a good guess as any can be.&nbsp; Also add that a 45 degree angle of i=
ncidence vs. vertical will add about 41% (sqrt(2)) more frictional damping,=20=
so angle of incidence is quite important to consider important, will help re=
ach the thresholds you discuss.&nbsp; And I think the potential terminal vel=
ocity will be limited proportional to the sqrt(mass), as mentioned previousl=
y, so stone will be 2-3 times iron upper size limit for similar shapes.<BR>
<BR>
Now does that sound right to you?&nbsp; It's quite late here...<BR>
Saludos, <BR>
Doug Dawn<BR>
Mexico<BR>
<BR>
<BR>
En un mensaje con fecha 01/08/2004 4:10:31 AM Mexico Standard Time, francisg=
raham_at_rocketmail.com escribe:<BR>
<BR>
<BLOCKQUOTE TYPE=3DCITE style=3D"BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT=
: 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"><BR>
Dear List,<BR>
 I was pondering what Ron had to say about hypersonic<BR>
impacts and other comments.<BR>
 From the Wilemette, Alnighito and Hoba meteorites,<BR>
it's safe to say the largest non-hypervelocity<BR>
impactors on Earth are about ~10 meters, as an order<BR>
of magnitude.<BR>
&nbsp; To avoid hypervelocity impact, the object must be<BR>
slowed by some dynamic pressure, which is proportional<BR>
to the density of the atmosphere x velocity squared.<BR>
The entry velocity is escape velocity for the planet;<BR>
For no hypervelocity impact the final velocity must be<BR>
less than the speed of sound in rock, which, in<BR>
comparison to the planet escape velocity, and for the<BR>
purposes of this crude proportionality calculation, is<BR>
close to zero.<BR>
&nbsp; The escape velocity squared is proportional to g<BR>
for the planet. This is 0.4 for Mars, approximately,<BR>
compared to Earth.<BR>
&nbsp; The desnity of the Mars atmosphere is about a<BR>
hundredth of Earth's, i.e., .01.<BR>
&nbsp; So the max dynamic pressure available on Mars is<BR>
about 0.4 x 0.01 =3D ` .004 compared to Earth.&nbsp; So a<BR>
MASS only .004 of the largest meteorites on Earth<BR>
could be brought below hypervelocity on Mars. <BR>
&nbsp; Since mass is proportional to radius cubed, the<BR>
largest meteorites on Mars to survive hypervelocity<BR>
impacts are therefore in the order of about 1 meter in<BR>
size. Since that is an approximate upper limit, we<BR>
would expect to find centimeter-size to 10 cm. size<BR>
meteorites in the Gusev strewnfield. <BR>
&nbsp; Is my thinking right on this? I admit I made a<BR>
great many handwaving assumptions and used a very tiny<BR>
envelope to write on the back of. Am I in the ball<BR>
park?<BR>
<BR>
Francis Graham<BR>
</BLOCKQUOTE><BR>
<BR>
</FONT></HTML>
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Received on Thu 08 Jan 2004 06:29:54 AM PST


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