[meteorite-list] Re: Meteorites on Mars

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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><HTML>Hi Francis, 
 
It's nice and passes the smell test, and I now realize we are looking for a=20=
threshold size limit for survival.  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  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.  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.  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. 
 
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, francisg=
raham_at_rocketmail.com escribe: 
 
<BLOCKQUOTE TYPE=3DCITE style=3D"BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT=
: 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> 
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 =3D ` .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 
</BLOCKQUOTE> 
 
</HTML>
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Received on Thu 08 Jan 2004 06:29:54 AM PST

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