[meteorite-list] Question Martian in 3-D

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 10 Aug 2009 21:41:22 -0500
Message-ID: <B84CB7C289A246239267012C6FF76043_at_ATARIENGINE2>

Hi, Randy, List,

    Timorous about questioning "experts," local or otherwise,
I find the notion that a meteorite could sit on (or near) the
surface of Mars undisturbed for four billion years to be most
unlikely, also the assumption that there was no denser
atmosphere for the last four billion years, also the assumption
that a denser atmosphere is required. I also feel that there's
a misconception about meteoritic re-entry here.

    Common sense makes us want to question the "soft landing"
of any iron meteorite, but every surficial iron meteorite cannot
be excavated or exposed from any great depth. First, only very
slow (relatively) moving objects penetrate the soil rather than
vaporize, so the depth of burial is always shallow. Second, one
has only to look at HOBA, now weighing in at 60 tons, but
estimated to have been 100 tons at landing (from the quantity
of iron shale in a roughly circular disc surrounding it. Presumably,
at "landing," it had a shape very much like the Martian iron
in question (but a lot bigger), and its thinner edges have oxidized
away, leaving the present "blocky" core.

    How did a 100 ton chuck of iron make a soft landing on the
Earth? (You tell me how HOBA did it; I'll tell you how BLOCK
ISLAND did it.) The answer is: "it flew," like the Space Shuttle
"flies" (my candidate for Scariest Glider of All Time, except for
the WWII Soviet "Flying Tank"). It flew at a steep angle, yes,
but it flew.

    Flight depends on the atmosphere, and the chief factor in
the difference between the Martian and Terrestrial atmospheres
is the scale height. The rate at which pressure declines with
altitude is characterized by the scale height, the altitude at
which pressure has dropped by a factor of "e" (nat. log. base =
2.718281828). The scale height of the Martian atmosphere
is about 11 kilometers; for the Earth, it's only about 6 kilometers.

    The formula for the scale height is H = ( k * T ) / ( M * g ),
where k = Gas constant = 8.314 J?(mole K)^-1, T = mean
planetary surface temperature in Kelvin degrees, M = mean
molecular mass of dry air (units kg?mole^-1), g = acceleration
due to gravity on planetary surface.

    Molecular mass of the Martian atmosphere is about 50% greater
then the Earth's "M," but "g" is only 38% of the Earth's. Planets with
lower gravity have "taller" atmospheres, if you want to remember
it the easy way. There are always "wrinkles" to ideal gas formulas.
At very high altitudes, the "air" is so thin that diffusion is easy,
so every species of gas molecule has "its own" scale height nearer
the top of the atmosphere. But Mars' atmosphere is almost entirely
carbon dioxide, so that factor doesn't change the results much.

    Atmospheric pressure on the surface of Mars varies from around
30 Pascals on Olympus Mons to over 1155 Pascals in the depths
of Hellas Planitia, with a mean surface level pressure of 600 Pascals.
This is less than 1% of the surface pressure on Earth (101,300
Pascals). The equivalent pressure in the atmospheres of the two
planets can be found in Mars' thin atmosphere at a height of
34-35 km, where the pressure is the same above either planet's
surface.

    Here's where it gets to be fun. The Martian atmosphere at 60
to 80 kilometers above the surface, or 100 kilometers, is DENSER
than the atmosphere of the Earth at that height. And that is the
range of heights at which most meteors "light up" or begin to ablate.
In fact, all Martian atmospheric densities at altitudes above 34
kilometers are greater than the density of the Earth's atmosphere
at the same height, due to the fact that the pressure falls off less
steeply than is the case in the Earth's atmosphere.

    So, the meteoroid that would "light up" at 60 km in the Earth's
atmosphere, will presumably "light up" at a higher altitude in the
Martian atmosphere. It may very well be slowed enough to terminate
its ablative flight at a higher altitude on Mars than the Earth because
of the increased density above 34 kilometers. But it would likely
"stagnate" at a lower altitude (for the same reason of density), then
have a shorter but slower "dark fall" in the lesser Martian gravity.

    For "normal" meteoritic fall, the problem becomes "Watch that
bottom step; it's a doozy!" But by the time the smaller meteorite
encounters (IF it survives that deep) the lower atmosphere where
the density is less than in Earth's atmosphere, it's usually already
lost most of its "cosmic" velocity and is traveling at sub-sonic speeds.
(The speed of sound is of course different for Mars' atmosphere
also.) It is in the most survivable phase of it re-entry by then.

    However, if the newly-arrived meteoroid is a lenticular or even
rectangular "chip" (helpfully arriving a low entry angle and/or a
slower-than-usual entry velocity), it will tend to stabilize in flight.
First surface ablation only improves its aerodynamic characteristics.
So, when one says that for such a landing, it is necessary that Mars
have a more substantial atmosphere, well... The fact is that Mars
HAS a more substantial atmosphere than the Earth, at least above
34 km. And that's where all the action is... or most of it.

    I'm quite certain that Mars has as many or more meteors in its
skies than the Earth. Besides having a more dense upper atmosphere,
Mars is in the right neighborhood for stray rocks. Shower meteors
in the skies of Mars have been photographed by the Spirit rover:
http://www.obspm.fr/actual/nouvelle/jun05/meteor.en.shtml

    As for not finding any "other meteorites" as big as this one, we've
found how many? ONE other meteorite, I believe. Pretty small sample
to generalize from, don't you think? And we've searched how much
of the planet's surface?

    I understand that the official NASA position is that a thicker
atmosphere is required:
http://news.prnewswire.com/DisplayReleaseContent.aspx?ACCT=104&STORY=/www/story/08-10-2009/0005075085&EDATE=
"Scientists calculate it is too massive to have hit the ground without
disintegrating unless Mars had a much thicker atmosphere than it
has now." Ah, yes, "scientists calculate..." The press release has
spoken.


Sterling K. Webb
-----------------------------------------------------------------------------------------------
----- Original Message -----
From: "Randy Korotev" <korotev at wustl.edu>
To: <meteorite-list at meteoritecentral.com>
Sent: Monday, August 10, 2009 12:50 PM
Subject: Re: [meteorite-list] Question Martian in 3-D


> Carl et al.
>
> Regarding the Block Island meteorite on Mars...
>
> I asked "Why does it have regmaglypts?" of our local Mars expert, Ray
> Arvidson, who is Deputy Principal Investigator of the Mars Exploration
> Rover Mission. He had mentioned the existence of the meteorite to me
> several weeks ago. He said that the fall happened "4 billion years
> ago," when Mars had a more substantial atmosphere. This makes sense
> to me because we've never seen a meteorite this size on the Moon. On
> the Moon meteoroids impact at several tens of kilometers per second,
> and vaporize. In order to survive as a whole rock, Block Island must
> have been decelerated by an atmosphere. (I'm sure that meteoroids
> hitting Mars are impacting at lower velocities than those hitting
> Earth-Moon, but I don't know the numbers.)
>
> The area where the meteorite was found is a deflation surface - like
> Roosevelt Co., NM, and places in Antarctica. It was buried for a long
> time and then exposed when the dust blew away. They know it's a
> deflation surface because the surface is "young" - the crater count is
> very low.
>
> Only after writing the above did I find some 3D glasses and actually
> looked at the image. Most of the "holes" don't look so much like
> regmaglypts to me. Maybe some are chemical weathering features.
> There will probably be some more info about this meteorite coming out
> later. Ray said that there is a great interest on what kind of
> chemical reactions it's experienced.
>
> Randy Korotev
> Washington University
>
>
>
>
> At 11:54 07-08-09 Friday, you wrote:
>>Pete, List,
>>Very interesting photo.
>>I have a question about it's morphology?
>>Why does it look like that? Why does it have so many holes / dents?
>>Given the atmosphere on Mars being so thin compared with Earth, I
>>thought Earths Atmosphere is what caused this type of erosion of
>>surface materials? It was my understanding that the material ablated
>>away as it passed through the atmosphere . If that is so then why does
>>it look the same on Mars.
>>Is it possible that maybe it already looked like this before it
>>entered Mars' atmosphere?
>>Just curious.
>>--
>>Carl or Debbie Esparza
>>IMCA 5829
>>Meteoritemax
>>
>>
>>---- Pete Pete <rsvp321 at hotmail.com> wrote:
>> >
>> >
>> >
>> > Hi, all,
>> >
>> > An incredible view of a Martian iron in fine detail!
>> >
>> > (note the full resolution link)
>> >
>> > http://www.nasa.gov/mission_pages/mer/images/mer20090806.html
>> > http://www.nasa.gov/mission_pages/mer/images/mer20090806.html
>> >
>> >
>> > It suggests red/green, but red/blue works fine.
>> >
>> >
>> > Cheers,
>> > Pete
>> > _________________________________________________________________
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Received on Mon 10 Aug 2009 10:41:22 PM PDT