[meteorite-list] Question Martian in 3-D

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Mon, 10 Aug 2009 23:10:15 -0400
Message-ID: <e51421550908102010hc177e7flb95141efbb888097_at_mail.gmail.com>

That was an enlightening read, Sterling. :)

Thanks for the detailed explanation!


On 8/10/09, Sterling K. Webb <sterling_k_webb at sbcglobal.net> wrote:
> 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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>>
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Received on Mon 10 Aug 2009 11:10:15 PM PDT