| Allen W. Sherzer | "A great man is one who does nothing but leaves |
| aws@iti.org | nothing undone" |
+----------------------101 DAYS TO FIRST FLIGHT OF DCX----------------------+
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Date: 13 Jan 93 06:56:36 GMT
From: "James N. Head" <jnhead@lpl.arizona.edu>
Subject: SNC meteorites
Newsgroups: sci.space,sci.astro,alt.sci.planetary
Why the SNCs are most probably from Mars
There has been in recent days some discussion of the SNC meteorites.
Put together, the articles have mentioned much of the geochemical
data underlying the SNCs = Mars rocks arguement, and some of the
dynamical and impact arguements as well. I thought some of you
might enjoy a summary, a history lesson really, of the evidence
supporting a Mars origin for the SNCs and the theoretical research
this idea inspired. This article should also tie together all the
loose threads--as well as provide a primer for the next person
who has a question on this topic, so you might want to keep a copy.
Sources:lecture notes, private discussions, Friday night beer hours,
various colloquia with H. Jay Melosh, Michael J. Drake, John S. Lewis,
Steven K. Croft, Robert B. Singer, and fellow grad students during my
first two years at LPL, U of A. They are not responsible for any
errors in what follows--all mistakes are mine. (I'm pretty sure I
got it right, though).
There are 8 SNC meteorites: 4 Shergotites, 3 Nahklites, 1 Chassignite.
The shergotites are basalts (lavas), the rest are either olivine or
pyroxene cumululates. Cumulates are the rocks you get when crystals
settle to the bottom (accumulate) of a magma chamber. Geochemical analysis yields the following information:
1)the SNCs were equilibrated at ~4.51Ga (U-Pb and Rb-Sr). This is
interpreted as indicating the time of late accretion or core formation.
This is the same age given by lunar and earth rocks, i.e., the SNC parent
body formed at about the same time as the earth and moon.
2)the whole rock Sm-Nd isotopic system was reset at approximately
1.3Ga. A thermal event this young is unique among the meteorites. Even the youngest moon rocks (that we've found) are >3.0Ga. This is interpreted
as the formation age of the SNCs. This is comparable to the age of the
Pike's Peak granite (=NOT age of Rocky Mtns).
3)the individual minerals show an event at ~180Ma (yes million) in the
U-Pb, Rb-Sr,and Ar-Ar clocks, i.e. those easiest to disturb. No record
of this event in the Sm-Nd dates. This is thought to represent the date of
the impact which ejected the SNCs from their parent body.
4)Rare earth element (REE i.e., the lanthanides) analysis indicates
the presence of garnet minerals in the source region of the shergotites
(the lavas). This indicates a source region pressure of >~40 kbars, about
the central pressure of the moon. This pressure is reached at depths of
about 120km on earth, 360km on Mars. Thus the SNC parent was almost
certainly larger than the moon.
5)The oxygen isotopic abundances indicate the all 8 SNCs came from the
same parent body. The abundances are distinct from the earth and moon
abundances, therefore the SNCs are not recaptured earth rocks. These
particular measurements are tricky to make (the lab apparatus has
a memory of earlier samples) but I've been told the results are reliable.
6)Noble gas signature of one SNC is "a dead ringer" for the noble
gas abundances measured for the Martian atmosphere by the Viking landers.
This is the coup-de-grace. The noble gas abundances of mars, earth,
venus, and the moon have been measured. They are unique (read
"diagnostic") to each planet. This particular rock had gas bubbles
trapped in glass. Apparently, the other SNCs don't have any trapped
gases, or have been too contaminated to get meaningful results. The
result for the one rock depends on a correction for earth weathering.
7)Cosmic ray exposure ages range from 1-10Ma. This means the SNCs were
in <1meter-sized chunks for about 1-10Ma preceding their fall to earth.
This all gives a (grossly oversimplified) history as follows:final accretion,
core formation(4.5Ga); partial melt of some Mars mantle material at depth,
some melt reaches surface, resets SM-Nd (1.3Ga); large impact ejects some of this material from Mars (200Ma); on-orbit collisions break SNCs down to ~<1m size(1-10Ma); SNCs fall to earth(recent).
I am told that at a LPSC meeting (held before the gas bubbles were discovered
and the noble gas measurements were made), Chuck Wood summarized much of the above data and concluded that if the SNCs weren't from Mars, he didn't have
the foggiest notion where else they could be from. This met with, to
put it mildly, profound skepticism from at least two rather vocal
theoretical physicists. One of them (Melosh) is a world class authority
on the physics of impact events, and was very relunctant to believe that
an impact could eject relatively unshocked material from a planetary
body. Melosh made a careful theoretical study of impacts....and found
a way to get unvaporized, unmelted, not even highly shocked material
accelerated to >5km/s (Mars escape velocity). The process is called
spallation and is described in Geology 13:144-148 (1985). See also
Vickery and Melosh (1987) Science 237:738-743. In brief, the part of the
shock wave associated with an impact near the surface (P=0) causes a slightly
collosal pressure gradient which can eject material at twice the particle
velocity. A lab example is fairly easy to make:whittle a knob at the end of
a dowel until the last ~1cm or so is barely still attached, then bang the other end with a hammer. The hammer blow sends a compression wave down the dowel, inducing a particle velocity vp. The end of the dowel will fly off at 2*vp
because of the interaction of the wave with the free surface. Only a very
small amount of material is "spalled" off the planet, no more than roughly
10% of the impactor mass. But it is theoretically possible.
By some coincidence, at the conference where Melosh first presented these
results, there were the first announcements of meteorites that had almost
certainly come from the moon (we have real moon rocks for comparison), thus
demonstrating that you can get rocks from A to B, even without a spaceship.
So you ask, are there any 200Ma craters on Mars overlying 1.3Ga terrain?
Maybe. Lyot (~50N 330W?) is the best bet as of the Vickery & Melosh paper.
There are 12 candidates in all. Note well:spallation does not require
very oblique impacts, so circular craters (incidence angle <~80) are
fair candidates.
Of course, we'd really like a sample return mission before laying this
whole question rest. But on the whole I think it would be a great
shock if such a sample disproved the SNCs=Mars rocks theory. For then we'd
*really* be at a loss.
Volcanic ejection:We had a homework on this...eruption velocity is due to
exsolution of gases. Most common gas in terrestrial volcanoes: H2O.
Enthalpy is conserved during expansion and converted to kinetic energy.
Then Vej = sqrt(2*h/u) where h = CpT= (7/2)*RT is the enthalpy and u is
the molar mass. For H2O at 1200C (earth upper mantle T) Vej ~1km/s.
This velocity is consistent with the maximum reported range (~10km) of
volcanic bombs (ballistic). The 100km plumes on Io are consistent with the enthalpy of SO2. To get Vej~5km/s at 1200C you need a real light gas, like
H2... but you'll have a hard time convincing me H2 is the most abundant gas
dissolved in the Martian mantle. For an H2O rich mantle, you
need an eruption T of ~7100C to get to Vej~ Mars escape. If you don't
know, this temperature is probably much higher than the central core T
of earth or Mars, and much much too high to be at all plausible for an
eruption temperature. So our natural geologic cannon is just too feeble
to get rocks off Mars.
More references:Impact Cratering as a Geologic Process (H.Jay Melosh, Oxford
Press) for spallation. Jagoutz & Wanke (1986) Geochemica et Cosmochemica
Acta p.~946 for isotopic studies of SNCs.
-JazzerJim
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Date: 13 Jan 93 08:10:15 EST
From: Chris Jones <clj@ksr.com>
Subject: Soviet space disaster?
Newsgroups: sci.space
In article <20639@ksr.com>, I wrote:
>(I have received email from Ken Schmahl which says that near the end of
>_Spycatcher_ there is a report of a Soviet cosmonaut being stranded in orbit
>and transmitting for hours before dying. My first impression is that this
>sounds a lot like the other unreputable reports I have read, but since I think
>this source is supposed to be more reliable than average, I'm going to give it
>a look).
I took a look at _Spycatcher_ in the library last night and I couldn't find
this reference. I read the index, and skimmed the last 200 pages of the book.
I'm fairly confident I would have picked out any reference that was at least a
paragraph or two long, but it's not a sure thing. So ... If anyone can give me
a more precise reference to *where* in _Spycatcher_ this story might be found,
I would appreciate it.
(While there, I also checked out Oberg's _Uncovering Soviet Disasters_ (that
may not be the exact title). He has more than a complete chapter on
spaceflight, and concludes that the only Soviet inflight fatalities have
occurred in the two announced incidents. He has quite a bit about unannounced
launch failures of unmanned rockets, some discussion about training accidents,,
etc., but he pretty much debunks the reports of other manned disasters.)
--
Chris Jones clj@ksr.com
------------------------------
Date: Wed, 13 Jan 93 09:11:46 EST
From: John Roberts <roberts@cmr.ncsl.nist.gov>
Subject: STS-54 launch
STS-54 launch nominal to main-engine cutoff and external tank separation.
Launch was at about 9:00 AM EST.
John Roberts
roberts@cmr.ncsl.nist.gov
------------------------------
Date: Wed, 13 Jan 93 11:06:48 GMT
From: Tero Siili <siili@sumppu.fmi.fi>
Subject: Voyager Titan & Saturn images in GIF or JPEG formats
Newsgroups: sci.space
Question: is there a FTP site, which would have Titan and possibly also Saturn
images in GIF or JPEG formats? I would like to use those for preparing
overheads and various reports.
I tried the AMES archive, but found nothing from /pub/SPACE/GIF.
All pointers - preferably via e-mail to one of the addresses below - are
appreciated.
Best regards,
Tero Siili
Tutkija Scientist
Ilmatieteen laitos Finnish Meteorological Institute
Geofysiikan osasto Department of Geophysics
PL 503 P.O. Box 503
00101 Helsinki SF-00101 Helsinki
Finland
Puh. (90) 192 9533 Tel.: +358-0-192-9533
Telekopio: (90) 192 9539 Telefax: +358-0-192-9539
FUNET: pouta::siili SPAN: 22104::pouta::siili
Internet: Tero.Siili@fmi.fi
X.400: c=fi;admd=fumail;o=fmi;giv=tero;sur=siili;
c=fi;admd=mailnet;pr=il;giv=tero;sur=siili;
--
Tero Siili
Tutkija Scientist
Ilmatieteen laitos Finnish Meteorological Institute
