This material is quoted verbatim posts made to sci.astro by James N. Head of the Lunar and Planetary Lab (jnhead@lpl.arizona.edu). The posts were made on August 15, and August 21, 1996, with the subject lines Answers and References. Many thanks to James for sharing his information with us!
This is intended to organize the answers to many of the Mars-related
questions circulating the past week into a coherent whole (if not hole).
This builds upon my post regarding SNC meteorites from several years
ago. My relevant research interests are Mars compositional mapping
and computer modeling of the fragmentation and ejection of meteorites.
Hopefully this will prove definitive enough as an apology for its length.
I gleefully take credit for any distortions of the many sources I cite.
Material I've not yet seen posted is labled "new." Note: replies from
Cagle will not be answered.
The Martian Meteorite Clan
SHERGOTITES:
Shergotty.....................Observed fall in 1865, basalt
Zagami........................Observed fall in 1962, basalt
LEW88516
EETA79001.....................entrained gases
ALHA77005
QUE94201
Yamato793605
NAHKLITES:
Nahkla........................Fell in 1911. Noted for killing a dog when
it landed. What are the odds.....?
"Lafayette"...................found in a drawer at Purdue in 1931
Governado Valadares
CHASSIGNITES:
Chassigny.....................Observed fall in 1815, dunite
and the joker
ALHA84001.....................orthopyroxenite, contains carbonate grains
For more geochemical information about the Shergotites (except for
Yamato) see Wadhwa, et al. (1994), GCA 58 4213-4229. For more information
about ALHA84001, including its identification as martian, see
Mittlefehdt (1994) Meteoritics 29, 214-221, and Treiman (1995)
Meteoritics (no vol., all I have is the manuscript). The classic
paper on this is the now somewhat dated review by McSween (1985)
"SNC meteorites: Clues to martian petrologic evolution?" Rev.
Geophys. 23 391-416.
Evidence For A Martian Origin
(NEW)For a more authoritative statement, see McSween's 1985 review.
This class of meteorites is noted for coherent associations distinct
from all other achondrites in terms of mineral chemistry, redox
state, oxygen isotopes, and radiometric ages. The ratio Fe/Fe+Mg
is high, indicating these rocks came from fractionated liquids in
a manner similar to earth rocks. Amphiboles are present, indicating
the presence of hydrous magmas, again similar to earth. The SNCs
equilibrated under relatively oxidizing conditions compared to other
meteorites.
The SNC meteorites lie along a line on an oxygen-isotope plot.
This means that all these stones shared a common oxygen reservoir.
This reservoir is distinct from the one shared by terrestrial
and lunar rocks and distinct from other meteorite types. The
oxygen isotope data was considered a test of whether ALHA 84001
was martian, even though it is not an S, N, or C (Mittlefeldt 1994).
These meteorites come from the same place, and that place is
*not* the earth or moon.
(NEW)The SNCs have crystallization ages between 180Ma and 1.3Ga. These
are uniquely young ages for meteorites, indicating a parent body
much larger than any asteroid (cf. the Eucrites, which are thought
to come from Vesta on spectrophotometric grounds). The rule of
thumb for planets is that larger ones retain heat and therefore
maintain volcanism longer than smaller ones in the absence of external
tidal forces. This suggests Mars, Venus, or earth as the parent body.
(NEW)Rare earth element (lanthanide series) analysis indicates that
garnet was in the source regions of the Shergotites. Garnet is
unstable at pressures less than about 40 kbars, a pressure attained
at ~120km depth in the earth, ~360km depth in Mars, and about
1700km (i.e., the middle) in the moon. This means the Shergotite
parent body is larger than the moon, i.e., earth, Venus, or
Mars.
One (maybe more now?) of the Shergotites appear to contain trapped
atmospheric gases, entrained at launch. The noble gas abundances and
isotopic ratios (nitrogen too) are "dead ringers" for those measured by
the Viking landers. These data are different from Venus' atmosphere.
These meteorites are from Mars.
The Joker
(NEW)ALHA84001 generated great excitement when it was recognized as
martian partly because it contains carbonate grains. That these grains
are not terrestrial is indicated by 1)the presence of cracks penetrating
some of them and 2)an age of ~3.6Ga. These grains are mineralogic
evidence for liquid water on Mars at some time in the past, corroborating
the morphologic evidence (Baker, The Channels of Mars) and previous
mineralogic data (the presence of iddingsite as an accesory phase in
Lafayette, dated to 300Ma, Swindle, et al (1995) LPSC 26 1385-1386).
And of course, we all know that carbonate rocks are good places to look
for fossils on earth. This thought has driven some of the development
for future Mars probes.
Ejection From Mars (Spallation)--(NEW)
Some have pointed out that tektites were melted at impact, how
then could material be accelerated to Mars' excape velocity
relatively unshocked? The short answer is spallation (Melosh
1984 Icarus 59, 234-260, 1989 Impact Cratering, pp. 71 et seq.).
The impact causes a detached shock wave (detached because of the
reflection of a rarefaction wave from the back of the projectile)
throughout the target material. Near the surface, the shock pressure
never gets very high because of interference. At the surface,
the pressure is always zero by definition (Mars sfc pressure is 6mbar,
12 orders of magnitude less than plausible shock pressures of a Mbar
during impact). This causes a strong pressure gradient near the surface
that can accelerate material to high velocity, even though the peak
pressure is never very large. Thus, very high speed, relatively
unshocked material. This phenomenon is observed in laboratory
tests (Gratz, et al. (1993) Nature 363 522-524).
Spallation is also invoked to explain the presence of intact meteorites
on the moon (several were found in the lunar samples). The shock wave
from the impact travels through the projectile as well as the target.
When the shock reaches the "back" of the projectile, we again have
interference effects, which will spall material off the back of the
projectile at high velocities. Since the projectile is moving very
fast relative to the target, the material spalled off the back is left
with little velocity relative to the target.
The relatively thicker atmospheres of earth and esp. Venus may not
necessarily be the great hinderence to ejecting material into space.
In a large enough impact (such that the fireball is comparable to the
atmospheric scale height) hole is blown through the top of the atmosphere,
giving a low-density path for spalled material to take. For earth,
"large enough" is ~220MT. The corressponding crater is about 3km across.
This suggests that tektites should only be associated with craters this
big (so far, that appears to be the case). I don't know what size impact
you need for Venus, but the presence of parabolic features centered on
~60 large craters suggests this has happened there as well. The parabolas
have been successfully modeled as re-entering ejecta from the impact
(see Vervack and Melosh, 1993, GRL). This phenomenon explains the
time dependence of the IR flux from the SL9 impacts at Jupiter, and
ionospheric disturbances observed following high-altitude nuclear tests
over the Pacific c. 1960. This is all somewhat speculative until we
recover a meteorite unequivacally from Venus or Earth.
Ejection from volcanoes fails because the enthalpy of the volcanic gas
is far too small. The energy source is the exsolution of the dissolved
gases--enthalpy is converted into kinetic energy. Thus, 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 untenable eruption T of ~7100C to get
to Vej~ Mars escape velocity.
Age of Ejection
There are two ways to date the ejection event. One way is to radio-
isotopically date rock material that melted during impact. The
one case I know of this being done
(on ALHA77005, Jagoutz 1989 GCA 53, 2429-2441)
gave an ejection age of 15Ma, plus or minus 15Ma, not so good
precision (though it was a very important result for other reasons). The
other way is cosmic ray exposure dating (CRE). Cosmic rays penetrate rock
to about a meter. In doing so they break nuclei to create spall products.
The abundance of stable spall products is proportional to the time of exposure
(e.g., how long the rock has been in space and ~1 meter in size), and the
cosmic ray flux, which has been measured. Measuring in the lab the
abundance of these spall products gives the CRE age. See Peter Henderson's
text "Inorganic Geochemistry" for more details.
Terrestrial Age
(NEW)This is determined in a way very similar to CRE ages. In this case it
is the abundance of unstable nuclides that gives the age. In space,
unstable nuclides are produced at a steady rate by the cosmic ray flux.
They decay at a rate given by the rate constant for that particular nuclide.
A steady-state abundance is achieved between these two processes. Once
on the earth, the meteorite is shielded from cosmic rays by the earth's
atmosphere and the abundance declines from the steady-state abundance.
The lower the abundance, the greater the terrestrial age. The chief
nuclides used are 14C and 36Cl. Again, refer to Henderson.
Orbital Dynamics
The most recent discussion I know of is Gladman et al. (1996) Science 271
1387-1392. In that paper, Gladman reports new resonances for Mars
ejecta on their way to earth, increasing the transfer efficiency. Keep
in mind that of the twelve martain clan meteorites, 4 are observed
falls within the last 200 years. Keep your eyes on the skies....
Oldest Life Signs on Earth
The oldest fossils of earth life date to about 3.5Ga. At the press
conference, this date was re-iterated. The earliest indicators of
life go back to 3.8Ga. Altered carbon isotopic ratios is the evidence,
since life is known to prefer one carbon isotope over another. Consider:
fossils of early earth life are found in the oldest rocks that one
might expect to preserve fossils. Altered carbon isotopic ratios are
found in the oldest rocks likely to preserve such. Moreover, these
oldest rocks date to the *end* of heavy bombardment (derived from the
lunar record) during which time the earth suffered impacts energetic
enough to boil the oceans (imagine the Hellas basin-forming impact
on the earth). That suggests to me that life got going on earth as soon
as conditions allowed, furthur suggesting that life might not be all
that difficult to get going, once favorable conditions predominate.
--
James N. Head | IMP Calibration Team
Lunar and Planetary Lab | So many pixels
jnhead@lpl.arizona.edu | So little time
This message contains questions asked by several other posters to sci.astro concerning James' posting. Here he tries to clear up any confusions ... MWR
First of all, a brief word about petrologic shock indicators. For a
more complete listing, see p.41 of Melosh's book. Planar shock features
appear at 5-40kbar for most major rock-forming minerals. High pressure
phases of qtz (stishovite and coesite) indicate shock pressures of 15-40kbar
and 30-50kbar respectively. Melting initiates around 50-65kbar and
vaporization initiates ~1000kbar. Some of the range in these indicators is
due to reverberations within an individual rock, convergence at corners,
around conretions, etc. Note that these indicators set upper limits to
shock pressure.
Thus it is an over simplification to call rocks shocked or unshocked. I tried
to avoid this with the euphanism "relatively unshocked." I should have
included the reference standard. Prior to the theoretical work on
spallation in impacts (~1981, spallation in nuclear tests was first described
c1960) it was thought that accelerating rock to lunar escape velocity
should melt it (min. shock pressure of 400+kbar), while acceleration to
Mars escape should vaporize it (min.shock P ~1000kbar). Hence Gene
Shoemaker's statement "If you blast a rock ofa Mars, it ain't gonna be
a rock anymore!" He, and many other geophysicists, were wrong. This is
manifest in the presence on earth of ~10 unequivically lunar meteorites.
As for the SNCs (SNC = S,N,C= Shegotites, Nahklites, Chassignites), the
Shergotites are shock damaged--the plagioclase has all been altered to
maskelinite, indicating shock pressures of about 400kbar. Chassigny shows
shock damage consistent with pressures ~100-150kbar, while the Nahklites
have no known shock indicators present, setting an upper limit of 5-40kbars.
These values are all called "relatively unshocked" because under the old
no-good standard theory (where the impactor had zero radius, the shock wave
rise time was instantaneous, and at the surface theoretical pressures were
simutaneously zero *and* 100kbar) these rocks would have to have been
shocked to 1000+kbars, and therefore vaporized. So yes, ALHA77005 and
EETA79001 contain shock melt, and some atmospheric gas was entrained,
but that is remarkably little damage for a rock ejected to Mars' escape
velocity. A discussion of shock metamorphism of the SNC's is in McSween's
review article.
JB>I have access to Geophys, but what is GCA? Where to find it?
GCA is Geochimica et cosmochimica acta. Despite the title it is an
english-language journal (after ~1960, that is).
While I am at it, LPSC is the Lunar and Planetary Science Conference,
held at the Johnson Space Center every March.
GRL stands for Geophysical Research Letters
----deleted----
jnh>>The SNC meteorites lie along a line on an oxygen-isotope plot.
JB>I'm uncomfortable with the idea that "This reservoir is distinct from
>the one shared by terrestrial and lunar rocks". The O isotopes on
>Earth show no monotonic trend. In fact, there is a tremndous interplay
>of O isotopes found on Earth as a function of paleo-oceanic events.
---deleted---
I am talking about a del17-del18 plot, on which all earth rocks
plot on a straight line indicating that differences in say, the 18/16O
ratio between two rocks is due to mass fractionation. Moon rocks
plot on the same line, indicating a common origin (and consistent
with the giant impact theory of lunar formation). The SNCs also fall
on a mass fractionation line of the same slope (1/2), but offset. Mass
fractionation will not get you from the earth-moon line to the SNC line.
The McSween paper also reviews O-isotopes, including a plot.
>
jnh>>(NEW)Rare earth element (lanthanide series) analysis indicates that
>>garnet was in the source regions of the Shergotites.
JB>So the ejecta material which is the source for the Mars meteor of
>current public interest comes from 360km deep holes on Mars?
No. Shergotites are basalts. The rock melts at depth, then finds
its way to the surface. The REE pattern indicates what other minerals
were present when the liquid was in TDE with the environment. For the
shergotites, garnet was present, indicating a depth of ~360km on Mars for
the source of the *lavas*. It was some time later that the lavas (solidified
and now called basalts) were ejected from very near the surface (meters).
-----------objection about shocks deleted, addressed above------------
>> The noble gas abundances and
>>isotopic ratios (nitrogen too) are "dead ringers" for those measured by
>>the Viking landers. These data are different from Venus' atmosphere.
>>These meteorites are from Mars.
>
>Again, a dubious statement. Noble gas abundances vary tremendously
>over the face of the Earth.
I thought that the troposphere kept gases well-mixed on earth (the
eddy diffussion coefficient is many orders of magnitude larger than
the molecular diffusion coefficient). From reading the ozone literature,
I remember that the He/Kr ratio was constant from earth's surface
to the homopause (~100km). Look at the del15N-Ar plot in McSween's
review. The del15 values for SNCs are unprecedented for earth rock's.
They are entirely consistent with Mars rocks. Historically, the first
indicator was actually in the Ar isotopic data. SNCs (or the Shergotites
at least) are heavily enriched in 40Ar, consistent with them being billions
of years older than the solar system or atmospheric contamination from
a planet with an atmospheric Ar40/36 ratio 10 times larger than earth's.
Mars is such a planet. Venus is not.
-------deleted-------
>>The Joker
>
>>(NEW)ALHA84001 generated great excitement when it was recognized as
>>martian partly because it contains carbonate grains. That these grains
>>are not terrestrial is indicated by 1)the presence of cracks penetrating
>>some of them
>
>???? I've seen thousands of pictures (thin section, SEM, core photo)
>of carbonate grains with cracks penetrating the grains.
These were cracks related to shock, presumably the ejection event, but
possibly an earlier, nearby impact on the parent body.
>What is it about these cracks in the
>ALHA84001 rock that makes it a "dead ringer" for cracked carbonate
>grains found on Mars? And where did we get cracked carbonate grains on
>Mars to test so that we can know for a fact that the cracking of
>carbonate grains on Mars is intrinsically distinct and differentiable
>from carbonate grain cracking on Earth?
I didn't say anything like this at all. I was discussing evidence that the
CO3 grains are pre-terrestrial (see original post, copied above).
-link between SNC mineral chemistry and evidence for water on Mars deleted-
>
>Of course, this all assumes that the meteors is question are actually
>from Mars. You can't say rocks A and B are from Mars because B has the
>Mars-like qualities of A and A, interestingly enough, has all the
>Mars-like qualities of B.
If the mineral chemistry says these rocks came from a place were water
was present, that is consistent with being from Mars. As to the suppossed
circularity of the argument, there is no such. The link is that entrained
atmospheric gases, trapped in impact shock melt, in at least one of the
Shergotites, closely resembles the atmospheric gas measurements made by the
Viking landers. The matches include the abundance of noble gases relative
to chondritic values, the ratios of the many isotopes of Xe, the ratio
of Ar40 to Ar36, and the 15/14N ratio. The values measured in these rocks
are a much better match to Mars' atmosphere then they are to earth. The
ejection times for Shergotites are all much less than that of ALHA84001,
so any argument that Mars' atmosphere has changed from the past must keep
that in mind. The shortest CRE time is ~0.5Ma.
---deleted---
>All of which
>is supposed to lead to the idea that the most likely scenario is that
>these meteors are actually Earth ejecta that is just falling back on
>us after millions of years in a near-Earth orbit.
A conclusion inconsistent with the noble gas geochemstry of the SNCs.
The McSween paper has many references to geochemical analysis of the
SNCs, so you can look into those for more details if needed.
>
>>Spallation is also invoked to explain the presence of intact meteorites
>>on the moon (several were found in the lunar samples).
>What does this have to do with the post?
Completeness. Someone had asked if any meteorites were found by the
astronauts on the moon. The answer is yes, though no one realized it
until the soil samples were examined back home. On earth, the atmosphere
slows down the meteor so that it can land without obliterating itself.
This mechanism clearly will not work on the moon. Spall provides an
explanation.
--deleted--
>And if Mars can be spewing out all of this material which makes its
>way the Earth so "easily", then I expect to see Earth ejecta on the
>Moon, eh?
I think you mean Mars ejecta on the moon. Whether spall can really
get material off the earth remains to be seen.
>>Age of Ejection
>See!!! Here it is again!!! Now we're going to measure the age of
>ejecta by radio-dating the rock material melted during impact!!
Objection addressed above
>
>And cosmic ray dating will be inaccurate for the purpose of dating
>ejection, too. All the rock on Mars to a depth of about 1 meter will
>have these CR events. So you don't know if the CRE age is the age that
>the rock has been in space or the age that the rock has been at the
>surface of the planet _plus_ the time it has been in space. We use CRE
>dating of paleolithic fragments here on Earth, too. It is amazing what
>you can tell about how long a rock was at the surface millions of
>years ago by use of this technique.
I think I mentioned (perhaps in another article) that 2pi CRE exposure
can be distinguished from 4pi exposure. The Paul Warren paper lists
both 2pi and 4pi exposure ages. For terrestrial work, I suspect the
relative abundances of spall products are very different given the shielding
effect of the atm. Or you sure you are refering to CRE and not fission
track?
>
>>Terrestrial Age
--deleted--
>But C14 dating is as old as the hills..... and it has its greatest use
>with estimating the end of biological processes. No space travel here,
>eh?
I was expressly refering to 14C and 36Cl created by (nuclear) spall processes
in space. The age cited for ALHA84001 with this method is 13,000 years,
so I do not understand your objection that "tens and hundreds of millions
of years are being considered."
>>Oldest Life Signs on Earth
>
>>The oldest fossils of earth life date to about 3.5Ga. At the press
>>conference, this date was re-iterated. The earliest indicators of
>>life go back to 3.8Ga. Altered carbon isotopic ratios is the evidence,
>>since life is known to prefer one carbon isotope over another.
>
>The carbon method of dating is not accurate at the Ga time frame. No
>one thinks the method can be accurate at that elapse time. And the
>trick is not that life "prefers one carbon isotope over another". The
>mechanism is quite different than a mere ingestion preference. If all
>of your evidence is grounded on such shakey basis, then all bets are
>"off" as to the varacity of the proposition that meteors from Mars
>fall to Earth, and that some of these meteors contain evidence of
>life.
As John Park has already figured out, I made no reference whatsoever to
radiocarbon dating in the proceeding paragraph. The 3.5 and 3.8Ga dates
come from other radio-isotope methods (U-Pb, Sm-Nd, Rb-Sr, K-Ar). I am
well aware of the maximum reliable age obtainable with an isotope system
(roughly 50,000 years for 14C, is it not?) I was refering to the well-
known tendency of living organisms to retain one stable isotope
of C over the other (either 12 over 13 or vice-versa). This biological
fractionation of Carbon 12 and 13 is used as a proxy for temperature
by paleoclimatologists. Look for Wally Broeker's book (Build Yourself a
Habitable Planet, or something like that).
>
>> Consider:
>>fossils of early earth life are found in the oldest rocks that one
>>might expect to preserve fossils. Altered carbon isotopic ratios are
>>found in the oldest rocks likely to preserve such.
>
>This is an absolutely inaccurate statement about altered carbon
>isotope ratios in the oldest rocks where fossils might be expected to
>be preserved.
I did not link the two. If I add "altered ratios" to the last sentence,
does that help?
The only earth rocks older than 3.8Ga I know of are the 3.96Ga gneisses.
from Canada (reported by Bowring et al in Geology, 1990). I have no
knowledge of altered Carbon 12-13 ratios in that rock. Perhaps you
know differently? The 3.8Ga rock in question has 12/13C ratios
that are altered in a way consistent with biological activity. This
was reported to me as background information by Chris McKay of NASA-
Ames Research Center. Chris has dedicated his career to examining the
question of the origin of life on earth, so I took his word as to the
oldest *indications* of life on earth being the altered 12/13C ratios
in the 3.8Ga rock and the earliest *fossils* occurring in a 3.5Ga rock.
Do you still think my statement is absolutely inaccurate? If so,
could you define precisely how it is inaccurate. There may be a
nuance I am missing, but the general statement and conclusions are
correct, I think.
>I'm begining to think you may not know what you are
>talking about.Or am I being scamed here? Is the joke on me?
Let me know what you decide.
--
James N. Head | IMP Calibration Team
Lunar and Planetary Lab | So many pixels
jnhead@lpl.arizona.edu | So little time