This material is quoted verbatim posts made to sci.astro by James N. Head of the Lunar and Planetary Lab (firstname.lastname@example.org). 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 email@example.com | 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 firstname.lastname@example.org | So little time