Martian Meteorites Discussion Summary Tabulated by Adeene Denton

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1 1 Martian Meteorites Discussion Summary Tabulated by Adeene Denton Greenwood et al., 2008 The Summary We began by reviewing the main findings of the Greenwood paper specifically, they identified the true deuterium/hydrogen ratio in martian meteorites. The three meteorites used were the Los Angeles, Alan Hills 84001, and Shergotty meteorites. The goal of this paper was to avoid measuring hydrogen that was added to the sample from terrestrial contamination in order to measure the bulk water content of Mars. From their resulting D/H ratios, they found two stages in the evolution of water in martian history, with a split in fractionation at 3.9 billion years ago. Before the 3.9 billion-year mark, Mars had significant early water loss followed by modest loss after the switch. The D/H ratios imply, however, that most of the water was lost by 3.9 billion years ago. The Hypotheses We moved on to possible hypotheses for explaining the change water loss, which must have had something to do with processes active in the earliest stages of Mars s history the loss of the magnetic dynamo (ALH is still magnetized, so we know it existed very early on), or possible early cessation of the late heavy bombardment. The biggest question we have is: are the ages of these meteorites correct? Different ages change how we reconstruct martian geologic history. The date of ALH84001 is specifically the problem, since that is our only really early meteorite, giving us our benchmark dates for the transition. However, even if the age of ALH is incorrect we still observe the measured split in fractionation between massive early loss and slower later loss. The Discussion The discussion turned to possible problems with their methods and how different researchers approach getting the most accurate data. The D/H ratios listed in the paper are measured in apatite grains; however, apatite can get re-set during a shock event, and the meteorite likely experienced several. Researchers looking for a D/H ratio at the time of formation of the rock have to avoid fractures and cracks, because fluids can infiltrate those cracks and instigate diffusion-based changes in crystal structure. The D/H ratio that Greenwood et al. measure is supposed to reflect magmatic water (as this is closest to the water budget of the martian mantle and thus most of Mars at the time), which depends on the pressure of the magma and other local properties. What they ve actually measured is a partition coefficient, a ratio of concentrations rather than a true value. Getting from D/H ratios to the actual characteristics of the magmatic water is difficult, as Ralph Miliken points out, since the partition coefficient depends on so many different system properties. We can only estimate a relative amount of water lost (some percent of the original), but since we don t have the original inventory we can t get at a volume. We can try to back-calculate from current values, which many research times have done. This is why we have Maven today so we can get better estimates of atmospheric lost rates in the present and thus make better estimates extrapolating backwards.

2 2 We finished by summarizing the hypotheses that result from this paper s findings how do we envision their demonstrated water loss actually happening? We know that such water loss is intimately tied to the loss of the atmosphere. We suggested that large impacts could be removing lighter gases, while turning off the martian dynamo could decrease Mars s ability to hold on to its atmosphere and water loss from degassing of the early magma ocean phase (very early on) could generate large changes in D/H ratios. There are different timescales for all of these possible hypotheses, which present different implications for early martian geologic history. Lapen et al., 2010 The Summary This subject of this paper directly follows from the previous one s issues with the ages of these meteorites, particularly our discussion of ALH Lapen et al. find a younger age for ALH84001 (from 4.5 Ga to Ga) and postulate that it might be linked to the sources of the shergottites, which are part of the younger groups of martian meteorites. The Hypothesis The authors tie their younger date for ALH84001 to a new interpretation of the evolution of Mars s magma ocean. ALH84001 has a source that is more enriched with incompatible elements compared to the shergottites, but they can still be broadly tied together to one ancient source. The authors conceptual model for a genetic tie between the two systems requires that the magma ocean s broad crystallization forms cumulates, but also traps smaller sections of liquids that crystallize later. Shergottite-type rocks are formed from the cumulates while ALH84001 forms from the residual trapped liquid that is left behind. The authors imply that the magma reservoirs producing volcanism on Mars (e.g., Tharsis and such) have been active for a long time because of this new age for ALH The Discussion We began by asking: how does this model actually work? Geochemical work suggests that we can use the same reservoir (here, the early magma ocean) to make both ALH84001 and the shergottites, but they don t align perfectly on a mixing line. Therefore, the differences between them cannot just be a crustal assimilant added to a magma reservoir. Figure 1: The new Lu-Hf isochron for ALH We found ourselves confused about the timeline outlined by the authors and the ease with which the paper tied ALH84001 to the shergottites. Ralph Miliken explained the multistep logic involved: in this conceptual model, the cumulates involved in magma ocean crystallization are in equilibrium with the residual trapped liquid. If those cumulates are later melted to produce the source magmas for shergottites, they represent a depleted endmember in terms of compositions. Previous investigations into the formation of the suite of shergottites have modeled a mixing process that generally requires a

3 3 depleted endmember as part of said process. The hypothetical magma ocean cumulate presented here is consistent with that preexisting shergottite model, and thus is palatable to the community. We were also confused by the author s decision to compare the values here to KREEP basalts, as it essentially shows that different planetary bodies can have very different paths towards differentiation. We concluded that the authors are demonstrating how a similar process (magma ocean crystallization) can have completely different results because of different planetary starting compositions. We can compare them to show how different degrees of crystallization manifest itself. Figure 2b: The aforementioned comparison between shergottites (red circles) and lunar KREEP basalts (blue squares). After the general discussion of the paper s concepts, we then walked through some guided discussion questions. First, the paper asserts that the primitive crust of Mars was completely obliterated, but why does that follow from getting a younger age of ALH84001? The paper assumes that this younger age means that the late heavy bombardment was much more efficient than the older age had previously suggested, because this is still the oldest sample we have. There are several issues with this, namely the problem of how and why we have the martian meteorites that we have. As we noted in our discussion, the residence times of meteorites in the solar system is not very long, so these martian meteorites were ejected fairly recently relative to their formation history on the planet. This includes ALH84001 it was not ejected during/immediately after its formation 4 Ga. Based on this, we decide that the new age of ALH does not rule out the destruction (or not) of an early crust, as there may be locations on mars that retain early crust that we simply don t have samples for. We then asked what processes could actually destroy the crust, or how likely Mars is to retain old crust. Sean Wiggins, our cratering expert, suggests that if the Borealis Basin (the hemispheric dichotomy-forming impact) was a real event then it could completely destroy and/or alter whatever surface existed at that point. An impact event that large would have lots of overturn, thick ejecta, etc. capable of making the new surface of Mars unrecognizable. The smaller major impact craters on Mars Hellas and Argyre wouldn t destroy the whole crust, but definitely a part of it. In the end, we note that one data point (ALH84001) is not a great way to make an assumption about the entirety of the martian crust. Second, what does the younger age of ALH84001 imply for the magnetic field strength of Mars before 400-Myr cooling? What we know about magnetization on Mars comes from measurements of ALH84001, which has a weakened magnetic field, as well as a magnetic field map for Mars derived from measurements by a magnetometer from the Mars Global Surveyor (which has fairly low spatial resolution). As such we know that there is remnant magnetization of the crust found only in

4 4 Noachian (ancient) terrain so we believe that a dynamo existed and was shut down by whatever happened at the Noachian-Hesperian transition that also eliminated most surface water on the planet. And as we discussed when thinking about the last paper, once Mars loses its magnetic field, it can t protect its atmosphere from stripping by solar radiation, so there is likely a connection. We conclude that a younger age for ALH84001 indicates a lengthier survival for the magnetic field that matches the magnetic field map produced by Mars Global Surveyor, but do not speculate further. Third, we discussed alteration processes on Mars in more depth, motivated by presenter Sierra Kaufman s expertise in mineralogy. In this case, the alteration process active on Mars that was measured in this paper preferentially depletes the samples in the Neodymium relative to Samarium. What is this process? Nd is slightly more incompatible than Sm because of a small difference in radii, but both are highly insoluble (resistant to weathering). Nd and Sm are concentrated in the apatite and whitlockite, which are phosphates that are then affected by weathering. Was it weathering on Mars or weathering on Earth? We do not have an answer for this question. We concluded that constraining Martian history is a difficult task, and without more data points we will continue to struggle. Moser et al., (2013) The Summary This paper tackled the problem of distinguishing between the age of crystallization and the age of the impact event for martian meteorites. Specifically, Pb-Pb values give a really old age, but mineral isochrons represent a much younger (Amazonian) age. How do we differentiate between these two different results? The Hypothesis Through their analysis, this paper develops a timeline for the formation of martian meteorites, stating that the source region for the magma was a depleted upper mantle during an ancient fractionation event (4 Gr), then crystallization ~180 my and launching towards earth 22 My. The Discussion We first discussed the author s choice of baddeleyite for dating purposes. Baddeleyite is ZrO2, a zircon oxide. We want to use zircon because zircon survives forever in a sample it has a higher condensation temperature so it resists change. The baddeleyite is in very small amounts, but making the extra effort to find it should lead to more exact results. The meteorite used was NWA5298, found in Morocco in We focused on the second figure, which shows the uncorrected Pb-Pb diagram it does not fall on the 4.1 Gyr reference line proposed by other research groups, so they are inconsistent. We initially found this figure confusing because the Pb-Pb is uncorrected so the ages don t have much meaning. Why is this plot shown? We realized with the help of Ralph Milikin that the plot is shown because it is directly tied to the true point of the paper, which is here to call out another paper Bouvier et al. that contends that all of the martian meteorites are 4 Gyr old. Figure 2 exists to point out that the minerals used by Bouvier have been mucked up but later geologic processes on Mars (FUBAR, according to Ralph), and so are likely not representative of crystallization ages since their structures

5 5 are more mobile and disposed to change. This is why the paper uses zircone instead, since it is much more difficult to mess up a zircon. The uncorrected data is shown a) because correcting the data requires certain assumptions that we want to avoid, and b) because even without correction we can tell that the baddeleyite data falls along the 0.2-Gyr line. The point is that all of the other minerals that have been previously used to date these meteorites cluster together and are generally not good data to use. We wondered why we haven t used zircons on ALH84001 if they re so amazing because we haven t found it yet. These features are so small and difficult to find that progress is slow and often by accident. We moved on to discussing the corrected data (which was corrected using urianium). With this plot we can track the original growth of the zircon as well as subsequent recrystallization and shock resetting via the points on the corrected data graph. The authors present a gradiation of values for shock resetting because the grains could have transmitted shock differently during the same shock event, but have fairly exact ages for the martian igneous event (the oldest date) and the meteorite s launch from Mars to Earth (the youngest date). Figure 2: The uncorrected Pb-Pb plots and the corrected U-Pb plots showing the usefulness of baddeleyite and the new timeline. Consider what does the 4.0 Gyr age that the French people are getting actually represent? There was some confusion about how a measured age could erroneously skew older rather than younger, since most alteration would skew it younger. They explain this discrepancy by saying that the crystallization age is much younger, but we can get an older age because of a mixture between terrestrial contamination and primitive martian lead in Pb-Pb dating. Crystallization ages may inherit an isotopic signature of a primitive mantle process, which is characteristic of the source region (where the magma came from) as opposed to the crystallization age. We try to correct for terrestrial contamination in our measurements, but are not always successful. We noted that this isn t just a martian problem we have similar issues with dating terrestrial igneous rocks.

6 6 Figure 3: A pictorial representation of the evolution of the Martian crust as seen by this shergottite. Lastly, we asked: why are all the Shergottites youngish? They re not identical ages, but they all cluster around a young age. We have a bias towards young basalts, but could it also be differences in survivability of material in space? If we consider Noachian crust vs. Hesperian/ Amazonian rocks, the Noachian rocks have been mucked up and are more friable ( brecciated megaregolith ), so if such rocks were ejected maybe they would fail to survive the journey to earth without breaking up. We also consider that maybe these were ejected from the same stack of lava flows, hence the clustering but we can t prove it. People are trying to find the impact crater that generated the SNCs, but it is incredibly difficult without know what size of crater is require to narrow down the search. People have tried using spectra, but that is a needle in a haystack approach. The French (i.e., Bouvier) have published a paper detailing the crater Mojave, a young crater southwest of Arabia Terra, as a source for the shergottites but base that on spectra, which we established as a sketchy justification in this case. They use the old age of the terrain to say that the shergottites are old, which is very obviously circular logic. The French also believe that all of the zircons have been melted during shock metamorphism, but the Moser paper accounted for that by using the zircons that have experienced the least amount of shock. We conclude that dating rocks, particularly those from another planet, is tricky business and should be handled with care. We suggest sample return missions from the martian surface to clear up some of these disputes once and for all. References Greenwood, J.P. et al., (2008). Hydrogen isotope evidence for loss of water from Mars through time. GRL 35, 5. Lapen, T.J. (2010). A younger age for ALH84001 and its geochemical link to Shergottite sources in Mars. Science 328, Moser, D.E. et al. (2013). Solving the Martian meteorite age conundrum using micro-baddeleyite and launchgenerated zircon. Nature 499, Nyquist, L.E., et al. (2001). Ages and geologic histories of Martian meteorites. Space Sciences Series of ISSI chronology and Evolution of Mars,