MARTIAN VOLATILE & ISOTOPIC SIGNATURES

Mars 2005 Sample Return Science Workshop
Presentation by Donald D. Bogard

I. INTRODUCTION

Data on martian volatiles comes from various sources:

Viking Atmosphere Measurements
Modest Ground-Based Spectra
Shock-Implanted Atmospheric Gases in martian (SNC) meteorites
Trapped Mantle(?) Gases in martian meteorites
Volatile-rich solid Phases in martian meteorites (Hydrates, Carbonates, Sulfates, etc.)

Figure 1 compares measurements of several volatile isotopic species made on Mars by Viking against measurements made in glass inclusion produced by impact in the EETA79001 martian meteorite. The close similarities between the two gas reservoirs across eight orders of magnitude in gas concentrations constitutes a strong argument for the martian origin of this meteorite.

Figure 2 compares the C/36Ar ratio against the N/36Ar ratio for several solar system reservoirs: the sun, lunar regolith, E (enstatite) chondrites, C1 and C3V carbonaceous chondrites, and the atmospheres of the earth, Venus, and Mars. The lower N/36Ar value for Mars is that actually measured by Viking, and the larger value is that estimated for early martian history prior to fractionated loss of nitrogen from the martian atmosphere. The ~5 order of magnitude spread in both of these ratios suggests that the ratio of noble gases to chemically-active volatiles on Mars could be quite variable, depending on the various sources for martian volatiles.

Data and models presented in this talk come from many sources. Apologies are given for the fact that no acknowledgments or literature citations are given.


II. ATMOSPHERIC VOLATILES:

Some Important Questions About Martian Atmospheric Volatiles:

  1. What Were the Sources of Martian Volatiles?
    (e.g., Mantle Outgassing, Solar, Comets, Cosmic Dust)
  2. How Do Isotopic Signatures Characterize Atmospheric Loss Mechanisms?
  3. How Much Water Was Lost and When?
  4. How Have Atmosphere-Surface Exchanges Buffered Volatile Loss?
  5. What Other Martian Processes Are Revealed in Atmosphere?
    (e.g., Outgassing, Exchange reactions, Nuclear interactions, etc.)

Some martian atmospheric components show isotopic mass fractionation that is indicative of significant early loss from the upper atmosphere:

2H/1H x5 Enriched Over Earth
15N/14N ~60% Enriched Over Earth
38Ar/36Ar ~30% Enriched Over Earth
Xe Isotopes 136/130 Enriched 16-25% Over Chondrites and Solar
13C/12C and 18O/16O Resemble Earth's, Resolution Poor

Several physical models have been offered to account for volatile loss and mass fractionation.

  1. Hydrodynamic Escape of H:
  2. Sputtering from Upper Atmosphere:
  3. Dissociative Recombination or Photochemistry: Martian atmospheric noble gases give conflicting isotopic patterns which indicate multiple origins for these volatiles. Figure 3 compares the isotopic compositions of Xe in the martian atmosphere, the earth, chondritic meteorites, and the Chassigny (martian) meteorite against the solar component, which plots as a horizontal line. The Xe composition of martian atmosphere (shock-implanted into EETA79001) is quite different from the Xe composition of Chassigny, and the latter closely resembles the solar composition. One can speculate that martian atmospheric Xe may have been derived by mass fractionation of either the chondritic or solar composition.

    Figure 4 compares the isotopic composition of Kr in the martian atmosphere, carbonaceous chondrites, and the solar composition against the earth's composition, the latter being plotted as a horizontal line. Martian atmospheric Kr resembles solar, except for an enhancement at mass 80, which is probably a neutron-capture effect on Cl. In addition, the 36Ar/38Ar ratio for the martian atmosphere may have a value as small as ~3.5, much lower than values of ~5.3-5.6 typical of other solar system reservoirs. Thus, Ar in the martian atmosphere resembles atmospheric Xe in that its lighter isotopes are relatively depleted compared to solar or chondritic components, whereas lighter isotopes of Kr are enriched compared to chondritic Kr and similar to solar Kr. The isotopic composition of martian Ne is too poorly known to make any detailed comparisons.

    Other volatile components measured in some martian meteorites are not believed to have been shock-implanted from the atmosphere, but rather to represent mantle gases. The solar-like Xe in Chassigny is one such component, and an analogous mantle component may exist for Ar. Figures 5 and 6 present the case for two components of martian N. Meteorite analyses shown on both plots define mixing lines that pass near the martian atmospheric composition measured by Viking and the composition of the earth's atmospheric N. Note that Figures 5 and 6 plot deviations of 15N/14N from a terrestrial standard (15N) against the 36Ar/N and 40Ar/N ratios, respectively.


    III. SURFACE VOLATILES:

    Some Important Questions About Martian Surface Volatiles:

    1. What are the martian, near-surface, volatile-rich minerals?
    2. When and how did these volatile-rich species form?
    3. What isotopic equilibria exist among rock-surface-atmosphere?
    4. Do organics exist in these phases?
    5. The questions above under atmospheric volatiles also apply.
    Measurements of martian meteorites have revealed several aspects about non-atmospheric martian volatile elements. The oxygen isotopic composition of silicates differs from other solar system reservoirs (Fig. 7). Small quantities of volatile-rich phases such as clays, carbonates, sulfates, amphibole, chlorite, and mica all have been reported present in martian meteorites. How and when did these phases form? Abundant carbonate (~1% (Ca,Mg)CO3) existing in ALH84001 probably formed from ground water. Because this martian meteorite was dated at 4.0-4.5 Ga old, the carbonate could derive from an early period of Mars. In contrast, water-bearing clay minerals in the martian nakhlite meteorites must be <1.3 Ga old, for that is the dated formation time of these rocks (see later discussion).

    An important question is whether O, C and other volatile elements within martian meteorites are in isotopic equilibrium or disequilibrium. Disequilibrium would indicate the requirement for more than one volatile source. To address this question, we must understand that the O18/O16 ratio naturally fractionates among phases in equilibrium, depending on the species present and the temperature. This is demonstrated in Figure 8 for CO2 and H2O in equilibrium with basalt. At 1000°C all three phases have very similar O18/ O16, but at 0°C this ratio in CO2 and H2O differs by more than 4%, or >40 per mill. (This isotopic difference would be written as O 18 >40%o, where indicates deviation of the O18/O16 ratio from a terrestrial standard. The notation delta O17 indicates deviation of O17 from the terrestrial fractionation line. The specific values of O18 for CO2 and H2O relative to the basalt depends on the CO2/H2O mixing ratio.

    Figure 9 is a plot of O17 vs. O18 for various martian phases. The sloped line is the expected mass fractionation line for species in equilibrium. Silicate samples from martian meteorites plot in the relatively narrow region of O18 =~3-5%. However, separated samples of the clay "mineral" called iddingsite plots at considerably higher values of O18, and may have a value of O 17 that falls above the martian fractionation line. Water collected by high-temperature pyrolysis of nakhlites shows a 18O similar to martian silicates, probably as a result of high-temperature exchange. These data suggest that martian surface water may not be in isotopic equilibrium with mantle-derived silicate material, and may be evidence for an additional surface component, with a composition possibly similar to that of CI chondrites (Fig. 9).

    Figure 10 compares the C 13/ C 12 ratio plotted as 13C in % notation compared to terrestrial carbonate for the carbon cycle of both earth and Mars. On earth, 13C/12C shows only modest fractionation effects <10% among the major reservoirs of carbonate, atmosphere, and mantle. Organic material on earth show a 13C value ~25% lower. With the exception of carbonate from ALH84001, the abundances of carbon-containing phases in martian meteorites are very low (sometimes only ~10 ppm), and the phase being measured in many cases is rather uncertain. Nevertheless, measurements of martian meteorites indicate a much wider range of 13C compared to the earth. In addition to the ALH84001 data, carbonates measured in other meteorites suggest a range of 13C at least as large as -5% to +30%. A sample with high-temperature C release was considered to represent mantle C and gave a 13C of ~-25%. The 13C measured by Viking for the martian atmosphere was ~-10%, but with the very large uncertainty of �50%o. One measurement in EETA79001 was interpreted to be atmospheric CO2 and gave 13C of ~40%.

    Both the oxygen and carbon isotopic compositions of martian materials could be consistent with two or more volatile reservoirs that are not in isotopic equilibrium. These different reservoirs could represent separate components accreted to Mars or components produced by mass fractionated loss processes discussed above. In addition the D/H (or 2H/11H) ratio measured in water released from certain igneous minerals contained in martian meteorites is greatly enhanced, up to about five times earth's ratio, and variable among different samples. The maximum enrichment 2H observed in martian meteorites is about the same as that observed in earth-based spectra taken of the martian atmosphere. This could imply that isotopic exchange has occurred between atmospheric hydrogen and water in these igneous minerals. Because some phases show _2H ratios only about twice that of the earth, the martian interior presumably contains a D/H ratio much lower than the atmosphere.

    To summarize the case with martian volatiles, isotopes of H, N, Ar, Xe, C, and O all suggest two (or more) distinct volatile components. One is interior and presumably reflects volatiles accreted with Mars. The others are probably surface and atmospheric components, produced either by heterogeneous late-stage accretion or mass fractionation during atmospheric loss, or both. Among the possibilities for accreted components are C1 chondrite material, comets, and cosmic dust. Because of fractionation mechanisms, most martian volatiles probably show a temporal variation in isotopic composition. Apparent existence of non-equilibrium volatile components on Mars in the recent past may indicates the lack of significant crustal subduction to mix these components. Analyses of martian-returned volatile-rich phases could greatly help in our understanding of these issues.


    IV. ISOTOPIC CHRONOLOGIES:

    Some Important Questions About Martian Isotopic Chronologies:

    1. What are the ages of major geological terrains on Mars?
      • What is the time scale of active volcanism?
      • What is the time scale of major impact cratering?
        (Must date surfaces to calibrate crater count ages.)
    2. Were early impacts related to water erosion of surface?
    3. What terrains were sampled by martian meteorites?
    4. How do initial ratios characterize rock petrogenesis?

    Because martian meteorites reveal groupings in their isotopic chronologies, we discuss them in this manner. Figures 11 and 12 show isochron plots of the 87Rb-87Sr and 147Sm-143Nd isotopic systems, respectively, for several shergottites, two nakhlites, and ALH84001. These figures can be used as reference for some of the characteristics listed below.


    Characteristics of Shergottite Meteorites:

    1. All are Appreciably Shocked
    2. They have model isotopic ages of ~4.5 Ga, which indicate an early initial differentiation of the planet. This also suggests minimal crust recycling compared to earth.
    3. Some Sm-Nd whole-rock "isochron" ages suggests ~1.2 Ga.
    4. All show Rb-Sr, Sm-Nd, & Pb-Pb ages of ~0.17 Ga.

    All shergottites are linked by the above characteristics. However, their mineralogies and initial isotopic ratios differ considerably. The latter suggests that they were not cogenetic 0.17 Ga ago, and they probably had a complex prior history. The nature of the ~0.17 Ga event is not completely defined and may have involved impact melting, igneous, and/or rock assimilation processes. Spectra signatures of the shergottites are similar to the Mars' uplands, suggesting that their composition may be common on Mars.

    Characteristics of Nakhlites & Chassigny

    1. Live 146Sm (halflife 103 Ma; decays to 142Nd) was present in the source rocks. The 142Nd/144Nd ratio is greater than the lunar value and suggests an even earlier mantle differentiation for Mars compared to the moon.
    2. All give isochron ages of 1.3 Ga by Rb-Sr, Sm-Nd, K-Ar, and/or U-Pb. (Does the similarity in this age and the ~1.2 Ga Sm-Nd model age of the shergottites suggest a relation?)
    3. None are significantly shocked

    Their initial isotopic ratios are similar, unlike shergottites. (See Longhi's presentation for the significance of initial ratios.)

    The Nakhlites & Chassigny are linked by the above characteristics. They likely had an igneous origin 1.3 Ga ago. They likely were ejected from Mars in a common event ~12 Ma ago, which is their cosmic ray (space) exposure age.


    Characteristics of ALH84001 (Orthopyroxenite with Carbonates)

    1. The Sm-Nd model age is ~4.57. Live 146Sm possibly existed.
    2. Rb-Sr & Sm-Nd Isochron Ages are ~4.5 Ga.
    3. A K-Ar (39Ar-40Ar) age is 4.0 Ga and may represent impact resetting.

    In many respects, ALH84001 differs from the other martian meteorites.


    A CHRONOLOGY SUMMARY:

    Isotopic chronologies signify very early mantle differentiation for Mars. However, most martian meteorites have rather young (<1.4 Ga) isotopic ages. Most of these ages appear igneous, but some may reflect effects of impact melting. Young isotopic ages are not obviously consistent with the interpretation from crater densities that most of Mars' surface is much older.


    V. ENERGETIC PARTICLE INTERACTIONS

    Some Important Questions About Energetic Particle Interactions:

    1. Martian meteorites give information about Mars' near-surface rocks. Questions about their ejection from Mars are: How? When? Where? and How many events?
    2. How can nuclear reaction products (stable & radioactive) in Mars surface samples help define:
    The past density of Mars' atmosphere if it experienced time variable shielding?
    Weathering Rates of Exposed Rocks
    Mixing Rates of Regolith

    To fully use the characteristics of martian meteorites to make inferences about Mars, we need to understand the origins of these samples. None of the martian meteorites resided for a significant period at the martian surface, for none show evidence for irradiation by cosmic rays on Mars. Exposure ages in space for the martian meteorites range over ~0.6-15 Ma, and show at least three and possibly as many as five groupings of these ages. Theoretical models suggest that relative large crater(s) are required to eject these meteorites into space (~10-100 km diameter, depending on the size of the objects ejected).

    Two viable models exist for explaining the space exposure ages of martian meteorites. One assumes that separate cratering events on Mars ejected those meteorites with a common exposure age. This explanation would require at least two events to eject the shergottites and one event to eject the nakhlites and Chassigny, and all three craters would have had to occur in relatively young (< 1.4 Ga) and presumably rare martian terrain. The second explanation assumes that all martian meteorites were ejected in one very large cratering event, and that the space exposure groupings were formed by later collisional break-up events in space. Figure 13 schematically shows one variant of this model whereby all shergottites were ejected ~0.18 Ga ago as a large shielded block, which was collisionally disrupted into smaller fragments ~3 Ma and ~0.6 Ma ago. This scenario permits only a single impact into young terrain, but requires a very large crater, which may not exist. Various combinations of these two exposure models and of the ejection time in model two can also be envisioned.

    Although the martian meteorites give no evidence of energetic particle irradiation on Mars, the present rarefied martian atmosphere allows entry of cosmic ray particles, which undoubtedly produce nuclear products at the martian surface. The quantity of the products produced are dependant upon the specific product and the total shielding offered by the atmosphere and surface material overlying the sample. Nuclear products produced by energetic primary and secondary particles show a maximum in their production at a shielding depth of ~0-50 g/cm2, whereas nuclear products formed by thermalized neutrons reach a maximum concentration at a shielding depth of ~200 g/cm2 (Figure 14).

    In principle, differences in production rates of various nuclear products as a function of shielding might be used to reveal certain aspects of martian surface history:

    1. An earlier epoch of a dense martian atmosphere. This possibility assumes that an early atmosphere was dense enough to effectively shield the surface, which because of the different chemical composition of atmosphere and rock, would greatly decrease the production of certain nuclear products over that time. Thus, if the atmosphere contains less stable nuclear products (e.g., 21Ne) than expected, that might indicate an extended period with a dense atmosphere. In addition, some ratios of cosmogenic nuclides (e.g., 80Kr/82Kr) are shielding dependant. It was noted above that atmospheric Kr trapped in EETA79001 seems to have an enhanced abundance of 80Kr.

    2. Regolith mixing rates. Measurements of core samples returned from the moon yielded significant information about the formation and mixing rates of fine-grained material within a few meters of the surface. Lunar surface rocks gave information on their times of excavation to the surface via impact cratering. Similar kinds of information can, in principle, be obtained from martian surface material.


    A Summary of Martian Meteorites

    Martian meteorites represent petrologically diverse rock types. Several contain volatile-rich alteration products and shock-implanted martian atmosphere. Returned samples from the martian surface, however, are likely to contain even greater quantities of these and other volatile-rich, alteration products, possibly including evaporate deposits. Most martian meteorites show relatively young formation or impact events, with ages of ~0.17 Ga and ~1.3 Ga. All show space exposure ages of ~0.6-15 Ma, in three or more age groupings.

    We can think of the martian meteorites as analogous to approximately three Mars sample return missions that collected only subsurface rocks within a limited area. Petrologically, martian meteorites represent five different rock types. These show three major groups of isotopic ages, ~0.17 Ga, ~1.3 Ga, and ~4.0-4.5 Ga (AlH84001), and at least three groups of space exposure ages. Meteorites sharing one or more of these characteristics have an implied linkage, and the greater the number of these three characteristics shared by two or more meteorites, the greater the probability that they derived from a common area of Mars, i.e., that they are equivalent to a single limited sample return. This relationship is illustrated by Figure 15, which plots the three parameters, rock type, formation age, and space exposure age against one another for those meteorites for which these data are available. In such a plot, the three sample groupings become obvious. Although the shergottites show two rock types (basaltic and lherzolitic), these are believe to be closely related. All shergottites show a common isotopic chronometer age of ~0.17 Ga, and all except EETA79001 have very similar space exposure ages. Each difference has a reasonable explanation that does not preclude all of the shergottite meteorites deriving from a limited area of Mars. Similarly, the nakhlites and Chassigny show similar isotopic formation ages and space exposure ages, even though Chassigny represents a different rock type. Meteorite ALH84001 by itself forms a third group. It is a different rock type with a distinctly different isotopic formation age, and it may or may not share a common space exposure age with group 2.

    If each of these three groups of martian meteorites indeed represents a separate impact ejection event from Mars (see above), then this perspective may give some insight to the diversity of subsurface rock types available in a limited sampling area of Mars.

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