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1.4 Cosmochemistry in an astrophysical context– relating the origin of the Solar System to processes of planet building elsewhere (Hansen, Lyons, McKeegan, Morris, Shuping, Wasson, Young)

1.4.1 The distribution of H₂O in protoplanetary systems

Figure 1.4.1. Results of finite difference model for water flow through an asteroid-like body in the early Solar System (after Young 2001) as a function of radius and time. Colors show intensity of water flux in the body m3/(m2s) as it evolves with time. Where water/rock recorded by isotopes and mineralogy is high, flux was protracted. Where water/rock is low, flux was fleeting. The total water content of the original body was uniform at 20 volume % but the water/rock preserved in a rock would depend on the original location it occupied in the body rather than the total water content of the body.

A key question related to the origin and evolution of terrestrial life is that of how water was accreted to Earth and, more generally, how rocky planets acquire their water. There are two main possibilities. One, the endogenous origin, is that the water accreted together with the planetesimals (ranging from asteroid to Mars in size). The other is that water present on Earth’s surface today is exogenous, having been delivered by comet impacts (e.g. Morbidelli et al 2000). At present there is skepticism about the cometary origin of water because the ratios of the heavy isotope of hydrogen to the light isotope, D/H, in the three comets measured thus far (Halley, hyakutake, and Hale-Bopp) are on the order of 2 times higher than in Earth’s oceans (Bockelee-Morvan et al. 2000). On the other hand, D/H would most likely have varied with radial distance from the Sun in the early Solar System, making the arguments based on D/H uncertain.

One way to distinguish between the exogenous and endogenous origins of water in rocky planets is to characterize the amount of water that was present in planetesimals. Lead-team investigators McKeegan, Wasson and Young have been studying the role that water played in the evolution of rocky precursors to planets in the Solar System through studies of the ways in which carbonaceous chondrite meteorites, vestiges of the planetesimals, have been altered by reactions with liquid and/or vaporous water (Choi et al. 1997; Choi et al. 1998; Young et al. 1999; Young 2001). This work builds on years of studies pertaining to the role of water in the evolution of planetesimals (asteroid-like precursors to planets) as evidenced by these primitive meteorites (e.g., Kerridge and Bunch 1979; Clayton and Mayeda 1984; Clayton and Mayeda 1999).

It is clear that the bodies from which at least some carbonaceous chondrites come from (now represented by the C-type asteroids) had significant amounts of water within them early in the history of the Solar System, but exactly how much water existed in primitive meteorite parent bodies depends on how the data are interpreted. This is an important question because if objects represented by the C-type asteroids had uniformly large amounts of water (as suggested by some workers, e.g. Young et al. 1999), then the implication is that water was plentiful in the building blocks of the planets in the Solar System (C-type asteroids are the most abundant type of primitive rocky body and the largest asteroid 1 Ceres is such an object). In this case, the origin of water in and on the terrestrial planets need not have been exogenous (i.e., from late introduction by comets) but instead could be the residues left over from much larger amounts of water that existed prior to melting and differentiation of the bodies (i.e., endogenous).

Alternatively, if large amounts of water were present in only a small fraction of the primitive building blocks of the planets (as implied by other interpretations of the meteorite data, e.g. Clayton and Mayeda 1984), then water would not have been nearly as plentiful during the planet building process and would not be expected to have survived the planet-forming processes in sufficient quantity to explain present-day abundances (on and within Earth and perhaps Mars).

There are two models put forward to explain the mineralogical and oxygen isotopic effects of reactions between water and rocks as evidenced in carbonaceous chondrite meteorites. One, the closed-system model put forward by Clayton and Mayeda (1984), states that the amount of water evidenced by the altered rock materials in a meteorite is an expression of the amount of water that existed in the entire parent body (that is to say, the "water/rock ratio" is a characteristic of the object). The implications of this interpretation of the data is that while a few carbonaceous chondrite parent bodies had on the order of 50 volume % water, most had substantially less (< 10%).

The other model, the open-system put forward by Young et al. (1999) and Young (2001) is that the amount of alteration of the meteorite depended upon where the particular sample came from in the parent body (Figure 1.4.1). In this model, the parent bodies were heterogeneous in mineralogy and oxygen isotopic characteristics but they all could have had on the order of 20 to 30 volume % water to begin with. If the open-system model is correct, it implies that water was a major constituent of proto-planets prior to their melting and differentiation.

Distinguishing between open-system alteration and closed-system alteration of a chondrite, and thus between large amounts of water in all bodies and large amounts of water in just a few bodies, requires analysis of

¹⁷O/¹⁶O (¹⁷O) and ¹⁸O/¹⁶O (¹⁸O)

by ultraviolet laser ablation combined with gas-chromatography isotope ratio mass spectrometry (Young et al. 1998). This novel technique is time-consuming but provides a combination of spatial resolution and precision that can not be obtained by any other method. Young and others carry out oxygen isotope ratio analyses of carbonaceous chonderite meteorites to search for signals that can be used to distinguish between open and closed-system reactions between the rocks and waters. Collection of data for characterization of a reasonable sampling of carbonaceous chondrite meteorites will take several years. The analyses will be carried out in Young’s stable isotope laboratory at UCLA.

Another way to examine the likelihood for exogenous and endogeous sources of water is to study the ways in which giant planets affect the delivery of water to regions where habitable, terrestrial-like planets are likely to form. Planetesimals formed beyond the "snow line" where water is in the solid state in a planetary system are harbingers of water (e.g., comets). Planetesimals formed inside of the snow line may be relatively dry. Giant planets interact gravitationally with planetesimals, in effect stirring them up. What is more, it is now clear that some giant planets in extrasolar systems may have migrated radially with respect to their stars, and it has been suggested that early on gaseous and icy giant planets in the Solar System may also have moved closer or further from the Sun (e.g., Thommes et al. 2002). Indeed it has been suggested that a mechanism for stopping the migration of a giant planet towards its central star is for it to encounter a sufficient number of asteroid-like or comet-like planetesimals (Murray et al. 1998). Giant planets are therefore expected to regulate the distribution of water-bearing planetesimals into regions of terrestrial planet formation.

The consequences of this new paradigm of moving giant planets for terrestrial planet formation are poorly understood. Hansen, in collaboration with Young, plans to perform a detailed investigation of giant planet-planetesimal interactions and the implications for the development of solar-like planetary systems. The challenge will be to cover both the large dynamic range in mass and the large number of particles necessary for a realistic description. The results will be examined in the context of the meteoritic evidence for the early evolution of our own Solar System.

1.4.2 The astrochemistry of protoplanetary systems and the meteorite record

An important step in recognizing signs of life, or the essential precursors to life, is characterization of the various abiotic pathways by which organic molecules are produced in protoplanetary environments, including in our own solar nebula (the protoplanetary disk that surrounded the Sun 4.6 Gyr ago). One potentially important process for forming organic molecules is photolysis. Lyons and Young have begun a program of research devoted to elucidating those characteristics of primitive meteorites that might be explained by photochemistry in the solar nebula. The goal is to get a better picture of the role that photochemistry might have played in determining the inorganic and organic chemistry of the nebula.

One of the most important clues to the origin of the Solar System is the presence of an excess of the ¹⁶O isotope of oxygen relative to the two heavier oxygen isotopes, ¹⁸O and ¹⁷O. This unusual distribution of O isotopes (by terrestrial standards), discovered by R.N. Clayton in 1973, was one of the reasons for suggesting that explosion of a nearby super nova might have triggered the collapse of molecular cloud material to form our Solar System (16O is a product of such an event). However, the connection between an overabundance of 16O inferred to derive from a super nova explosion has not been observed in populations of "presolar" mineral grains that represent pristine ejecta from stars found in meteorites. The variability in ¹⁶O apparently has another explanation that remains elusive.

What is clear is that the ¹⁶O enrichment is telling us something about conditions that prevailed in the very young solar nebula. Whether these conditions are relevant to the subsequent development of life on Earth is unknown, but they are almost certain to be central to our understanding of other young planetary systems and may be clues to the photochemistry that took place early in the Solar System.

Clayton (2002) made the suggestion that self shielding by CO in the early solar nebula could have been the origin of the anomalous array of 18O/16O and ¹⁷O/¹⁶O in primitive Solar System materials (the slope-1 line on a plot of ¹⁷O vs. ¹⁸O where 17O = per mil deviation in ¹⁷O/¹⁶O relative to standard mean ocean water and d18O is defined similarly). The efficacy of the photodissociation of CO as a means for producing slope-1 lines in oxygen three-isotope space is underscored by the detection of slope-1 oxygen isotope ratios in interstellar CO (Sheffer et al. 2002). A major component of the CO self shielding hypothesis is that the starting materials for all solids in the early solar nebula were rich in ¹⁶O, with isotopic compositions comparable to the most ¹⁶O-rich calcium-aluminum-rich inclusion minerals.
Calculations by Lyons and Young show that self shielding by CO of a stellar flux of UV photons illuminating regions above and below the midplane of the solar nebula (the protoplanetary disk that became the Solar System) should have been sufficient to produce several Earth masses of oxygen with large depletions in ¹⁶O relative to the starting over time scales > 10³ yrs. The most likely sink for the ¹⁷O and ¹⁸O-rich oxygen liberated by photolysis of CO would have been adsorption onto solid dust grains followed by surface reactions to produce water, as described for molecular clouds by Yurimoto and Kurimoto (2002). Settling of these dust grains and radial transport toward the accreting star (the Sun) brings this source of ¹⁶O-depleted oxygen into the nascent inner Solar System where it can react with gases, minerals, and liquids that form planet precursors.

	Figure 1.4.2. Calculated rate of C18O photodissociation (white contours) in a protoplanetary disk representing the solar nebula as a function of radial distance from the star (R) and height above the midplane (Z). After calculations by Young and Lyons (2003).

Experiments show that exposure of ices containing C compounds to UV photons can produce complex organic compounds (Schutte 2002). The model put forward by Lyons and Young implies that ices, primarily composed of water, were important in determining the oxygen isotopic composition of rocky bodies in the Solar System. It also implies that irradiation of condensed materials, including ices, by UV photons could have been an important process, and that the UV photon flux could have come from the central proto-Sun if the ices existed well above the midplane. If shown to be correct, there is the possibility that the origin of organic molecules and ¹⁶O anomalies in primitive meteorites are both telling us about the photochemistry of the early solar nebula, and perhaps protoplanetary environments in general.

Theoretical tests of this model will be conducted in the next several years. A key refinement will be development of molecular shielding functions. The effects of mutual and self shielding by H, H₂ and CO were included in the original calculations by using the H₂ and CO density-dependent shielding functions of van Dishoeck and Black (1988). These functions were based on calculations appropriate for molecular cloud environments, but should be redone for applications in protoplanetary disks. The calculations will ultimately be extended to investigate the implications for D/H and N isotopes.
If photochemistry at distal regions in protoplanetary disks is common in the Galaxy and recorded in the isotopic compositions of meteorites, then the process should be evident in disks surrounding young low-mass (i.e., solar-like) stars elsewhere. Lead-team members Shuping and Morris will test this suggestion by examining the protoplanetary disks in the Orion Nebula. The relative abundances of carbon monoxide isotopomers in the disks can be measured directly using millimeter and submillimeter interferometers. Interferometry is required because the target disks are typically small; spatial resolution on the order of, or better than, an arcsecond is required to match or resolve nearby disks such as those found in Orion and Taurus. Resolution of up to 0.1" is achievable with the Submillimeter Array and with the CARMA array (soon to be in operation).

CO emission has been reported widely in star-forming cores, but only the new generation of interferometers will be capable of measuring the isotopomers of CO in compact protoplanetary disks. By measuring them initially at 230 GHz (the J=2-1 rotational line), where sensitivity is greatest, Shuping and Morris would establish which disks are most promising for measurements of the rarest isotope, C¹⁷O. Then higher-lying lines would be measured in order to provide information on optical depth and the temperature structure in the disks. The ultimate goal is to determine whether there has been any significant selection, via photodissociative processes, for the more abundant isotopomer relative to the CO abundances in the nearby molecular clouds from which the star and disk formed.

Another approach to take to investigate isotope abundances in disks is to observe the near-infrared (4.6 and 2.3 µm) rotational-vibrational absorption by CO of light from the central star as seen through the disk. While C¹³O and C¹⁸O are both routinely observed toward protostellar sources at 4.6 µm, the C¹⁷O isotope has not yet been detected. Absorption line observations can only be made for protostellar disks with just the right inclination to our line of sight (60 - 80 degrees), and with central stars that are sufficiently bright at 1-5 µm. There are a handful of protostars in Taurus and Ophiuchus which satisfy these conditions. High-resolution near-infrared spectroscopy of the CO lines at both 2.3 and 4.6 µm can be carried out using the Near-Infrared Spectrometer (NIRSPEC) at the Keck Observatory (adaptive optics is not required). In addition to the isotope ratios, these observations will also enhance our understanding of the overall abundance of CO in protostellar disks, both in the gas-phase and as ices condensed onto dust grains (e.g. Shuping et al. 2001, Boogert et al. 2002).

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