Photochemistry in the Early Solar System - ACS Symposium Series

Chapter 8

Photochemistry in the Early Solar System Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 4, 2015 | http://pubs.acs.org Publication Date: February 15, 2008 | doi: 10.1021/bk-2008-0981.ch008

Robert N. Clayton Enrico Fermi Institute, University of Chicago, Chicago, IL 60637

Isotope-selective photodissociation of gaseous carbon monoxide is a well-known process in molecular clouds. This process may have been important in the early solar system, with the nascent Sun as the source of ultraviolet radiation. The consequent self-shielding gives rise to the non-mass-dependent oxygen isotope variations observed in primitive meteorites. This model implies that the solar oxygen isotope ratios, O/ O and O/ O, are about 5% smaller than those ratios in the Earth and other inner solar system bodies. A similar effect may also have occurred for nitrogen, through photolysis of N . These photochemical processes lead to disequilibrium chemistry in the formation of solid bodies in the Solar System. 18

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Introduction The importance of chemistry in astronomical settings has been increasingly recognized in recent years. This is especially true for the formation of stars (including our Sun) within dense molecular clouds. Molecular clouds have masses in the range 10 -10 solar masses, densities on the order of 10—10 particles/cm , and temperatures of 10-20 K. They are common birthplaces of 2

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© 2008 American Chemical Society In Chemical Evolution across Space & Time; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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142 new stars, as is seen today in the Orion molecular cloud. Their principal molecules are H and CO, reflecting the cosmic abundances of the elements H, C, and O, and the stability of these molecules. At the low temperatures of molecular clouds, only those exothermic reactions without activation energy can occur. These are ion-molecule reactions, with ionization produced by either stellar ultraviolet light and X-rays or by galactic cosmic rays. An example of a cosmochemically important ion-molecule reaction, with implications for isotopic variability, is as follows (/): Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 4, 2015 | http://pubs.acs.org Publication Date: February 15, 2008 | doi: 10.1021/bk-2008-0981.ch008

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C + CO 12

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The difference in zero-point energy between CO and CO (288 J/mole) is similar in magnitude to the mean kinetic energy of molecules at 10 Κ (125 J/mole), so that the equilibrium constant for this reaction is 32, i.e., a substantial enrichment of C in CO. However, a separate process also affects the C / C ratio of CO in molecular clouds, acting in the opposite direction: isotope-selective photodissociation of CO by ultraviolet starlight. This process preferentially dissociates the less abundant species, CO, due to the phenomenon of self-shielding (2), which will be discussed below in the context of formation of the solar system. Photochemistry is also important in present-day planetary atmospheres. The best-known abiotic example is the production of ozone (0 ) in the Earth's stratosphere, initiated by the photodissociation of 0 by ultraviolet sunlight: 13

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0 —^->20 2

0 + 0 + M - » 0 , + M, 2

resulting in a ratio of 0 / 0 that is many orders of magnitude greater than the equilibrium value. On Venus, water vapor has been photodissociated, accompanied by hydrogen escape from the planet, resulting in a very dry atmosphere and a D/H ratio in hydrogen that is about 100 times greater than that on Earth. Photodissociation of N in the atmosphere of Mars, with consequent preferential loss of N , has produced a N / N enrichment of more than 60% (5). In Jupiter and the other gas giant planets, the principal carbon reservoir is methane, but its photodissociation leads to production of other hydrocarbons, such as ethane and acetylene. The examples in the previous paragraph all describe the effects of ultraviolet sunlight on planetary atmospheres. These chemical effects are largely confined to the atmospheres, since short-wavelength light is strongly absorbed by molecules. This same property has led to the view that ultraviolet photochemistry was probably not important in the much more massive solar nebula: the cloud of gas and dust from which the Sun and planets formed. As a 3

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In Chemical Evolution across Space & Time; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 4, 2015 | http://pubs.acs.org Publication Date: February 15, 2008 | doi: 10.1021/bk-2008-0981.ch008

143 consequence, almost all published models of solar nebular chemistry are based on ordinary, thermally-activated reactions, governed by equilibrium chemical thermodynamics. In this chapter, I wish to explore the possible role of photochemistry (an inherently disequilibrium process) in formation of the planets and smaller bodies in the solar system. As was seen in the planetary examples above, the small differences in chemical properties of the stable isotopes of abundant light elements, especially carbon, nitrogen, and oxygen, provide key information regarding nebular chemistry. The rapid attenuation of light in a cloud of dust and gas requires that the irradiation region be continuously replenished, so that chemistry and dynamics are intimately interrelated.

Meteorite Evidence

Oxygen It is difficult to decipher early solar-system history from observations of the planets themselves, since most of their earliest records have been obliterated by subsequent processing. However, the meteorites, mostly derived from asteroids, have remained largely unchanged since their constituent minerals formed in the presence of the nebular gas, and thus provide the best information on early chemical and physical processes, such as evaporation and condensation, or chemical exchange between gas and condensed phases. Probably the most informative objects in meteorites are the refractory, calcium-aluminum-rich inclusions (CAIs). They are sub-millimeter- to centimeter-sized objects found in all types of primitive (chondritic) meteorites. On the basis of their uranium/lead radiometric ages, they are believed to be the first-formed "rocks" in the Solar System (4). Their chemical compositions are consistent with equilibrium condensation as solids from a gas of solar composition at high temperatures (~ 1700 K). The major mineral phases are spinel (MgAl 0 ), pyroxene (Mg, Ca, Al, Ti silicate), melilite (another Mg, Ca, Al silicate), and anorthite (CaAl Si 0 ). They are enriched in refractory (less volatile) trace elements, such as the rare-earth elements, by a factor of 15-20 (5), reflecting their high temperature of condensation. The abundances of the three stable isotopes of oxygen exhibit a pattern not seen in any terrestrial rocks (6). On earth, ratios of abundances of isotopes, such as 0 / 0 and 0 / 0 , vary by a few percent as a result of differences in bond energies caused by differences in isotopic mass. In such mass-dependent effects, variations in 0 / 0 are accompanied by variations in 0 / 0 of one-half the magnitude. In 2

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144 10 ALLENDE


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CAi minerals Dark Inclusions

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*>0(%ore!. SMOW) Figure 1. Oxygen three-isotope diagram showing isotopic compositions of minerals from Allende CAIs (CCAM = carbonaceous chondrite anhydrous minerals). The other line, labelled TF (terrestrial fractionation) is the locus of data points for terrestrial rocks and waters (data not shown). The bulk oxygen isotopic composition of the Earth is indicated by point E; the expected oxygen isotopic composition of the Sun is indicated by point S. The ^notation on the axes is defined as: £ Ό = 1000

where χ = 17 or 18, and R = ~τι— ; standard s - Standard Mean Ocean Water, and is (0,0) in this plot.


145 meteoritic CAIs and chondrules (less refractory spherules), the ratio ΟΙ Ο remains almost constant, whereas 0 / 0 and 0 / 0 vary by several percent. The contrast between the meteoritic "non-mass-dependent" (NMD) fractionation and the terrestrial "mass-dependent"fractionationis illustrated in Figure 1. The statistical mechanical theory underlying the well-known massdependent isotope effects was presented 60 years ago by Urey (7) and by Bigeleisen and Mayer (8). Some fundamentally different process is required in order to produce the NMD effects seen in primitive solar system materials. The first proposal was that CAIs had inherited excess 0 derived from supernova nucleosynthesis (9), but this proposal failed due to the absence of related nuclear effects in. magnesium and silicon, to which the "anomalous" oxygen is chemically bound. A second proposal was based on the experimental demonstration of NMD isotope effects in the synthesis of 0 from 0 (10). This phenomenon has been well documented both in the laboratory (10) and in the Earth's atmosphere (11, 12). It has not yet been shown whether a similar NMD process may have been important in early solar system chemistry, although such a process has been proposed (13). The third proposal (14% currently under evaluation, is the isotope-selective photodissociation of CO, the most abundant oxygen-bearing species in molecular clouds, and probably in the solar nebula (15) . Ultraviolet photolysis of carbon monoxide occurs through absorption in narrow lines, corresponding to specific rotational and vibrational levels of several electronic transitions. The absorption wavelengths are isotope-dependent due to the effect of isotopic mass on the vibrational and rotational energies. As a consequence, the absorption lines due to C 0 , the most abundant isotopologue, do not overlap those due to C 0 and C 0 . Because of the large differences in isotopic abundance ( 0/ 0 = 2500; 0 / 0 = 500), the absorption lines of C 0 become saturated (optically thick), while the absorption lines of C 0 and C 0 remain unsaturated. Therefore, the photodissociation of C 0 occurs predominantly close to the light source (stars or Sun), while the dissociation of C 0 and C 0 predominates in the part of the gas cloud that is more remote from the light source. This process of "isotopic self-shielding" has been modelled quantitatively for molecular clouds (16) , and has been directly observed (17). In the case of molecular clouds, it has been assumed that a cloud is irradiated by ambient ultraviolet starlight, so that CO molecules in the periphery of the cloud are dissociated in proportion to their original isotopic abundances. However, photodissociation in the cloud interior favors the rarer, unshielded isotopic species: C 0 , C 0 , and C 0 . In the case of the solar nebula, three different irradiation geometries have been proposed: 18

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direct inheritancefromthe parent cloud (18),


146 2.

irradiation of the surface of the solar nebular disk, by the Sun and nearby stars, at a radial distance of tens of astronomical units (1 AU = 1.5 χ ΙΟ km)fromthe Sun (19), and irradiation by sunlight very near the young Sun (~ 0.05 AU) (14).

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In the first two of these scenarios, it has been proposed that the rare-isotopeenriched atomic oxygen was quickly trapped by reaction with H to form water ice, which was transported sunward to enrich the inner solar system reservoir in 0 and 0 . In the third scenario it was proposed that the rare-isotope-enriched atomic oxygen was quickly trapped by reaction with metals to form solid mineral grains, which were transported outward by a stellar wind (20). Photochemical self-shielding of CO has two observable consequences: (l)the "anomalous" oxygen isotope patterns observed in chondrules and CAIs, and (2) the apparent overabundance of oxidized species (such as Fe in silicates). The self-shielding model also leads to a striking prediction of the oxygen isotopic abundances in the present-day Sun. Self-shielding enhances the abundances of atomic 0 and 0 in the interior of the nebular cloud, with the implication that the initial isotopic composition must lie at the 0-rich end of the mixing line in Figure 1. Thus, it is expected that there is about a 5% deficit in the solar ratios 0 / 0 and 0 / 0 relative to the values in the Earth and other inner solar system bodies. Existing measurements of the solar isotopic composition are not precise enough to detect the predicted difference, but forthcoming analyses of the solar wind, made by the NASA Genesis mission, should resolve this effect. Two attempts have been made to estimate the solar oxygen isotopic composition by measurement of solar wind atoms implanted in metal grains on the surface of the Moon, with conflicting results. Hashizume and Chaussidon (21% analyzing grains from an ancient lunar soil, found a composition enriched in 0 relative to terrestrial compositions, whereas Ireland et al. (22% analyzing grains from a young lunar soil, found a composition depleted in 0 . The reason for this difference is unknown. The oxygen isotopic record in CAIs appears to reflect a two-stage history: (1) primary condensation from a gas of solar chemical and isotopic composition, corresponding to point S in Figure 1, and (2) a secondary exchange process, that is mineral-specific, moving the oxygen isotopic composition upward along the slope-1 line of Figure 1. Objects forming at lower temperatures, such as chondrules and planetary precursors, appear to have seen only this second environment, in which reactive 0 and 0 atoms have been enriched by the photochemical shielding. The sequence in the preceding paragraph suggests a solution to the long standing problem of understanding the oxidation state of primitive meteoritic materials. The CAIs are highly reduced, containing no primary Fe , and containing T i (along with the more common Ti ) in pyroxene (23% consistent 2


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147 with equilibrium with a hydrogen-rich solar gas. On the other hand, matrix olivine crystals in the same meteorite have compositions that are much more oxidized, containing abundant ferrous iron. This matrix is also greatly enriched in the rarer, heavy isotopes of oxygen, relative to the CAIs (and hence, relative to the solar composition). The oxygen isotope enrichment is clearly a disequilibrium phenomenon, which suggests that the oxidation of iron was also a disequilibrium process. An astrophysical model of the chemical effects of photodissociation of CO (24) showed that the abundances of oxygen-bearing molecules, such as OH and H 0 , may differ from equilibrium values by many orders of magnitude. 2

Nitrogen If photodissociation was an important process for CO in the early Solar System, it may also have been important for N , an isoelectronic and isobaric molecule, which had an abundance in the gas phase lower than CO by only a factor of six. Nitrogen is less useful than oxygen as a tracer of Solar System chemistry, for a number of fundamental reasons: (1) it has only two stable isotopes, so that there is no simple way to distinguish between mass-dependent and abundance-dependent isotope effects; (2) the homonuclear character of the N molecule renders it invisible to most astronomical observational techniques; (3) it is not chemically trapped as a major element in meteoritic minerals. Nitrogen is, however, a major constituent of many organic molecules, both in space (25) and in meteorites (26). The range of N / N observed in Solar System materials exceeds a factor of three (Figure 2), which is much larger than can be accounted for by lowtemperature ion-molecule chemistry (25), and which therefore suggests that isotope-selective photochemistry may occur in N , as it does in CO. As in the case of oxygen, the stable isotope ratio, N / N , in the Sun is not yet well known. It will also be measured in the Genesis solar wind sample. If the isotopic self-shielding mechanism applies to nitrogen, the Sun should show a lower N / N ratio than the Earth, but there is no obvious meteoritic sample of unfractionated solar composition, as there is for oxygen in the CAIs. A possible candidate for such a primitive sample is the refractory mineral osbornite (TiN) found in a primitive carbonaceous chondrite (27). This mineral has a ratio of N / N = 2.36 χ 10~ , 36% lower than the ratio in the terrestrial atmosphere. The Allende meteorite, from which many of the CAI oxygen data have come, does carry a nitrogen component that is depleted in N by at least 9% (28), but this is not likely to be carried within the CAIs. Similar nitrogen isotope ratios were observed in several types of iron meteorite (29). Two samples that may represent the Solar System's primordial nitrogen isotope abundances are: 2

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148 (1) ammonia in the atmosphere of Jupiter, and (2) solar wind ions implanted into lunar mineral grains. The former (30), based on measurements of the Galileo Probe Mass Spectrometer, gave δ Ν = -380 ± 80%o relative to Earth's atmosphere; the latter (31), based on ion microprobé measurements of correlated hydrogen and nitrogen isotope abundances in lunar soil minerals, gave δ Ν < -240%o. The isotopic compositions of oxygen (Figure 1) and nitrogen (Figure 2) are given in the conventional δ-notation, as deviations in permil (%o, parts per thousand) in 0 / 0 , 0 / 0 , N / N from the corresponding terrestrial standards (SMOW for oxygen, AIR for nitrogen). These values, coupled with the highest ratio seen in a bulk meteorite (32), show that the N / N ratios in early Solar System materials varied by a factor of three, well beyond the range of any known chemical kinetic or equilibrium process. 15

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Photochemistry in the Early Solar System - ACS Symposium Series

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