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NITROGEN IN PLANETARY MATERIALS

Nitrogen forms a variety of compounds that are signifi cant in the inter- materials, resistant to dissolution in acids and solvents. Their survival in interstellar space, transport and processing in the protoplanetary stellar medium, star formation, planetary formation and the evolution disk and their distribution through asteroidal parent bodies make it of life. Through the study of planetary materials, we recognize three likely that presolar grains also survive alteration on the parent bodies generations of nitrogen and nitrogen-bearing compounds: primary, of primitive . secondary and tertiary. Species from different sources are recognized Organic Material: The is rich in organic mole- by characteristic isotopic compositions or specifi c modes of occurrence cules formed by several different processes, such as ion– reac- tions on the surface of icy grains. The effect of such reactions is to (e.g. Grady and Wright 2003). increase the heavier over the lighter one, producing material with characteristic isotopic signatures (e.g. Herbst 2003). Inventories PRIMARY NITROGEN of interstellar (e.g. www.astro.uni-koeln.de/cdms/molecules) indicate that there are ~60 N-bearing species, most of which are organic Primary nitrogen is nitrogen that has been inherited directly from the in nature. As a result, a variety of N-bearing organic species are avail- molecular cloud from which the evolved. The isotopic able for incorporation into parent bodies, and these species composition of primary nitrogen refl ects the original source reservoirs may be subsequently altered during parent-body evolution. These inter- and is preserved in presolar grains, organic molecules and high-tem- stellar species include molecules (e.g. , amines, carboxylic perature condensates. acids) that are important both as building blocks and as intermediates Presolar grains are found in primitive meteorites. These grains origi- in the chain of evolution from simple to complex molecules, which nated from other regions of the molecular cloud, beyond the proto- ultimately led to the formation of DNA. Although precursor molecules planetary disk from which our Solar System formed ~4.567 billion years have been identifi ed, there have been no confi rmed observations of ago. Presolar grains were fi rst recognized because they host isotopically amino acids in the interstellar medium. was thought to have unusual noble gases (Zinner 2007). Subsequent characterization has been discovered in the interstellar medium (Kuan et al. 2003), but the shown them mainly to be grains of (SiC) and , conclusion was later overturned (Jones et al. 2007). plus oxides and, possibly, nanometre-sized diamonds. The isotopic High-temperature condensates: Primary nitrogen is also present composition of the grains themselves is often, but not always, unusual, within highly reduced thought to be grains that condensed because the grains come from stars which experienced different nuclear at high temperatures as the presolar cooled (Meibom et al. 2007; processes from our (e.g. carbon-burning stars, supernovae). Their Feng et al. 2012). These minerals include osbornite (TiN), sinoite isotopic composition is related to the production of individual species (Si2N2O) and nierite (Si3N4), and they occur in varying abundances in astrophysically distinct locations. Nitrogen is formed in stars through within the primitive classes. They are characterized by an 14 the CNO cycles (FIG. 1A), as a result of which the abundance of N is isotopically light nitrogen composition, and they are interpreted as 15 gradually increased over that of N. If a star reaches the end of the having the original composition of the solar nebula (Meibom et al. 14 15 CNO cycles, it will attain a N/ N ratio of 5000, which corresponds 2007). to a δ15N value of –945‰. In contrast, grains produced during outbursts are more enriched in 15N, and reach a 14N/15N ratio ~ 5, which SECONDARY NITROGEN means a δ15N value of +54,000‰. Second-generation nitrogen-bearing species are those produced on FIGURE 1B is a summary of nitrogen (and carbon) isotope compositions parent bodies by alteration (aqueous or thermal) of primary nitrogen- of presolar grains; where fi elds overlap in C versus N space, the grains bearing species. Such processes may lead to modifi cation of the primary can be separated by silicon isotope composition (Nittler 2003). It is species, for example, the graphitization of amorphous carbon during clear that the isotopic extremes generated by nucleosynthetic reactions metamorphism, and may or may not alter the original isotopic com- in different types of stars are inherited by the materials produced within positions. The most primitive types of meteorites, the CI, CM and CR the stars, and these materials can be identifi ed as discrete grains within (FIG. 2), contain ~500–5000 ppm nitrogen, with overall δ15N meteorites. FIGURE 1B shows that we can identify the astrophysical loca- ranging from +15‰ to +150‰ (see Fogel and Steele 2013 this issue). tion in which a particular group of presolar grains was produced. The presolar grains that have been extracted from meteorites are refractory

A Relationship between FIGURE 1 B astrophysical processes and presolar grains. (A) Schematic of CNO cycles showing production of 14N and 15N (ADAPTED FROM AUDOUZE AND VAUCLAIR 1980) (B) Isotopic composition of SiC grains from the Murchison primitive . The four quadrants are labelled according to the fi elds in which the products of specifi c astrophysical processes would fall. FIGURE ADAPTED FROM ZINNER (2007) AND SOURCES THEREIN

ELEMENTS 373 OCTOBER 2013 CosmoELEMENTS

The , FIGURE 2 which fell in 1969, An additional, and so far unique, example of tertiary nitrogen is that contains ~2 wt% organics, some of which appear to derive directly from interstellar which occurs in many Martian meteorites, where clasts of glass act as organic compounds. Specimen is ~15 cm hosts of gases trapped from the Martian atmosphere (see Fogel and across. Steele 2013).

TERTIARY NITROGEN SUMMARY Tertiary nitrogen species are introduced into meteorites by implanta- Through the study of nitrogen in meteorites, we can identify and track tion, mainly during exposure to cosmic and solar radiation, during several generations of nitrogen-bearing species. Primary material both residence at the parent-body surface and transit in formed in stars and molecular clouds and became incorporated into inter planetary space. the protoplanetary nebula from which the Sun and Solar System formed. Secondary nitrogen resulted from the alteration of primary species on Cosmogenic (or spallation) nitrogen is produced by the interac- parent bodies. Some meteorites also contain tertiary nitrogen implanted tion of cosmic rays with silicate minerals, where the main target ele- by solar or cosmic radiation. Lunar and asteroidal surfaces also record ments are 16O and 24Mg. Both 14N and 15N are produced, with a 14N/15N tertiary nitrogen, and Martian meteorites carry samples of the planet’s ratio of approximately 1 (Reedy 1981; Silberberg et al. 1998), which atmosphere, trapped inside grains/melt pockets at the time the mete- translates to a δ15N value of +270,000‰. The abundance of cosmogenic orite was ejected from the surface of Mars by impact. On Earth, the nitrogen is related to the length of time that material is exposed to structural and isotopic signatures of original parental materials have radiation – the exposure age of a sample – and is generally been modifi ed by tectonic cycling and biological processing. Only in very low, at the parts per billion level. meteorites can we look back through the different generations of nitrogen to understand the reservoirs from which planetary nitrogen Solar nitrogen: The solar wind was originally sourced. is a stream of highly energetic particles that “blows” from the Monica M. Grady ([email protected]) surface of the Sun. The energetic The Open University, UK nature of the particles causes them to become implanted within grains when they impact a plan- etary surface. Two types of mate- REFERENCES rial have acted as collection sur- Audouze J, Vauclair S (1980) An Kuan Y-J, Charnley SB, Huang H-C, faces for the solar wind from Introduction to Nuclear Tseng W-L, Kisiel Z (2003) which measurements have been Astrophysics. D. Reidel, Dordrecht Interstellar glycine. Astrophysical Journal 593: 848-867 made: lunar soils and collectors Feng L, El Goresy A, Zhang J, Hao J, from NASA’s Genesis mission. Boyet M, Lin Y (2012) Excess 36S in Marty B, Chaussidon M, Wiens RC, Examination of grains lawrencite and nitrogen isotopic Jurewicz AJG, Burnett DS (2011) A 15 from Apollo soil samples revealed compositions of sinoite from N-poor isotopic composition for Almahata Sitta MS-17 EL3 chon- the Solar System as shown by that they had glassy rims (FIG. 3) rd Glassy rim on the surface drite fragment. 43 Lunar and Genesis Solar Wind samples. FIGURE 3 produced by a combination of of a grain of lunar soil. Planetary Science Conference, Science 332: 1533-1535 Abstract 1766 The rim is produced by solar wind and cosmic and solar radiation. Meibom A, Krot AN, Robert F, cosmic ray bombardment. IMAGE COURTESY Nitrogen from these grains had Fogel ML, Steele A (2013) Nitrogen in Mostefaoui S, Russell SS, Petaev MI, OF SARAH NOBLE, NASA/MARSHALL SPACE δ15N values as low as about extraterrestrial environments: Gounelle M (2007) Nitrogen and FLIGHT CENTER (HTTP://LUNARNETWORKS. –240‰ (Frick et al. 1988; Clues to the possible presence of carbon isotopic composition of the BLOGSPOT.CO.UK/2010/03/UNDERSTANDING- life. Elements 9: 367-372 Sun inferred from a high-tempera- REACTIVITY-OF-LUNAR-DUST.HTML) Hashizume et al. 2000), which ture solar nebula condensate. was inferred to have been Frick U, Becker RH, Pepin RO (1988) Astrophysical Journal 656: L33-L36 implanted by radiation. Solar wind record in the lunar regolith - Nitrogen and noble gases. Nittler LR (2003) Presolar in 18th Lunar and Planetary Science meteorites: Recent advances and Conference, Proceedings, pp 87-120 scientifi c frontiers. Earth and The Genesis mission exposed an Planetary Science Letters 209: array of collector plates (FIG. 4) to Grady MM, Wright IP (2003) 259-273 Elemental and isotopic abundances the solar wind for a period of just of carbon and nitrogen in meteor- Owen T, Mahaffey PR, Niemann HB, over 2 years. Analysis of nitrogen ites. Space Science Reviews 106: Atreya S, Wong M (2001) Protosolar in the plates found that the solar 231-248 nitrogen. Astrophysical Journal 15 15 553: L77-L79 wind was depleted in N, with δ N Hashizume K, Chaussidon M, Marty about –400‰ (Marty et al. 2011). B, Robert F (2000) Solar wind Reedy RC (1981) Cosmic-ray pro- This finding fits well with the record on the Moon: deciphering duced stable : various nitrogen isotope composition of the presolar from planetary nitrogen. production rates and their implica- Science 290: 1142-1145 tions. 12th Lunar and Planetary second-largest nitrogen reservoir in Science Conference, Proceedings, the Solar System: the atmosphere Herbst E (2003) Isotopic fraction- pp 1809-1823 15 ation by ion-molecule reactions. of Jupiter, which has δ N values of Space Science Reviews 106: 293-304 Silberberg R, Tsao CH, Barghouty AF about –375 ± 100‰ (Owen et al. (1988) Updated partial cross sec- 2001). It also confi rmed that the Jones PA, Cunningham MR, Godfrey tions of proton-nucleus reactions. PD, Cragg DM (2007) A search for Astrophysical Journal 501: 911-919. nitrogen implanted into the outer biomolecule signatures in layers of grains from the lunar rego- The Genesis spacecraft Sagittarius B2 (LMH) with the Zinner E (2007) Presolar grains. In: FIGURE 4 lith was almost certainly from the during integration, Australia Telescope Compact Array. David AM (ed) Meteorites, solar wind. showing the different arrays of Monthly Notices of the Royal and Planets. Treatise on collectors. IMAGE COPYRIGHT NASA (HTTP:// Astronomical Society 374: 579-589 Geochemistry, Volume 1, Elsevier, CURATOR.JSC.NASA.GOV/SEH/GENESIS.CFM) Chapter 1.02

ELEMENTS 374 OCTOBER 2013