Interplanetary Dust Particles

Interplanetary Dust Particles

Interplanetary dust particles (IDPs) are among the most primitive materials in the solar system. Large enrichments of deuterium relative to hydrogen have been found in interplanetary dust particles and are interpreted as due to material formed in chemical reactions in interstellar molecular clouds.

High-Resolution Isotope Imaging of IDPs

Nitrogen and deuterium enrichments are common in many IDPs and are generally believed to have resulted from chemical isotopic fractionations in interstellar molecular clouds. However, little has been known about the internal distribution of isotopic anomalies in IDPs and the phases that carry these anomalies. The Washington University NanoSIMS can carry out high-resolution isotopic measurements at a sub-micrometer scale. We have, therefore, been using this instrument to study the isotopic structures of interplanetary dust particles.

Nitrogen isotopic imaging shows that many IDPs contain discrete hotspots that are strongly enriched in ¹⁵N, up to ~1300. However, some IDPs also contain larger regions with more modest enrichments in ¹⁵N, leading to average bulk N isotopic compositions that are ¹⁵N-enriched. Carbon isotopic compositions are normal in most IDPs, but two ¹⁵N-rich hotspots have correlated ¹³C anomalies; these are the first observations of C isotopic anomalies in IDPs. Hydrogen isotopic distributions are similar to those of N: D anomalies are present both as distinct D-rich hotspots and as larger regions with more modest D enrichments.

Oxygen isotopic imaging shows the presence of abundant presolar silicate grains in some of the IDPs. The oxygen isotopic compositions of the grains are similar to those of presolar oxide and silicate grains from primitive meteorites. In addition, C and O isotopic imaging led to the discovery of the first observed presolar grains of corundum and SiC in an IDP, presolar phases that are common in primitive meteorites.

Based on their N isotopic compositions, IDPs can be divided into two groups. One group is characterized as being 'isotopically primitive' and consists of those IDPs hat have anomalous bulk N isotopic compositions. These particles typically also contain numerous ¹⁵N-rich hotspots, occasional C isotopic anomalies and abundant presolar silicate grains. In contrast, the other 'isotopically normal' IDPs have normal bulk N isotopic compositions, and generally contain few, if any, presolar phases. Thus, isotopically interesting IDPs can be identified on the basis of their N isotopic compositions for further study. However, the distinction does not appear to extend to H isotopic compositions: D anomalies are as common in normal IDPs as they are in those characterized as isotopically primitive, based on their N isotopes.

IDPs from Space-Exposed Aerogel

Aerogel is the medium of choice for the intact capture of small particles in space, because it is capable of decelerating high-velocity projectiles without substantial melting or other modification of their component materials. After space-exposed aerogel is returned to the laboratory, the first step of analysis will be a non-destructive optical evaluation of impact features and their basic classification. For a more detailed analysis of the projectile residues, however, it is necessary to extract the samples from the surrounding aerogel for mounting on a suitable substrate. We are developing a sample preparation routine that allows the routine measurement of trace element concentrations and isotopic compositions of aerogel-captured particles in the ion microprobe. Since the aerogel capture mechanism in low Earth orbit is so fundamentally different from IDP collection in the stratosphere, it would be of interest to compare the distribution of isotopic anomalies and trace element concentrations in both sets of samples. These measurements could, for example, give clues to the effect of atmospheric entry heating on particle properties.

In a first step, we have made the first SIMS isotopic measurement of interplanetary dust that was captured in aerogel outside the Mir Space Station. Further work will be important for the return of dust samples from space experiments that use aerogel as a capture medium.

CR Chondrites: Effects of Parent Body Processing

(Click to see enlargement)

Like interplanetary dust particles, the CR chondrite Renazzo contains abundant isotopic anomalies in both hydrogen and nitrogen, attesting to the primitive nature of this meteorite. Renazzo has been suggested as a promising analog for IDPs because it is the only meteorite known to have D-rich hotspots and, like IDPs, it shows evidence for several distinct D carriers. We are using the NanoSIMS to carry out isotopic imaging studies of Renazzo matrix material in order to compare the nature and distribution of primitive matter in this meteorite with that found in IDPs. Questions that we are trying to address with this work include determining whether the isotopic characteristics of IDPs and primitive meteorites are similar, whether the phases carrying the isotopic anomalies are the same in both materials and what role parent body processing has played.

Like IDPs, Renazzo matrix material contains ¹⁵N-rich hotspots and also larger regions with so-called 'bulk' enrichments in ¹⁵N (see Figure). However, the overall levels of enrichment are somewhat lower than the corresponding ranges that have been observed for ¹⁵N-rich hotspots and bulk compositions in IDPs. Carbon isotopic imaging revealed two C- and Si-rich grains with large ¹³C excesses whose ¹²C/¹³C ratios suggest that they are mainstream SiC grains, consistent with the presence of this presolar phase in many primitive meteorites. However, extensive oxygen isotopic imaging of numerous matrix fragments failed to locate any presolar silicate or oxide grains in Renazzo, although such grains have been found in high abundance in other primitive meteorites and IDPs.

CR chondrites are among the most primitive meteorites known and have largely escaped thermal metamorphism. They have, however, experienced low temperature aqueous alteration. This alteration would probably not destroy refractory presolar phases such as diamonds and SiC, but could alter presolar silicate grains or re-equilibrate their oxygen isotopic compositions.

Our work to date suggests that Renazzo is one of the more pristine representatives among primitive chondrites and its matrix material shares many similarities with IDPs. However, the aqueous alteration it experienced affected the abundance and distribution of at least some of its presolar phases. Continued work will focus on constraining the effects of such parent body processing on the distribution of presolar phases by examining other members of the CR chondrite group, such as LEW 85332, which has a higher bulk ¹⁵N enrichment than Renazzo.

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Interplanetary dust particles (IDPs) are among the most primitive materials in the solar system. Large enrichments of deuterium relative to hydrogen have been found in interplanetary dust particles and are interpreted as due to material formed in chemical reactions in interstellar molecular clouds.

High-Resolution Isotope Imaging of IDPs	Nitrogen and deuterium enrichments are common in many IDPs and are generally believed to have resulted from chemical isotopic fractionations in interstellar molecular clouds. However, little has been known about the internal distribution of isotopic anomalies in IDPs and the phases that carry these anomalies. The Washington University NanoSIMS can carry out high-resolution isotopic measurements at a sub-micrometer scale. We have, therefore, been using this instrument to study the isotopic structures of interplanetary dust particles. Nitrogen isotopic imaging shows that many IDPs contain discrete hotspots that are strongly enriched in ¹⁵N, up to ~1300. However, some IDPs also contain larger regions with more modest enrichments in ¹⁵N, leading to average bulk N isotopic compositions that are ¹⁵N-enriched. Carbon isotopic compositions are normal in most IDPs, but two ¹⁵N-rich hotspots have correlated ¹³C anomalies; these are the first observations of C isotopic anomalies in IDPs. Hydrogen isotopic distributions are similar to those of N: D anomalies are present both as distinct D-rich hotspots and as larger regions with more modest D enrichments. Oxygen isotopic imaging shows the presence of abundant presolar silicate grains in some of the IDPs. The oxygen isotopic compositions of the grains are similar to those of presolar oxide and silicate grains from primitive meteorites. In addition, C and O isotopic imaging led to the discovery of the first observed presolar grains of corundum and SiC in an IDP, presolar phases that are common in primitive meteorites. Based on their N isotopic compositions, IDPs can be divided into two groups. One group is characterized as being 'isotopically primitive' and consists of those IDPs hat have anomalous bulk N isotopic compositions. These particles typically also contain numerous ¹⁵N-rich hotspots, occasional C isotopic anomalies and abundant presolar silicate grains. In contrast, the other 'isotopically normal' IDPs have normal bulk N isotopic compositions, and generally contain few, if any, presolar phases. Thus, isotopically interesting IDPs can be identified on the basis of their N isotopic compositions for further study. However, the distinction does not appear to extend to H isotopic compositions: D anomalies are as common in normal IDPs as they are in those characterized as isotopically primitive, based on their N isotopes.

IDPs from Space-Exposed Aerogel	Aerogel is the medium of choice for the intact capture of small particles in space, because it is capable of decelerating high-velocity projectiles without substantial melting or other modification of their component materials. After space-exposed aerogel is returned to the laboratory, the first step of analysis will be a non-destructive optical evaluation of impact features and their basic classification. For a more detailed analysis of the projectile residues, however, it is necessary to extract the samples from the surrounding aerogel for mounting on a suitable substrate. We are developing a sample preparation routine that allows the routine measurement of trace element concentrations and isotopic compositions of aerogel-captured particles in the ion microprobe. Since the aerogel capture mechanism in low Earth orbit is so fundamentally different from IDP collection in the stratosphere, it would be of interest to compare the distribution of isotopic anomalies and trace element concentrations in both sets of samples. These measurements could, for example, give clues to the effect of atmospheric entry heating on particle properties. In a first step, we have made the first SIMS isotopic measurement of interplanetary dust that was captured in aerogel outside the Mir Space Station. Further work will be important for the return of dust samples from space experiments that use aerogel as a capture medium.

CR Chondrites: Effects of Parent Body Processing (Click to see enlargement)	Like interplanetary dust particles, the CR chondrite Renazzo contains abundant isotopic anomalies in both hydrogen and nitrogen, attesting to the primitive nature of this meteorite. Renazzo has been suggested as a promising analog for IDPs because it is the only meteorite known to have D-rich hotspots and, like IDPs, it shows evidence for several distinct D carriers. We are using the NanoSIMS to carry out isotopic imaging studies of Renazzo matrix material in order to compare the nature and distribution of primitive matter in this meteorite with that found in IDPs. Questions that we are trying to address with this work include determining whether the isotopic characteristics of IDPs and primitive meteorites are similar, whether the phases carrying the isotopic anomalies are the same in both materials and what role parent body processing has played. Like IDPs, Renazzo matrix material contains ¹⁵N-rich hotspots and also larger regions with so-called 'bulk' enrichments in ¹⁵N (see Figure). However, the overall levels of enrichment are somewhat lower than the corresponding ranges that have been observed for ¹⁵N-rich hotspots and bulk compositions in IDPs. Carbon isotopic imaging revealed two C- and Si-rich grains with large ¹³C excesses whose ¹²C/¹³C ratios suggest that they are mainstream SiC grains, consistent with the presence of this presolar phase in many primitive meteorites. However, extensive oxygen isotopic imaging of numerous matrix fragments failed to locate any presolar silicate or oxide grains in Renazzo, although such grains have been found in high abundance in other primitive meteorites and IDPs. CR chondrites are among the most primitive meteorites known and have largely escaped thermal metamorphism. They have, however, experienced low temperature aqueous alteration. This alteration would probably not destroy refractory presolar phases such as diamonds and SiC, but could alter presolar silicate grains or re-equilibrate their oxygen isotopic compositions. Our work to date suggests that Renazzo is one of the more pristine representatives among primitive chondrites and its matrix material shares many similarities with IDPs. However, the aqueous alteration it experienced affected the abundance and distribution of at least some of its presolar phases. Continued work will focus on constraining the effects of such parent body processing on the distribution of presolar phases by examining other members of the CR chondrite group, such as LEW 85332, which has a higher bulk ¹⁵N enrichment than Renazzo.


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