Stardust grains can provide useful information about the Solar System environment before the Sun was born. Stardust grains show distinct isotopic compositions that indicate their origins, like the atmospheres of red giant stars, asymptotic giant branch stars, and supernovae (e.g., Bose et al. 2010). It has been argued that some stardust grains likely condensed in classical nova outbursts (e.g., Amari et al. 2001). These nova candidate grains contain 13C, 15N and 17O-rich nuclides which are produced by proton burning. However, these nuclides alone cannot constrain the stellar source of nova candidate grains. Nova ejecta is rich in 7Be that decays to 7Li (which has a half-life of ~53 days). I want to measure 6,7Li isotopes in nova candidate grains using the NanoSIMS 50L (nanoscale secondary ion mass spectrometry) to establish their nova origins without ambiguity. Several stardust grains that are nova candidate grains were identified in meteorite Acfer 094 on the basis of their oxygen isotopes. The identified silicate and oxide stardust grains are <500 nm in size and exist in the meteorite surrounded by meteoritic silicates. Therefore, 6,7Li isotopic measurements on these grains are hindered because of the large 300-500 nm oxygen ion beam in the NanoSIMS. I devised a methodology to isolate stardust grains by performing Focused Ion Beam milling with the FIB – Nova 200 NanoLab (FEI) instrument. We proved that the current FIB instrument cannot be used to prepare stardust grains smaller than 1 𝜇m due to lacking capabilities of the FIB. For future analyses, we could either use the same milling technique with the new and improved FIB – Helios 5 UX or use the recently constructed duoplasmatron on the NanoSIMS that can achieve a size of ~75 nm oxygen ion beam.
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on how one interprets diverse observations. Quantifying if, when, or where lightning occurs would provide novel information about Venus’s atmospheric dynamics and chemistry. Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–70 km above the surface) for up to an Earth-year. For decades, spacecraft and ground-based telescopes have searched for lightning at Venus, using many instruments including magnetometers, radios, and optical cameras. Two surveys (from the Akatsuki orbiter and the 61-inch telescope on Mt. Bigelow, Arizona) observed several optical flashes that are often attributed to lightning. We expect that lightning at Venus is bright near 777 nm (the unresolved triplet emission lines of excited atomic oxygen) due to the high abundance of oxygen as carbon dioxide. However, meteor fireballs at Venus are probably bright at the same wavelength for the same reason. Here we derive power laws that quantify the rate and brightness of optical flashes from meteor fireballs at Venus. We calculated that meteor fireballs are statistically likely to cause bright optical flashes at rates that are consistent with published observations. Small meteors burn up at altitudes of ~100 km, roughly twice as high above the surface as the clouds. Therefore, we conclude that there is no concrete evidence that lightning strikes would be a hazard to missions that pass through or dwell within the clouds of Venus.
The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM.
Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage.
Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates.