Helium and neon in comet 81P/Wild 2 samples from the NASA Stardust mission

• Published in: Meteoritics & planetary science Vol. 54; no. 1; pp. 3 - 53
• Main Authors: Palma, R. L., Pepin, R. O., Westphal, A. J., Füri, E., Schlutter, D. J., Gainsforth, Z. S., Frank, D. R.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01-01-2019; Wiley
• Subjects: Aerogels; Comet heads; Cometary coma; Cosmic rays; Ejecta; Ejection; Galactic cosmic rays; Gases; Helium; Interstellar; Irradiation; Isotope ratios; Neon; Particle tracking; Proton flux; Rare gases; Sciences of the Universe; Solar proton flux; Solar protons; Solar wind; Spallation; Stardust Mission; Trays; Wild 2 comet
• ISSN: 1086-9379; 1945-5100
• DOI: 10.1111/maps.13189

Helium and neon distributions are reported for a variety of Stardust comet 81P/Wild 2 samples, including particle tracks and terminal particles, cell surface and subsurface slices from the comet coma and interstellar particle collection trays, and numerous small aerogel blocks extracted from comet cells C2044 and C2086. Discussions and conclusions in several s published during the course of the investigation are included, along with the relevant data. Measured isotope ratios span a broad range, implying a similar range for noble gas carriers in the Wild 2 coma. The meteoritic phase Q‐20Ne/22Ne ratio was observed in several samples. Some of these, and others, exhibit 21Ne excesses too large for attribution to spallation by galactic cosmic ray irradiation, suggesting exposure to a solar proton flux greatly enhanced above current levels in an early near‐Sun environment. Still others display evidence for a solar wind component, particularly one C2086 block with large abundances of isotopically solar‐like helium and neon. Eighty‐nine small aerogel samples were cut from depths up to several millimeters below the cell C2044 surface and several millimeters away from the axis of major track T41. A fraction of these yielded measurable and variable helium and neon abundances and isotope ratios, although none contained visible tracks or carrier particle fragments and their locations were beyond estimated penetration ranges for small particles or ions incident on the cell surface, or for lateral ejecta from T41. Finding plausible emplacement mechanisms and sources for these gases is a significant challenge raised by this study.