Detection of solar wind-produced water in irradiated rims on silicate minerals

Abstract

The solar wind (SW), composed of predominantly ∼1-keV H + ions, produces amorphous rims up to ∼150 nm thick on the surfaces of minerals exposed in space. Silicates with amorphous rims are observed on interplanetary dust particles and on lunar and asteroid soil regolith grains. Implanted H + may react with oxygen in the minerals to form trace amounts of hydroxyl (−OH) and/or water (H 2 O). Previous studies have detected hydroxyl in lunar soils, but its chemical state, physical location in the soils, and source(s) are debated. If −OH or H 2 O is generated in rims on silicate grains, there are important implications for the origins of water in the solar system and other astrophysical environments. By exploiting the high spatial resolution of transmission electron microscopy and valence electron energy-loss spectroscopy, we detect water sealed in vesicles within amorphous rims produced by SW irradiation of silicate mineral grains on the exterior surfaces of interplanetary dust particles. Our findings establish that water is a byproduct of SW space weathering. We conclude, on the basis of the pervasiveness of the SW and silicate materials, that the production of radiolytic SW water on airless bodies is a ubiquitous process throughout the solar system. solar wind radiolysis | prebiotic water | cosmic dust | astrobiology | aberration-corrected scanning transmission electron microscopy T here are two principal space weathering processes, solar wind (SW) ion irradiation and micrometeorite impacts, that produce rims on exposed mineral surfaces (1). Our focus here is on SW irradiation because of its possible connection to the production of water and hydroxyl radicals (2-5). Space-weathered rim thicknesses vary with the densities of implanted solar flare (SF) tracks, and track densities depend on the exposure ages of individual interplanetary dust particles (IDPs) in space. On lunar soil grains and grains on surfaces of IDPs, rims are typically 75-150 nm thick with SF track densities of 10 10 -10 11 cm −2 that are consistent with 10 4 -to 10 5 -y SW exposure ages (6, 7) ( Fig. 1 B-D). Rims on asteroid Itokawa regolith grains are 30-60 nm thick with SF track densities of 5 × 10 9 cm −2 that are consistent with ∼10 3 -y exposure ages (8) (Origins and Properties of Rims on IDPs, Asteroid Itokawa, and Lunar Soil Grains). This correlation between amorphous rim thicknesses and SF track densities indicates that SW irradiation is the primary mechanism for amorphous rim formation. We examine rims on surface grains in IDPs because they are solely due to SW irradiation, whereas rims on lunar soil grains are due to SW irradiation, impact vapor deposition, or a combination of both (9), and remote observations suggest that if water is indeed produced in rims on lunar soil grains, it is not efficiently retained (10). Typical 5-to 25-μm diameter chondritic porous (CP) IDPs are low-density aggregates of predominantly submicrometer-sized grains, and they are collected in the stratosphere (11) (Fig. 1 A-D and Origins of CP IDPs). Chemical ana...

In situ experimental formation and growth of Fe nanoparticles and vesicles in lunar soil

“…; Bradley et al. ). Features such as chemically heterogeneous grain rims have been attributed to both micrometeorite impacts and irradiation processes.…”

Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life

“…Water is more or less ubiquitous on Earth and in other parts of the Solar System (Bradley et al, 2014;Küppers et al, 2014); it may be present within the atmospheres, subsurface, rocks and regolith, polar ice sheets, glaciers, and/or subsurface oceans of planetary bodies, in vapour plumes extruded into space, and -indeed -within space itself. 8 Whereas here on Earth, we tend to be most familiar with water in its bulk-liquid phase, in both terrestrial and extraterrestrial environments, it can also be present in a variety of forms.…”

“…See Waite and colleagues (); Nimmo and colleagues (); Tosca and colleagues (); Campins and colleagues (); Sohl and colleagues (); Carter and colleagues (); Martínez and Renno (); and Bradley and colleagues ().…”

The negligible chondritic contribution in the lunar soils water

“…The possible oxidation of SW H into water during silicate melting could also be considered as a possible source for this mantellic water. Indeed, production of water by SW implantation is now considered as a ubiquitous process in the solar system (13,19) and one of the possible mechanisms for bringing water to the Moon's surface. However, its contribution relative to chondritic or cometary sources is still debated (20).…”