This post was written by Rhian Jones.
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In the early days of the Solar System, 4.6 billion years ago, the new Sun was surrounded by a disk of gas and dust called the Solar Nebula. Current models for formation of the Solar System propose that the Solar Nebula was divided into two parts by massive Jupiter. Closer to the Sun, where the Earth formed, dust in the disk was mostly rocky, while beyond Jupiter the disk contained ices as well as rocky material. If this idea is correct, the Earth did not contain a lot of water when it originally formed and water must have been added later, for example from comets. The question of how and when the Earth got its water is currently a major puzzle.
Ordinary chondrites, the most common type of meteorites, are linked to the inner part of the protoplanetary disk where there was little ice. But several people have noticed things about ordinary chondrites that suggest that the asteroids they come from did in fact contain ice. We have published a new paper, Lewis et al. (2022), in which we took a closer look at the mineral feldspar in ordinary chondrites, to see what it can tell us about the presence of water on asteroids. We found a sequence of mineral reactions that record how fluids moved through the rocks when asteroids were heated, a few million years after they formed.
In this scanning electron microscope image of the ordinary chondrite Avanhandava, we see that the mineral plagioclase (plg) is being dissolved into a porous, spongy looking texture and a new mineral, albite (ab) is replacing the original plagioclase. Plagioclase and albite are both minerals in the feldspar group: plagioclase is a calcium-rich feldspar and albite is a sodium-rich feldspar. The texture of the feldspar minerals is a clear indicator that fluids were present. The other minerals in the image are pyroxene (px), clinopyroxene (cpx), and small white grains of oxide minerals.
Evidence like this shows that there was indeed water in the parent asteroids of ordinary chondrites, when they first formed from dust and ice in the disk. Although little of the water remains today, it left a tell-tale trace in grains of feldspar. If the inner Solar System was made up of ordinary chondrite materials, the Earth may have started out with more water than would be expected from a simple picture of a divided disk.
The full citation for this paper is below, and it is available to download from the journal website (subscription required):
Lewis, J.A., Jones, R.H. and Brearley, A.J., 2022. Plagioclase alteration and equilibration in ordinary chondrites: Metasomatism during thermal metamorphism. Geochimica et Cosmochimica Acta, 316, pp.201-229. https://doi.org/10.1016/j.gca.2021.10.004
Earth & Solar System 