We reside within a relatively interior position within the Milky Way galaxy which hinders our ability to understand its structure. Nonetheless, astrophysical observations of other galaxies in unison with spectroscopic measurements have produced a model for the Milky Way as a grand design, barred, spiral arm galaxy, with either two or four arms. Viewing through the plane of the Milky Way is not possible with any current astrophysical technique. However, perhaps terrestrial geology can help where current observations of our stellar environment cannot. During the orbit of our solar system around the galactic centre, Earth will have seen different cosmic surroundings, as a function of the solar system's orbit (240 km/s) that is faster than the spiral arm's density waves (210 km/s). Specifically, if the terrestrial impact record, or proxies for it, in some cryptic way reflect perturbations on the gravity field of the local solar system, then Earth may act as a geological orrery, with some interesting implications. Here we explore various models for the design of the Milky Way and compare these to geological proxies proposed by some as indicators for impact flux, through the deep time record within our planet. Isotope signatures in zircon are statistically coherent with a four-armed spiral model. However, even better correspondence is shown between the terrestrial isotopic record and more complex atomic hydrogen models of the galaxy.

Supplementary material:https://doi.org/10.6084/m9.figshare.c.7294700

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