Doing geology by looking up, doing astronomy by looking down (with a focus on the Umbria–Marche pelagic limestones of Italy)

Abstract Astronomy and geology: the first deals with myriad, enormous, unreachable objects seen at great distances and at very low resolution; the other investigates one tiny (by comparison) object, Earth, at close range, from planetary scale down to almost atomic resolution in the laboratory. Planetary Science, studying solar system objects, to some extent bridges the gap between astronomy and geology. Astronomers mostly look up; geologists mostly look down. This paper lists cases where studies in one of those fields have contributed to understanding in the other. The central focus is on investigations of the Cretaceous and Paleogene pelagic Scaglia limestone in the Italian Apennine Mountains where geologic studies bear on five classes of solar system objects. The classes are separated by three orders of magnitude in size: the Moon‐forming impactor, bolides that form craters on Earth, meteorites, meteor‐forming grains, and extraterrestrial dust. The paper closes by calling attention to LIGO, the Laser Interferometer Gravitational‐Wave Observatory, in which astronomical discoveries made by looking down have yielded a geological discovery done by looking up. LIGO has detected distant collisions involving black holes and neutron stars by measuring infinitesimal distortions of the Earth, which in turn have shown that at least some of Earth's supply of heavy elements has been created by collisions of pairs of neutron stars—a major contribution to geochemistry and cosmochemistry.