A key question about the early history of the Solar System is whether the giant planets formed roughly at the distances from the Sun they presently occupy, or, as some theories predict, much closer to the Sun. The discovery of other solar systems with radically different configurations of planets has made this question more pressing, since it appears that the configuration of the Solar System might be atypical.

In the podcast, Hal Levison explains why the Trojan asteroids of Jupiter offer us the best opportunity to discriminate between the various models of Solar System evolution. And that is why a spacecraft called Lucy is now well on its way to a rendezvous with these asteroids.

Hal Levison is the Principal Investigator of the Lucy mission. He studies the dynamics of astronomical objects and, in particular, the formation and long-term behavior of solar system bodies. He is one of the original proponents of the Nice model (named after the city where it was conceived), a scenario that proposes the migration of the giant planets from an initial compact configuration closer to the Sun to their present positions. He is Chief Scientist in the Department of Space Sciences at the Southwest Research Institute in Boulder, Colorado.

The first multicellular animals to build reefs lived in the Early Cambrian around the time of the Cambrian explosion. They were sponges called archaeocyaths. In the podcast, Sara Pruss suggests that the rise of the archaeocyaths fostered an increase in animal diversity. But they were relatively short-lived, and when they died out in the Middle Cambrian, the diversity declined. Over geological time, reef-building organisms appear and disappear again and again until the corals we have today appeared in the Middle Triassic, about 240 million years ago.

Pruss is currently trying to understand why reefs are such a persistent feature of the geological record, despite the environmental stresses imposed on them. She is a Professor of Geosciences at Smith College.

Water can have a dramatic effect on the style of an eruption. In the podcast, Michael Manga explains how the most powerful eruptions, such as the 2022 Hunga Tonga eruption, occur when hot magma comes into contact with water and suddenly generates vast quantities of steam. Water dissolved in magma as it rises to the surface and depressurizes can also drive destructive volcanic eruptions. Manga also talks about water-driven volcanism on Mars and on the icy moons of Jupiter and Saturn.

Manga is a Professor in the Earth and Planetary Science department of the University of California, Berkeley.