Lunar Magnetism, Space Weathering, and Icy Satellite Interiors
An enduring mystery since Apollo is that, in spite of the Moon's lack of a global magnetic field, the surface is nevertheless dotted with regional magnetic fields strong enough to be detected from orbit. Did the Moon once have an intrinsic global field that magnetized parts of the crust but has since decayed away? This is a question of fundamental importance to understanding the formation and evolution of solid planetary bodies, and yet it remains unanswered due in part to limitations in our knowledge of these crustal magnetic anomalies. Adding to the puzzle, many of these magnetic anomalies are accompanied by enigmatic optical features, known as swirls, which may hold the key to understanding "space weathering"—a process by which airless bodies change color over time due to exposure to solar wind and micrometeoroids. Here we show both that swirl morphology provides information about the structure of the underlying magnetic sources, and that the color of the lunar surface varies systematically with latitude in a way that allows us to distinguish between the effects of solar wind ion and micrometeoroid bombardment, addressing a decades-old problem in remote sensing, and aiding in the interpretation of the spectra of airless bodies throughout the solar system.
The remarkable diversity of the outer solar system's satellites provides important clues about the formation and evolution of the solar system. Many of the satellites have surprisingly young surfaces, owing in some cases to on-going geologic activity. Moreover, the existence of subsurface oceans within some of the satellites raises the intriguing possibility of extant habitable environments in the outer solar system. Determining the properties of their ice shells and the structures of their deep interiors places fundamental constraints on how the icy satellites formed and evolved, and on what governs their behavior today. Using gravity and topography data from Cassini, we develop analytical models showing that Titan's ice shell may be very rigid and therefore unlikely to be geologically active. In contrast, we also model the internal structure of the tiny, but highly geologically active moon Enceladus, and find that a subsurface liquid ocean is likely.