From pinpoints of light to geologic worlds: the magic of photometry

Before the age of space exploration, astronomers were limited to Earth-based telescopes to understand the composition and physical nature of celestial bodies. Through clever techniques – viewing different planetary longitudes, or observing at a variety of solar illumination angles, for example – planetary scientists were able to reveal a surprising amount of information from the pinpoints of light or blurry disks observed through their instruments. But as space missions transformed the planets, their moons and rings, and small bodies into geologic worlds, ground-based observations were eclipsed by more tangible techniques such as geologic mapping and geophysical modeling.

But even dedicated missions rarely provide the full picture. Spacecraft tours often lack specific viewing geometries, context (the “big picture”), long-term temporal coverage, and even key spectral regions. Planetary scientists now realize that the optimal scientific return comes from combining spacecraft measurements with Earth-based observations (including those from instruments in Earth-orbit). This strategy is especially important for the outer Solar System, where seasonal changes such as cloud formation and volatile transport occur over decades. A full picture requires observations even after the mission is completed. Historical observations can further extend the temporal timeline.

Planetary science was further transformed by augmenting spacecraft measurements with the quantitative models that show not just variations on the surface in composition and albedo, but in roughness and textural properties, offering clues that might indicate regions of recent geologic activity. Often these models enable characterization of structures below the spatial resolution limit of cameras.

The power of combining ground-based and mission data has led flight projects to recognize the importance of a “support” team of Earth-based observers working with mission teams for continuity and follow-up. Such a team will be important for Europa Clipper in order to provide a rapid response to understand the nature of possible sporadic activity. Teams should be in place even before a mission starts: an organized ground-based observing program of the ice giants and their moons will both focus the scientific goals of future missions to these bodies and define a baseline for seasonal or other changes.

Government sponsorship acknowledged.