Planetary Motion
What is the evidence? Measurements of variations (“wobbles”) in a planet’s spin around its axis; measurements of gravitational anomalies on a planet—regions with increases or decreases in mass—inferred from how a spacecraft’s velocity changes as it orbits a planet
What does it tell us? How much a planet wobbles depends on the tilt of its rotation axis and how mass is distributed within the planet; this means that variations in a planet’s movement as it spins tell us about the internal properties of a planet, such as the size and density of its core and whether the core is molten or solid. Learning about the properties and evolution of the core of planets like Mercury will help scientists understand more about Earth and its place in the solar system.
Key limitations: Spacecraft remote-sensing operations and tracking may be constrained by its orbital position, orientation, time of day, and scheduling issues
Example: Inside Mercury
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As it orbited Mercury, the MESSENGER spacecraft collected data, including how mass was distributed on the planet and how the thickness of the crust varied. Based on the planet’s spin and how the planet’s gravity affected the spacecraft as it orbited around it, MESSENGER data helped scientists model the internal structure of Mercury, and determine that, like Earth, Mercury has a solid, metallic inner core. Mercury’s inner core makes up about half of its entire core (total core radius 2,004 km). As shown in the diagram, Mercury’s core is huge, taking up about 85 percent of the planet’s volume.
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