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The radius of the core of Mercury is approximately 75% of that of the entire planet, which is a much larger fraction of the planet than for Earth. Like Earth, Mercury has a core that is at least partially liquid. However, unlike Earth, the size of the solid inner core is not known. Figure courtesy of NASA and APL. |
As previously discussed for Questions 1 and 3, Mercury is known to have a very large iron-rich core and a global magnetic field; this information was gathered by the Mariner 10 flybys. More recently, Earth-based radar observations of Mercury have also determined that at least a portion of the large metal core is still liquid to this day! Having at least a partially molten core means that a very small but detectable variation in the spin-rate of Mercury has a larger amplitude because of decoupling between the solid mantle and liquid core. Knowing that the core has not completely solidified, even as Mercury has cooled over billions of years since its formation, places important constraints on the thermal history, evolution, and core composition of the planet.
However, these constraints are limited due to the low precision of current information on Mercury's gravity field from the Mariner 10 flybys, currently the only gravity measurements available for Mercury. Fundamental questions about Mercury's core remain to be explored, such as what is the composition of the core? A core of pure iron would be completely solid today, due to the high melting point of iron. However, if other elements, such as sulfur, are also present in Mercury's core, even at only a level of a few percent, the melting point is lowered considerably, allowing Mercury's core to remain at least partially molten. Constraining the composition of the core is intimately tied to understanding what fraction of the core is liquid and what fraction has solidified. Is there just a very thin layer of liquid over a mostly solid core or is the core completely molten? Addressing questions such as these can also provide insight into the current thermal state of the interior of Mercury, which is very valuable information for determining the evolution of the planet from its early formation to the present day.
Using the laser altimeter, MESSENGER will verify the presence of a liquid outer core in Mercury by measuring the planet's libration. Libration is the slow 88-day wobble of the planet around its rotational axis. The libration of the rocky outer part of the planet will be twice as large if it is floating on a liquid outer core than if it is frozen to a solid core. By radio tracking of the spacecraft, MESSENGER will also determine the gravity field with much better precision than accomplished with the Mariner 10 flybys. The libration experiment, when combined with improved measurements of the gravity field, will help to provide constraints on the size of the core and how much of it is liquid, crucial information for understanding the structure of Mercury's core.
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