Mercury’s Molten Core how do we know… and what does it mean? Lindsay Johannessen PTYS 395 All images courtesy of NASA/JPL and Science Magazine

The Basic Facts: Mercury’s Mass: (Earth=1) Density: 5.43 (g/cm^3) Gravity: (Earth=1) Orbit Period: (Earth days) Rotation Period: (Earth days) Eccentricity of Orbit: Was previously thought to have solidified, cooled iron core New evidence states otherwise. Mercury’s core is partially molten!

Astronomers already knew… …Mercury had a pre-existing magnetic field. …A dynamo-like construction would be needed to form/sustain said magnetic field. …With a dynamo construction in Mercury, the core would not be composed of just Iron, but at least a certain percentage of lighter elements. What Astronomers Didn’t Know… …at first was how to accomplish these goals without waiting for MESSENGER to land. …the accuracy of two calculations: the tilt of the spin axis and gravitational field at the poles and equatorial regions.

What did they do? Using radar echoing from NASA/JPL’s DSS-14 antenna at Goldstone, CA, the Green Bank Telescope (GBT) in WV, and the Arecibo Telescope in Puerto Rico Data was taken at each location. With signals being sent out, there were two telescopes recording data from each respective signal. With the ‘speckle’ patterns in the data being recorded and analyzed, the astronomers were able to detect oscillations, or ‘forced librations’ in the rotation of Mercury.

What does Mercury’s spin rate have to do with anything…? By calculating the slight variation in the rotation period of Mercury, astronomers are able to compare the results of their radar echoing to those of theorized spin rates of solid bodies. These comparisons are of huge significance. After obtaining these variations in Mercury’s rotation, astronomers found a distinct pattern in oscillations they call ‘forced’ or ‘longitudinal’ librations.

These librations, when compared to those of a solid body, are of much greater amplitude than they should be. This data observation gives evidence of a molten core. The inner core and the crust of the planet must be decoupled, or somehow loosely connected via a fluid substance in order for this amplitude of oscillation to occur. - Notice the eccentricity of the orbit. How does this uneven gravitational pull effect the rotation libration when the body is not completely solid?

-Compare with boiled egg test…and add a gravitational force. Take into account that scientists now have evidence to support the dynamo theory for Mercury. This helps explain the magnetic field which is now theorized to be continuous, not residual.

Problems. Dundundun Is it actually possible for this tiny planet to still have a hot core? - Not as far as modern science has theorized. So if the core isn’t hot enough, can it still somehow be molten? - If sulfur is present, YES. Can sulfur be there in the core of the planet closest to our sun? - All experimentation and theory point to NO. During the formation of the solar system, the heavier elements gravitated towards the sun to form the terrestrial planets. Sulfur, not being one of those, would need to have an occupancy of 0.1% weight fraction in Mercury’s core to keep the outside mantle from freezing. This, as small as it seems, is still too far from theory.

What answers are we hoping MESSENGER will bring? - MESSENGER will acquire mineralogical and compositional data to help discover why Mercury is as dense as it is. - MESSENGER will closely study Mercury’s gravitational field to confirm the outer core is molten. - MESSENGER will study Mercury’s exosphere to determine its characteristics more in-depth and the processes by which it came about.