Large longitude libration of Mercury reveals a molten core.
Observations of radar speckle patterns tied to the rotation of Mercury establish that the planet occupies a Cassini state with obliquity of 2.11 +/- 0.1 arc minutes. The measurements show that the planet exhibits librations in longitude that are forced at the 88-day orbital period, as predicted by theory. The large amplitude of the oscillations, 35.8 +/- 2 arc seconds, together with the Mariner 10 determination of the gravitational harmonic coefficient C22, indicates that the mantle of Mercury is decoupled from a core that is at least partially molten.
Margot
J L
JL
Department of Astronomy, Cornell University, 304 Space Sciences Building, Ithaca, NY 14853, USA. jlm@astro.cornell.edu
Peale
S J
SJ
Jurgens
R F
RF
Slade
M A
MA
Holin
I V
IV