Radar measurements of Mercury's obliquity and librations

Evidence for a global magnetic field at Mercury has raised perplexing questions regarding the existence and nature of the planet's core. The problems related to Mercury's core are of great interest because of their implications on how planets evolve thermally and on how they generate magnetic fields. Peale (1976) showed that the measurement of four quantities, including the planet's obliquity and the amplitude of its longitude librations, could place constraints on the size and state of the core.

Our observations are designed to characterize Mercury's interior by obtaining precise measurements of its rotational state. We illuminate the planet with a radar signal and record the echoes at two distant telescopes.

Roughly 30 measurements of the spin state of Mercury have been obtained with the Goldstone and Green Bank Telescopes since May 2002. Our measurements provide the first experimental proof that Mercury is in a Cassini state, a three-order of magnitude improvement in the knowledge of the spin orientation, and a compelling detection of the longitude librations. Our libration data show that the core of Mercury is molten.

Existing coverage

This diagram shows the positions of Earth and Mercury in the ecliptic plane at the times of our measurements. 01: 2002 May 13 02: 2002 May 22 03: 2002 Jun 02 04: 2002 Jun 12 05: 2003 Jan 13 06: 2003 Jan 23 07: 2003 May 31 08: 2003 Jun 01 09: 2003 Sep 18 10: 2003 Sep 19 11: 2003 Sep 20 12: 2004 Mar 31 13: 2004 Dec 12 14: 2004 Dec 18 15: 2004 Dec 19 16: 2005 Mar 13 17: 2005 Mar 14 18: 2005 Mar 16 19: 2005 Mar 18 20: 2006 Jun 29 21: 2006 Jul 12 22: 2008 Jul 05 23: 2008 Jul 06 24: 2009 Jan 12 25: 2009 Jan 13 26: 2009 Jan 14 27: 2009 Jun 19 28: 2010 Jan 02 29: 2010 Jan 03 30: 2010 Jan 10 The dashed line is the line of nodes and the dotted line is the line of apsides. Each measurement is particularly sensitive to rotations of the spin vector about the line of sight. The best geometries for future observations are those that are orthogonal to previous observations.