Mercury Science Options

[ljk4-1]

Astrophysics, abstract
astro-ph/0511419

From: Stan Peale [view email]

Date: Mon, 14 Nov 2005 22:21:56 GMT (314kb)

The proximity of Mercury's spin to Cassini state 1

Authors: S. J. Peale

Comments: 23 pages,12 figures, In press in Icarus

In determining Mercury's core structure from its rotational properties, the value of the normalized moment of inertia, $C/MR^2$, from the location of Cassini 1 is crucial. If Mercury's spin axis occupies Cassini state 1, its position defines the location of the state. The spin might be displaced from the Cassini state if the spin is unable to follow the changes in the state position induced by the variations in the orbital parameters and the geometry of the solar system. The spin axis is expected to follow the Cassini state for orbit variations with time scales long compared to the 1000 year precession period of the spin about the Cassini state because the solid angle swept out by the spin axis as it precesses is an adiabatic invariant. Short period variations in the orbital elements of small amplitude should cause displacements that are commensurate with the amplitudes of the short period terms. By following simultaneously the spin position and the Cassini state position during long time scale orbital variations over past 3 million years (Quinn {\it et al.}, 1991) and short time scale variations from JPL Ephemeris DE 408 (Standish, 2005) we show that the spin axis will remain within one arcsec of the Cassini state after it is brought there by dissipative torques. We thus expect Mercury's spin to occupy Cassini state 1 well within the uncertainties for both radar and spacecraft measurements, with correspondingly tight constraints on $C/MR^2$.

http://arxiv.org/abs/astro-ph/0511419

[edstrick]

Note that Mercury is actually brightest when it is at superior conjunction, on the far side of the sun. It's not a whole lot further from Earth than when at maximum elongation, is 100% instead of 50% illuminated, and, maybe most importantly, is near zero phase angle where backscatter and the hiding of soil-grain shadows makes the surface dramatically brighter than at phase angles of even 30 or 15 degrees, much less 90 deg.

The Venus cloud-haze is forward scattering, so, together with the much greater size change of the Venusian disk between crescent and gibbous phase, results in a rather fat crescent being the geometry where Venus appears brightest.