Excitation of Mars polar motion

Abstract

In this paper, computations of Mars' polar motion are presented for Mars models with three homogeneous layers as a function of the radius of the inner core and for different excitation possibilities (atmosphere, ice caps, quakes). We estimated the amplitude of the two polar motion normal modes, i.e. the Chandler wobble and the Inner Core wobble, resulting from atmospheric excitation and for a reasonable interval of damping factor values. We show that the signature of the inner core in the polar motion is very small and is unlikely to be detected with the present observational precision. We further investigated the possibility of exciting the normal modes of Mars' polar motion through Marsquakes and show that the predicted quake moments are not large enough to excite polar motion to an observable level.

Atmospheric and oceanic excitation of the rotation of a three-layer Earth

“…The low frequency case is discussed in Dehant et al (2005), in the case of Mars. In the nearly diurnal approximation, the matrix is:…”

Rotation of the Terrestrial Planets

“…The ICW has a long period in a corotating frame and the FICN a long period in an inertial frame. Extended Liouville theories for terrestrial planets with a solid elastic mantle, a liquid outer core, and an elastic inner core show that the solution for polar motion shows resonances at the eigenfrequencies of these four rotational normal modes (see, e.g., Dehant et al, 1993Dehant et al, , 2005Mathews et al, 1991Mathews et al, , 2002. An important consequence of these resonances is that for frequencies close to the resonant frequencies, rotation variations can be amplified.…”

“…From the spectral amplitude in the pressure recordings of the Viking landers near the CW period, Yoder and Standish (1997) estimated a CW amplitude of about 10 mas and possibly as large as 50 mas with ice loading excitation included. Dehant et al (2006) estimated the power spectral density of the atmospheric excitation close to the CW period from the LMD GCM (Forget et al, 1999) and determined a CW amplitude between 30 and 100 mas for values of the quality factor Q between 80 and 300 (for estimates of the Q factor, see Smith and Born, 1976;Yoder, 1982). Other excitation sources such as marsquakes are too small (Golombek, 2002;Golombek et al, 1992) to excite polar motion to an observable level .…”

Astrophysics in 2006

“…Continuing copious data provided evidence during the past year that Mars also has (a) polar caps that sublime from bottom to top rather than top to bottom (Keiffer et al 2006), (b) aurorae like terrestrial ones (Lundin et al 2006), (c) an ionosphere that expands in response to solar flares (Mendillo et al 2006), (d) glaciers (Furget et al 2006, which is actually a calculation pertaining to past epochs when the obliquity of the Martian ecliptic was 45°or more), and (e) two sorts of X-ray emission, fluorescence and charge exchange, both to be blamed on the Sun (Dennerl et al 2006). Marsquakes are not the main exciter of its Chandler and Inner Core Wobbles (Dehant et al 2006). And the "Mars face" looks rather different in images taken at different angles and resolutions (Neukum 2006), though the most easily deluded author still sees the bits of terrain that made up the nostrils, eyebrows, lips, and chin.…”