摘要:

A long period analysis of solar system orbital evolution, correct to fourth order in orbital eccentricities and inclinations (Bretagnon, 1974), and an improved value of the planet's moment of inertia (Reasenberg, 1977) have been incorporated in a recalculation of the obliquity oscillations of Mars. A linearized solution predicts a maximum oscillation amplitude of 13.6° centered on a long‐term average value of 24.4°. A numerical integration of the obliquity for the past 107years is also presented. Epochs of minimal oscillation like the present occur at intervals of the order of 4

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00237

年代:1979

数据来源: WILEY

摘要:

The present gravitational quadrupole moment of Mars contains a 6½% nonhydrostatic contribution from the uncompensated Tharsis construct. Prior to the Tharsis uplift, the rate of spin axis precession was nearly equal to the frequency of a minor term in the expression for the orbital precession of Mars developed by Bretagnon (1974). In such a situation, small changes in theJ2of Mars could have caused the spin axis precession rate to drift repeatedly through the orbital frequency. We show that even minor, low‐amplitude orbital components can have a significant impact on the obliquity oscillations due to secular spin‐orbit resonance. Our approach to this problem is in three stages. First, we examine a number of processes which have the potential to alter the spin axis precession rate, discussing both the expected magnitude and time scales involved. Of these, Tharsis plays a dominant role, but other processes, i.e., differentiation, mantle convection, etc., may also have significantly influenced the planet's quadrupole moment in the past. Second, a theory describing the consequences of passage through a secular spin‐orbit resonance is developed. The problem is solved analytically for the case of uniform orbital precession. It is then shown that the resonance behavior is almost undisturbed by the presence of other orbital terms (i.e., nonuniform precession), even if these terms are appreciably larger than the resonance terms. Third, numerical integrations of the full equations of motion for Mars are presented that demonstrate the resonance and indicate that the obliquity of Mars may have intermittently been as low as ∼9° and as high as 46° (or more) in its ear

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00243

年代:1979

数据来源: WILEY

摘要:

Extensive sampling has revealed a new feature of the Mono Lake geomagnetic excursion. The previously known eastward swing in declination and steepening of inclination are preceded by an even greater swing to westerly declination and shallow inclination. The duration of the entire excursion is estimated to be about 1000 years. Excellent agreement of paleomagnetic directions between four sites shows the excursion is a real expression of the geomagnetic field. Interpretation of natural remanent magnetization/anhysteretic remanent magnetization (NRM/ARM) ratios for one site indicates the field intensity may have fallen well below and then increased well above the dipole field intensity, suggesting the excursion is a manifestation of the nondipole field. The source can be modeled by a radial eccentric dipole at high northern latitudes and pointing outward during the first part of the excursion, and near the equator and pointing inward during the latter part. Movement of the source appears localized, displaying a complex pattern of eastward, westward, and even northward drift. The average moment of the hypothetical eccentric dipole during the excursion is comparable with the largest calculated for the 1945 field, and the maximum moment is almost twice as great.

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00261

年代:1979

数据来源: WILEY