[Show abstract][Hide abstract] ABSTRACT: Pluto may be the only known case of precession-orbit resonance in the solar system. Pluto's precession caused by Charon about their orbital plane might have a period of 250.8 Earth years, the same as the orbital period of the Pluto-Charon system about the Sun. A Pluto flyby mission might refute or provide more evidence for the resonance. It is not clear how the planet would get in to such a resonance. Present-day Earth-based is observations appear to rule out Pluto's being in a resonance associated with half of its orbital period about the Sun unless Pluto has a large nonhydrostatic component to its flattening.
Journal of Geophysical Research 05/1999; · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mars may have substantially changed its average axial tilt over geologic time due to the waxing and waning of water ice caps through the phenomenon of climate friction (also called obliquity-oblateness feedback). Depending upon Mars' climate and internal structure, water caps of the order of 1017–1018 kg cycling with the obliquity oscillations could have either increased or decreased the average obliquity by possibly tens of degrees. This is in contrast to previous results, which indicated that 1017 kg carbon dioxide caps only increased the axial tilt. Since the south polar cap appears to be mostly uncompensated, Mars may be largely rigid on the obliquity timescale. Further, Mars may be a water-rich planet so that there is a large phase angle between insolation forcing and the size of the obliquity-driven water caps. A stiff, water-rich planet indicates the obliquity may have decreased over the eons. Such a decrease might account for the apparent youthfulness of the polar layered terrain, the idea being that fewer volatiles were available to be cycled into and out of the terrain at high obliquity because of more even insolation between equator and pole, so that the movement of volatiles produced thin layers or perhaps no layers at all. As the obliquity decreased, the insolation contrast between high and low latitiudes increased, and more volatiles might have shuttled into and out of the polar regions, forming the observed thick layers. In another but perhaps less likely scenario, Mars' average obliquity may have either increased or decreased until it became “stuck” at its present value of ∼24°. In this case the idea is that Mars' climate dynamics altered as the average tilt changed. Once the rate of increase in tilt caused by the deformation of the solid planet equaled the rate of decrease caused by the caps, the obliquity evolution ceased, leaving Mars at its present tilt.
Journal of Geophysical Research 01/1999; 104:30765-30771. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: There is an optimal size for the delivery of small asteroids from Mars to the Earth by Yarkovsky thermal drag. Basaltic asteroids with radii of about 6 m take on the average 185 million years (Myr) for their semimajor axes to shrink by 0.52 AU, assuming circular orbits and ignoring planetary perturbations and collisions. All other sizes take longer. Bigger objects are slower because they are more massive, and smaller objects are slower because they are more isothermal. These results are based on treating the asteroids as spheres and solving the heat conduction equation using spherical Bessel functions. The small near-Earth asteroids show a concentration of sizes in the thermal drag range; thus some of them may come from Mars as survivors of gravitational mechanisms which eliminate them on the 10 Myr timescale. The possible role of thermal drag in Mars-Earth delivery will remain speculative until it is included in numerical integrations of the orbits of small asteroids.
Journal of Geophysical Research 01/1998; 103:1725-1732. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photon thrust from the solar heating of the LAGEOS I satellite appears to explain much of the eccentricity variations seen in the satellite's orbital elements. We invoke a thermal model of LAGEOS I in which the photon thrust from solar heating is directed along the satellite's spin axis and functionally depends only on the cosine of the angle between the Sun's position and the spin axis. We calibrated the amplitude of the force from the 1980-1983 equivalent along-track acceleration derived from the observed orbital perturbations; during this time the spin axis position is assumed to be known and to be that at orbit injection. The photon thrust from this simple thermal model, plus later spin axis positions obtained from Sun glint data (which show LAGEOS I to be processing), give reasonable agreement with the observed along-track acceleration in the time period of 1988-1995. Thus much of the eccentricity variations seem to be due to thermal thrust and do not have a geophysical origin (atmospheric tides) as has been proposed. However, our solar heating model does not appear to explain the highest peaks and deepest troughs seen in the along-track acceleration, indicating the need for a better thermal model and consideration of other forces, such as that due to anisotropic reflection.
Journal of Geophysical Research 01/1997; 102:585-590. · 3.17 Impact Factor