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When Saturn's orbital elements are slightly changed from their present values, the motion of the jovian planets enters a large, robustly chaotic zone associated with the 2:5 mean-motion near-resonance between Jupiter and Saturn. The transition to chaos is through large increases in the secular frequency associated with Saturn's perihelion, while the other secular frequencies do not change appreciably. The chaotic zone is closer in the orbital elements' phase space to their current values than the range taken on by these values. It is plausible, therefore, to suspect that passages through such chaotic zones did occur throughout the history of the Solar System. Key Words: celestial mechanics; chaos; outer planets; resonances.

From several indicators, Venus has had a low level of tectonism and volcanism, compared to Earth, since resurfacings 500-1000 My ago. This behavior is generally ascribed to Venus having a very dry upper mantle and crust. Most indicators constrain only the last [approximately]20% of Venus's evolution. Consistent with the dryness - leading to a stiffer, more impenetrable lithosphere, thence smaller length scale convection and smaller percent partial melts - is the deficiency of 40Ar in the atmosphere: 24([+ or -]10)% of the generated 40Ar, based on an average K/U of 7220([+ or -]1220) and primordial mantle abundance 21([+ or -]3) ppb of U (contrasting to 52 [+ or -] 9% for the Earth). This potassium/argon constraint applies to the entire history of Venus. Water must have been expelled from the upper mantle within the first few 100 My, since all argon would have been expelled at the same time. The condition that the upper mantle must have been dry throughout Venus history also favors episodic overturns, in which most of the argon now in the atmosphere would have been out-gassed. U abundances at the lander sites indicate partial melts of about 4%. If we assume the frequency of episodes declined with the rate of heat generation, then there have been eight, the most recent at about 0.75 Ga. Each episode scavenged the K + Ar from 2-2.5% of the mantle and produced about 1-km average thickness of resurfacing. The 3.3 ppb of radiogenic argon in the atmosphere sets an upper limit on the potassium in the crust, and hence the crustal thickness: 22 [+ or -] 10 km; assuming a 10-km decay depth, it also implies 1.0% average surface density of [K.sub.2]O. The present heat delivery from the mantle of this model is 24 mW/[m.sup.2], about 20% higher than the upper limit for stagnant lid convection (C. C. Reese et al. 1998, J. Geophys. Res. 103, 13,643-13,657). Reductions of the mantle U and K/U and increase of the atmospheric argon by one standard deviation each would reduce the heat delivery to 16 mW/[m.sup.2]. Key Words: argon; Venus; potassium; evolution.

We report on numerical simulations exploring the dynamical stability of planetesimals in the gaps between the outer Solar System planets. We search for stable niches in the Saturn/Uranus and Uranus/Neptune zones by employing 10,000 massless particles many more than previous studies in these two zones - using high-order optimized multistep integration schemes coupled with roundoff error minimizing methods. An additional feature of this study, differing from its predecessors, is the fact that our initial distributions contain particles on orbits which are both inclined and noncircular. These initial distributions were also Gaussian distributed such that the Gaussian peaks were at the midpoint between the neighboring perturbers. The simulations showed an initial transient phase where the bulk of the primordial planetesimal swarm was removed from the Solar System within [10.sup.5] years. This is about 10 times longer than we observed in our previous Jupiter/Saturn studies. Next, there was a gravitational relaxation phase where the particles underwent a random walk in momentum space and were exponentially eliminated by random encounters with the planets. Unlike our previous Jupiter/Saturn simulation, the particles did not fully relax into a third Lagrangian niche phase where long-lived particles are at Lagrange points or stable niches. This is either because the Lagrangian niche phase never occurs or because these simulations did not have enough particles for this third phase to manifest. In these simulations, there was a general trend for the particles to migrate outward and eventually to be cleared out by the outermost planet in the zone. We confirmed that particles with higher eccentricities had shorter lifetimes and that the resonances between the jovian planets 'pumped up' the eccentricities of the planetesimals with low-inclination orbits more than those with higher inclinations. We estimated the expected lifetime of particles using kinetic theory and even though the time scale of the Uranus/Neptune simulation was 380 times longer than our previous Jupiter/Saturn simulation, the planetesimals in the Uranus/Neptune zone were cleared out more quickly than those in the Saturn/Uranus zone because of the positions of resonances with the jovian planets. These resonances had an even greater effect than random gravitational stirring in the winnowing process and confirm that all the jovian planets are necessary in long simulations. Even though we observed several long-lived zones near 12.5, 14.4, 16, 24.5, and 26 AU, only two particles remained at the end of the [10.sup.9]-year integration: one near the 2: 3 Saturn resonance, and the other near the Neptune 1 : 1 resonance. This suggests that niches for planetesimal material in the jovian planets are rare and may exist either only in extremely narrow bands or in the neighborhoods of the triangular Lagrange points of the outer planets.

We report on numerical simulations designed to understand the distribution of small bodies in the Solar System and the winnowing of planetesimals accreted from the early solar nebula. The primordial planetesimal swarm evolved in a phase space divided into regimes by separatrices which define their trajectories and fate. This sorting process is driven by the energy and angular momentum and continues to the present day. We reconsider the existence and importance of stable niches in the Jupiter/Saturn zone using highly accurate numerical techniques based on high-order optimized multistep integration schemes coupled to roundoff error minimizing methods. We repeat the investigations of W. M. Weibel et al. (Icarus 83, 382-390, 1990) with one hundred thousand massless particles - nearly [10.sup.3] time more particles than our 1990 investigation. Previous studies of the Jupiter/Saturn zone have employed only hundreds of particles, usually starting on circular and zero inclination orbits. By employing [10.sup.5] particles on both inclinded and eccentric orbits, we can perform a near-exhaustive search for test particle stability as a function of initial orbital elements. The increase in the numbers of test particles also facilitates robust statistical inference and comparison with analytic results. In our simulations, we observed three stages in the planetesimal dynamics. At the start of the simulation many planetesimals are quickly eliminated by close approaches to Jupiter or Saturn. Next there is a gravitational relaxation phase where the surviving particles are exponentially eliminated by random gravitational encounters with Jupiter or Saturn. Finally, the only long-lived particles in the simulation were initially located either at a Lagrange point or in an orbit nearly commensurable with Jupiter or Saturn. We conclude that although niches for planetesimal material are rare, extremely high-accuracy long-duration simulations employing many particles will be able to capture even the qualitative nature of early Solar System planetesimal evolution.