AKirkwood gap is a gap or dip in the distribution of thesemi-major axes (or equivalently of theorbital periods) of the orbits ofmain-beltasteroids. They correspond to the locations oforbital resonances withJupiter. The gaps were first noticed in 1866 byDaniel Kirkwood, who also correctly explained their origin in the orbital resonances with Jupiter while a professor atJefferson College inCanonsburg, Pennsylvania.[1]
For example, there are very few asteroids with semimajor axis near 2.50AU, period 3.95 years, which would make three orbits for each orbit of Jupiter (hence, called the 3:1 orbital resonance). Other orbital resonances correspond to orbital periods whose lengths are simple fractions of Jupiter's. The weaker resonances lead only to a depletion of asteroids, while spikes in the histogram are often due to the presence of a prominentasteroid family(seeList of asteroid families).
Most of the Kirkwood gaps are depleted, unlike themean-motion resonances (MMR) ofNeptune or Jupiter's 3:2 resonance, that retain objects captured during the giant planet migration of theNice model. The loss of objects from the Kirkwood gaps is due to the overlapping of the ν5 and ν6secular resonances within the mean-motion resonances. The orbital elements of the asteroids vary chaotically as a result and evolve onto planet-crossing orbits within a few million years.[2] The 2:1 MMR has a few relatively stable islands within the resonance, however. These islands are depleted due to slow diffusion onto less stable orbits. This process, which has been linked to Jupiter andSaturn being near a 5:2 resonance, may have been more rapid when Jupiter's and Saturn's orbits were closer together.[3]
More recently, a relatively small number of asteroids have been found to possess higheccentricity orbits which do lie within the Kirkwood gaps. Examples include theAlinda andGriqua groups. These orbits slowly increase their eccentricity on a timescale of tens of millions of years, and will eventually break out of the resonance due to close encounters with a major planet. This is why asteroids are rarely found in the Kirkwood gaps.
The most prominent Kirkwood gaps are located at mean orbital radii of:[4]
Weaker and/or narrower gaps are also found at:
The gaps are not seen in a simple snapshot of the locations of the asteroids at any one time because asteroid orbits areelliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps does not differ significantly from the neighboring regions.[5]
The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit, for instance. Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby. (For example, an 8:3 resonance for asteroids with a semi-major axis of 2.71 AU).[6]
The main or core population of the asteroid belt may be divided into the inner and outer zones, separated by the 3:1 Kirkwood gap at 2.5 AU, and the outer zone may be further divided into middle and outer zones by the 5:2 gap at 2.82 AU:[7]
4 Vesta is the largest asteroid in the inner zone,1 Ceres and2 Pallas in the middle zone, and10 Hygiea in the outer zone.87 Sylvia is probably the largest Main Belt asteroid beyond the outer zone.
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