E-belt asteroids

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Hungaria asteroids

The E-belt asteroids were the population of a hypothetical extension of the primordial asteroid belt proposed as the source of most of the basin-forming lunar impacts during the Late Heavy Bombardment.[1]

E-belt model

The E-belt model was developed by William F. Bottke, David Vokrouhlicky, David Minton, David Nesvorný, Alessandro Morbidelli, Ramon Brasser, Bruce Simonson and Harold Levison.[1] It describes the dynamics of an inner band of the early asteroid belt within the framework of the Nice model.

Location and stability

The extended-belt

outer planets would have been in a more compact configuration with nearly circular orbits.[2] With the planets in this configuration the ν6 secular resonance would be located outside the asteroid belt.[3] Stable orbits would have existed inside 2.1 AU and the inner edge of the primordial asteroid belt would have been defined by Mars-crossing orbits.[4]

Late Heavy Bombardment

During the migration of the giant planets the ν6 secular resonance would have moved inward as Saturn moved outward.[5] Upon reaching its current location near 2.1 AU the ν6 secular resonance and other related resonances would destabilize the orbits of the E-belt asteroids. Most would be driven onto planet-crossing orbits as their eccentricities and inclinations increased. Over a period of 400 million years impacts of the E-belt asteroids yield an estimated 9-10 of the 12 basin-forming lunar impacts attributed to the Late Heavy Bombardment.[1]

Hungaria asteroids

As their orbits evolved many of the E-belt asteroids would have acquired orbits similar to those of the

Hungaria asteroids with high inclinations and semimajor axis between 1.8 and 2.0 AU.[6] Because orbits in this region are dynamically sticky these objects would form a quasi-stable reservoir.[1] As this population of the E-belt asteroids leaked from this reservoir they would produce a long-lived tail of impacts after the traditional end of the late heavy bombardment at 3.7 billion years ago.[7] A remnant representing roughly 0.1–0.4% of the original E-belt asteroids would remain as the current Hungaria asteroids.[1]

Evidence of extended belt

Problems with alternative sources of LHB

Evidence for the

siderophile elements in these samples shows a better match for impactors from the inner Solar System than for comets.[8]
Studies of the dynamics of the main asteroid belt during giant planet migration have significantly limited the number of impactors originating from this region. A rapid alteration of Jupiter's and Saturn's orbits is necessary to reproduce the current orbital distribution.[3] This scenario removes only 50% of the asteroids from the main belt producing 2–3 basins on the Moon.[4]

Support for E-belt as source of LHB

Examination of samples recovered from the Moon indicates that the impactors were thermally evolved objects.[6] E-type asteroids, an example of this type, are uncommon in the main belt[9] but become more common toward the inner belt and would be expected to be most common in the E-belt.[6] The Hungaria asteroids, which are a remnant of the E-belt in this model, contain a sizable fraction of E-type asteroids.[10]

The decay of the population of E-belt asteroids captured onto Hungaria like orbits produces a long-lived tail of impacts which continues past the LHB. The continuation of the bombardment is predicted to generate basin-forming impacts on the Earth and Chicxulub-sized craters on the Earth and Moon.[1] Impact craters on the Moon and impact spherule beds found on the Earth dated to this period are consistent with these predictions.[1]

The E-belt model predicts a remnant population will remain on Hungaria-like orbits. The initial population of E-belt asteroids was calculated based on the population of potential basin-forming impactors remaining among the Hungaria asteroids.[8] The result was consistent with calculations based on the recent estimates of the orbital density of the main asteroid belt before the planetary migration.[4]

References

  1. ^
    S2CID 4423331
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  2. PMID 15917802
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  3. ^
    S2CID 8950534
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  4. ^
    S2CID 34283891
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  5. S2CID 38040202
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  6. ^ a b c Bottke, W. F.; Vokrouhlicky, D.; Nesvorný, D.; Minton, D.; Morbidelli, A.; Brasser, R. (March 2010). "The E-Belt: A Possible Missing Link in the Late Heavy Bombardment" (PDF). 41st Lunar and Planetary Science Conference: 1269.
  7. S2CID 43100552
    .
  8. ^
    S2CID 4423331
    .
  9. ^ Lang, Kenneth. "Asteroid distribution of spectral type with distance". Tufts University.
  10. doi:10.1016/j.icarus.2009.06.004
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