E-belt 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]

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.

The extended-belt asteroids were located between the current inner boundary of the asteroid belt and the orbit of Mars with semi-major axis ranging from 1.7 to 2.1 astronomical units (AU). In the current Solar System most orbits in this region are unstable due to the presence of the ν₆ secular resonance.^[1] However, prior to the giant planet migration described in the Nice model the 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]

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]

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 for the Moon does not support comets from the outer planetesimal belt as the source of the basin-forming lunar impacts. The size frequency distribution (SFD) of ancient lunar craters is a similar to the SFD of main belt asteroids instead of that of comets.^[4] Samples recovered from the Moon containing impact melts have a range of ages rather than the sharp spike expected if comets produced the LHB.^[8] Analysis of highly 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]

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]