Spitzer Evidence for a Late-heavy Bombardment and the Formation of Ureilites in η Corvi at ~1 Gyr

We have analyzed Spitzer and NASA/IRTF 2-35 μm spectra of the warm, ~350 K circumstellar dust around the nearby MS star η Corvi (F2V, 1.4 ± 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ~3 AU from the central star, in the system's terrestrial habitability zone. Spectral features due to ultra-primitive cometary material were found, in addition to features due to impact produced silica and high-temperature carbonaceous phases. At least 9 × 10¹⁸ kg of 0.1-100 μm warm dust is present in a collisional equilibrium distribution with dn/da ~ a ^-3.5, the equivalent of a 130 km radius Kuiper Belt object (KBO) of 1.0 g cm³ density and similar to recent estimates of the mass delivered to the Earth at 0.6-0.8 Gyr during the late-heavy bombardment. We conclude that the parent body was a Kuiper Belt body or bodies which captured a large amount of early primitive material in the first megayears of the system's lifetime and preserved it in deep freeze at ~150 AU. At ~1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10 km s^-1 with a rocky planetary body of mass <=M _Earth at ~3 AU, delivering large amounts of water (>0.1% of M _{Earth 's Oceans}) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO.