Rapid collisional evolution of comets during the formation of the Oort cloud

The Oort cloud(1) of comets was formed by the ejection of icy planetesimals from the region of giant planets-Jupiter, Saturn, Uranus and Neptune-durin g their formation(2). Dynamical simulations(3,4) have previously shown that comets reach the Oort cloud only after being perturbed into eccentric orbi ts that result in close encounters with the giant planets, which then eject them to distant orbits about 10(4) to 10(5) AU from the Sun (1 AU is the a verage Earth-Sun distance). All of the Oort cloud models constructed until now simulate its formation using only gravitational effects; these include the influence of the Sun, the planets and external perturbers such as passi ng stars and Galactic tides. Here we show that physical collisions between comets and small debris play a fundamental and hitherto unexplored role thr oughout most of the ejection process. For standard models of the protosolar nebula (starting with a minimum-mass nebula) we rnd that collisional evolu tion of comets is so severe that their erosional lifetimes are much shorter than the timescale for dynamical ejection. It therefore appears that colli sions will prevent most comets escaping from most locations in the region o f the giant planets until the disk mass there declines sufficiently that th e dynamical ejection timescale is shorter than the collisional lifetime. On e consequence is that the total mass of comets in the Oort cloud may be les s than currently believed.