The Fall of the Nice Model
Nader Haghighipour

In an effort to explain the probable rapid increase in the number of collisions on the surface of Moon (a controversial topic known as the Late Heavy Bombardment), in a series of three articles published in Nature in 2005, it was suggested that approximately 880 million years after their formation, Jupiter and Saturn crossed a 2:1 mean-motion resonant state at which point planetesimals beyond the orbit of Neptune (a disk of trans-Neptunian objects) became unstable, and these bodies were scattered throughout the solar system. Through interaction with giant planets, some of these scattered objects reached the inner regions and became the cause of the LHB. Despite many fundamental issues in its physics and the mechanics of its implementation, this model, known as the Nice model (named after the city of Nice, the place of its inception), gained a great deal of popularity. Recent findings on the ratio of the amount of Highly Siderophile Elements (HSE) in Earth and in Moon's mantle have added to these complexities to the extent that the entire idea of the instability, especially the time of its occurrence were brought into question. A direct consequence of these new findings is that Earth accreted significantly more mass compared to moon after the moon-forming event, a process that, as stated by the creator of the Nice model, cannot be explained by the 880 million year instability (Morbidelli et al, 2017, LPSC abstract 2298, available on ADS). These authors suggest that if the giant planets' instability happened at all, it must be immediately after the formation of these planets and that the end process of the formation of terrestrial planets can explain the lunar cratering record and the amount of HSEs in the lunar mantle without needing any giant planet instability. I will review the post-formation evolution of our solar system and the few lunar cratering measurements that led to the LHB hypothesis. I will also present the thought process that resulted in the development of the Nice model and discuss its weaknesses and issues. Finally, I will review the implications of the new findings of the amount of HSEs in the lunar mantle for the post-formation evolution of the solar system.