Alessandro Morbidelli on the Structure of the Solar System

Fast Moving Front Commentary, May 2011

Alessandro Morbidelli

Article: Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets

Authors: Gomes, R;Levison, HF;Tsiganis, K;Morbidelli, A
Journal: NATURE, 435 (7041): 466-469, MAY 26 2005
Addresses: CNRS, Observ Cote Azur, BP 4229, F-06304 Nice, France.
CNRS, Observ Cote Azur, F-06304 Nice, France.
MCT, ON, BR-20080090 Rio De Janeiro, Brazil.
(Addresses have been truncated)

Alessandro Morbidelli talks with and answers a few questions about this month's Fast Moving Fronts paper in the field of Space Science.

SW: Why do you think your paper is highly cited?

This paper was revolutionary in the sense that it proposed a totally new view of the evolution of our solar system and provided the first (and still unique) reasonable explanation of the so-far mysterious Late Heavy Bombardment of the terrestrial planets.

SW: Does it describe a new discovery, methodology, or synthesis of knowledge?

It's a new discovery, because we found a dynamical evolution of our planetary system never envisioned before. But it is also a synthesis of knowledge, because—to be fair—all the basic ingredients were already known. The originality of our work was to put all pieces together in a coherent scenario that matches observational constraints. This was not easy to do.

SW: Would you summarize the significance of your paper in layman's terms?

"This paper was revolutionary in the sense that it proposed a totally new view of the evolution of our solar system and provided the first (and still unique) reasonable explanation of the so-far mysterious Late Heavy Bombardment of the terrestrial planets."

Before our work, the solar system was considered a "boring place" where planets have been turning around the Sun on the same orbits since their formation. Our paper provides a paradigm shift, by showing that the planets underwent a phase of dynamical instability during which their orbits changed significantly.

So, the planets did not form on their current orbits. The structure of the Solar System as we know it today, originated from this phase of planet's orbital instability, which happened 3.9 billion years ago, i.e., about 600 million years after the formation of the planets themselves.

SW: How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?

I am a planetary scientist. I have always been involved in studies on planet formation and dynamical evolution. The Late Heavy Bombardment was the major flaw for all existing models. The Late Heavy Bombardment is an intense spike in the bombardment rate of the Moon and the terrestrial planets that occurred approximately 600 My after planet formation, i.e., at a time when the existing models predicted a quite life for the planets, characterized by a bombardment rate comparable to that at the present time.

The fact that a bombardment spike happened, implies that something dramatic must have happened in the structure of the solar system at that time. In fact, to have a spike in the bombardment rate so late after planet formation, it is necessary that a reservoir of small bodies remained stable somewhere in the solar system for 600 My and then suddenly became unstable. So, we started to investigate how this could have been possible. This led us to investigate possible instabilities in the motion of the planets and mechanisms to trigger such instability at the appropriate time.

When we finally could put all the pieces together, and the scenario that emerged appeared to be coherent and consistent with observational constraints, it was an amazing feeling of achievement and satisfaction.

SW: Where do you see your research leading in the future?

Tens of papers have been published by our group, but also other scientists, on the model that we proposed in the 2005 paper. These works refine the original model and confront it, with success, to a series of new observational constraints that we did not consider in 2005. This year a new paper by our group (Walsh et al., 2011) will appear in Nature, linking the formation and early evolution of the giant planets to the initial conditions of the 2005 model.

The terrestrial planet formation is also worked out in the framework of the same scenario. Thus, the overall consistency and coherence of our understanding of the formation and evolution of the solar system is broadening, and will improve further in the forthcoming years.

SW: Do you foresee any social or political implications for your research?

Not really. This is fundamental research: useless from practical purposes but extremely fascinating as it deals with the question of our origins.End

Alessandro Morbidelli
Director of Research
Observatoire de la Côte d'Azur
Nice, France



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