According to a model published in 2005 (the “Nice Model”), a major rearrangement of the giant planets in the solar system took place about four billion years ago. As a result, an exterior disk of icy planetesimals with a total mass of 20-50 Earth masses was scattered by the outward migrating ice giants (Uranus and Neptune). In addition, the asteroid belt between Mars and Jupiter was stirred up so that the terrestrial planets were bombarded by both asteroids and icy planetesimals. This offered an explanation for something called the Late Heavy Bombardment (LHB) – a transient spike thought to exist in the lunar cratering rate 4 Gyr ago.

Around 2010, the most widespread opinion was that this bombardment was mostly caused by the icy planetesimals, i.e., objects of the same kind as comet nuclei. A team was formed at SRC with the intention to investigate in detail how this cometary bombardment would have affected Mercury, the Earth and Mars in addition to the Moon. One particular issue was the origin of water in Mars, because the red planet might have been formed essentially dry and picked up its water mostly during the LHB. The team also included other colleagues from Copenhagen, Nice, Rome and Warsaw.

We developed a numerical method to compute the minimum distance between the orbits of the planets and those of the comets, and we used this to derive the statistical collision rates by Monte Carlo simulation. Finally, the orbits of 100,000 comets were accurately traced for nearly a million years by numerical integration allowing for planetary perturbations, and collision probabilities with the planetary targets were estimated.

This work led to a broad range of scientific results. We showed that current models for the physical evolution of comet nuclei fail to yield a good fit between the observed inclination distribution of Jupiter Family comets and the one predicted by the secular dynamics. The capture rate necessary to keep the Jupiter Family in a steady state was found to be compatible with a relatively low mass of the source population in the scattered disk. We also concluded that asteroidal impactors dominated the LHB cratering of the Moon and planets as also implied by geochemical evidence. Moreover, the volatile delivery to the Earth and Mars by LHB comets was found to be much less than the estimated water inventories of these planets.

In a subsequent review of Mars’ early water we concluded that most of it likely arrived via stray planetesimals during the time before the Earth’s Moon-forming impact. A corresponding resurfacing event of smaller scale on Mars, also causing an important loss of early water, is the still hypothetical Borealis-forming impact. Moreover, it was shown that the km-sized comets that would have populated the exterior disk until the LHB would not survive against collisional destruction – a conclusion in apparent conflict with the pristine nature of comets. The resolution of this dilemma likely consists of a revision of the timing of the planetary rearrangement in the Nice Model. This is nowadays preferred to have happened after only 1/10 of the waiting time until the LHB.

Website created as part of the project: http://moid.cbk.waw.pl/. This application allows to calculate the MOID (Minimum Orbit Intersection Distance) between two elliptical orbits. MOID is a parameter used in astronomy to evaluate potential close proximity and collision between two celestial bodies, in particular between the Earth and the individual asteroid. This parameter is defined as the distance between two nearest points located in the osculating orbits of two celestial bodies.