Cometary impact rates on the Moon and planets during the late heavy bombardment
1 P.A.S. Space Research Centre, Bartycka 18A, 00-716 Warszawa, Poland
2 Dept. of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
3 IAPS-INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
4 IFAC-CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy
5 Département Lagrange, University of Nice – Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
Received: 22 July 2016
Accepted: 9 October 2016
Context. The Nice model predicts that the trans-planetary planetesimal disk made a large or even dominant contribution to the cratering in the inner solar system during the late heavy bombardment (LHB). In the presence of evidence that lunar craters and mare basins may be mainly of asteroidal origin, there is a dilemma of the missing comets that is not yet resolved.
Aims. We aim to revisit the problem of cometary impact rates on the Moon and the terrestrial planets during the LHB with a flexible model, allowing us to study the influences of physical destruction of comets, the mass of the primordial disk, and the distribution of this mass over the entire size range.
Methods. We performed a Monte Carlo study of the dynamics of the cometary LHB projectiles and derive the impact rates by calculating individual collision probabilities for a huge sample of projectile orbits. We used Minimum Orbit Intersection Distances (MOIDs) according to a new scheme introduced here. Different calculations were performed using different models for the physical evolution of comet nuclei and for the properties of the primordial, trans-planetary disk.
Results. Based on the capture probability of Jupiter Trojans, we find a best fit radius of the largest LHB comet impacting the Moon for a low-mass primordial disk. For this disk mass, the LHB cratering of the Moon, Mercury and Mars were dominated by asteroids. However, some smaller lunar maria were likely preceded by comet impacts. The volatile delivery to the Earth and Mars by LHB comets was much less than their water inventories.
Conclusions. There is no excessive cometary cratering, if the LHB was caused by a late planetary instability in the Nice Model. The Earth and Mars obtained their water very early in their histories. The Noachian water flows on Mars cannot be attributed to the arrival of LHB-related H2O or CO2.
Key words: comets: general / Earth / Moon / planets and satellites: surfaces
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