Stop Hitting Yourself: Did Most Terrestrial Impactors Originate from Terrestrial Planets?

Project: Research project

Project Details


We propose to study the influence of debris released by giant impacts on the late formation and evolution of the inner solar system, using dynamical methods and results of impact simulations.

Although the asteroid belt is the main source of impactors in the inner solar system today, it contains only 0.05 Lunar masses of material. While the asteroid belt would have been much more massive when it formed, it is unlikely to have had greater than a Lunar mass since the formation of Jupiter and the dissipation of the solar nebula. By comparison, giant impacts onto the terrestrial planets during the late stage of accretion typically released debris equal to several per cent of the planet's mass into heliocentric orbit. The Moon-forming impact on Earth, for example, released over a Lunar mass of debris, more than has ever been contained in the asteroid belt. The Borealis basin impact on Mars released more debris at once, than the present day asteroid belt.

Escaping impact debris is less long lived than the main asteroid belt, as it is injected on unstable, planet-crossing orbits. This same factor however also increases the impact probability with the terrestrial planets and asteroids. With such a large amount of mass it seems highly likely that these now-extinct populations of impactors should have played an important role in the evolution of the solar system, much as now-extinct radionuclides played an important role in the thermal histories of solar system bodies.

One of the most obvious effects of re-impacting debris is the production of craters. The Moon, Mars, Mercury and the largest asteroids may be expected to display evidence of re-impacting bombardments in their crater record, subject to their thermal state and subsequent surface re-processing. However, in understanding the cratering history of the Solar System, the contribution of giant impact debris has been almost universally neglected, and constraining this contribution constitutes our primary task.

In addition to the production of surface craters, large impacts can punch through the lithosphere and exhume material from large depths. This is particularly pertinent for re-impacts onto a progenitor body which will have a deep magma ocean after the giant impact. Depending on the size and composition of the target, this magma ocean may form a flotation crust as on the Moon and perhaps Mercury, that would act as an insulating blanket. Impactors that punch through or disrupt this crust may allow the magma ocean to cool faster. Conversely, for magma oceans that freeze from the bottom up, the energy input from re-impacts may keep them liquid for longer. This feeds back into the cratering record as craters will only be recorded once there is a stable, solid surface. In this sense the dynamics of giant impact debris can in principle become a chronometer of planetary solidification. The influence of long term re-accumulation on planetary thermal evolution is our second task.

Our third task is to study re-impacting debris as an aspect of terrestrial satellite formation. The Moon, Phobos and Deimos likely formed from circumplanetary debris disks. These disks did not exist in isolation. They, and the satellites that form from them, would have been subject to re-impacts by returning debris from their formative giant impacts. Whether such disk impacts would have aided or hindered satellite formation is unclear.
Effective start/end date4/18/163/31/20


  • NASA: Goddard Space Flight Center: $643,000.00


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