The Earth’s surface is subject to great change over geological time periods, due to the movement of tectonic plates, and volcanic activity, as well as long-term weathering and erosion. As a result, craters caused by meteorite, asteroid and comet bombardment long ago is gradually eradicated. We therefore look to the Moon’s cratered surface to provide clearer evidence of the bombardment history of the Earth/Moon binary. That cratering history is then compared to other planets and objects in the inner solar system, allowing astronomers to discern patterns in cratering over long time periods. One of the most significant events is the late, heavy bombardment. Following a period of relative quiet after the formation of the planets, this mass bombardment was thought to have occurred about 3.9 billion years ago:
“Competing models of meteorite-impact rate for the first 2 billion years (Ga) of Earth and Moon history. Note that Earth is believed to have formed about 4.55 Ga before present. Two hypotheses are shown: exponential decay of impact rate (dashes); and cool early Earth–late heavy bombardment (solid curve).” [see right-hand graph] (1).
More recently however, there has been a gradual realisation that this was not a sudden, dramatic event, but rather a sustained period of impacts by what were some colossal bodies:
“Recent high-resolution orbital data and images, more refined techniques for studying small lunar, terrestrial, and other impact samples and a better understanding of their ages, and improved dynamical models based on orbital and sample data have caused a paradigm shift in how we think about the lunar impact rate … The long-held idea of a “lunar cataclysm” at ~3.9 Ga is being replaced by the idea of an extended lunar bombardment from ~4.2 Ga to 3.5 Ga.” (3) Read More…
The two moons of Mars have always presented planetary scientists with something of a mystery. These tiny moons, Phobos and Deimos, whizz around Mars at no great height at all: Phobos whips around the red planet in less than 8 hours, at a height of only 3,700 miles – the closest of any moon to its parent planet. I say ‘parent’ advisedly because a new theory of the origin of these peculiar little moons suggests that they emerged from a major impact between mars and a dwarf planet. It has generally been assumed that they were captured asteroids, but the relative circularity of their orbits argued against such a capture. Work on the possibility of a catastrophic origin was carried out last year by two separate teams of researchers, after decades of battling intense scepticism within the scientific community (1). An important finding of the modelling at that time was that the resultant debris would circulate around the red planet at a relatively low altitude, which is in keeping with the orbits of the two extant moons.
More recently, further computer modelling of various impact scenarios carried out by one of those teams has narrowed down the range of masses of an impactor to about the size of Pluto. The resultant debris field was initially far more extensive than the two moons left today: