Earth's moon has been blasted by a lot of space rocks in its 4.5-billion-year history. While these impacts have left a near-uniform distribution of craters across our satellite's surface, planetary scientists have shown that one of its sides – the side that faces Earth – bears significantly larger scars than the other. How can this be?
Measuring Impact Craters Is Harder Than You Think
To explain why craters on one side of the Moon are bigger than others, researchers led by Katerina Miljković – a planetary scientist at the Institut de Physique du Globe de Paris who specializes in impact basins – first needed to clarify what it means for a crater to be "big" in the first place.
Historically, determining a crater's true size has proven to be more difficult than one might expect. It seems obvious, for example, that a basin's size should be defined by its diameter, or its depth. But craters can fill up with dirt or lava. Their walls can crumble, and their rims can erode. (It's not for nothing that scientists commonly refer to impact craters as "transient" cavities.) Some impact basins even contain multiple "rims" – which one should be measured, then, to determine a hollow's true breadth?
Notably, all of these issues are associated with measurements made on the surface of impact basins. But the best indication of a crater's size may be buried underground.
When an asteroid makes contact with a rocky body like the Moon, it gouges out huge quantities of material from its crust and upper mantle. It stands to reason, therefore, that a good way to determine the true size of a basin would be measure the effect of the impact on the Moon's "crustal thickness" at that particular spot; if the crust beneath Crater 1 is thinner over a broader area than the crust beneath Crater 2, we can assume that Crater 1 is the bigger basin. But to measure crustal thickness you need an instrument that can see underground. And that's where NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission comes in.
The Biggest Basins
The results from the Agency's recent GRAIL mission provided Miljković and her colleagues with crustal thickness data for the entire Moon. The detail, she tells io9 via e-mail, was unprecedented, allowing her team to not only "investigate morphological subsurface structures of large craters and basins," but "measure their sizes, unambiguously, for the first time."
In doing so, the researchers discovered that while the Moon's nearside hemisphere and farside hemisphere each possess 12 craters with regions of crustal thinning greater than 200km in diameter (below, circled in black), the nearside craters are, consistently, significantly larger. The team describes its findings in the latest issue of Science[emphasis added]:
Whereas there are eight basins on the nearside hemisphere with diameters greater than 320 km, only one of this size is found on the farside, and this basin straddles the western limb of the Moon. Simulations of the Moon's impact bombardment by near-Earth asteroids show that the difference in cratering rate between the nearside and farside hemispheres should be less than 1% for a large range of impact conditions. With a uniform cratering rate, there is less than 2% probability that eight basins with diameters greater than 320 km would form on the nearside and only one such basin on the farside.
Why the disparity? About 4-billion years ago, a disproportionately large number of asteroids swept through the inner solar system, colliding with Mercury, Venus, Earth and Mars in an event known as the Late Heavy Bombardment. Earth's Moon took a pretty serious beating, too. So much so, in fact, that the Late Heavy Bombardment is commonly referred to as the "Lunar Cataclysm."
Miljković and her team argue that volcanic activity that also occurred during this cataclysmic period resulted in the crust and upper mantle being hotter on the Moon's nearside than on its farside. This heat, reason the researchers, made the Moon's nearside geology more susceptible to expanding readily outward following an asteroid impact. On the Moon's cooler (and therefore less pliant) farside, crust beneath the crater rim was prone to collapsing inward, "resulting in a diameter of thinned crust that is smaller than the transient crater diameter."
Simulations confirmed this. Above, on the left, the researchers use conditions representative of the cooler, farside hemisphere to model the first two hours following a collision with a 30-kilometer asteroid impacting at 10 km/second about 4-billion years ago; while the simulation on the right shows the first two hours following an impact on the warmer, nearside hemisphere. The models suggest that an impact on the nearside could have formed basins with diameters twice as wide as those generated by similar impacts on the Moon's farside hemisphere. (NB: The simulation on the left is cycling about twice as fast as the one on the right, but the gist is clear: The geologic deformation on the right is much more extensive.)
Studies like this one certainly help scientists paint a clearer picture of the Moon's history, but they can also tell us a lot about the evolution of the solar system as a whole. For instance, Miljković's team argues that, because the temperature profile beneath the Moon's nearside is not representative of the Moon overall, the true magnitude of the Late Heavy Bombardment has likely been overestimated. Likewise, a better understanding of geological processes on the Moon can come in handy when making sense of impact basins on other planets, like Mars, Mercury, Venus, or even Earth.
Whatever the planetary body, Miljković tells io9 that data provided by missions like GRAIL is crucial. "To be able to verify the numerical and theoretical work, we must have planetary mission data to compare and contrast," she says. "The advantage of our work is that with [GRAIL's] gravity data mixed in with the [Lunar Reconnaissance Orbiter's] topography data, we were able to tie the surface measurements with the subsurface in great detail. Future work should be expanded to other planetary bodies, too."