But because it was believed erosion caused some craters to disappear, it was difficult to find an accurate impact rate and determine whether it had changed over time. A way to sidestep this problem is to examine the Moon, which is hit by asteroids in the same proportions over time as Earth.
Using LRO data, the team was able to assemble a list of ages of all lunar craters younger than about a billion years. They did this by using data from LRO's Diviner instrument, a radiometer that measures the heat radiating from the Moon's surface, to monitor the rate of degradation of young craters. During the lunar night, rocks radiate much more heat than fine-grained soil called regolith.
This allows scientists to distinguish rocks from fine particles in thermal images.
Ghent had previously used this information to calculate the rate at which large rocks around the Moon's young craters—ejected onto the surface during asteroid impact—break down into soil as a result of a constant rain of tiny meteorites over tens of millions of years. By applying this idea, the team was able to calculate ages for previously un-dated lunar craters. When compared to a similar timeline of Earth's craters, they found the two bodies had recorded the same history of asteroid bombardment.
The reason for the jump in the impact rate is unknown, though the researchers speculate it might be related to large collisions taking place more than million years ago in the main asteroid belt between the orbits of Mars and Jupiter. Such events can create debris that can reach the inner solar system.
Ghent and her colleagues found strong supporting evidence for their findings through a collaboration with Thomas Gernon, an Earth scientist based at the University of Southampton in England who works on a terrestrial feature called kimberlite pipes. These underground pipes are long-extinct volcanoes that stretch, in a carrot shape, a couple of kilometers below the surface, and are found on some of the least eroded regions of Earth in the same places preserved impact craters are found.
Gernon showed that kimberlite pipes formed since about million years ago had not experienced much erosion, indicating that the large impact craters younger than this on stable terrains must also be intact.
Using data for craters on the melt surfaces and for small-diameter bright ejecta craters, an absolute model age of 1 Ma is estimated. This age is considerably younger than that estimated by other studies and probably represents a maximum age. How old is this crater? A scientist would be tempted to conclude that the ejecta blanket is old, and the secondary impact is young, with a long time having passed between.
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The series of events could have been relatively rapid. Another assumption, that of incoming impactor rate, is also non-empirical. Nobody has lived long enough to watch how fast new impactors arrive on a surface.
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Scientists can deduce the order in which some impacts occurred, by looking at craters on top of other craters. Plescia and Mark S.
Crater Count Dating: Self-Secondaries Reduce Age Estimates
Can Crater Dating Be Tested? A million years is still a long age estimate, but notice they are calling it a maximum age — it could be far less. Let this be a lesson on all dating methods. I will climb back out of the rabbit hole eventually with lots of good stories about the geology of many different planets, but I'm going to have to tell those stories bit by bit. It all begins, appropriately, with the history of impact basins on the Moon. I think that's appropriate because the Moon is where the study of planetary geology started, even before the Space Age.
Scientists find increase in asteroid impacts on ancient Earth by studying the Moon
The familiar face of the Moon contains dark splotches, the maria. Look at the maria with a telescope, and you can see that they're flat plains that appear to fill low-lying areas. And most of those low-lying areas are circular basins rimmed by mountainous ridges. We know now that these are impact basins, places where asteroids slammed into the Moon. Arguably the most beautiful of the Moon's basins is Orientale, whose glory we really couldn't appreciate until the Space Age, because its eastern edge just peeks over the visible near side of the Moon.
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Here is a lovely view from Lunar Reconnaissance Orbiter. The big basins are old, so a lot has happened to them since they formed. So much, in fact, that some of them can be difficult to spot in photographs, even though they're very big all of the ones discussed here are kilometers wide or more. But when you have topographic data and gravity data, you can make maps that cause hidden basins to leap out at you, like they do in this map of lunar crustal thickness. Basins have thin crust blue and green , and are often rimmed by thick crust red and orange. Geologists take maps like the one above and draw circles around basins and give them names.
Some of them are classical names that Moon fans should recognize, things like Imbrium and Crisium and Orientale. Other names are unfamiliar; these are the basins that were not obvious until we got topography and gravity data.
Often, these previously hidden basins are named by mentioning two craters or other features that occur on their edges -- things like Freundlich-Sharonov and South Pole-Aitken. So far, this is all stamp collecting. We've named some basins.
Well, now that we've identified where the Moon's big basins are located, we can start delving into the history of the Moon. Which one happened first, and which one happened later? When did all of this stuff happen? What order the lunar basins formed in is a question I have a lot when looking at these global maps of the Moon, and I hadn't found an online resource for it.
So I am hereby making one, based on a new paper that's in press in the Journal of Geophysical Research by Caleb Fassett, Jim Head, and another five coauthors: Basin names are in caps if they are in caps on the map above, to help you locate them. It lists the basins from oldest to youngest, based on crater counting -- basins with more, bigger craters on top of them are older. For a few basins, you can actually see one basin overlap another; that's noted in parentheses.