What secrets lie hidden in the craters of the Moon?

Nils Charles Prieur
"We often call craters the fossils of the Universe", says Nils Chrles Prieur. He has studied the Moon's history. Photo: Ida Arff Tarjem/UiO Bruk bildet.

What secrets lie hidden in the craters of the Moon?

What can give us valuable insight if we one day decide to inhabit other planets? Nils Charles Prieur looked to the Moon for answers.

Over 400 years ago, Galileo Galilei pointed his telescope towards our nearest neighbour - the Moon. He discovered that the landscape was far from a perfect sphere, but characterised by indentations and circular depressions. Galileo chose to call these craters.

Craters are formed when celestial bodies such as comets and asteroids smash into planets or their moons. Huge forces are at play and the extinction of the dinosaurs shows that the effect of such an impact may even change the course of the history of the earth.

What is less known however, is that these 'traces of the past' actually have a story to tell about the celestial bodies they encounter.

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New knowledge about the development of craters

When we think of craters, we imagine that it is the speed and size of the comet or asteroid that mainly determines the size of the resulting crater. These are important factors, but far from the only ones.

"Most studies that have investigated how craters develop have more or less ignored the influence of the target's properties on the size and appearance of the crater, in other words its morphology", says the recently graduated PhD Nils Charles Prieur at the Centre for Earth Development and Dynamics (CEED) at UiO's Department of Geosciences.

"We found that properties such as in particular friction, as well as how densely packed the soil particles are and how porous the surface material is, have a surprisingly big influence on the resulting size of the crater," Prieur continues.

The fossils of the Universe

The craters on the Moon are visible fom Earth on clear nights. Photo: NASA/JPL

If you look up at the Moon on a clear and starry night, you can make out the contour of thousands of craters. Prieur also looked to the Moon for answers.

"The Moon was a natural place to start. We have an extensive crater register, thousands of pictures, and thanks to the Apollo and Luna program we have samples and measurements from the top layer of the surface."

And this is highly valuable information. Thanks to radioactive isotopes, i.e. different varieties of unstable elements that break down over a specific period of time, it is possible to estimate the age of the Moon's surface where the samples were taken.

These measurements can be linked to the density of craters of a particular size within the defined area; the older the surface, the more craters of a larger size, and vice versa. As a result, the number of craters within a certain area can give us an estimate of the age of the surface.

"Using this method, scientists have identified a period from about 4.1 to 3.8 billion years ago when the Moon, and thus also the Earth, was heavily bombarded by asteroids and comets. The conditions on Earth could not have been particularly hospitable!", he says.

In other words, craters can provide us with a peek back into the history of planetary bodies.

"We often call craters the fossils of the Universe," Prieur smiles.

Which planet is the best for colonisation?

As previously mentioned, Prieur discovered that it is possible to say something about the surface of a planet or moon by studying its craters. This can provide essential information if humans decide to inhabit other planets in the future.

"By studying craters one can get an indication of, for example, whether there is water present in the surface layers of a moon or planet. This would represent a natural place to set up a colony," Prieur emphasises.

Age-determination of planetary bodies using craters can be extended to the solar system as a whole.

"A young solar system contains a lot of remnants after the planets were formed, and this poses an increased risk of impacts. In this case, I would choose a different solar system to colonise," he laughs.

Detective work

The red lines to the left illustrate cross sections across a few craters on the Moon’s surface . In the next figure (right), elevation values have been extracted from these cross sections (red) and compared to equivalent numerical simulations (in black). Illustration: Nils Charles Prieur

But how do you investigate something that is so far away, in both time and space? Unlike Galileo Galilei, Prieur was not forced to crank down in front of a telescope at hours on end. Instead, he cranked down for hours on end in front of a computer, feeding numbers into numerical models and simulations. He had to wait for three to four weeks to discover whether the models he had run were correct, which they did not always turn out to be.

"It is really a sort of detective work trying to find the best combination of surface target parameters and the ratio between them, to reproduce the craters that we can actually observe on the Moon. We ran simulations for around 70 observed craters of different size and eventually got very good results, " he says pleased, pointing to a number of graphs showing the correspondence in size between the simulated and observed craters. They fit almost like a hand-in-glove.

Still a lot to discover

The obvious disadvantage of space research is that it is somewhat challenging to easily venture off to the Moon, Mars or elsewhere in the solar system (unless your name is Elon Musk, it may seem).

"One of the main problems is that we actually know very little about the actual properties of the surface of the moons and planets in the solar system. How porous is the surface up there? And how does this relate to other properties, such as friction?", Prieur asks.

In the laboratory, researchers have shot projectiles onto different types of materials in an attempt to recreate the impact of an asteroid or comet. However, such experiments have certain limitations. For example, not all types of materials can readily be used, including stone, and it is a challenge to upscale from a few inches in the laboratory to hundreds of meters and kilometres, as on the surface of the Moon.

"We must therefore make a number of assumptions when we run the simulations and there is room for improvement," Prieur says.

A new golden age of space exploration

"This is groundbreaking research that nobody has done before. Prieur has developed a new type of tool that we can utilise and build upon. This is basic, but future-oriented research that puts us one, if not several steps ahead of others in the same field, "says Associate Professor at CEED, Stephanie Werner, who was Prieur's main supervisor.

In the summer of 2020, the Mars2020 rover of the American space organisation NASA starts its journey towards the red planet. The samples taken here can tell something about the properties of the surface.

"We want to apply the methods also for Mars," she adds.

Prieur also looks beyond the horizon of the Moon:

"I want to build on my work and use the same methods for the moons of Jupiter."

In 2022, the European Space Agency (ESA) launches JUICE ("JUpiter ICy moons Explorer") which has a scheduled arrival time in 2029. This will give us, and scientists like Prieur, an opportunity to take a closer look at Jupiter's three biggest moons Ganymede, Callisto and Europe, where researchers believe there is the possibility of water - and maybe life.

"I think we are venturing off into a new golden age of space exploration, and I hope the research we are doing can contribute to this development," concludes Prieur enthusiastically.

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