Written by Jonathon Leonard
A group of scientists in France have approached the problem of the age of the moon at a whole different angle and have totally changed our previous estimates of its age. The study was published in the prestigious journal Nature and has received a fair bit of media attention. They team used computer simulation to re-estimate the timing of the huge celestial collision between Earth and a Mars sized body that created the moon.
Before we get into the details of the study, let's first have some background information.
At some stage after the formation of the solar system (called the condensation of solids), there was a huge collision with Earth by a body similar to the size of Mars. This collision significantly changed the structure of Earth's mantle and core as well as causing the formation of the moon by a large chunk flying off post-collision.
Before this study, all attempts to date this huge collision were done using dating from radioactive elements. Much of these dates of collision were between 30 and 50 million years after condensation of solids in the solar system (that is, the formation of the solar system). However, this team of scientists took an entirely different approach to dating the moon-forming impact. They used a series of computer simulations that predicted the formation of the solar system.
The computer simulations initially used classical ideas on the early solar system, such as similar orbits of the planets as they have today and a large area of matter around the sun that contained all the building blocks of the solar system. However they found this simulation had problems. For example, the sizes of Mars were all wrong.
Instead the scientists changed the initial conditions so that the disk of matter around the sun that contained the building blocks of the universe was much smaller. After a few adjustments, they achieved simulations of our solar system similar to our current system.
Yet even after making these successful simulations, there were still a range of viable simulations that predicted a moon forming collision at a wide range of dates. So which predictions are more likely to be right? Well to do this, the group of scientists measured the abundance of a group of elements in the mantle that are high siderophile (which means easily dissolve into iron). During the huge moon-forming impact, these elements should pretty much all have sunk into the mantle since they dissolve into iron so easily. Thus the presence of highly siderophile elements (HSEs) that aren't in the core must be from after the impact, called late accreted mass. By comparing the amount of HSEs in the mantle to those we find in meteorites that have a similar composition to the early Earth, we can take a good guess at how much late accreted mass the Earth has gained.
Now comes the fun part. The scientists compared the late accreted mass measured on Earth to the values predicted in the simulations. The values where the simulations consistently agree with our measured values (within an acceptable uncertainty) was between 67 and 126 million years after the formation of the solar system. In fact, out of the large number of different simulations, not one had the correct late accreted mass that had an impact earlier than 48 million years after the formation of the solar system. The scientists estimated that there is only a 0.1% chance that the big impact occurred before 40 million years after condensation, a huge result considering this was smack bang in the middle of many previous predictions.
So what does this result mean?
Well firstly, it means that the moon formed much later than we previously expected. It also means that the disk of matter that revolved around the early sun and contained all the building blocks of the planets has a much smaller area than previous thought. Finally the wrong radio dating figures could mean that these figures may have come from elements before the collision, meaning that the moon-forming impact didn't totally reset the mantle as previously thought.
Perhaps the biggest lesson from this study should be that to solve a problem in science, it is often useful to look at it from a completely different perspective. This is where Einstein achieved much of his genius (look up the Equivalence Principle from General Relativity for example). Those successful in science (and life!) aren't necessarily the most intelligent, but the most creative and willing to look outside the box for a solution.
Read the full article here: http://www.nature.com/nature/journal/v508/n7494/pdf/nature13172.pdf