Deciphering the impact record at different times and different scales on the Moon

Our Solar System has different populations of planetary bodies - the innermost part closest to the Sun is dominated by the large rocky terrestrial worlds Mercury, Venus, Earth-Moon, and Mars. Beyond the orbit of Mars, the asteroid belt hosts small rocky and metal-rich bodies, then there is the giant planet zone (Jupiter, Saturn, Uranus, Neptune), and beyond that the outer Solar System which is dominated by ice-rich bodies.

This structure has not always been fixed - in the earliest part of Solar System history some bodies migrated inwards towards the Sun and others migrated outwards towards the far icy limits. This dynamic environment often resulted in hypervelocity high-energy collisions between bodies. Where collisions occurred, impact craters were formed: these craters are seen on every solid surface and occur at all spatial scales.

The number, size, shape, and products of these craters provide us with knowledge about ages of planetary surfaces, the structure and stability of planetary crusts, and the nature of the impactors that caused them. We can use the rates at which impact craters were formed to inform us about past Solar System dynamical processes. This helps us understand how other exo-planetary systems around other stars evolve.

We can also use this knowledge to understand the risk from impacts on Earth in the more recent past, and ultimately at the present day.

Our project will use samples from the Moon to probe Solar System scale impact processes. The Moon is an ideal place to test these questions as it has a very ancient surface compared to planets like the Earth, Venus and Mars which have been recently geologically active. Thus, as the Moon is located so close to the Earth, it is a great laboratory to understand how our own planet must have been affected by impacts in the past.

The Moon is also a relatively accessible field locality - we have already collected many samples from its surface through human and robotic missions, and have some samples that are naturally delivered to us here on Earth as meteorites. In our project, we will measure the chemistry and age of these different types of lunar samples to understand when different size lunar impact craters formed and what types of impactors were colliding with the Moon.

In particular, we will investigate the timing of very large impact basin formation (craters bigger than 150 km in diameter) before 3.9 billion years ago, to test if they were formed over a gradual period of time, or if they formed in a very short window known as a 'cataclysm'. We will also investigate crater formation over more recent periods of the Moon's history, during the last 2 billion years, in the window when life on Earth was starting to proliferate.

For all these different time periods we will also ask what types of impactors collided with the Moon - did they originate from early planets that broke apart, from material in the asteroid belt, or from further out from the icy parts of the outer Solar System? This will help us understand how material has moved around the Solar System at different times in the past, shedding light on dynamical processes in the Solar System throughout the past 4.5 billion years.