The Moon’s surface is absolutely covered in impact craters. These range in size from the behemoth South Pole-Aitken basin, which is a staggeringly ~2500 km in diameter and 13 km deep, all the way down to microscopic impact craters on glass beads that are less than a millimetre in diameter. No matter the size, the one thing all these impact craters have in common is that that they were created when something smashed into the Moon at high speed on the order of 15 km/sec or more (that’s pretty fast when you consider a bullet out of shotgun travels at about 0.9 km/sec).
Impacts have changed the surfaces of all rock and ice planetary bodies in the Solar System, including the Earth. However, they are so well preserved on the Moon that we can use lunar impact craters and lunar impact rocks that were made when these craters formed, to reveal the history of impact bombardment in whole of the inner Solar System through the past 4.5 billion years.
When did the Moon get bombarded?
We know that a lot of large projectiles hit the Moon early in its history before 3.7 billion years ago, and that the rate of bombardment rapidly declined after ~3.5 billion years ago. But there are many gaps in our knowledge. Our understanding of the Moon’s impact history comes from (i) studying the makeup and absolute age dates of rocks and soils brought back by the Apollo and Luna missions and (ii) understanding the relative age of when one impact crater formed compared with another (e.g., seeing what one crater occurred on top of another crater to build up a scale of what is older and what is younger).
What has been causing such damage to the Moon and why do we care?
There are several different types of projectiles that have been hitting the Moon through time. These include:
- ‘Primordial’ material left over from the formation of the Moon and the Solar System – dust and small bodies that impacted the Moon very early in its history (probably from 4.5 to 4 billion years ago).
- Asteroids – Asteroidal projectiles originate from material that has migrated in from the asteroid belt and beyond. These types of projectiles are large enough to create cm to kilometre size impact craters. Asteroids include a range of varieties including types that have differentiated (i.e., formed a crust, a mantle and a core) and those that have never been melted and represent primitive (very ancient) Solar System material. If a fragment of an asteroid survives the impact event, this material is termed a meteorite. Meteorites that have come from primitive parent bodies are known as chondritic, and those from differentiated parent bodies are known as achondritic. Achondritic meteorites include a wide range of different types of stones including iron meteorites, martian meteorites, meteorites from the crusts of objects like the asteroid Vesta.
- Comets – small icy rocky dusty bodies that orbit around the Sun and travel on orbits from the inner Solar System out into outer Solar System (known as short period comets). Some even travel outside of the Solar System’s elliptical plane (known as long period comets). Cometary projectiles are large enough to create cm to kilometre size impact craters on the Moon, but are not thought to easily survive entry through the Earth’s atmosphere.
- Planets – it is possible that impacts on the Earth, Mars, Venus and Mercury could have spalled off (i.e., ejected) pieces of rock that may have later impacted the Moon (and survived this impact event). If such material could be identified on the Moon’s surface it may provide us with key records of ancient terrestrial geological processes (ancient fossils?) or fragments of the venusian crust!
- Dust – the Solar System is full of dust sized particles left over from impacts between planets and asteroids. When dust and micrometeorids hit the Earth’s atmosphere they burn up and form bright streaks of light we call shooting stars. However, as the Moon does not have a protective atmosphere, small dust particles rain down straight onto the lunar surface – most are instantly melted and form glassy constructs in the lunar regolith, while others form small millimetre to micrometer sized impact crater pot holes on rocks and soil particles.
It is important to understand what sort of projectiles have been striking the Moon back through time as this provides us with information about dynamical processes in the Solar System. If we can understand what types of impactors caused an enhancement in the lunar impact record at 3.9 Ga (often called the lunar cataclysm), then this knowledge might reveal information about what types of Solar System processes were occurring at this time. For example, some researchers have suggested that the lunar cataclysm was caused when a small planet or large asteroid broke up and its debris was thrown into an Earth-Moon crossing orbit. Other researchers have suggested that the cataclysm was caused when the orbits of the gas giant planets were suddenly altered, and that a dramatic change in orbit meant that large amounts of outer Solar System asteroids and comets were thrown into the inner Solar System, causing widespread damage to the asteroid belt and all inner Solar System planets. This hypothesis is known as the ‘Nice Model’ (named after Nice the city, not the term of affection!). However, these hypotheses need further evidence to support, or refute such ideas.
How do we know what types of projectiles have been hitting the Moon in the past? The new paper we have published as a collaboration between UK and USA teams now explores this in detail.
We know what types of meteorites we find on Earth at the present day. We can classify collected stones and use this information to tell us about what types of asteroid parent bodies they come from. However, it is possible that the type of meteorites we find at the present day are not the same types that were hitting the Earth and Moon in the past. To understand the timing and sources of ancient bombardment on the Moon we must look at lunar samples returned by the Apollo and Luna missions, or that have been collected here on Earth as lunar meteorites (see overview here). There are then two methods used to work out what types of projectiles were hitting the Moon at these times:
- To analyse the chemistry of the rock. There are two main groups of elements that help to reveal impactor source. Siderophile elements (e.g., Ni, Fe, Co, Ir, Au, Pt, Ru, Os, etc.,) can be used to trace if the projectile was an asteroid, and if so what type of asteroid it was. Light volatile elements like hydrogen and carbon species (e.g., H2O, CO, CO2, H2, CH4, HCN, N2, etc.) help to reveal if the sample was formed, or affected, by a volatile-rich cometary impact.
- To locate and classify fragments of meteorites in the rocks themselves. Fragments that survive impact onto the Moon are very rare yet several have been found in lunar rocks and soils. These samples are very important as they can be analysed and classified as originating from certain types of asteroid parent bodies.
Different groups of researchers around the world are examining lunar rocks and soils to search for evidence of the timing and sources of lunar impacts. Renewed exploration of the Moon in the future should also be prepared to locate and classify different meteorite groups to identify their parent bodies and timing of delivery. These data will help us better understand the geological history of the Moon and wider processes in the Solar System that have likely effected all rocky planets like the Earth, Mars, Venus and Mercury, informing us about our own planet’s past, and understanding how impacts may have affected the development and proliferation of life here on Earth throughout the past 4.5 billion years.
The full paper is online (Open Access at ) Joy K. H., Crawford I. A., Curran N. A., Zolensky M. E., Fagan A. L., and Kring D. A. (In Press) The Moon As An Archive Of Small Body Migration In The Solar System. Earth Moon and Planets DOI: 10.1007/s11038-016-9495-0
- Find out about the Centre for Lunar Science and Exploration at http://www.lpi.usra.edu/exploration/
- This blog post is adapted from one I previously wrote at https://moonzooblog.wordpress.com/2011/02/21/big-bangs-in-the-solar-system/