Hello all. I have written a few Moon Zoo blogs about different aspects of lunar geology and the geological history of the Moon, here, I thought I would share with you the lunar research topic I am currently working on. I hope that it is interesting and if you have any questions please do let me know. Katie Joy
The Moon’s surface is absolutely covered in impact craters (Figure 1). These range in size from the behemoth South Pole-Aitkin 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 the whole of the inner Solar System through the past 4.5 billion years.
Figure 1. Impact craters and basins on the Moon are clearly seen in topography maps of the Moon. The gigantic South Pole-Aitkin basin (labelled SPA) is located on the southern part of the far side of the Moon. Credit: Clementine mission/NASA.
When did the Moon get bombarded?
We know that a lot of large projectiles hit the Moon early in its history, 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) 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 when one crater occurred on top of another crater to build up a scale of what is older and what is younger).
Data from both sources have been combined to develop a lunar cratering curve – where the number of craters on a region of the Moon’s surface (as measured by crater counting like you are doing in the Moon Zoo crater survey task) is compared with the absolute rock age of that surface. A graphical representation of this curve is shown in Figure 2. As we have rocks from only nine known localities on the Moon, the scientific calibration of this curve is poor before ~4 billion years ago and for more recently after 3 Ga (see Figure 2). So, there are still some big questions about how many impacts have been hitting the Moon through time and if there has been one or more ‘spikes’ in the impact rate; when the Moon was pummelled by impacting projectiles in a very short period of time. One of these spikes is thought to have happened at ~3.9 Ga and is known as the lunar cataclysm (Figure 2). Other smaller spikes may also have occurred at other times in the Moon’s past.
Figure 2. The grey line indicates a possible lunar cratering chronology (number of impact craters ?1 km per km2 that have formed on the Moon’s surface at different times, based on a set ‘Size-Frequency Distribution’ for the projectiles). The black calibration points are derived from age dating samples of impact basins and lava flows (A = Apollo mission sampled lava flow, L = Luna mission sampled lava flows). The blue points are best guess calibration ages of impact craters and basin based on rock ages of material found at Apollo landing sites rather than in the crater itself. The red question marks refer to questions about the parts of the lunar impact curve where we have no calibration data at all. Credit: K. Joy adapted from the Stoffler et al. (2006) chapter in the New Views of the Moon book and from David Kring/LPI’s lunar impact flux diagram <http://www.lpi.usra.edu/nlsi/science/>
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 Solar System – dust and small bodies that impacted the Moon very early in its history from 4.5 to 4 billion years ago (Ga).
- 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.
- 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 a big spike in the lunar impact record at 3.9 Ga (e.g. 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 giants (Jupiter, Saturn etc.,) 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 are at the moment just good guess work – and now we 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?
Well, 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.
Therefore, 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 blog here). Some of these rocks were formed in impact craters, and dating these impact rocks using radiometric dating techniques tells us when impacts to the Moon took place. There are then two methods used to work out what types of projectiles were hitting the Moon at these times:
(1) 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 (see Figure 3).
(2) To locate and classify fragments of meteorites in the rocks themselves. Fragments that survive impact onto the Moon are very rare (Figure 4). 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.
Figure 3. Astronaut photograph of lunar soil sample from the Apollo 16 mission to the Moon. This soil, named 61220, has high levels of carbon and other volatile species and is thought by researchers at NASA’s Johnson Space Center to have been impacted by a comet. Credit: Image and further information about soil 61220 taken from LPI.
Figure 4. Photograph of a meteorite fragment that has been found on the Moon. This is Bench Crater meteorite that is from a carbonaceous chondrite parent asteroid body. The fragment is very small – about 5 × 3 mm in size. The dark opaque areas represent a fine grained matrix. The large brown mineral grains are attributed to clays (phyllosilicates) that have been converted to a ferromagnesian phase that is intermediate in composition to the minerals olivine and pyroxene (information credit Dr. Mike Zolensky). Credit: photograph of Bench Crater meteorite by K. Joy.
Different groups of researchers around the world are examining lunar rocks to search for evidence of the timing and sources of lunar impacts. We hope that these data will help us to not only better understand the geological history of the Moon, but to understand wider processes in the Solar System that have likely effected all rocky planets like the Earth, Mars, Venus and Mercury. This information will help us to understand the history of our own planet, and address how impacts may have affected the development and proliferation of life here on Earth throughout the past 4.5 billion years.
Few things to remember:
- A projectile or an impactor is a term given to an object that strikes the Moon. This object may be a comet, an asteroid, or even dust
- If a projectile is an asteroid, surviving fragments are called meteorites
- If a projectile is smaller than ~50 m the object is often called a meteoroid and surviving fragments are also called meteorites
- If a projectile is smaller than ~1 cm the object is called a micrometeoroid and surviving fragments are called micrometeorites
- Is a projectile is dust sized, surviving fragments are called interplanetary or interstellar dust particles
Types of impact crater shapes – the shape and structure of impact craters reveals what sizes meteorite or comet formed the crater
Understanding the number and size of impact events on the Moon at the present day – something that is very important for planning safe future lunar habitats and bases