This is a very busy section for crater chains/secondary impacts near the Lunar North Pole at 72.51 N, 121.37 W
The impact formation at top right is especially interesting:
It is somewhat of a mystery to me on how it formed. You can see what appears to be an ejecta blanket that perhaps moved in a W-E direction giving the surface a sculpted and scraped appearance. That would be fine except for the line of craters that have made a chain of impacts on the left edge of this formation that would seem to have come in from a N-S or S-N direction. Notice there is no surface ejecta sculpting to the left of this chain.
What are your thoughts?
Tom128 is a regular contributor to the Moon Zoo forum
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
Back in June Moon Zoo forum member Toban posted this unusual looking crater in the Alpine Valley east of Plato from the LRO strip M104497175LE. Toban wondered if what we were seeing was “old” lava. He asked:
“Was the impact so deep, that this has “opened” the ground a long time ago?! I think there is no liquid under the moon surface… so maybe it is very old or it’s not lava…”
It reminded me of another unusual crater posted by Geoff a week earlier
which IreneAnt identified as “an impact into a not-completely-solidified melt sheet” and she also provided a link to a paper showing experimental impacts into viscous targets and noted that there were more examples of this type of crater in the same area (Aristarchus – strip M111904494RE.)
Alternatively could Toban’s crater be a very eroded crater or a “ghost” crater? Eroded craters have been worn and eroded by a history of micrometeorite impacts so that their original form is hard to make out. Ghost craters are craters which have subsequently been filled with lava leaving only a “ghost” of a rim visible when the sun is at a sufficiently low angle. Here are a couple of examples of ghost craters:
Toban thought this unlikely because his crater has a very definite unbroken ring structure with a clear boundary which has captured some of the boulders that have rolled down into the ring area.
Close up of Toban’s crater – the section around 10 o’clock
IreneAnt confirmed that Toban’s crater was another impact into viscous material and provided a link to another paper which describes laboratory experiments to study the morphologies of craters produced by impacts into various viscous materials using different impact velocities. IreneAnt draws our attention to Figure 4b of this paper which shows an impact into a clay-oil mixture producing a crater with no central peak and poorly defined rim topography. A similar type of crater, known as a “splosh crater” is much more common on Mars where the geology, history, atmosphere and gravity produce craters which are multi-lobed with splashed rather than rubbly ejecta. This type of Martian crater was formed when a meteorite hit an area rich enough in water to turn the impact site into runny mud. The lunar version, however, did not involve water but was formed because the impact site rocks had melted enough to behave like a viscous liquid. IreneAnt, therefore, suggests the term “melt splosh crater” to describe the lunar version to avoid confusion with the Martian water based splosh craters.
Although not quite the same thing the lunar versions were still formed more from a splash than a crash. Geoff found more examples of lunar viscous impact craters around King Crater on LROC strip M115529715LE.
These craters were formed when parts of the Moon were covered in molten lava and the splashes have been “frozen” as the lava cooled. So we should be able to find craters in various stages depending on the consistency of the lava impacted. Look out for them and post your finds in Toban’s thread.
Jules is a volunteer moderator for the Moon Zoo Forum.
The Boulder Tracks thread is one of the most popular within the Moon Zoo forum and we have some amazing tracks posted there.
Boulder tracks are important to the Moon Zoo project – the following quote is from one of the Moon Zoo team members:
One of the main reasons we are asking Moon Zoo users to search for scars left behind by tumbling boulders is to help support future lunar exploration initiatives. Boulders that have rolled down hillsides from crater walls, or massifs like the Apollo 17 landing site, provide samples of geologic units that may be high up a hillside and thus difficult to access otherwise by a rover or a manned crew vehicle. If mission planning can include traverses to boulders that have rolled down hills, and we can track these boulders back up to the part of hillside from where they have originated, it provides a neat sampling strategy to accessing more geological units than would have been possible otherwise… Thus we hope to use Moon Zoo user data to produce a map of known boulder tracks (and terminal boulders) across the Moon. – Katie Joy
Recently ElisabethB (Els) posted the tracks shown below. Quite an amazing variety of track sizes and shapes! The track on the bottom appears to have mounds inside the track caused by the shape of the boulder that created the track.
Also, some of the tracks have craters overlapping them which may have been caused by the same impact. The original impact would have sent boulders bouncing and rolling along the regolith but would also have sent boulders upwards and they would have eventually fallen back and created craters.
The area where these tracks are found is Montes Alpes / Vallis Alpes.
Sun Angle: -62.71°
Scale: 0.51 meters / pixel
Zoom Level: 3
One of the great things about the Moon Zoo forum is the way you can learn new things from questions posed by users.
At the end of May 2010, Darrin Cardani started a thread called What causes erosion on the Moon?.
This is an interesting question! There is no weather on the Moon (as we Earthlings define weather) as there is no atmosphere and no liquid water.
When the Moon was young, erosion was mainly due to impacts (from comets, asteroids and meteorites) and volcanism. Today the main causes of erosion are micro-meteorites, the solar wind, moonquakes and degradation of rocks by the temperature change as the surface alternately heats up and cools down.
Recently, a new user to the Moon Zoo forum, astrodel, posted in the “What causes erosion on the Moon?” thread with a reference to an article in Nature about superficial weathering on the Moon. I couldn’t access the original article as it requires payment but dug around a bit and found a reference to “Lunar Swirls”!
The term Lunar Swirls describes unusual sinuously-shaped features on the lunar surface. They have been described as looking like the swirls on the top of a mug of coffee when cream is poured in and slightly stirred! The following image shows an example of a lunar swirl in Oceanus Procellarum and is the largest and longest swirl on the near side.
All the lunar swirls found so far appear to be associated with magnetic anomalies on the lunar surface but their formation is still a bit of a mystery. There are three different models for swirl formation according to “The Lunar Swirls” document (see References at the end of this posting).
- The solar wind deflection model.
- The cometary impact model.
- The meteoroid swarm model.
A short quote from this document may help explain what swirls actually are:
At the resolution of current data, the swirls appear to overprint the topography on which they lie, indicating that they are quite thin or a surface manifestation of an underlying phenomenon that is manipulating normal surface processes. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain may be less prominent and exhibit simpler shapes such as single loops or diffuse bright spots. – [The Lunar Swirls; A White Paper to the NASA Decadal Survey]
Two of the swirls on the far side of the Moon are directly opposite the centres of two large near side impact basins, Mare Imbrium and Mare Orientale, so there appears to be some connection with a large impact causing a swirl to appear on the opposite side of the Moon.
Swirls show up because they “weather” a lot slower than surrounding terrain. If the original impacts that formed the near side impact basins also somehow caused the magnetic domains on the antipodes to form with swirls above them then these swirls are very old. Mare Imbrium formed about ~3.8 billion years ago so the swirl on the opposite side (“butterfly” swirl) must have been formed then and is still protecting the surface from weathering. This is one of the many mysteries of lunar swirls.
In the early 1970s NASA put two small satellites in orbit around the Moon to measure Earth’s magnetic tail (the solar wind blowing against Earth’s magnetic field creates a “tail” that stretches more than a million miles away from Earth) and these satellites also measured the magnetic field of the Moon. To the scientists’ surprise they found that there were strange magnetic domains all over the Moon in no particular order. They also found that the strongest magnetic fields were above lunar swirls.
These magnetic domains may help to prevent the solar wind from weathering the surface so the albedo remains high.
If you happen to spot a lunar swirl please post it here: TLP Project – Lunar Swirls
“Butterfly swirl” in Mare Ingenii (directly opposite Mare Imbrium).
[LROC WAC M103439292MC]