Crater chains are just one type of feature we are looking out for in Moon Zoo but they are proving to be rather elusive. A number of processes can leave a chain of craters (a Catena):
- Boulders from a larger impact are flung out and form smaller secondary craters where they land. The larger the impact the further away the ejected boulders are thrown. In really large impacts ejecta can be hurled hundreds of kilometres away. Some boulders land on their own forming isolated small craters; a group of boulders landing together can form a ridged herringbone pattern and a string of boulders landing together can form a chain of craters which fall radially to their parent crater. Individual craters in the chain are also usually elongate in shape (due to the shallow angle of impact) with irregular rims and the ejecta appears ‘splashed’. Isolated secondary craters are often difficult to identify but craters in chains are easier to spot and can, therefore, be studied as secondary craters.
- The impact of a fragmented meteorite or comet might also have created a crater chain.
- Some crater chains are thought to be volcanic in origin but it can be difficult to distinguish between the two types of crater chains. Volcanic crater chains are lined up along a common fault or set of faults, and each crater produces a blanket of volcanic ejecta. As you would expect, volcanic crater chains are not assocated with a parent crater.
There are many – too many – examples of three or four small craters in a line and while these may well be secondary craters from a larger impact they are not especially noteworthy. Some of the more famous crater chains are very impressive and contain over 20 smaller craters. It would be quite something if we could find an example like that.
These are some of our contenders so far:
Posted by Forum member Caro:
Moon Zoo team member Irene Antonenko says:
“The wispy, feathery texture associated with crater chains is formed when the ejecta deposits of the craters in the chain interfere with each other during their emplacement (kind of like point-source waves interfering in a wave tank.)”
Discussion on the forum concluded that the impact angle was so low that the top part of the impactor sheared off and bounced to form the “tail.”
If you want to read more about Crater Chains try these links:
Finding a good, clean example of a long crater chain, whether volcanic or impact in nature, is this week’s challenge!
Jules is a volunteer moderator for the Moon Zoo Forum.
In Anthony’s Looking For Change blog he says that we should be looking out for any interesting surface features that may have been freshly disturbed or contain a vent from which outgassing has occurred. “TLP” projects have been set up in the forum for each of the features mentioned, except for Deep Seated Fractures and another feature, “Atmospheric” phenomena (which was not mentioned by Anthony).
Deep seated fractures can be found on some images from LROC but “Atmospheric” phenomena will probably not be found. Some images taken during the Apollo project had fuzziness along the horizon when it was expected that the horizon would be sharp-edged. This may have been caused by dust particles suspended above the lunar surface by electrostatic charging.
Deep Seated Fractures
The one feature which Anthony mentioned and which is not covered in the other TLP Project threads is deep seated fractures and he uses an example from the floor of Tycho crater:
Tycho Crater floor, from LROC image M114031031LE
One of the Moon Zoo users, ElisabethB (Els), found something similar near Proclus crater:
Sun Angle: -76.23°
Scale: 0.50 meters / pixel
Zoom Level: 3
Apollo 17 astronauts saw and sketched what they called “bands”, “streamers” and “twilight rays” which were visible just before lunar sunset or sunrise. These rays were also seen by astronauts on the Apollo 8, 10 and 15 missions.
from Nasa Media Library
On the side of the Moon in daylight, the solar radiation knocks electrons out of atoms and molecules in the regolith. A positive charge builds up which is sufficient to loft particles 1 micron and smaller in size above the surface. These particles can go up more than a kilometre.
The Moon seems to have a tenuous atmosphere of moving dust particles. We use the word ‘fountain’ to evoke the idea of a drinking fountain: the arc of water coming out of the spout looks static, but we know the water molecules are in motion. In the same way, individual bits of moondust are constantly leaping up from and falling back to the Moon’s surface, giving rise to a “dust atmosphere” that looks static but is composed of dust particles in constant motion.
[quote byTimothy J. Stubbs, of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center]
It is believed that the dust on the night side of the Moon is negatively charged due to electrons from the solar wind flowing around the Moon onto the night side. So at the Moon’s terminator between the positively charged dust of the daylight side and the negatively charged dust of the night side, there could be flows of dust which may resemble auroras.
The four links below from NASA discuss various aspects of this phenomena:
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
Time for a bit of light relief.
While looking for sinuous rilles, lava tube skylights, grabens and bits of discarded spacecraft, and in between counting boulders and measuring craters, forum members have also found several possible treasure hoards obviously left by previous visitors to the Moon……
You might be fooled into thinking that these crosses are not the mark of Lunar Pirates at all but that they have been caused by the forces of Lunar geological processes. Maybe the stresses involved in crater formation on boulders landing heavily after being flung out over hundreds of metres just happened to cause them to crack by chance to form an “X.” Or you might suggest that the LRO camera took the image when the angle of the Sun was just right to catch a rocky rim and cast an “X” shaped shadow across the crater floor. You might even surmise that the albedo of the Lunar regolith just happens to resemble an “X” when viewed in a certain light.
But you would be wrong! We know different!
There’s treasure to be had, they obviously didn’t cover their tracks very well and we are on to them!
We are keeping the coordinates to ourselves but can you see where “X” marks the spot in these images?
Jules is a volunteer moderator for the Moon Zoo Forum.