Another Look into Daguerre Crater with LROC
In April of 1972 the Apollo 16 CSM Casper piloted by Thomas K. Mattingly orbited the Moon while Commander John W. Young and LM Orion pilot Charles M. Duke landed in the Descartes region and conducted three EVAs on the lunar surface.
Onboard the CSM was a sophisticated array of cameras that photographed the lunar surface during the 64 orbits. The panoramic photograph AS16-4511 (P) top right shows a 2 km crater near the western edge of Daguerre crater, 11.53 S, 33.11 E. I marked the WMS Image Map photo strip M121993376RE area in green, shown below and a LROC photograph of the crater is shown above, top left.
Here is a short summary about the crater from, “APOLLO OVER THE MOON: A VIEW FROM ORBIT (NASA SP-362)” on page 118:
“It shows the striking bilateral symmetry of the rays of a small (2-km diameter) crater in the floor of the large crater Daguerre in Mare Nectaris. Continuous areas and narrow filaments of light-gray ejecta extend from the crater across the dark mare surface through 270°, but are entirely absent in the southern 90° sector. Within the crater, dark material occurs on the southern crater wall while the remaining walls are bright. (The reader may wonder about the material whose reflectivity cannot be observed because it lies in shadow on the east wall of this crater. Until the area is observed under high Sun conditions, we are forced to make the simplifying assumption that it is bright because most of the materials visible elsewhere in the walls are bright.) This crater probably resulted from the impact of a projectile traveling from south to north along an oblique trajectory. Its pattern of ejecta distribution is similar to that of small craters produced by the impact of missiles along oblique trajectories at the White Sands Missile Range, N. Mex. Some observers postulate that the dark material is a talus deposit of mare material that has fallen into the crater.-H.J.M. Another geological explanation is that the unusual pattern may be due to an intrinsic characteristic of the local terrain, probably an abrupt lateral change in the composition of the bedrock within the area that was excavated. F.E.-B. ”
Taking a second look at the crater with LROC, we can now see which of the two geologists was correct with a stunning picture of the dark material spreading out in a ray pattern and also cascading over the crater wall towards the crater floor.
Here is an initial analysis made by MZ team member IreneAnt about the crater and its apparent oblique impact:
“If you look carefully at the LROC image, you can see an ejecta herring bone pattern in the dark wedge portion outside the crater. I think this is good evidence that this dark wedge is ejecta. You can also see from the LROC image, that this dark wedge matches up very well with a layer of dark material in the crater wall. So, there isn’t really a missing section of ejecta here, it’s just a different colour; in the black area, you hit and excavated black material, while in the light areas, you hit and excavated light material. So, it seems that Farouk El-Baz was right and the ejecta pattern may have been caused by variations in the pre-impact target.
The thing that I find really interesting here is the implication for low angled impacts. From the Apollo images it seems clear that there is a section of the ejecta deposit that is missing, thus indicating a low angled impact. However, when you look at the higher resolution LROC image, you can see that there is, in fact, dark ejecta in the “missing” section, so this is not a low-angled impact. So, the two images tell very different stories. This casts a doubt on all “low-angle” impact craters identified based on missing ejecta sections. It may just be that higher resolution data is needed to see some ejecta of a different composition.”
Irene continues her analysis of the crater talking more about the lateral shift in the lunar material from lighter to darker:
“These kinds of horizontal variations are more common on the Moon than was originally thought. This is what my research revolves around. What probably happened here is that a small mare flow was confined by topography (hills and such) or was just too small to flow very far (think how a little bit of milk spilled on a flat table will only flow so far). Another possibility is that this is a pyroclastic deposit. Sometimes volcanoes can explode like a shaken bottle of pop. When this happens, drops of lava cool as they fly from the vent and land on the ground as glass beads. These kinds of deposits stay near the source vent, since glass beads don’t flow as well as liquid lava does. One final possibility is that this is a pond of impact melt from the Daguerre impact event. Impact melt would have been confined the same way as a lava flow.”
Irene presents very interesting information on the dark colored ejecta and iron:
“You can learn a bit more by looking at the composition of the material in and around the crater. In this image, you can see the iron content (calculated using data from the Clementine mission) overlain on a Lunar Orbiter image of this exact crater. The colours represent various iron contents (in Weight % FeO), as identified by the colour bar on the right. The shading represents topography through the use of shadows (sun is shining from the right). You can see that most of Daguerre’s floor has relatively high iron content (greens and yellows), while the rim of Daguerre (top right of the image) is relatively low in iron (blues). Our LROC crater seems to have excavated mostly material that is less iron-rich (light and dark blues), except for the wedge of high iron (greens and yellows) at the bottom of the crater. This high iron wedge corresponds exactly to the dark ejecta section you can see on the LROc image.”
I marked the wedge section with white lines and circled the approximate outer rim of the crater. The white rectangle in the crater indicates no iron content.
“If you zoom in, you can see that the iron-rich wedge at the bottom of the image extends into the crater interior. Remember that only parts of the crater interior are in shadow. To get a feel for the extent of the entire crater, complete in your mind the circle that is suggested by the right edge of the shadow section. Then you will see that the green wedge does indeed extend inside the crater. It is possible that the iron data is registering the land slide of dark material that we can see down the crater wall in the LROC image.
There is a high likelihood that the dark layer, which was the source of the ejecta that created this iron-rich wedge, is basalt. It is also possible that it may be a pyroclastic deposit, where the glass beads re-crystallized into basalt-type minerals after they solidified. The dark ejecta wedge, is expected have the same composition as this source layer.
The lighter ejecta material is highland material. It is lighter in color because of its lower iron content. It was emplaced when this impact crater excavated either sections of Daguerre’s crater wall and slump terraces or some pre-existing highland ejecta from an older and large crater. There’s not enough information here right now to tell which of these was excavated by our LROC crater, though. Lots more work to be done!”
I would like to thank Irene for taking the time out from her busy schedule to make this outstanding analysis of the crater. Forum moderator Jules and I also had some fun with a speculative comparison of the crater using the photographs of Apollo 16 and LROC.
Tom128 is a regular contributor to the Moon Zoo Forum
Irene Antonenko is a Moon Zoo Team member