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.