Dark Haloed Craters
Dark haloed craters are windows to the volcanic history of the Moon. This blog entry was inspired by Thomas’s choice of Image of the Week where he highlights impact craters that have dark material surrounding the crater hole itself.
Dark haloed craters provide us with key insights into what must have been a dramatic and violent volcanic period of lunar history. To probe a little further here is a little background about eruptive volcanism on the Moon:
Between about 4 billion years ago and 3 billion years ago the lunar mantle underwent a period of partial melting where magma was generated at depth and then propagated up through fracture networks and magma conduits towards the lunar surface. It has been proposed that some of this lava was very rich in gases like carbon monoxide that may have caused rapid upward movement (maybe on the scale of one to several days) and caused dramatic pyroclastic eruptions at the lunar surface. These fire-fountaining events are similar to, but on a much larger scale than, eruptions witnessed at volcanoes in Hawaii, with some plumes of lava being thrown up to 40 km in height above the surface of Moon!
Other lavas that were less gas-rich would have migrated to the lunar surface more slowly, and could have erupted more gently, forming long lava flows that travelled great distances from their volcanic vent site. These lava flows are thought to have the consistency of runny motor oil and easily flowed into topographic lows like impact craters. Sometimes large quantities of lava must have flowed in channel networks – forming rivers of fire across the lunar surface. It must have been a dramatic time, but as available heat sources were diminished in the lunar interior, less magma was generated and by about 1 billion years ago we believe the Moon’s eruptive volcanic history came to a close.
So how do dark haloed craters fit into this story? Well they actually help address two separate lunar science questions as there are two types of dark haloed craters to keep an eye out for in Moon Zoo images (although we would please like you to classify them using the same button!)…
1. Volcanic eruption sites – these are rare places where pyroclastic beads, the ‘airfall’ deposits of lunar volcanoes, are concentrated on the lunar surface and form mantles around their source vents. These beads are a little bit like the volcanic ash or the Pele’s tears glass that gets erupted from volcanoes on Earth and you can read more about these types of dark halo mantles at:
- Explosive Volcanic Eruptions on the Moon.
- The Pyroclastic Volcanism Project at the US Geological Survey.
- Moon Zoo Thomas’s guide to pyroclastic craters.
- Pyroclastic bead features seen in LROC NAC images.
2. Crater excavation sites – you are far more likely to spot these types of dark haloed craters in Moon Zoo images. The dark haloed craters provide us with a neat view down through a series of geological layers. These are formed when an asteroid or comets smashes into the Moon, punches through an overlying light coloured layer (probably an ejecta blanket material from a nearby highland impact crater) and excavates material from below that is darker in colour. This darker material is likely to be a lava flow that was buried at depth and that is now revealed by the impact cratering process.
Here’s a nice diagram of this process can be seen at where the lower image shows a schematic of what two dark haloed impact craters look like from side-on. You can also view a 3D perspective of this process.
We call these types of buried lava flows cryptomaria as they would otherwise be hidden from view if we had not have spotted the tell tale signs of dark haloed craters. By mapping the location and extent of dark haloed craters we can therefore map out the distribution of buried lava flows at depth across the Moon and get a much better idea of the amount of ancient volcanism on the Moon. This in turn helps to shed new light on the Moon’s thermal and magmatic history, helping us to understand geological processes on small rocky planetary bodies.
Good examples of these types of impact formed dark-haloed craters spotted by Moon Zoo users include:
- http://www.moonzoo.org/examine/AMZ100185s (top right)
- http://forum.moonzoo.org/index.php?topic=209.msg4111#msg4111 (lovely collection of small craters at left centre)
- http://forum.moonzoo.org/index.php?topic=209.msg4270#msg4270 (right centre)
- Other good examples are provided on the Moon Zoo How to Take Part Tutorial.
So please do keep an eye our dark haloed craters on your Moon Zoo lunar exploring! Thanks to Irene Antonenko for providing helpful guidance about this topic.