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]