The lunar surface is marked by strange patterns known as swirls that can be kilometers long and are unknown on any other object. More than 50 years after we landed on the Moon, the cause of these swirls remains unsolved, but a new crucial piece of information has been discovered, pointing the finger towards a more active lunar environment than generally imagined.
The swirls can be seen as relatively bright patterns standing out against the dark background of the lunar “seas” (actually basaltic plains). Although they have been studied for decades, efforts to understand them have made little progress, with magnetic fields, cometary impacts, and meteoroid swarms unconvincingly evoked.
Now a paper in Geophysical Research Letters points out that, contrary to previous observations the swirls tend to be concentrated at lower elevations. That provides a major hint as to what is causing them. “We believe this correlation with topography argues for highly mobile dust transport across the lunar surface,” the paper concludes.
On a different world, such a finding might seem entirely unsurprising. Wind might have picked up dust and deposited it in valleys. However, wind in the traditional sense requires an atmosphere, which the Moon lacks. Therefore mobile dust requires some more complex explanation.
Dr Deborah Domingue of the Planetary Science Institute and co-authors studied two swirls in detail, both located on the appropriately named Mare Ingenii (Sea of Cleverness). In each case, the location of the swirls was compared with digital elevation models. These revealed the swirls sit in topographic lows, which appear to collect dust of micron-size or smaller. “However,” the paper notes; “In order to trap lunar dust grains, there must first be a mechanism for migrating lunar dust across the Moon's surface.”
The explanation, the authors think, may lie in the solar wind, made up of charged particles pushed out by the Sun. The electrostatic effects of these hitting the lunar surface may be enough to lift tiny dust grains and initially keep them hovering until the upward force declines and gravity takes over. When this happens it seems, the lighter-colored dust particles congregate in lower-lying areas.
There have been hints of dust migrations on the Moon before. The lunar Surveyor missions detected a glow on the horizon 30 centimeters (12 inches) high. Instruments left behind by Apollo 17 recorded signs of both vertical and horizontal dust migration.
No one had investigated whether such forces were sufficient to produce patterns on the scale of the swirls. However, the authors say numerical simulations show that a crater just 1 meter (3 feet) deep could accumulate dust grains thanks to the more complex electric fields around the edge. Meanwhile “dust ponds” have been found in craters on the asteroid Eros.
In 1979, an association was found between the swirls and magnetic anomalies within the lunar crust, but the relatively weak link didn't explain their cause. Although the association with magnetic anomalies has attracted most of the attention, the paper suggests magnetic influences on the solar wind may be secondary to the topography, explain the inconsistency of the relationship.
Unanswered questions include the typical sizes of the dust grains in swirl and non-swirl areas and the magnetic properties of tiny iron particles. Future Moon-missions, no longer such a distant idea, could answer these.