July 19, 2012
Bubble Bubble – Swirl and Trouble
Swirls in Mare Ingenii, far side of the Moon
The Moon, unlike Earth, has no global magnetic field but many surface locales of limited extent (tens of kilometers across) are magnetized. In many instances, these small areas of high magnetic intensity are associated with unusual patterns of surface brightness (albedo, or degree of reflectance) that occur in curved, blotchy or other strange “swirl-like” shapes. First observed by telescope, lunar scientists have been puzzled by the possible origin of what they imaginatively named “swirls.”
An example of a lunar swirl is a feature named Reiner γ (pronounced “Reiner gamma”), a bright splotch in southern Oceanus Procellarum, the dark mare region of the western near side. The name indicates that initially this feature was thought to be an isolated peak of highland material that juts up through the mare (lowercase Greek letters were assigned to such prominences in the old nomenclature.) However, even at very low sun elevations, close examination shows that this bright patch does not cast a shadow. It is simply a bright patch on the surface, one with diffuse and nebulous edges, yet clearly more reflective than the surrounding dark mare material. It does not appear to be associated with any crater or other surface feature. It’s as though someone smudged a finished painting of the lunar surface.
During later Apollo missions, orbiting vehicles released “subsatellites” (small spacecraft that continued to orbit the Moon long after the crews had left for home) carrying instruments to measure the Moon’s magnetic field. Interestingly, they found a very strong magnetic field enhancement around the Reiner γ feature. Moreover, numerous other swirls were found elsewhere on the Moon, especially on the floor of the huge South Pole-Aitken (SPA) basin in Mare Ingenii on the far side, and on the eastern limb of the Moon near Mare Marginis. Each newly seen swirl was found to be associated with a magnetic anomaly. However, the converse statement is not true – not all magnetic anomalies have associated swirls.
Two principal models emerged to explain these relations. One model held that the swirls and the magnetism were contemporaneous – the swirls were surficial deposits caused by the scouring of the surface during the impact of a comet. In this model, the cometary coma (i.e., the dense gaseous “atmosphere” surrounding the icy nucleus) struck the Moon at high velocity, scouring the surface and increasing its brightness while at the same time embedding the soil with a strong magnetic field caused by the creation of an impact-generated plasma (high temperature, low density matter).
The other model suggested that the magnetic anomalies pre-dated and were the cause of the swirls. The lunar surface darkens and becomes redder with time owing to exposure to the solar wind (the stream of energetic particles – mostly protons – from the Sun). Strong, localized magnetic fields serve as protective “bubbles” that caused the incident solar wind to flow around these tiny areas, darkening the edges of the field bubbles with enhanced flow but preserving the inner zones (which were shielded from the solar wind) as bright patches. Thus, the bright parts of the swirls are areas that have not undergone “weathering” by the solar wind while the dark parts are zones that have experienced excessive space weathering.
Magnetic bubble created in the laboratory (RAL/Univ. York)
It remained uncertain whether this postulated “magnetic bubble” effect would actually work but recent experiments suggest that these bubbles might well operate on the Moon. Scientists from the UK’s Rutherford Appleton Laboratory, creating a “solar wind tunnel” to observe the interactions of streaming plasma and confined magnetic bubbles, successfully produced a magnetic bubble under simulated space conditions. They have compared the flow field around the laboratory magnetic bubble with the observations from orbiting spacecraft of lunar surface magnetism and find that the solar wind would be diverted around these magnetic anomalies on the Moon. If solar wind darkening is the primary process that darkens the surface, we may have an explanation for the creation of the bright swirls.
The astute reader will note that while this bubble model might account for the origin of the swirls, it begs the question about what caused the magnetic field anomalies in the first place. That remains a mystery. It was noted many years ago by my colleague Lon Hood of the University of Arizona that many of the magnetic anomalies on the Moon are at the antipodes (i.e., 180° away from the center) of some of the youngest, large impact basins on the Moon. The largest concentration of both surface magnetic anomalies and swirls are on the floor of the large SPA basin, near Mare Ingenii on the lunar far side. This area is directly antipodal to the large, young Imbrium basin on the near side. Likewise, the Mare Marginis swirls and magnetic fields are antipodal to the Orientale basin on the western limb (the last of the large lunar multi-ring impact basins). Furthermore, as basins tend to cover the entire Moon, one can find a basin near the antipode of almost any given feature (note well: the swirl that started all this hubbub, Reiner γ, isn’t antipodal to anything in particular). But an even more significant issue is that while the basin antipodal association of many swirls is intriguing, it does not explain why we should see a zone of enhanced magnetization at such locations. Igneous intrusion, concentration of impact-generated plasmas and converging ballistic ejecta have all been proposed but no specific mechanism seems to emerge as the magnetic field creating event.
We are left with a continuing and highly unsatisfactory situation – a possible explanation for the development of surface swirls on the Moon and of their association with magnetic field bubbles, but we still don’t understand the origins of these fields, the cause of their shapes and intensities and how they fit into the continually vexing problem of lunar magnetism in general. Two steps forward and one step back. Lunar science marches on.
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I wonder if these magnetic bubbles would provide any reduction in surface radiation. If so they may be desirable places to locate some of the early bases.
Comment by Thomas Matula — July 19, 2012 @ 5:28 pm
Tom,
I wonder if these magnetic bubbles would provide any reduction in surface radiation
The most dangerous radiation to human and other lifeforms are the high energy galactic cosmic rays and the heavy nuclei from solar particle events. These magnetic anomalies are not strong enough to deflect such radiation. In any event, we want to locate a lunar base close to usable resources, which on the Moon, means close to deposits of water. Such locations are in the polar areas where there are no evident magnetic anomalies.
Comment by Paul D. Spudis — July 19, 2012 @ 6:44 pm
Two steps forward and one step back. Lunar science marches on.
Isn’t that the way of all science?
It would be interesting to drop a couple of Curiosity-sized probes onto those swirls to see what they are made of.
Comment by JohnD — July 20, 2012 @ 9:05 am
“-desirable places to locate-”
The only desirable place is underground. The problem is excavating. Transporting 1000 ton TBM’s (tunnel boring machines) to the moon is problematic.
http://gizmodo.com/5894751/giant-metal-moles-are-tunneling-through-london
Project Plowshare, which used nuclear weapons to excavate was not a very good idea on Earth- but might be the answer to constructing a moon colony.
The Nuclear Industry should be looking to relocate IMO.
Comment by GaryChurch — July 20, 2012 @ 10:56 am
“-reduction in surface radiation-”
One way to provide radiation shielding for a surface moonbase would be to process lunar ice into water and fill up a double layer plastic dome. 14 feet of water will provide protection equal to earth’s atmosphere at 18,000 feet above sea level according Dr. Eugene Parker. Why not grow food in it while we are at it? And if you made it out of clear plastic you would get a nice view of the landscape. But would it be difficult to keep from freezing and boiling?
Comment by GaryChurch — July 20, 2012 @ 11:06 am
Interesting article–as usual!
Q: Could it be that the magnetic anomalies are the result of metallic meteor or asteroid impacts (the resultant magnetic fields would then cause the swirls), and that the swirl magnetic anomalies thus possibly mark high value deposits of metals, including PGM’s?
Comment by Warren Platts — July 20, 2012 @ 7:39 pm
Comment by Thomas Matula — July 19, 2012 @ 5:28 pm
“I wonder if these magnetic bubbles would provide any reduction in surface radiation. If so they may be desirable places to locate some of the early bases.”
Comment by Paul D. Spudis — July 19, 2012 @ 6:44 pm
“The most dangerous radiation to human and other lifeforms are the high energy galactic cosmic rays and the heavy nuclei from solar particle events. These magnetic anomalies are not strong enough to deflect such radiation.”
When I read the article I had two questions, Mr. Matula beat me to the first one (too bad about the answer). So I will move on to the second question.
The article suggests two explanations for the magnetic fields. The first one listed is as follows:
“One model held that the swirls and the magnetism were contemporaneous – the swirls were surficial deposits caused by the scouring of the surface during the impact of a comet. In this model, the cometary coma (i.e., the dense gaseous “atmosphere” surrounding the icy nucleus) struck the Moon at high velocity…”
These cometary cores would presumably contain high concentrations of water ice and other volatiles (Carbon, Nitrogen). If this explanation were to be the correct, one would these locations be another potential lunar source of such resources? Would the cometary core survive the impact intact? If so, how deeply would they be buried?
Comment by Joe — July 20, 2012 @ 7:45 pm
Warren,
the swirl magnetic anomalies thus possibly mark high value deposits of metals, including PGM’s?
Such has been proposed (most recently, HERE), but we really don’t know the answer. Much about the magnetic history and properties of the Moon remain mysterious.
Comment by Paul D. Spudis — July 21, 2012 @ 2:06 pm
Joe,
high concentrations of water ice and other volatiles (Carbon, Nitrogen). If this explanation were to be the correct, one would these locations be another potential lunar source of such resources? Would the cometary core survive the impact intact? If so, how deeply would they be buried?
Not likely, mostly because volatile substances are not stable at mid- to equatorial latitudes, where the magnetic anomalies are all found. We find them at the poles because the colder temperatures up there allow them to be preserved over time.
Comment by Paul D. Spudis — July 21, 2012 @ 2:08 pm
Paul,
Pity they are too weak. And yes, the habitats do need to be a close to water as feasible which is why they will be at the poles.
Comment by Thomas Matula — July 21, 2012 @ 7:58 pm
Gary,
The big question would be how the low gravity and lack of water in your average lunar rocks would effect the effectiveness of making underground cavities with nuclear devices.
A more practical solution would be to adopt the technology that currently exists for tunnel boring to the lunar environment and than power the tunnel borers with a nuclear reactor. Not only would you be able to construct kilometers of underground chambers using borers but you should also be able use the cuttings to produce an atmosphere to fill the tunnels. Like the turtle they work slow but steady.
Its also a solution that could be used early in the lunar industrialization process since the borers could be made of any size needed and they could be tele-operated from Earth with only a small repair team on site. A 2.5 meter borer would be a good size to start with.
Comment by Thomas Matula — July 21, 2012 @ 8:12 pm