Debate on the origins of swirls on the lunar surface has arisen again, making this concept one of lunar science’s most enduring enigmas. Swirls are complex albedo (reflectance) markings on the surface of the Moon. They consist of bright and dark surface markings that have complex curved and linear shapes. The Moon has no global magnetic field, but localized, anomalously strong magnetic fields have been mapped at various localities around the Moon and the swirls appear to be associated with these anomalies.
Ideas about the origin of swirls fall into two broad camps. One concept holds that the magnetic anomalies pre-date the swirls and that these magnetic anomalies formed in early lunar history, during the time when the largest impact craters on the Moon (multi-ring basins) were created (prior to about 3.8 billion years ago). The anomalies, induced by some process associated with basin formation (although the exact mechanism is unknown) serve to protect parts of the lunar surface from the aging and darkening effects of solar wind exposure. Thus, the swirls represent areas of the Moon that have been shielded from the solar wind for millions of years and display both dark and light markings (depending upon the local, highly variable strength of the magnetic field).
The other idea proposes that as a result of the impact of a cometary coma, the swirls and the magnetic fields are created at the same time. Comets are icy objects from the outer Solar System; occasionally one will work its way into the inner Solar System through some chance gravitational encounter with one of the giant planets, usually Jupiter. Deflected comets travel very fast, hitting the Moon at speeds as high as 80-100 kilometers/second (asteroidal debris hits the Moon at speeds around 20 km/second). Because impact energy increases as the square of the impact velocity, you can see that cometary impacts would be very energetic.
A cometary core consists of two main parts, the nucleus (a mixture of mostly water ice (> 90 percent) and rocky fragments— the term “dirty snowball” is sometimes used to describe it) and the coma. The coma is a mini-atmosphere of tenuous gases and plasmas formed by the solar heating of the icy nucleus. Both parts of the comet hit the Moon; the nucleus forms a fresh impact crater, just as the collision of an asteroidal rocky body does, but it’s bigger due to its high speed. The mechanics of the collision of the coma with the Moon are poorly understood— the coma has an extremely low density, yet because it has mass, its collision with the Moon must result in some physical effects.
Newly published calculations describe some of the effects of the collision of a tenuous gas and plasma cloud with the Moon; this work suggests that the production of surface swirls may be the result. To understand how, we must consider the structure of the uppermost surface of the Moon. The Moon is covered by fine-grained, silicate powder that is very loosely packed in the uppermost surface. Such a delicate structure is called “fairy castle” and results in the brightening of reflectance when the uppermost surface is observed near zero phase (i.e., when the eye of the observer is aligned with the direction of solar rays). This brightening is called the coherent backscatter effect and is the result of the positive, constructive wave interference of reflected light off a very fine-grained, loosely piled dust layer. (By the way, this same constructive interference effect occurs at radio frequencies and was how the Mini-RF radar experiment found ice in the polar dark areas of the Moon.)
The lunar swirls show very strong forward scattering (i.e., diffuse scattering when observed in front of solar incident rays) but low backscattering (the reverse). From this phenomenon, we infer that such surfaces are more tightly packed than the normal, undisturbed lunar regolith (i.e., the normal “fairy castle” structure has been destroyed by some process). One way to achieve this closer-than-usual packing is to have a cometary coma hit and interact with the uppermost surface layer, compressing the dust grains into a very tight space, while simultaneously inducing a magnetic field into the iron-bearing dust grains. Some support for this concept comes from the observation of the terrain surrounding and beneath the Apollo Lunar Modules—the near-LM surface (seen from orbit) is noticeably brighter than the surrounding undisturbed terrain. In this case, the plume from the LM descent engine swept away the loose, fairy-castle overburden of dust, exposing the more tightly packed zone of regolith beneath it.
The swirls on the Moon seem to overlie every unit upon which they are found (suggesting a young age) and show no compositional differences with adjacent geologic units (suggesting a physical, rather than a compositional difference). In the old swirl model, the magnetic fields surrounding the swirls guard them from solar wind sputtering, making them appear brighter than the usual lunar surface, which is darkened by the process. But the surface is also darkened by micrometeorite bombardment, which produces impact-melted glass that darkens and reddens the surface. The magnetic fields would not protect the regolith from this process. Moreover, not every magnetic field anomaly displays swirls, and a few swirl deposits are not associated with a magnetic field anomaly. These relations tend to support the comet impact idea.
However, the biggest objection to the comet model is that swirls are not common on the Moon; in fact, they are quite rare (globally distributed in about a dozen locations). We have reason to believe that the number of cometary and asteroidal impacts on the Moon are roughly equal, so if comets make swirls, why aren’t the swirls all over the place? Comet model advocates get around this by claiming that the normal impact churning of meteorite bombardment destroys coma-produced anomalous packing very quickly, so the swirls that we see are all extremely young, on the order of a couple of million years or less (young for the Moon). But that rationalization seems a bit ad hoc— I would still expect to see many more swirl deposits on the Moon than are evident if they were produced by cometary impact.
In short, we still don’t understand the origin of these enigmatic features. Both the “magnetic” and the “cometary” models of origin have points for and against them. In science, such a situation usually means that neither explanation contains the full story. Continued study of these features may yet yield that critical piece of data, or some as-yet unimagined insight will crack the conundrum of lunar swirls. Often in science, it’s not the lack of data that holds us back—sometimes it’s a paucity of imagination.