Usually our anaglyph images - which require inexpensive, readily available red-cyan glasses to view in 3D - highlight mountains, canyons, and craters on the Moon; that is, craggy, complex features with lots of ups and downs. This time around, however, the features appear so flat that they appear to be airbrushed onto the surface.
Lunar swirls remain mysterious nearly 50 years after Apollo astronauts viewed them from lunar orbit. They range from complex and delicate, often with a "dark lane" separating two delicate brighter areas, to shapeless blobs (some of which might not actually qualify as swirls - the jury is still out). In all cases, the swirls are brighter than the surrounding lunar regolith, the rocky, dusty mix of materials that stands in for soil on the Moon.
For decades, only a few swirls were known. Reiner Gamma, on the nearside (the side of the Moon that always faces toward Earth), is by far the best known and most distinct. With the advent of sophisticated robotic lunar orbiters in the 1990s, however, their bold loops and delicate ribbons began to turn up across large areas of the lunar surface. The largest swirl field, more or less centered on Mare Ingenii, spans 1000 kilometers across the farside and includes more than 80 distinct swirl features painted on the surface.
The Mare Marginis swirl field straddles the line between nearside and and farside on the Moon's eastern limb. It includes at least 400 swirl features draped over its topography. The western Mare Marginis swirl we highlight in today's Featured Image is on the nearside, but just barely.
The Apollo 15, 16, and 17 missions each included a magnetometer - an instrument for detecting and measuring magnetic fields on the Moon from lunar orbit. In the 1970s, scientists examining their data found early hints that the swirls and localized magnetic fields go together. Lunar orbiters since the 1990s returned data that helped scientists confirm the link between magnetic fields and swirls.
The Moon's localized magnetic fields bear little resemblance to Earth's magnetic field, which is global and is generated by the spinning dynamo of its molten iron core. The Moon has no global field. Localized fields could, however, be remnants of a time soon after it formed when the Moon, too, had a global magnetic field.
Earth's magnetic field is one reason it has life on its surface. The magnetic field deflects radiation from the Sun, creating a safe, habitable bubble around our planet. It acts a little like a "deflector shield" does in science fiction tales.
Much about swirls remains enigmatic. Nevertheless, scientists have put forward theories to account for them. The leading theory, based on the most current data, is that the localized magnetic fields shield areas of the lunar surface. Solar wind particles cannot get through the magnetic fields to strike and darken the surface; so, while the landscape around them grows steadily darker under the continuous daylight barrage of solar wind particles, the swirl areas remain bright.
Scientists and engineers have proposed at least two low-cost lunar-orbiting missions to investigate the localized magnetic fields of the swirls. The key, they say, is to get down close to the surface. One mission would lower instruments on a tether; the other would see a cubesat swoop over the lunar surface above the prominent Reiner Gamma swirl at an altitude of just five kilometers. Other scientists would like to land a rover with a magnetometer to chart the Reiner Gamma fields on the surface. A rover would, of course, be more costly than a small orbiter.
Some engineers and scientists suggest that a better understanding of swirls might lead to safer space travel. It might help us to figure out how to shield interplanetary spacecraft, both piloted and robotic, from space radiation that can harm astronauts and equipment. Magnetic shields might also protect outposts on other worlds. Perhaps future lunar settlers will build habitats on the swirls under the naturally occurring localized magnetic shields.
Check out the long anaglyph image below. Our Featured Image is located very near the center of this strip of the lunar surface. North is toward the top. Note how the swirl looks painted on. And be sure to look at the links to Related Featured Images below, which explain and display Reiner Gamma and many other lunar swirls.
Related Links for this Anaglyph
Related Featured Images on Swirls
Swirls Across the Moon An excellent overview of how swirls might work and where they are be found, with a link to a scientific paper for readers seeking more details.
Reiner Gamma Constellation Area of Interest Reiner Gamma was considered a prime exploration target for astronauts during the cancelled Constellation Program in 2009.
Lunar Swirls at the Mare Ingenii Constellation Area of Interest Mare Ingenii on the farside of the Moon was another possible Constellation landing site.
The Swirls of Mare Ingenii An update from 2012 on the Mare Ingenii swirls.
Blewett, D. T., Coman, E. I., Hawke, B. R., Gillis-Davis, J. J., Purucker, M. E., Hughes, C. G. (2011) Lunar Swirls: Examining crustal magnetic anomalies and space weathering trends, Journal of Geophysical Research, 116 (EO2002)
Bamford, R. A., Kellett, B., Bradford, J., Todd, T. N., Benton, Sr., M. G., Stafford-Allen, R., Alves, E. P., Silva, L., Collingwood, C. , Crawford, I. A., R. Bingham (2014), An exploration of the effectiveness of artificial mini-magnetospheres as a potential solar storm shelter for long term human space missions, Acta Astronautica, 105, 385-394
Glotch, T. D., Bandfield, J. L., Lucey, P. G., Hayne, P. O., Greenhagen, B. T., Arnold, J. A., Ghent, R. R., Paige, D. A. (2015) Formation of lunar swirls by magnetic field standoff of the solar wind, Nature Communications, 6:6189
Denevi, B. W., Robinson, M. S., Boyd, A. K., Blewett, D. T. (2016), The distribution and extent of lunar swirls, Icarus, 273, 53-67
Robinson, M. S., Thangavelautham, J., Anderson, B. J., Deran, A., Lawrence, S. J., Wagner, R., Ridenoure, R., Williams, B., Dunham, D., Babuscia, A., Cheung, K.-M., Genova, A. L. (2018), Swirl mission concept: unraveling an enigma, Planetary and Space Science, in press
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