Mysterious Lunar Swirls

Jun 27, 2006
The Reiner Gamma swirl, photographed by the ESA's SMART-1 lunar orbiter
The Reiner Gamma swirl, photographed by the ESA's SMART-1 lunar orbiter.

Picture this: A cup of coffee, steaming and black. Add a dollop of milk and gently stir. Eddies of cream go swirling around the cup. Magnify that image a million times and you've got a Lunar Swirl.

Lunar swirls are strange markings on the Moon that resemble the cream in your coffee—on a much larger scale. They seem to be curly-cues of pale moondust, twisting and turning across the lunar surface for dozens of miles. Each swirl is utterly flat and protected by a magnetic field.

What are they? "We don't know," says Bob Lin of UC Berkeley, who has been studying the swirls for almost 40 years. "These things are very strange."

One of the swirls, Reiner Gamma, can be seen through a backyard telescope. It lies near the western shores of Oceanus Procellarum (the Ocean of Storms) and looks at first sight like a strangely disorganized crater. Indeed, that's what most astronomers thought it was until 1966 when NASA's Lunar Orbiter II spacecraft flew overhead and photographed Reiner Gamma from point blank range. Whatever it was in that grainy black and white photo, it was not a crater.

A magnetic map of Reiner Gamma obtained by NASA's Lunar Prospector spacecraft in the 1990s
A magnetic map of Reiner Gamma obtained by NASA's Lunar Prospector spacecraft in the 1990s.

Before long, two more swirls were found on the Moon's farside. They lie directly opposite the nearside impact basins Mare Imbrium (the Sea of Rains) and Mare Orientale (the Eastern Sea). Impacts on one side of the Moon, it seemed, made swirls on the other side. No one could explain how.

The mystery deepened in 1972 when Lin and colleagues discovered that the swirls were magnetized. "It was an accidental discovery," he recalls. As often happens in science, "we were trying to learn about something completely different."

Their target was Earth's magnetic tail, a ropey pasta of magnetic force fields extending from Earth more than a million miles into deep space. The solar wind blowing against Earth's magnetic field makes the tail, and in the days of Apollo not much was known about it.

To study the tail, "we built two small satellites and asked NASA to put them in orbit around the Moon." The Moon is a great place to sample the Earth's magnetotail, he explains, because the Moon passes through the tail once a month as it orbits Earth.

NASA said yes, and two "sub-satellites" were deployed by the crews of Apollo 15 in 1971 and Apollo 16 in 1972. "The astronauts pushed a button and the satellites were shoved into space by a spring," says Lin. Free of the Service Module (the Apollo mothership), they orbited the Moon, gathering data collected by onboard electron detectors and magnetometers.

"We learned a lot about Earth's magnetic tail," says Lin. But they learned even more about the Moon:

As the sub-satellites flew just 60 miles above the lunar terrain, they passed in and out of strange magnetic domains. Magnetic force fields were sprouting out of the lunar surface, reaching up and affecting the satellites' sensors. "We realized that the crust of the Moon must be magnetized," he recalls. It wasn't a global magnetic field like Earth's, but rather a crazy-quilt of magnetic patches.

The strongest fields were located above Lunar Swirls. "The swirls have magnetic fields measuring a few hundred nano-Tesla (nT) at ground level," says Lin. (Earth's magnetic field, for comparison, is 30,000 nT.) "If you walked around a swirl with a magnetic compass, the needle would swing back and forth in a confusing way. You'd quickly get lost because the magnetic fields are so jumbled."

Lin believes these strange fields are an important clue to the origin of swirls, and he offers this possibility:

"Almost four billion years ago, the Moon had a liquid iron core and a global magnetic field. Suppose an asteroid hit the Moon. The blast would make a cloud of electrically conducting gas ('plasma') that would sweep around the Moon, pushing the global magnetic field in front of it. Eventually, the cloud would converge at a point directly opposite the impact, concentrating the magnetic field at that point." Eons later, the Moon's core cooled and its global magnetic field faded away. Only the strongest, tangled patches remained--the swirls.

This idea provides an explanation for the light, creamy appearance of swirls. According to some researchers, moondust is darkened by long exposure to solar wind. Maybe the swirls are light because they get less exposure: their magnetic fields deflect solar wind. If so, lunar swirls are merely a shadow of the magnetic forces arching above them.

It all sounds neat and tidy, but there's a problem: While two of the lunar swirls are directly opposite an impact basin, one is not: Reiner Gamma. The prototype swirl doesn't fit!

"It's a real mystery," acknowledges Lin.

More clues are on the way. NASA is returning to the Moon, eventually with people but first with robot scouts. Leading the way is Lunar Reconnaissance Orbiter (LRO), due to launch in 2008. Among other things, LRO will make detailed 3D maps of the whole Moon using a state-of-the-art camera and a laser. Its view of the swirls should be breathtaking.

Another NASA instrument, the Moon Mineralogy Mapper, is hitching a ride to the Moon onboard India's Chandrayaan-1 spacecraft, also due to launch in 2008. Using an infrared spectrometer, "M-cubed" will survey the lunar terrain and tell us in fantastic detail what minerals are in the ground. The whole Moon will be surveyed--including swirls.

What are swirls made of? Are they truly flat? How does the cream differ from the coffee? Questions to ponder over your next cup of joe…

Source: Science@NASA, by Dr. Tony Phillips

Explore further: Grad student's aTmCam offers cosmic insight for dark energy survey

add to favorites email to friend print save as pdf

Related Stories

First evidence of dynamo generation on an asteroid found

Oct 11, 2012

About 4.6 billion years ago, the solar system was little more than a tenuous disk of gas and dust. In the span of merely 10 million years, this soup evolved to form today's massive, complex planets. In the ...

'Deflector Shields' protect the Lunar Surface

Jul 19, 2012

(Phys.org) -- Scientists from RAL Space at the Rutherford Appleton Laboratory have solved a lunar mystery and their results might lead the way to determining if the same mechanism could be artificially manipulated ...

O, Pioneers! (Part 1): The motes in God's eye

Feb 27, 2012

March 2012 marks the fortieth anniversary of the launch of one of the most extraordinary spacecraft ever constructed - Pioneer 10 - the first true deep space probe. The story of the Voyager spacecraft is well ...

Juno spacecraft prepares to launch

Aug 01, 2011

In one week, NASA is scheduled to launch the Juno spacecraft on a mission to Jupiter, the largest planet in our solar system, to answer some fundamental questions about the gas giant and, in turn, about the ...

Recommended for you

Hubble sees 'ghost light' from dead galaxies

14 hours ago

(Phys.org) —NASA's Hubble Space Telescope has picked up the faint, ghostly glow of stars ejected from ancient galaxies that were gravitationally ripped apart several billion years ago. The mayhem happened ...

When did galaxies settle down?

20 hours ago

Astronomers have long sought to understand exactly how the universe evolved from its earliest history to the cosmos we see around us in the present day. In particular, the way that galaxies form and develop ...

Image: Hubble views the whirling disk of NGC 4526

22 hours ago

This neat little galaxy is known as NGC 4526. Its dark lanes of dust and bright diffuse glow make the galaxy appear to hang like a halo in the emptiness of space in this image from the NASA/ESA Hubble Space ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.