Mysterious Lunar Swirls

http://science.nasa.gov/headlines/y2...unarswirls.htm

Mysterious Lunar Swirls
NASA Science News
June 26, 2006


June 26, 2006: 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.

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
exposu 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 Chandraayan-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...