23Jul2012

Magnetic Shields Under Moon’s Surface.

The most striking "lunar swirl", known as "Reiner Gamma" can be seen from the Earth using a good telescope. The Reiner Gamma swirl is to the left of the image near to the crater Reiner after which it is named (Credit: NASA)

Although they had been observed through a telescope prior to the Apollo era, pale “lunar swirls” were an enigma… a poorly understood region of magnetic fields buried in the lunar crust.

Now, unmanned lunar missions such as the Lunar Prospector, JAXA Kaguya and India’s Chandrayaan-1 have taken a much closer look at these regions with an eye towards future lunar missions. Is it possible these magnetic anomalies could somehow be utilized as a type of magnetic shield? Could these miniature “magnetospheres” provide a safe haven for tomorrow’s space travelers? Scientists from RAL Space at the Rutherford Appleton Laboratory say yes. 

The process which created these lunar magnetic “hot spots” may very well be able to be manipulated into providing a naturally sheltered environment, much like Earth’s own magnetosphere shields us. By combining space data and laboratory scale experiments using a “Solar Wind Tunnel”, the research team was able to identify the manner in which small scale magnetic anomalies form and how they could be employed to deflect solar wind particles encountering the lunar surface.

“When we first tried the experiment in the Solar Wind Tunnel and it worked, it was very exciting,” says lead scientist Dr. Ruth Bamford of the Centre for Fundamental Physics and RAL Space at the Rutherford Appleton Laboratory.

“The active force which deflect the solar wind particles is electric not magnetic. The electric field is created naturally by the edges of the Moon’s magnetic ‘bubbles’”, Dr. Bamford explains. “What matters is the ‘gradient’ in the magnetic field, rather than the overall size of the magnetic bubble. So they can be as small as you like — as long as the gradient is steep enough.”

However, there is still work to be done. Scientists will still need to clarify how “mini-magnetospheres” are formed and what causes the hole within them. Once that is established, they can further refine their experiments to determine if the same mechanism can be recreated.

This image shows a truly miniature, magnetosphere recreated in the laboratory. The plasma “bubble” shown here is a mere inch across in total. The purple glow, seen in the photograph, is the stream of super-heated hot gas particles (plasma) of the Solar Wind Tunnel. (Credit: RAL Space & Uni. of York)

Referring to the above image, you’d expect this stream to scorch the metal target. Without a magnetic field it would. But the experiment shows how instead what happens is a thin barrier or “skin” is formed of the plasma itself, within which a tiny cavity in the pseudo- solar wind is formed, which holds back the hazardous particles, so protecting the target except for at the magnetic poles. On the moon this cavity formation prevents the bombarding particles of the solar wind from weathering or darkening the surface color, keeping it white.

The barrier width is about 100th the width that such a weak magnetic field (known to exist on the Moon’s surface) could possibly bend a heavy, fast moving, positively charged proton ion out of the way. So something else must be deflecting the ions into such a narrow “skin”. That is an electric field the origin of which although complicated has been theoretically determined by the team and found to match the values seen in space by spacecraft and in the laboratory recreation.

How do we know where these magnetic hot spots appear? Once upon an epoch, the lunar soil was white, but it is widely accepted that it turned dark from continuous exposure to the charged particles of the solar wind. It has been theorized that swirls like Reiner Gamma may have formed as a result of magnetic shielding, but scientists were at a loss to explain the process. Because the Moon has very weak magnetic fields, it is a process that could have involved hundreds of millions of years.

According to Dr. Bamford, “Close to the Moon’s surface, the strength of a magnetic anomaly is likely to be very irregular, featuring overlapping ‘cavities’ and ‘gradients.’ We cannot know the precise arrangement without going there to see for ourselves”, but the result on the surface would be a corresponding pattern of retarded and accelerated “space weathering,” visible as areas of lighter material separated by darks “lanes.”

It has taken almost 4 billion years for these patterns to emerge – a long life-time of solar winds bombarding the lunar surface. Thankfully, the idea of how they formed took far less time to confirm. Through experiments done in the laboratory with the University of York in the U.K. using their “Plasma Wind Tunnel”, the particles were effectively “corralled” by a narrow electrostatic field – producing the desired result of a magnetic shield.

“We still need to determine quite how effective this mechanism would be at deflecting the real hazardous higher energy particles.” says Dr. Bamford. “The jury is still out on that one, but such an active shield could make the difference between survivable and certain death for astronauts on their way to Mars”.

Original Story Source: Science and Technology News Release.

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