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How Big Waves Go Rogue
Animation of Giant Iceberg Collision as Seen From Space
Earth’s Magnetic Field Is 3.5 Billion Years Old

Posted: 05 Mar 2010 03:45 PM PST

rogue_wave2

An extra-tall wave struck a cruise ship off the Mediterranean coast of Spain this week, claiming two lives and injuring one person on board. Though the wave may not qualify as a "rogue wave," it could have been created by the same forces.

To officially be rogue, the wave's height must be more than double the "significant wave height" of the area, which is calculated by averaging the height of the tallest third of all the nearby waves.

The wave measured 26 feet tall and shattered plate-glass windows at the bow of the vessel. Still, it wasn't very tall compared to some of the waves oceanographer Libe Washburn of UC Santa Barbara has seen.

"I was surprised it was really that damaged by a 26-foot-high wave," Washburn said. "Twenty-six feet isn't that big."

Until recently, scientists were skeptical that rogue waves even exist, because evidence of them was mostly anecdotal. More often called "freak waves," these monsters of the sea were confirmed only six years ago by satellite images and extensive studies carried out by MaxWave, a research group funded by the European Commission.

Waves over 100 feet tall have been spotted by oceanographers, scientists and vessel passengers. The highest wave ever recorded was 112 feet tall, spotted in the Pacific by a U.S. Navy tanker in the 1920s. Now, whenever large ships get lost at sea and never return, many are quick to speculate they were victims of rogue waves.

Rogue waves occur in the open ocean in a number of ways. One common cause is when two smaller waves coalesce to produce a very large wave for a short time.

wave_crest "You get waves that add up — smaller waves that constructively interfere and for a short time produce a very large wave," Washburn said. "When they add up, they can make an extra high crest and an extra deep trough."

Another way rogue waves propagate is when an ocean wave encounters a very strong current that's running counter to the direction of the wave, according to Washburn. The Agulhas Current, which flows down the eastern coast of South Africa, is notorious for producing rogue waves.

"It's very dangerous at the Agulhas," Washburn said. "Even if you're on a big ship, that doesn't mean you're any safer."

Storm-related wind is a factor as well. Strong winds transfer energy into the waves, creating interactions between them. Large waves take energy from smaller ones, creating a bigger and bigger wave, said oceanographer Peter Challenor of the National Oceanography Centre in the United Kingdom, in an interview with Agence France-Presse.

Photo: NOAA

Image: NASA

Animation of Giant Iceberg Collision as Seen From Space

Posted: 05 Mar 2010 02:27 PM PST

iceberg

The collision in early February of the 60-mile-long B-9B iceberg with the protruding tongue of the Mertz Glacier in East Antarctica is captured here in a series of satellite radar images.

The crash created a second massive iceberg nearly 50 miles long and 25 miles wide, named C-28. The name means that it's the 28th glacier since 1976 that has broken off from the quadrant of Antarctica that faces Australia.

The two icebergs have since drifted into a polynya, which is an area of open water that's surrounded by sea ice but stays unfrozen for much or all of the year. The bergs are obstructing the ocean circulation created by the polynya, and could deprive local marine life of oxygen if they don't move.

The images were taken by the synthetic-aperture-radar instrument aboard the European Space Agency's Envisat satellite.

Images: ESA

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Earth’s Magnetic Field Is 3.5 Billion Years Old

Posted: 05 Mar 2010 11:29 AM PST

magnetic_field

Evidence for the existence of Earth's magnetic field has been pushed back about 250 million years, new research suggests. The field may therefore be old enough to have shielded some of the planet's earliest life from the sun's most harmful cosmic radiation.

Earth's magnetic field was born by 3.45 billion years ago, a team including researchers from the University of Rochester in New York and the University of KwaZulu-Natal in South Africa report in the March 5 issue of Science.

That date fallsduring life's earliest stages of development, between the period when the Earth was pummeled by interplanetary debris and when the atmosphere filled with oxygen. Several earlier studies had suggested that a magnetic field is a necessary shield against deadly solar radiation that can strip away a planet's atmosphere, evaporate water and snuff out life on its surface.

"I think it's a magnificent piece of work, a real landmark," says geophysicist David Dunlop of the University of Toronto, who was not involved in the research. "It pushes the boundary back about as far back as you could reasonably expect to measure on Earth."

The researchers measured the magnetic strength of certain rocks found in the Kaapvaal craton of South Africa, a geologic region known to date back more than 3 billion years.

Just finding old rocks wasn't enough, though. "It's a Goldilocks theory of finding rocks," says John Tarduno of the University of Rochester, a coauthor of the new study. Iron minerals record the strength and direction of the magnetic field that was present during their formation. But when rocks are heated in subsequent geological processes, they can lose or overwrite that record.

"We had to find a rock that had just enough iron to record a magnetic signature, but not so much that it would be affected by later chemical changes," Tarduno says.

The Greenstone Belt in South Africa had rocks that were just right: crystals of quartz less than two millimeters long with nanometer-sized bits of iron-containing magnetite embedded in them.

"Quartz is the perfect capsule," Tarduno says. "It's not affected by later events, but it has these [iron] inclusions in it."

Tarduno and his colleagues had studied similar rocks in 2007 and found that a magnetic field half as strong as today's was present 3.2 billion years ago. Using a specially designed magnetometer and improved lab techniques, the team detected a magnetic signal in 3.45-billion-year-old rocks that was between 50 and 70 percent the strength of the present-day field, Tarduno says.

"When we think about the origin of life, there are two threads to follow," Tarduno says. "One obviously is water. But you also have to have a magnetic field, because that protects the atmosphere from erosion and the complete removal of water." Mars may be dry today because it lost its magnetic field early on, he adds.

To determine if the early magnetic field was enough to hold back the rain of radiation, the team needed to know what the sun was doing. Tarduno and Eric Mamajek, an astronomer at the University of Rochester, used observations of young sunlike stars to infer how strong a solar wind the Earth was up against.

magnetic_field_2 The young sun probably rotated more quickly than it does today, Tarduno says. This quick rotation powered a strong magnetic field, which heated the sun's atmosphere and carried away mass and angular momentum in a strong solar wind of charged particles. The team calculated that the point where the Earth's magnetic field cancels out the solar wind would be only about five Earth radii above the planet's center, less than half of the 10.7 radii it is today.

The amount of radiation regularly reaching Earth from the sun 3.45 billion years ago would be comparable to what rains down on the planet during the most powerful solar storms today, Tarduno says. The aurora borealis, caused by solar wind particles accelerating along Earth's magnetic field, would have been visible as far south as present-day New York City.

The study "can be used to guide our searches for other life-bearing planets" as well, says astronomer Moira Jardine of the University of St. Andrews in Scotland. Astronomers might want to focus more on older, less active stars or search for planets with their own magnetic fields, she says.

Despite the fact that no extrasolar planets with magnetic fields have ever been detected, Jardine and Tarduno remain optimistic. "It's just another parameter we need to think about," Tarduno says.

Images: 1) J. Tarduno, R. Cottrell. 2) NASA.

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