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Powerful October Blast from Sun Scorches Solar System - 2004-07-09

The solar system is still feeling the effects of a record-breaking storm on the sun that blasted shock waves through it last October. Scientists say similar events on the sun over billions of years might explain why Mars lost its water and most of its atmosphere.

At one time, scientists thought the space between Earth and Sun was a vacuum. We now know that the Sun fills it with gusts of hot, electrically charged atomic particles called the solar wind that is pushed out from the Sun's corona, or atmosphere.

Sometimes this wind blows very hard, as it did in about 12 major bursts that hurled billions of tons of electrified gas outward at supersonic speeds in late October. A fleet of U.S., European and Japanese solar observation satellites recorded it as the biggest blast since measurements began.

University of Michigan space scientist Thomas Zerbuchen had this to say about the event.

"The events during this time period were record breaking in many ways," he says. "They included the most intense flare ever recorded, the fastest blast wave ever recorded at the sun, clocked at the speed of five million miles per hour (8 million km/hour)."

Even eight months after the solar surge, it is still spreading outward past the planets at nearly 2.5 million kilometers per hour. The distant U.S. Voyager-Two spacecraft launched in 1977 has detected it more than 11 billion kilometers away.

Scientists say the solar electrical surge shook Earth's protective magnetic field, which blocks the worst affects of the radiation. The impact was still severe enough to cause the rerouting of aircraft, force space station astronauts into a shielded part of the outpost, cause a power failure in Sweden, and disrupt long distance radio communications.

California Institute of Technology physicist Edward Stone says these are mild effects compared to what has occurred to Mars over the ages, because it is has a weak magnetic field.

"The irradiation can affect the surface of Mars when these events occur because Mars has so little protection," he explains. "Bare objects that are not protected by the magnetic field and the atmosphere of Earth can be affected by the irradiation over long periods of time, which can alter the composition of material on their surfaces."

Scientists who wonder where Mars' water went may now have an answer. Thomas Zerbuchen at the University of Michigan says successive blasts of charged solar particles over billions of years might gradually have broken it into its component oxygen and hydrogen molecules. He says they also could have depleted Mars' atmosphere.

"This atmosphere is only roughly one percent as dense in terms of pressure as the one on Earth," he notes. "During a space storm simulated here by our colleagues at Nagoya University [in Japan], the blast wave compressed the atmosphere and substantial parts of the upper atmosphere escape from the planet into space."

Scientists continue to track the solar blast wave so they can determine when it meets the edge of the solar system. This is called the heliosphere and is the boundary with interstellar space, where the charged solar particles pile up against a similar wind from nearby stars. The collision is expected to occur late this year or early next and may generate extremely low frequency radio signals that will provide a better understanding of the size of the sun's domain.

Edward Stone says the size of the heliosphere varies because of the impact of the solar wind.

"These blast waves temporarily push the boundary out and then it actually rebounds after a year or two," he says. "But there is a longer term thing that happens with the 11-year solar cycle because the solar wind itself changes its pressure over the 11-year cycle and the heliosphere breathes in and out with the 11-year cycle of solar activity, and then punctuated by these transient events, which push the boundary out for maybe a year or two before it relaxes back in."

Mr. Stone says the October solar flare might have been the last big storm from the sun for a while as its activity diminished from the 11-year peak that occurred in 2000.