Scientists probing the outer reaches of our solar system have hit upon an unusual phenomenon much closer to home. Instruments aboard a NASA spacecraft have detected fast-moving hydrogen atoms emanating from the moon. The atoms, which originated as protons from the sun, may help scientists study the lunar surface and other solar system objects in greater detail than believed possible.
The moon is continuously being pelted by hazardous radiation. Most of it comes from the solar wind, a stream of protons moving at about 1.6 million kilometers per hour in all directions from the sun. When these particles hit the moon, most stick to the lunar surface. But researchers suspected that a small number picked up electrons and bounced back into space as fast-moving hydrogen atoms. However, no one had detected these rebounding particles, until a team at the Southwest Research Institute (SwRI) in San Antonio, Texas, activated an instrument aboard NASA's Interstellar Boundary Explorer (IBEX) last December.
IBEX was designed to monitor the interstellar boundary, the region of space several billion kilometers away from Earth, where the outward pressure of the solar wind is balanced by the influence of incoming particles from all across the Milky Way. The SwRI team, led by space physicist David McComas, was starting up IBEX's high-speed particle detector just as the moon passed in front of its field of view. Suddenly, the instrument started picking up signals galore from high-speed hydrogen atoms streaming from the moon. "We were still testing the instrument," McComas says. "We hadn't even gotten into full science mode."
When they did, McComas and colleagues determined that as many as 10% of the solar protons that hit the moon bounce back into space as hydrogen atoms. Those atoms, the team explains this week in Geophysical Research Letters, are not electrically charged, so they can't be influenced by magnetic fields from the sun or Earth.
That means the particles can travel a long way undisturbed, such as back from the asteroids or the moons of Mars or even Jupiter, says physicist Yue Chen of Los Alamos National Laboratory in New Mexico. And because so many of them are reflected, they can help scientists build a portrait of the type of surface that reflected them, such as how dusty, rocky, or even icy it is.
One of the key questions in the evolution of planetary systems is how surfaces interact with the stellar wind, says physicist Larry Paxton of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. So these findings "will help us develop our understanding of the processes that shaped our world and the other planets."