Streams of electrically charged particles from the Sun and outer space are turning out to be major players in our solar system's cosmic drama. They help shape planetary atmospheres, sterilize exposed surfaces, and even bring possibly life-supporting oxygen to an icy Jovian moon.
In their latest act, solar protons (the nuclei of hydrogen atoms) allow our own supposedly arid moon to make enough water every day to offset evaporation and maintain trace amounts of moisture all over its surface. Forget those bone dry textbook images. In September, several research teams made the case for a damp moon in Science online. A decade of data from several spacecraft showed water concentrations up to a liter per ton of lunar "soil". The European Space Agency (ESA) confirmed that finding with a study of its own announced last month. That's still pretty dry. But MIT planetary scientist Benjamin Weiss says that, given it might be easy to recover the water, "I think this makes it a promising resource for astronauts."
The water trick works like this. Lunar "soil" is a loose mix of dust grains that soaks up protons in the so-called solar wind. Trapped between the dust grains and absorbed, the solar protons react with the oxygen there to make water. They continuously make enough to offset the water that evaporates back into space. A European-supplied instrument on board India's Chendrayaan-I craft now orbiting the Moon found the data to back up this scenario.
They also contained a surprise. One in five of those incoming positively charged solar protons bounced back without joining the water project. Instead, it picked up an electron, became electrically neutral, and escaped the grasp of the magnetic fields that guide the electrically charged solar wind. Stas Barabash with the Swedish Institute of Space Physics says that's something "we didn't expect to see."
This potentially opens a whole new field of space observations – using the paths of electrically neutral hydrogen atoms such as light rays to form images. Photons leaving the lunar surface fly away on straight paths unaffected by magnetic fields or lunar gravity. Those paths (light rays) point directly back to their source. ESA explained that the bounce-back hydrogen atoms also travel in straight lines unaffected by magnetic fields or weak lunar gravity. Images of the surface can be made by tracing each atom back to its source. Areas emitting the most hydrogen will show up the brightest. Dr. Barabash and his team members are honing their skills at such image making.
Meanwhile, cosmic rays work some potentially life-sustaining molecular magic on Jupiter's moon, Europa. Scientists think its icy surface covers a vast ocean. They also believe that cosmic particles undergo reactions in the surface ice to produce oxygen. But it's been hard to imagine how to get that oxygen through the ice into the water to support oxygen-breathing life. Now Richard Greenberg from the University of Arizona in Tucson has shown it's theoretically possible for processes that renew the surface ice to work fast enough to maintain ample oxygen in the covered over ocean. Explaining his scheme at an American Astronomical Society meeting in Puerto Rico last month, he estimated there would be enough oxygen to support 3 billion kilograms of "macrofauna."
Where would our solar system be without the subtle influence of those speeding interplanetary particles?
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