IBEX basks in fast hydrogen atoms from the Moon

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 19 June, 2009

Very fast hydrogen atoms emanating from the Moon have been detected for the first time by NASA’s Interstellar Boundary Explorer (IBEX) spacecraft following decades of speculation regarding their existence.

IBEX is set to map the complex interactions occurring at the edge of the Solar System where the solar wind slams into the interstellar material from the rest of the Galaxy. Fortunately our planet’s strong magnetic field shields us from the millions of kilometres per hour strong solar wind, but the Moon, with its relatively weak magnetic field, has no such protection, allowing the solar wind to slam onto the Moon’s sunward side. Most of the solar wind particles become embedded into the lunar surface, while a few scatter off in random directions, becoming neutral by picking up electrons from the lunar surface.

IBEX's primary goal is to map the complex interactions of the solar wind with interstellar material at the boundary of the Solar System and interstellar space.

During the spacecraft commissioning phase the mission team turned the IBEX-Hi and IBEX-Lo instruments on, which between them measure atoms with speeds from about 160 thousand to four million kilometres per hour. From its vantage point in space, IBEX sees about half of the Moon – one quarter of it is dark and faces the nightside while the other quarter faces the dayside.

“Just after we got IBEX-Hi turned on, the Moon happened to pass right through its field of view, and there they were,” says IBEX principal investigator David McComas. “The instrument lit up with a clear signal of the neutral atoms being detected as they backscattered from the Moon.”

Based on their observations, the IBEX team estimate that just ten percent of the solar wind ions reflect off the dayside of the Moon as neutral atoms, while the remaining 90 percent are embedded in the lunar surface. Variations in the lunar surface characteristics, such as rocks, craters and dust influence the particles’ direction of scatter and the percentage of particles that become embedded in the surface.

IBEX made the first detection of neutral atoms coming from the Moon. The colour-coded data toward the bottom shows the neutral particles and geometry measured at the Moon on December 3, 2008. The neutral atoms are summed in 6 degree bins with the lunar direction indicated by the white arrow. Image: Southwest Research Institute.

The new result will also help scientists to understand how particles are recycled throughout the Solar System and beyond, from the minor bodies in our Solar System, to exoplanets and even protostellar nebula. As the solar wind and other charged particles impact dust, asteroids, moons and so on on their journey through space, they cause backscattering and reprocessing as neutral atoms. These atoms can travel vast distances before they are stripped of their electrons and become ions, and the complicated process begins again.

As to the mission's primary goal, at the end of the summer the IBEX team is expected to release the first all-sky map of the energetic processes occurring at the edge of the Solar System, which so far “doesn’t look like any of the models,” hints McComas.

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