As part of MESSENGER's Comprehensive Performance Test, the Neutron Spectrometer (NS) element of the Gamma-Ray and Neutron Spectrometer instrument underwent a thorough checkout last week. In this week's annotated Webcam image, an engineer tapes a neutron source (Americium-241 boron) to the painter's shelf on a stepladder. The ladder was then positioned to put the source a few inches from the NS. The NS has three different detectors, so the ladder and source were moved around to check each detector. (Click here for large and small time-lapse movies of the test.)

MESSENGER's Neutron Spectrometer will help mission scientists unravel Mercury's early history by providing compositional information about the planet's crust. Specifically, NS will map variations in the fast, thermal, and epithermal neutrons Mercury's surface emits. When cosmic rays collide with Mercury's crust, they dislodge neutrons and produce gamma rays. Some of the newly released neutrons shoot directly into space; these are known as fast or hot neutrons (epithermal neutrons).

Other neutrons collide with neighboring atoms in the crust before escaping into space. If a neutron collides with a small atom (approaching the size of a neutron), it will lose energy and be detected as a slow, cool (or thermal) neutron. Scientists can look at the ratio of epithermal to thermal neutrons across the surface of Mercury to estimate the amount of hydrogen - possibly locked up in water molecules - and other elements. (This same approach has been used to infer the presence of sub-surface water on Mars.)

By combining the analysis of NS results with measurements of gamma rays emitted by the surface, scientists can map the abundance of major elements that form the crust. One of the mission's major goals is to reconstruct the formation and evolution of Mercury's crust by synthesizing results from all of the spacecraft's science instruments.

Last updated: November 6, 2003