The purpose of the gamma ray and neutron spectrometer is to provide information about the elements that make up Mercury’s surface crust. More exactly, it will provide information about the uppermost tens of centimeters of the crust. This instrument measures the numbers and energies of gamma rays and neutrons that reach the MESSENGER probe as it passes near the planet.
(Image Credit: K. Garver) To understand this process, it is first important to know what neutrons and gamma rays are, and why they are present near Mercury. Neutrons are electrically neutral particles that, together with positively charged protons, make up the centers or nuclei of atoms. Gamma rays are little packets of high energy light. Gamma rays are more energetic than the visible light we see — even more than ultraviolet light or x-rays.
Both neutrons and gamma rays in the vicinity of Mercury can be produced by the “cosmic rays” that continually bombard Mercury’s surface. Cosmic rays are mostly protons from our Sun or other more distant sources in the universe. They easily reach Mercury’s surface because, unlike Earth, the planet lacks an atmosphere to intercept them.
These cosmic rays are moving so fast that, when they collide with an atom on the surface, they can dislodge neutrons from the atom’s nucleus. Some of the dislodged neutrons can then collide with the nuclei of other atoms, giving energy to them. When these nuclei return to their normal energy state, they can give off gamma rays.
The neutrons and gamma rays that travel far enough from Mercury can then be intercepted by MESSENGER and counted. Additionally, gamma rays can be produced by the decay of radioactive elements like uranium and thorium on the surface. The flux of these neutrons and gamma rays is higher at lower altitudes, so detection by GRNS is most effective when MESSENGER is orbiting Mercury.
GRNS detects gamma rays with a high-purity germanium semiconductor crystal, from which electric charge pulses are collected, caused by interaction with the gamma rays. The charge collected is proportional to the gamma ray energy. GRNS detects neutrons with scintillators, which are materials (lithium-rich glass for low-energy neutrons and boron-rich plastic for higher-energy neutrons) that give off a tiny pulse of light when struck by a gamma ray or neutron. The brightness of each light pulse is directly related to the energy of the neutron. So the instrument counts gamma rays and neutrons and measures their individual energies at the same time.
How do neutrons and gamma rays tell us what Mercury’s surface is made of? The key is in the fact that GRNS measures the energy of each gamma ray and neutron that it detects.
The energy of a gamma ray is a kind of signature of the element from which it came. For example, oxygen nuclei can emit gamma rays of a certain energy (about 6 MeV), while iron atoms emit gamma rays of a completely different energy (about 7.6 MeV). Therefore, scientists can use data from this instrument to determine which elements are present on the surface.
Moreover, the number of gamma rays detected with a particular energy tells us how much of that element is present. For example, if there are a lot of gamma rays detected at about 7.6 MeV, you would expect to find a high concentration of iron on the surface of Mercury. So GRNS can help to tell us which elements are present and the relative concentrations of those elements.
The gamma-ray spectrum from the surface of the asteroid, Eros,
indicates the presence of Iron, Potassium, Silicon, and Oxygen.
Note: the location or specific energy of a peak indicates the presence of an element, and the height of the peak tells us about how much of the element is present.
(Image credit: JHU/APL/NASA, from the NEAR mission)
The MESSENGER GRNS germanium detector yields much narrower energy peaks than shown in the above figure for the NEAR XGRS, which uses a sodium iodide scintillator to detect gamma rays. The GRNS thus has much less background under the peaks and can distinguish peaks much closer together.
Fast-moving neutrons, on the other hand, lose energy by colliding with atomic nuclei, in the way that a cue ball on a pool table gives up its energy to the balls that it hits. The lighter the nuclei that it collides with, the more energy a neutron loses. The lightest nucleus – consisting of a single proton – is that of the element hydrogen. Since hydrogen is particularly effective at slowing the neutrons down, a large increase of slow-moving neutrons or a large decrease of neutrons of higher energy (due to this slowdown) can indicate the presence of hydrogen-rich materials such as water ice. So the numbers of fast and slow-moving neutrons detected by GRNS are clues to the relative abundance of light and heavier nuclei on the planet’s surface.
Gamma ray and neutron spectrometers are not new to solar system exploration, but have not been employed very often. The Lunar Prospector (1998) mission employed these instruments, using scintillator detectors, to aid in the discovery of water ice near the poles of our Moon and to map many elements over the lunar surface.
Prospector Neutron Spectrometer data
from the north pole of the Moon, showing
evidence of water ice (dark blue to magenta).
(Image credit: Los Alamos National Laboroatory)
In addition, gamma-ray and neutron spectrometers, with similar detectors to those in MESSENGER GRNS, were used in the Mars Odyssey (2001) mission to map the distribution and concentration of many elements and water ice on the Mars surface.
Mars Odyssey data
representing gamma rays
from the element potassium, which is about twice as abundant on
Mars as it is on Earth.
(Image credit: NASA/JPL/University of Arizona)