[meteorite-list] MESSENGER: Measuring Mercury's Surface Composition

[Ron Baalke]

http://messenger.jhuapl.edu/soc/highlights.html

MESSENGER Science Highlights from Mercury's Orbit
Measuring Mercury's Surface Composition
May 3, 2011

MESSENGER carries a Gamma-Ray Spectrometer (GRS) that is capable of
measuring and characterizing gamma-ray emissions from the surface of
Mercury. Just like radio waves, visible light, and X-rays, gamma rays
are a form of electromagnetic radiation, but with higher energies than
those other types of radiation. Gamma rays coming from Mercury carry
information about the concentrations of elements present on its surface,
so observations from the MESSENGER GRS are being used to determine the
surface composition of the planet. These results will then be applied to
studying the formation and geologic history of Mercury.

Sources of Gamma Rays

Figure 1.Sources of gamma-ray emission from the surface of a planetary
body. These gamma rays can be grouped into two categories, those
resulting from natural radioactivity, and those resulting from
interactions between the surface and galactic cosmic rays. Reproduced
from Encyclopedia of the Solar System, 2nd Edition, Academic Press.

Planetary gamma rays can be grouped into two categories: gamma rays
produced during radioactive decay and those produced by galactic
cosmic-ray interactions (Figure 1). Naturally occurring radioactive
elements (e.g., thorium, uranium, potassium) can be found on the
surfaces of all of the terrestrial planets. These elements emit gamma
rays as part of their natural radioactive decay process.

Stable elements (e.g., iron, silicon, and oxygen) do not spontaneously
release gamma rays, so they are not normally detectable with a gamma-ray
spectrometer. However, because Mercury lacks a substantial atmosphere,
its surface is constantly bombarded by galactic cosmic rays. Cosmic rays
are primarily high-energy protons, and when they collide with the
surface they produce neutrons that subsequently excite elemental nuclei
through such processes as neutron scattering and neutron capture. The
normally stable nuclei, converted to unstable ?excited states,? emit
gamma rays to shed the extra energy they received from the neutrons as
they return to their stable forms. Each element emits gamma rays at
diagnostic energies during this process, enabling the MESSENGER GRS to
determine the surface abundances of such elements from a spectrum of
gamma-ray flux versus energy.

The MESSENGER Gamma-Ray Spectrometer

The MESSENGER GRS detects gamma rays having energies from 300 to 8000
keV with a cylindrical block of high-purity germanium (HPGe). When a
gamma ray enters the HPGe crystal, it ionizes germanium atoms, and a
measurement of the number of ionized electrons reveals the deposited
gamma-ray energy. The ionization signal in the HPGe crystal is very
small and can be overwhelmed by the thermal motion of germanium atoms.
To reduce this ?noise? associated with the thermal motion, the HPGe
crystal is cooled to cryogenic temperatures. Such cooling requires that
the MESSENGER GRS instrument include a mechanical cooler and heat
radiator in order to keep the crystal temperature near 90° Kelvin (below
-300° Fahrenheit).


Figure 2. Two example gamma-ray spectra acquired by the MESSENGER
Gamma-Ray Spectrometer, with gamma-ray count rates shown as a function
of energy (keV, or kilo-electron volt, is a unit of energy). To the left
is shown a gamma-ray spectrum collected while MESSENGER was far from the
planet; to the right is a spectrum obtained close to the surface (less
than 2000 km altitude). ?BG? denotes background gamma-ray peaks. Two
particular gamma rays, at 1460-keV resulting from potassium and at
1779-keV resulting from silicon, are highlighted, as they show clear
enhancements near the surface. These data demonstrate the presence of
potassium and silicon on Mercury's surface. Other unlabeled peaks in the
gamma-ray spectra in this energy range result from galactic cosmic-ray
interactions with the spacecraft and detector material.

The MESSENGER GRS is now collecting gamma rays from Mercury's surface.
As an example of data acquired by the GRS, Figure 2 compares gamma-ray
spectra taken far from Mercury (left) with spectra obtained close to
Mercury (right) in the energy range 1000 to 2000 keV. This energy range
includes examples of the two types of gamma rays discussed above.
Potassium gamma rays, at an energy of 1460 keV, are emitted during the
radioactive decay of potassium atoms. Silicon gamma rays, at an energy
of 1779 keV, result from neutron-inelastic-scattering reactions of
cosmic rays with silicon atoms. Both types of gamma rays show larger
intensities near Mercury and therefore indicate the detection of these
elements from Mercury's surface. Other elements within the detection
capability of GRS include iron, titanium, oxygen, thorium, and uranium.
Converting the measured gamma-ray intensities to elemental
concentrations requires a detailed analysis that accounts for factors
such as reaction probabilities, detection efficiencies, variable viewing
geometries, and background gamma rays. The MESSENGER Science Team is
carrying out these analyses determine the composition of Mercury's
surface materials and their implications for planetary formation and
evolution.

For more information on MESSENGER's Gamma-Ray Spectrometer (GRS), see
http://messenger.jhuapl.edu/instruments/GRNS.html