by bystander » Mon May 02, 2011 4:33 pm
Measuring Mercury's Surface Composition
Science Highlights from Mercury Orbit | 2011 May 03
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
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).
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.
To Ngoc Van's Central Pit
NASA JHU-APL CIW | MESSENGER | 2011 May 02
The two largest craters in this image are Burns (43 km in diameter) and To Ngoc Van (71 km in diameter). To Ngoc Van contains an irregularly shaped pit, similar to those seen inside of the craters
Beckett,
Picasso, and
Gibran, among others.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features.
Beautiful Bartok
NASA JHU-APL CIW | MESSENGER | 2011 May 03
The complex crater Bartok, named for the Hungarian composer and pianist Bela Bartok, contains a prominent central peak. Chains of
secondary craters formed by Bartok's ejecta can be seen across the entire image.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features.
Turn, Turn, Turn
NASA JHU-APL CIW | MESSENGER | 2011 May 04
This dramatic view of Mercury's limb includes Beethoven basin, which can be seen towards the center of the image. Compare Beethoven's appearance and distance from the
terminator here to its appearance in
this image, taken ten days before, to get a sense of Mercury's rotation rate.
This image was acquired as part of MDIS's limb imaging campaign. Once per week, MDIS captures images of Mercury's limb, with an emphasis on imaging the southern hemisphere limb. These limb images provide information about Mercury's shape and complement measurements of topography made by the Mercury Laser Altimeter (MLA) of Mercury's northern hemisphere.
Abedin's Ejecta
NASA JHU-APL CIW | MESSENGER | 2011 May 05
The long shadows created near the terminator accentuate the topography of
Abedin's continuous ejecta blanket and secondary crater chains.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features.
Meet Joe Green
NASA JHU-APL CIW | MESSENGER | 2011 May 06
The crater Verdi was named in 1979 for the nineteenth century Italian composer Giuseppe Verdi. This crater, first imaged by Mariner 10, is an example of a complex crater, with a central peak structure and terraced walls.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
100 Orbits and Counting
MESSENGER Mission News | 2011 May 06
Later today, MESSENGER will begin its 100th orbit around Mercury. Since its insertion into orbit about the innermost planet on March 17, the spacecraft has executed nearly 2 million commands.
The data gathered so far include more than 70 million magnetic field measurements, 300,000 visible and infrared spectra of the surface, 16,000 images, and 12,000 X-ray and 9,000 gamma-ray spectra probing the elemental composition of Mercury’s uppermost crust.
“As the primary orbital phase of the MESSENGER mission unfolds, we are building up the first comprehensive view of the innermost planet,” states MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “The surface is unraveling before our eyes in great detail, and the planet’s topography and gravity and magnetic fields are being steadily filled in. As the Sun becomes increasingly active, Mercury’s extraordinarily dynamic exosphere and magnetosphere continue to display novel phenomena.”
MESSENGER continues its science-mapping phase in orbit around Mercury. All spacecraft systems remain safe and healthy, and all science instruments are on and continue to collect data according to the baseline observation plan.
“Over the next several weeks, MESSENGER’s subsystems and instruments will experience their hottest temperatures yet as the spacecraft crosses between the planet’s surface and our Sun at high noon close to the planet, preceded by hour-long eclipses near local midnight with only the spacecraft battery to keep the spacecraft alive in the dark of Mercury’s night,” notes MESSENGER Project Scientist Ralph McNutt.
“All of this was planned in great detail more than seven years ago, as was the orbit insertion burn that went so flawlessly,” he adds. “Theory is one thing and reality another, and the sense of thrill leading to ‘firsts’ is always followed by a sense of relief, especially in the challenging environment of interplanetary space, far from home.”
With less than one-sixth of its primary orbital mission completed, MESSENGER is already rewriting our books on what is known (and unknown) regarding the innermost planet, McNutt says. “By exploring our near — and far — neighbors in our solar system, we touch new knowledge, new understanding, and new wonderment about not only our own origins and place but of the other worlds circling the stars we see in our night sky.”
[url=http://messenger.jhuapl.edu/soc/hl_050311.html][size=120][b][i]Measuring Mercury's Surface Composition[/i][/b][/size][/url]
Science Highlights from Mercury Orbit | 2011 May 03
[quote]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.
[float=right][img3="[i][b]Figure 1.[/b] 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.[/i]"]http://messenger.jhuapl.edu/soc/highlights/composition01.jpg[/img3]
[img3="[i][b]Figure 2.[/b] 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.[/i]"]http://messenger.jhuapl.edu/soc/highlights/composition02.jpg[/img3][/float][b][i]Sources of Gamma Rays[/i][/b]
Planetary gamma rays can be grouped into two categories: gamma rays produced during radioactive decay and those produced by galactic cosmic-ray interactions [i]([b]Figure 1[/b])[/i]. 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.
[b][i]The MESSENGER Gamma-Ray Spectrometer[/i][/b]
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).
The MESSENGER GRS is now collecting gamma rays from Mercury's surface. As an example of data acquired by the GRS, [b][i]Figure 2[/i][/b] 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 [url]http://messenger.jhuapl.edu/instruments/GRNS.html[/url]. [/quote]
[url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=486][size=120][b][i]To Ngoc Van's Central Pit[/i][/b][/size][/url]
[i]NASA JHU-APL CIW | MESSENGER | 2011 May 02[/i]
[quote][float=left][url=http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EW0211938059G_web.png][img]http://messenger.jhuapl.edu/gallery/sciencePhotos/picsMed/EW0211938059G_web.png?1304353570[/img][c][i]Click on image to enlarge[/i][/c][/url][/float]
The two largest craters in this image are Burns (43 km in diameter) and To Ngoc Van (71 km in diameter). To Ngoc Van contains an irregularly shaped pit, similar to those seen inside of the craters [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=127]Beckett[/url], [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=382]Picasso[/url], and [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=317]Gibran[/url], among others.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features. [/quote]
[url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=487][size=120][b][i]Beautiful Bartok[/i][/b][/size][/url]
NASA JHU-APL CIW | MESSENGER | 2011 May 03
[quote][float=left][url=http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EN0212191757M_web.png][img]http://messenger.jhuapl.edu/gallery/sciencePhotos/picsMed/EN0212191757M_web.png?1304429042[/img][c][i]Click on image to enlarge[/i][/c][/url][/float]
The complex crater Bartok, named for the Hungarian composer and pianist Bela Bartok, contains a prominent central peak. Chains of [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=3&search_type=and&image_id=180][b]secondary craters[/b][/url] formed by Bartok's ejecta can be seen across the entire image.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features. [/quote]
[url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=488][size=120][b][i]Turn, Turn, Turn[/i][/b][/size][/url]
NASA JHU-APL CIW | MESSENGER | 2011 May 04
[quote][float=left][url=http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EW0212175990G_web.png][img]http://messenger.jhuapl.edu/gallery/sciencePhotos/picsMed/EW0212175990G_web.png?1304515898[/img][c][i]Click on image to enlarge[/i][/c][/url][/float]
This dramatic view of Mercury's limb includes Beethoven basin, which can be seen towards the center of the image. Compare Beethoven's appearance and distance from the [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=2&gallery_id=2&image_id=343][b]terminator[/b][/url] here to its appearance in [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1&gallery_id=2&image_id=482][b]this image[/b][/url], taken ten days before, to get a sense of Mercury's rotation rate.
This image was acquired as part of MDIS's limb imaging campaign. Once per week, MDIS captures images of Mercury's limb, with an emphasis on imaging the southern hemisphere limb. These limb images provide information about Mercury's shape and complement measurements of topography made by the Mercury Laser Altimeter (MLA) of Mercury's northern hemisphere. [/quote]
[url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=489][size=120][b][i]Abedin's Ejecta[/i][/b][/size][/url]
NASA JHU-APL CIW | MESSENGER | 2011 May 05
[quote][float=left][url=http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EW0212547521G.map.png][img]http://messenger.jhuapl.edu/gallery/sciencePhotos/picsMed/EW0212547521G.map.png?1304601654[/img][c][i]Click on image to enlarge[/i][/c][/url][/float]
The long shadows created near the terminator accentuate the topography of [url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1&gallery_id=2&image_id=471][b]Abedin's continuous ejecta blanket[/b][/url] and secondary crater chains.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features. [/quote]
[url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=490][size=120][b][i]Meet Joe Green[/i][/b][/size][/url]
NASA JHU-APL CIW | MESSENGER | 2011 May 06
[quote][float=left][url=http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EW0212807461G.map_web.png][img]http://messenger.jhuapl.edu/gallery/sciencePhotos/picsMed/EW0212807461G.map_web.png?1304689565[/img][c][i]Click on image to enlarge[/i][/c][/url][/float]
The crater Verdi was named in 1979 for the nineteenth century Italian composer Giuseppe Verdi. This crater, first imaged by Mariner 10, is an example of a complex crater, with a central peak structure and terraced walls.
This image was acquired as part of MDIS's high-resolution surface morphology base map. The surface morphology base map will cover more than 90% of Mercury's surface with an average resolution of 250 meters/pixel (0.16 miles/pixel or 820 feet/pixel). Images acquired for the surface morphology base map typically have off-vertical Sun angles (i.e., high incidence angles) and visible shadows so as to reveal clearly the topographic form of geologic features. [/quote]
[b][i]Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington[/i][/b]
[url=http://messenger.jhuapl.edu/news_room/details.php?id=170][size=120][b][i]100 Orbits and Counting[/i][/b][/size][/url]
MESSENGER Mission News | 2011 May 06
[quote]Later today, MESSENGER will begin its 100th orbit around Mercury. Since its insertion into orbit about the innermost planet on March 17, the spacecraft has executed nearly 2 million commands.
The data gathered so far include more than 70 million magnetic field measurements, 300,000 visible and infrared spectra of the surface, 16,000 images, and 12,000 X-ray and 9,000 gamma-ray spectra probing the elemental composition of Mercury’s uppermost crust.
“As the primary orbital phase of the MESSENGER mission unfolds, we are building up the first comprehensive view of the innermost planet,” states MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “The surface is unraveling before our eyes in great detail, and the planet’s topography and gravity and magnetic fields are being steadily filled in. As the Sun becomes increasingly active, Mercury’s extraordinarily dynamic exosphere and magnetosphere continue to display novel phenomena.”
MESSENGER continues its science-mapping phase in orbit around Mercury. All spacecraft systems remain safe and healthy, and all science instruments are on and continue to collect data according to the baseline observation plan.
“Over the next several weeks, MESSENGER’s subsystems and instruments will experience their hottest temperatures yet as the spacecraft crosses between the planet’s surface and our Sun at high noon close to the planet, preceded by hour-long eclipses near local midnight with only the spacecraft battery to keep the spacecraft alive in the dark of Mercury’s night,” notes MESSENGER Project Scientist Ralph McNutt.
“All of this was planned in great detail more than seven years ago, as was the orbit insertion burn that went so flawlessly,” he adds. “Theory is one thing and reality another, and the sense of thrill leading to ‘firsts’ is always followed by a sense of relief, especially in the challenging environment of interplanetary space, far from home.”
With less than one-sixth of its primary orbital mission completed, MESSENGER is already rewriting our books on what is known (and unknown) regarding the innermost planet, McNutt says. “By exploring our near — and far — neighbors in our solar system, we touch new knowledge, new understanding, and new wonderment about not only our own origins and place but of the other worlds circling the stars we see in our night sky.” [/quote]