* Astronomy

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RE: MESSENGER

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This image was captured on the 29th September, 2009, by the Narrow Angle Camera (NAC) aboard the Messenger spaceprobe, when it was 15,100 kilometres away from Mercury.
The image shows the Rembrandt basin on Mercury.
The image scale is 390 metres/pixel

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This image was captured on the 29th September, 2009, by the Narrow Angle Camera (NAC) aboard the Messenger spaceprobe, when it was 14,600 kilometres away from Mercury.
The image shows the southern hemisphere of Mercury. This area of Mercury's surface had not been imaged at high-resolution prior to MESSENGER's third Mercury flyby.

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This image was captured on the 29th September, 2009, by the Narrow Angle Camera (NAC) aboard the Messenger spaceprobe, when it was 15,300 kilometres away from Mercury.
The image shows a 480-kilometer-long scarp cutting across the upper right corner. In the expanded view a crater can be seen to overlap the scarp.
The image scale is 400 meters/pixel

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MESSENGER begins revealing Mercury's secrets
Lately, the discoveries made about the solar system have been astronomical, especially concerning the smallest and inner-most planet, Mercury.
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging, or MESSENGER, spacecraft was designed by the Hopkins Applied Science Laboratory to collect data about Mercury's core structure, magnetic field, high density, geological history, atmosphere and poles, and ultimately transmit the information back to Earth.

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This image was captured on the 29th September, 2009, by the Wide Angle Camera (WAC) aboard the Messenger spaceprobe, when it was 16,300 kilometres away from Mercury.
The image shows Mercury in true and enhanced Colours.
The image scale is 5 kilometres/pixel

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MESSENGER Spacecraft Reveals More Hidden Territory on Mercury
A NASA spacecraft's third and final flyby of Mercury gives scientists, for the first time, an almost complete view of the planet's surface and provides new scientific findings about this relatively unknown world.
The Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft, known as MESSENGER, flew by Mercury on Sept. 29. The probe completed a critical gravity assist to remain on course to enter into orbit around Mercury in 2011. Despite shutting down temporarily because of a power system switchover during a solar eclipse, the spacecraft's cameras and instruments collected high-resolution and colour images unveiling another 6 percent of the planet's surface never before seen at close range.
Approximately 98 percent of Mercury's surface now has been imaged by NASA spacecraft. After MESSENGER goes into orbit around Mercury, it will see the polar regions, which are the only unobserved areas of the planet.

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Sean Solomon, MESSENGER Principal Investigator,
The Carnegie Institution of Washington, Washington, D.C.

Image 1.1

A MESSENGER colour observation of Mercury obtained as the spacecraft approached the planet for its third and final flyby on 29 September 2009. The 1000, 700, and 430 nm filters were combined in red, green, and blue to create this colour image (approximately 5 km/pixel resolution), the last that will be acquired until MESSENGER goes into orbit around Mercury in March of 2011. Only 6% of Mercury's surface in this image had not been viewed previously by spacecraft, and most of the measurements made by MESSENGER's other instruments during this flyby were made prior to closest approach. The observations nonetheless revealed fresh surprises.

Presenter #2

Ronald J. Vervack, Jr., MESSENGER Participating Scientist,
The Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

Image 2.1

Illustration of the major source and loss processes that generate and maintain Mercury's exosphere. The panels on the left summarize the three primary sources of exospheric material. Photon-stimulated desorption occurs when solar photons excite surface-bound atoms or molecules, releasing them to the exosphere. Sunlight also heats the surface, causing atoms and molecules to evaporate. These are both low-energy processes, so most of the released material reaches only low altitudes and usually returns to the surface. Ion sputtering occurs when ions from the solar wind or Mercurys magnetosphere impact the surface, "knocking off" atoms and molecules. Meteoroid vaporisation occurs when incoming meteoroids, generally small dust particles, impact Mercury's surface at high speeds, causing the surface material to vaporise. Both ion sputtering and meteoroid vaporisation are high-energy processes, and the released material can reach high altitudes. All material in the exosphere is accelerated in the anti-sunward direction by radiation pressure; atoms and molecules at sufficiently high altitudes for this force to overcome the gravitational influence of the planet enter Mercurys neutral tail. Neutral constituents in the tail either escape the Mercury system or are ionised by solar radiation. The ionised material can also escape along open magnetic field lines, but some of the ions are returned to the surface by Mercurys magnetosphere.

Image 2.2

Comparison of neutral sodium observed during MESSENGER's second and third Mercury flybys. The left panel shows that emission from neutral sodium in Mercury's tail, which extends away from the planet in the anti-sunward direction, was a factor of 10-20 less than during the second flyby. This difference is due to variations in the pressure that solar radiation exerts on the sodium as Mercury moves in its orbit. During the third flyby, the net effect of radiation pressure was small, and the sodium atoms released from Mercury's surface were not accelerated anti-sunward as they were during the first two flybys, resulting in a diminished sodium tail. These predictable changes lead to what are effectively "seasonal" effects on the distribution of exospheric species.

Image 2.3

Comparison of the neutral sodium observed during the second and third Mercury flybys to models. The top left and right panels show the same observations as does Image 2.2, but the colour scale for the third flyby has been stretched to show the distribution of sodium more clearly. As in previous flybys, the distinct north and south enhancements in the emission that result from material being sputtered from the surface at high latitudes on the dayside are seen. The lower two panels show Monte Carlo models of the sodium abundance in Mercury's exosphere for conditions similar to those during the two flybys. These models illustrate that the "disappearance" of Mercurys neutral sodium tail is consistent with the change in conditions. Observations of the sodium exosphere and tail throughout Mercury's orbit during MESSENGER's orbital mission phase will enable such "seasonal" effects to be studied. Refinement of models similar to these will lead to an improved understanding of the source and loss processes and their variations among Mercurys different exospheric "seasons."

Image 2.4

Observations of calcium and magnesium in Mercury's neutral tail during the third MESSENGER flyby. The distribution of neutral calcium in the tail appears to be centered near the equatorial plane and the emission rapidly decreases to the north and south as well as in the anti-sunward direction. In contrast, the distribution of magnesium in the tail exhibits several strong peaks in emission and a slower decrease in the north, south, and anti-sunward directions. These distributions are similar to those seen during the second flyby, but the densities were higher during the third flyby, a different "seasonal" variation than for sodium. Studying the changes of the seasons for a range of species during MESSENGER's orbital mission phase will be key to quantifying the processes that generate and maintain the exosphere and transport volatile material within the Mercury environment.

Image 2.5

First observations of emission from ionised calcium in Mercurys tail region compared with simultaneous observations of neutral calcium. Neutral calcium is rapidly converted to ionised calcium by sunlight, explaining the generally rapid decrease of neutral calcium away from the planet. The high degree of correlation between the two observed distributions reflects the rapid conversion of neutrals to ions and demonstrates that ionised calcium represents a significant fraction of the overall calcium abundance. Simultaneous measurement of the abundances of calcium neutrals and ions is therefore necessary to determine accurately the total calcium abundance in Mercurys exosphere. This situation is in contrast to that for sodium and magnesium, which are ionised much more slowly. The significantly longer lifetime for neutral magnesium may explain why its abundance is more widely distributed in the tail region than calcium (Image 2.4).

Presenter #3

David J. Lawrence, MESSENGER Participating Scientist,
The Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

Image 3.1

Schematic view of Mercurys interior showing its large, iron-rich core, which constitutes at least ~60% of the planets mass. Observations from Earth and by MESSENGER at visible and near-infrared wavelengths have shown that Mercurys surface has a very low concentration of iron (Fe) in silicate minerals, leading to the common view that Mercurys surface and crust are generally low in iron. A puzzle for investigations of Mercurys formation and evolution is how a planet with such a large Fe-rich core could form with such an Fe-poor surface?

Image 3.2

Areas on Mercury's surface measured with the MESSENGER Neutron Spectrometer (NS) during the first and third Mercury flybys (M1 and M3, respectively) are shown as circles. The spacecraft ground tracks for M1 and M3 are indicated by the black and blue lines, respectively. A mosaic of Mercurys surface in cylindrical projection is shown as background. The inset is a schematic illustration of how thermal neutrons are used to probe the iron (Fe) and titanium (Ti) content of Mercury's surface. Fe and Ti capture thermal neutrons very efficiently, so low fluxes of thermal neutrons indicate high abundances of these elements.

Image 3.3

Modelled and measured neutron counting rates for M1. The solid lines show predicted neutron counting rates for three different composition models: low Fe and Ti (blue), high Fe and Ti (red), and highest Fe and Ti (green). The low Fe and Ti model is similar in composition to the lunar highlands. The high and highest Fe and Ti models are similar in composition to lunar basalts from Mare Fecunditatis (Luna 16) and Mare Tranquillitatis (Apollo 11), respectively. A spacecraft manoeuvre was executed at 19:00 UTC that enabled the NS to measure an enhanced signal of thermal neutrons. The NS data (black circles) show that Mercurys surface fits the model with high Fe and Ti abundances, in contrast to previous inferences by many that Mercurys surface is low in Fe and Ti.

Image 3.4

Modelled and measured neutron counts for M3. While the data stop prior to 21:50 UTC because of the spacecraft safeing event that shut off all data collection, enough NS data were returned to again show that Mercurys surface fits the model with high abundances of Fe and Ti. These results from both M1 and M3 demonstrate that Mercurys surface has a significantly higher Fe+Ti content than had commonly been appreciated. Models for Mercury's formation and crustal evolution must be revised to take this finding into account.

Presenter #4

Brett Denevi, MESSENGER Imaging Team member and Postdoctoral Researcher,
Arizona State University, Tempe, Ariz.

Image 4.1

Combined image coverage map of Mercury after Mariner 10 and MESSENGER's first two flybys of Mercury. Although 90% of Mercury's surface had been imaged after MESSENGER's second flyby, there was a gap in longitudinal coverage centered at about 60° E.

Image 4.2

Image coverage map of Mercury after the third MESSENGER flyby. The approach trajectory of the third flyby allowed the longitudinal gap to be seen as a part of a mosaic created from 58 images. The nearly complete (98%) coverage now leaves unimaged only portions of the polar regions before MESSENGER is placed into orbit about Mercury in March 2011. Despite the now-complete equatorial coverage, it is worth noting that illumination conditions were far from uniform during the various flybys, and images of many areas did not favor observations of surface texture and topography. Imaging of these areas from orbit under more optimum lighting conditions and higher resolution will improve our understanding of the evolution of Mercury's surface.

Image 4.3

Selected features revealed during MESSENGER's third flyby. This enhanced-colour view was created with a statistical technique that highlights subtle colour variations seen in the 11 filters of MESSENGER's wide-angle camera that are often related to composition. Merged with images from the higher-resolution narrow-angle camera, the two sets of observations tell the story of the geology of the area and the compositional differences of the features observed. This region, viewed in detail for the first time during the third flyby, appears to have experienced a high level of volcanic activity. The bright yellow area near the top right is centred on a rimless depression (Image 4.4) that is a candidate site for an explosive volcanic vent. The 290-km-diameter double-ring basin in the center of the image has a smooth interior (Image 4.5) that may be the result of effusive volcanism. Smooth plains, thought to be a result of earlier episodes of volcanic activity, cover much of the surrounding area. Image resolution is 1 km/pixel.

Image 4.4

Detailed view of the irregular depression in Image 4.3. This region of high reflectance was just barely seen on the limb during MESSENGER's second flyby, but without enough detail to characterize it as anything other than a bright spot. A more favourable viewing angle reveals this bright spot to be an irregular rimless depression approximately 30 km across surrounded by highly reflective material. Its features are distinctly different from those of impact craters and, though its origin remains ambiguous, it is suspected to be volcanic. The high-reflectance halo surrounding this enigmatic feature is distinct in colour (see Image 4.3) and may represent a pyroclastic deposit greater than 150 km in diameter.

Image 4.5

Detailed view of the interior of the double-ring basin in Image 4.3. This spectacular 290-km-diameter double-ring basin seen in detail for the first time during MESSENGER's third flyby of Mercury bears a striking resemblance to the Raditladi basin, observed during the first flyby. This still-unnamed basin is remarkably well preserved and appears to have formed relatively recently, compared with most basins on Mercury. The low numbers of superposed impact craters and marked differences in colour across the basin suggest that the smooth area within the innermost ring may be the site of some of the most recent volcanism on Mercury. MESSENGER's final flyby of Mercury brings to the fore the importance of the orbital phase of the mission that begins 18 March 2011!

Source NASA

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NASA is hosting a media teleconference just now, to announce scientific findings and release new images from the third and final flyby of Mercury by the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft, known as MESSENGER.
MESSENGER successfully flew by Mercury on September 29, 2009, gaining a critical gravity assist that will enable it to enter orbit about Mercury in 2011.

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MESSENGER Mercury Flyby