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MESSENGER Teleconference: More 'Hidden' Territory on Mercury Revealed
10.29.08
 
Introduction

NASA's Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) is the first mission sent to orbit the planet closest to the sun. On Oct. 6, 2008, the probe flew by Mercury for the second time this year, using the planet’s gravity for a critical assist needed to keep the spacecraft on track for its orbit insertion around the planet three years from now. During this month's Mercury pass MESSENGER’s cameras captured more than 1,200 high-resolution and color images of the planet -- unveiling another 30 percent of Mercury’s surface that had never before been seen by spacecraft and gathering essential data for planning the overall mission.



Marilyn M. Lindstrom, MESSENGER Program Scientist
NASA Headquarters, Washington


Image 1.1
MESSENGER image of Mercury > Larger image

The MESSENGER spacecraft successfully completed its second flyby of Mercury on October 6 en route to becoming the first spacecraft to orbit the innermost planet starting in 2011. Precision solar sailing achieved the desired gravity-assist trajectory to within 0.6 km of the targeted 200-km altitude. All seven instruments operated as planned. They returned 650 MB of data and 1287 images and mapped another 30% of the surface never before viewed by spacecraft. This image was the first downloaded and released, on October 7; more than 30 images have since been released.



Brian J. Anderson, MESSENGER Deputy Project Scientist
The Johns Hopkins University Applied Physics Laboratory


Image 2.1
Trajectories of probes that observed Mercury > Larger image

This view of Mercury from above its north pole shows the trajectories along which Magnetometer observations were made by the Mariner 10 (blue) and MESSENGER (tan) spacecraft. The MESSENGER data from flyby 2 provide the only data to date from the planet’s western hemisphere and are therefore key to constraining the geometry of the planet’s internal magnetic field.

Image 2.2
Graph of magnetic field strength observed by MESSENGER > Larger image

The magnetic field strengths measured during MESSENGER’s first (blue) and second (orange) flybys show a striking similarity in the maximum field measured. The observations are displayed versus distance along the planet-Sun line; closest approach (CA) occurs at about three-fourths of a Mercury radius to the night side of the planet. The magnetopause and bow shock crossings occurred where they were expected, so for this comparison the distance scale for flyby 1 has been stretched so that these boundaries are coincident. Near CA, the flyby 2 data yield a field strength that is only a few percent lower than that obtained from flyby 1 observations. This remarkably close agreement means that the planetary magnetic moment is very nearly centered and is strongly aligned with the rotation axis, to within a tilt of 2 degrees. This result favors models for Mercury’s magnetic field generation that predict a magnetic moment aligned with the rotation axis.

Image 2.3
Graph of magnetic field strength observed by MESSENGER > Larger image

This figure shows the angle that the magnetic field made with the northward direction for the outbound passes through the magnetopause and bow shock for flyby 1 (blue) and flyby 2 (orange). Although the magnetic field strengths were comparable for the two flybys, the direction of the field outside the magnetosphere, imposed by the solar wind, was opposite, northward for flyby 1 and southward for flyby 2. The two encounters therefore present us with a nearly ideal “controlled” experiment to contrast Mercury’s magnetosphere under these two opposite extremes in its interaction with the solar wind.

Image 2.4
Illustration of Mercury magnetic field > Larger image

This figure illustrates the profound difference in magnetic connection between Mercury and the solar wind when the magnetic field in the solar wind is southward (left) as for flyby 2 versus northward (right) as for flyby 1. These views from the Sun show a notional cross section of the magnetic lines of force in the dawn-dusk meridian plane. For southward solar wind magnetic fields, the solar wind and planetary magnetic fields are connected over the poles and Mercury’s magnetosphere is tightly coupled and strongly driven by the solar wind. By contrast, for northward solar wind magnetic fields, the magnetosphere is “closed” and there is minimal inter-connection between the solar wind and planetary magnetic fields.

Image 2.5
Illustration of MESSENGER path and magnetism observations > View animation

This animation is a zoom-in of the flyby 2 outbound magnetopause crossing, showing nine seconds of magnetic field data at high resolution sampled 20 times per second. Data were obtained at the point shown by the spacecraft in the lower left as it crossed the magnetopause (white trace). The strong magnetic field directed inward at the magnetopause (18:49:14 UT) indicates that Mercury has the most intensely driven magnetosphere system ever observed in situ.



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


Image 3.1
Image showing MESSENGER spectrum analysis > Larger image

Example sodium and calcium emissions detected by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on the MESSENGER spacecraft during the second Mercury flyby. The histograms represent typical observations in the tail region of Mercury’s exosphere from calcium (left panel) and sodium (right panel) atoms. Known as “spectral lines,” these emissions have been scaled to approximately the same peak level for ease of comparison; however, the sodium emission is much brighter than that of calcium. Each emission occurs at a unique wavelength, with that of sodium in the “yellow” part of the visible spectrum and that of calcium in the “blue” part. The sodium emission is actually two very closely spaced emissions that are usually termed the D lines of sodium. The peaks of the two emissions are just separated (indicated by the D2 and D1 labels) in the figure. These are the same emissions that produce the yellow glow in sodium vapor lamps often used in street lighting. Although both sodium and calcium in Mercury’s exosphere have been observed with ground-based telescopes on Earth, this is the first time that measurements of the two species have been obtained simultaneously. Atoms in the exosphere heavier than hydrogen and helium predominantly originate from the surface of Mercury, and a number of processes contribute to their release from the surface material. Differences in the spatial and temporal distributions of the exospheric constituents therefore provide insight into the relative importance of the processes that generate and maintain Mercury’s exosphere.

Image 3.2
Image showing MESSENGER spectrum analysis > Larger image

Example magnesium emission detected by the MASCS instrument on the MESSENGER spacecraft during the second Mercury flyby. The histogram represents a typical observation in the tail region of Mercury’s exosphere from magnesium atoms. These MASCS measurements mark the first time that magnesium has been detected in Mercury’s exosphere. In this case the emission occurs at a wavelength that is in the ultraviolet part of the spectrum. Magnesium has not been observed from ground-based telescopes partly because it emits at ultraviolet wavelengths, which are completely obscured by the Earth’s atmosphere. Because these atoms primarily originate at the surface of Mercury, the detection of magnesium in the exosphere provides evidence that magnesium is a component of surface material, something that has been expected for years but until now had not been proven. As with calcium and sodium, the distribution of magnesium in Mercury’s exosphere is a result of the processes that release the magnesium atoms from the surface and can provide valuable clues to the relative importance of each process.

Image 3.3
Image showing MESSENGER spectrum analysis > View animation

This animation illustrates how MASCS observed Mercury's tail during the flyby. As MESSENGER flew toward Mercury on the inbound leg of its trajectory, the spacecraft rotated back and forth about the Sun-Mercury line. This nodding caused the MASCS instrument viewing direction (i.e., the direction the instrument is pointed) to move alternately north and south, building up an image in a whiskbroom fashion. At two points, the scanning sequence was briefly interrupted while the spacecraft reoriented to take an image; MASCS observations continued during these times, but the observation direction changed. After all the observations have been processed, they are then converted to an image of the emission intensity. In these images, the color scale indicates relative intensity as a function of spatial position.

Image 3.4
Image showing MESSENGER spectrum analysis > Larger image

Image of the spatial distribution of sodium emission in the tail region of Mercury, which extends away from the planet in the anti-sunward direction. In the figure, north is up and the Sun is to the left. The color scale represents the relative brightness of the emission in the tail. Because the observed emission intensity is related to the number of atoms along the line of sight, images such as this one are a measure of the density of the emitting species. The small-scale structures in these images may be artifacts of the viewing geometry and should not be given too much weight. More important are the broad-scale features that are composed of numerous observations and are therefore a better representation of the overall emission structure. The sodium emission shows two broad peaks that are located close to the planet to the north and south, and there is less emission near the equatorial region.

Image 3.5
Image showing MESSENGER spectrum analysis > Larger image

Image of the spatial distribution of calcium emission in the tail region of Mercury. In contrast to the sodium emission image in Image 3.4, the calcium emission is mostly symmetric about the equatorial region and less bright near polar regions. The spatial variations between the calcium and sodium distributions indicate that the processes controlling these two species are likely different.

Image 3.6
Image showing MESSENGER spectrum analysis > Larger image

Comparison of images of the sodium emission in the tail region of Mercury during MESSENGER’s first and second flybys. The sodium emission was less symmetric during flyby 1, with a larger region of emission in the north relative to the south. During the orbital phase of the mission, the MASCS instrument will regularly measure emissions from atoms and molecules. Mapping the distributions of species on a daily basis, in conjunction with the information provided by the other instruments on MESSENGER, will constrain the processes that generate and maintain the exosphere as well as provide information on the composition of the surface from which the exospheric species originate.



Maria T. Zuber, MESSENGER Co-Investigator
Massachusetts Institute of Technology


Image 4.1
MESSENGER altimeter measurements > Larger image

Image 4.2
MESSENGER altimeter measurements > Larger image

Image 4.3
MESSENGER altimeter measurements > Larger image

The above three figures show a ~1,600-kilometer-long section of the Mercury Laser Altimeter (MLA) profile from MESSENGER’s second Mercury flyby superimposed on an approach image mosaic acquired by the Mercury Dual Imaging System (MDIS) during MESSENGER’s first Mercury flyby. The blue dots indicate the spacecraft ground track, and the yellow dots show altimetry points. This hemisphere has about 70 percent of the range in topography sampled during flyby 1, and so this part of the equatorial hemisphere is smoother than that sampled last January. Near longitude -97 degrees there is a wrinkle ridge nearly 1 kilometer high that indicates horizontal shortening of the crust, possibly the result of global contraction associated with cooling of the interior (Image 4.2). In the longitude range -115 degrees to -120 degrees the instrument sampled several craters of different depths with tilted floors (-0.5 to -0.2 degrees) that may have been the result of deformational processes (Image 4.3).

Image 4.4
MESSENGER altimeter measurements > Larger image

Image 4.5
MESSENGER altimeter measurements > Larger image

Image 4.6
MESSENGER altimeter measurements > Larger image

These figures show a 400-kilometer-long section of the Mercury Laser Altimeter (MLA) profile from MESSENGER’s second Mercury flyby superposed on a high-resolution departure mosaic acquired by the Mercury Dual Imaging System (MDIS) during the same encounter. Near the center, the profile crosses two craters of comparable size but different depths. From these images it is apparent that the shallower crater has been filled, probably by volcanic material (Image 4.5). By making such measurements systematically over the surface, it will be possible to measure the volumes of volcanic material erupted over Mercury’s history (Image 4.6).



Mark S. Robinson, MESSENGER Co-Investigator
Arizona State University


Image 5.1
Mariner 10 image of Mercury > Larger image

Image 5.2
MESSENGER images of Mercury > Larger image

Image 5.3
MESSENGER images of Mercury > Larger image

Coverage maps of Mercury. Mariner 10 photographed approximately 45 percent of Mercury’s surface during three flybys in the mid-1970s (Figure 5.1). MESSENGER has now seen about 80 percent of Mercury from two encounters this year (14 January and 6 October, Figures 5.2 and 5.3). The narrow regions outlined in blue are the sunlit crescents seen as MESSENGER approached Mercury during each flyby. The larger areas outlined in orange are the sunlit portions of the surface seen as MESSENGER departed. The second flyby on October 6 filled in most of the areas that had never before been imaged by spacecraft. Between Mariner 10 and MESSENGER we have now mapped about 90 percent of Mercury at a resolution of 1 kilometer. Because of the fast encounter velocity and Mercury’s slow rotation, the lighting angle within the global mosaic varies from high noon to just over the horizon, resulting in a non-uniform look at the planet. After MESSENGER enters orbit about Mercury in 2011, a higher-resolution (250 m/pixel) global mosaic will be built up with more uniform illumination.

Image 5.4
Images of Mercury > Larger image

Mercury in color. The Mercury Dual Imaging System (MDIS) has 11 narrow-band spectral filters covering visible and near-infrared wavelengths (400 to 1050 nm). The specific colors of the filters were selected to discriminate among common minerals. Three-color images (480 nm, 560 nm, 630 nm) were combined to produce an approximation of Mercury’s true color as might be seen by the human eye (left). From this rendition of Mercury it is obvious that color differences on the surface are slight. Statistical methods that utilize all 11 filters in the visible and near-infrared highlight subtle color differences (right) and aid geologists in mapping regions of different composition. What do the exaggerated colors tell us about Mercury? The nature of color boundaries, color trends, and brightness values help MESSENGER geologists understand the discrete regions (or “units”) on the surface. From the color images alone it is not possible to determine unambiguously the minerals that comprise the rocks of each unit. During the brief flybys, MESSENGER’s other instruments sensitive to composition lack the time needed to build up adequate signal or gain broad areal coverage, so only MESSENGER’s camera is able to acquire comprehensive measurements. Once in obit about Mercury, MESSENGER’s full suite of instruments will be brought to bear on the newly discovered color units to unlock their secrets.

Image 5.5
Image of Mercury > Larger image

Color context image showing equatorial regions on the side of Mercury seen during MESSENGER’s departure from Mercury on October 6, 2008. Mercury’s true colors are very subtle, but statistical methods can take advantage of the ability of MDIS to take pictures in 11 narrow color bands spread across visible and near-infrared wavelengths (400 to 1050 nm). Such methods greatly enhance color differences in the materials at Mercury’s surface, providing insight into the compositional variations present on Mercury and the geologic processes that created those color differences. The bright yellow halo around Kuiper crater (60 km diameter; just below center) stands out as bright and colorful mostly because of its young age. When the crater was formed, material from beneath the surface was ejected outwards and formed the bright surrounding blanket. As time passes this young material will suffer micrometeorite impacts and the ravages of solar wind bombardment and will gradually become darker and redder. Several hundred million years from now Kuiper’s distinctive rays will fade into the background. The wispy blue linear features are rays from distant young impact craters. The bright orange material (left arrow) in the floor of Lermontov crater (160 km diameter) was most likely deposited by volcanic eruptions in Mercury’s distant past. A mysterious dark bluish material is seen in many places on Mercury, and new data from the second flyby reveal a small area with a high concentration (right arrow). Once MESSENGER goes into orbit about Mercury its full complement of instruments will examine these diverse locations and help unravel the processes that accompanied formation and later modification of Mercury’s crust. The width of this scene is 4000 km, and north is up. The white box indicates the boundaries of Image 5.6.

Image 5.6
Image of Mercury > Larger image

Highest-resolution color images obtained. After MESSENGER made its closest approach to Mercury, flying just 200 km (124 miles) above the surface, and as soon as the sunlit side of Mercury was fully in view, MDIS captured the highest-resolution color images ever obtained of Mercury (500 m/pixel). This area was also seen by Mariner 10, whose lower-resolution two-color images hinted at the variety and nature of regions of different colors, and hence composition, on Mercury. Viewed here at the full resolution and in enhanced color, the relationship between the relatively young smooth plains on the left and older, dark blue material on the right is clear. The younger smooth plains cover the lower parts of rougher pre-existing topography and infill older craters, like the 120-km diameter Rudaki crater lower-left of center. Dark, relatively blue material was ejected from the 105-km-diameter crater on the right side of the image, covering older smooth plains. A relatively young, small crater then excavated through this blue material to reveal the smooth plains beneath. The total width of the scene is 620 km, centered at 4 degrees South, 310 degrees East.

Image 5.7
Image of Mercury > View animation

A series of five 11-band images was captured by MESSENGER just after the spacecraft crossed the night/day line (the “terminator”). At the beginning of the movie, it is dawn in that region of Mercury and the Sun is just off the horizon. The long shadows cast by crater walls exaggerate the ruggedness of the terrain and highlight variations in topography. Though Mercury’s true colors are subtle, the 11 color bands of MDIS were combined in a statistical method used to highlight differences in color units. Older, low-reflectance, and relatively blue material is encroached by younger, relatively red smooth plains. Several lobate scarps or cliffs are observed, which are places where compressional stresses caused Mercury’s crust to fracture and shorten.

Image 5.8
Image of Mercury > View animation

As MESSENGER completed its second successful flyby of Mercury, the Narrow Angle Camera (NAC), part of MDIS, took images of the planet every 5 minutes as it receded in the distance. In total, 124 images were snapped during this time and were compiled sequentially to produce this movie. At the start of the movie, MESSENGER was about 93,000 kilometers (about 58,000 miles) from Mercury, and the first image has a field of view of about 2,500 kilometers (about 1,500 miles) in width. At the end of the movie, the MESSENGER spacecraft was about 288,000 kilometers (179,000 miles) from Mercury. (EN0131787738M through EN0131824960M).