MESSENGER reveals more 'hidden' territory on Mercury
Wed Oct 29, 2008 at 17:59 UTC
Gliding over the battered surface of Mercury for the second time this year, NASA's MESSENGER spacecraft has revealed even more previously unseen real estate on the innermost planet, sending home hundreds of photos and measurements of its surface, atmosphere, and magnetic field.
The probe flew by Mercury shortly after 00:40 UTC on October 6, 2008, completing a critical gravity assist to keep it on course to orbit Mercury in 2011 and unveiling 30 percent of Mercury's surface never before seen by spacecraft.
"The region of Mercury's surface that we viewed at close range for the first time this month is bigger than the land area of South America," says Sean Solomon, MESSENGER principal investigator and the director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. "When combined with data from our first flyby and from Mariner 10, our latest coverage means that we have now seen about 95% of the planet."
MESSENGER's science instruments worked feverishly through the flyby - cameras snapped more than 1,200 pictures of the surface, while topography beneath the spacecraft was profiled with the laser altimeter. "We have completed an initial reconnaissance of the solar system's innermost planet, enabling us to gain a global view of Mercury's geological history and internal magnetic field geometry for the first time," Solomon continues.
The comparison of magnetosphere observations from MESSENGER's first flyby in January with data from the probe's second pass has provided key new insight into the nature of the planet's internal magnetic field and revealed new features of Mercury's magnetosphere.
"The previous flybys by MESSENGER and Mariner 10 provided data only on Mercury's eastern hemisphere," explains Brian Anderson, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "The most recent flyby gave us our first measurements on Mercury's western hemisphere, and with them we discovered that the planet's magnetic field is highly symmetric."
"This seemingly simple result is significant for the planet's internal field because it implies that the dipole is even more closely aligned with the planet's rotation axis than we could conclude before the second flyby," says Anderson, who is deputy project scientist. "Even though the rigorous analyses of these data are ongoing, we expect that this result will allow us to limit the theories of planetary magnetic field generation to those that predict a strongly rotationally aligned moment."
The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) observed the extended tail, night side, and day side regions of Mercury's thin atmosphere - known as an exosphere - searching for emission from sodium, calcium, magnesium, and hydrogen atoms.
"The MASCS observations of magnesium are the first-ever detection of this species in Mercury's exosphere," explains MESSENGER participating scientist Ron Vervack of APL. Preliminary analysis of the sodium, calcium, and magnesium observations suggests that the spatial distributions of these three species are different and that the distribution of sodium during the second flyby is noticeably different from that observed during the first flyby.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
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.
Credit: NASA/Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
This figure shows about a 1,600 kilometer-long (1,000 mile-long) section of the MLA profile from MESSENGER's second Mercury flyby superimposed on a portion of the NAC approach mosaic from the mission's first Mercury encounter. The blue line indicates the spacecraft ground track, and the yellow dots show the altimetry data points; the blue arrow shows the spacecraft's direction of travel. This hemisphere has about 70% of the range in topography sampled by MLA during the first Mercury flyby and so this part of the equatorial hemisphere is smoother than that sampled last January. Near longitude -97° (263°E) there is a wrinkle ridge nearly 1 kilometer high (yellow arrow and white box containing a magnified view) that indicates horizontal shortening of the crust, possibly the result of global contraction associated with the cooling of the interior. In the longitude range of -115° to -120° (245°E to 240°E), the instrument sampled several craters of different depths with tilted floors (tilts of -0.5° to -0.2°; example indicated with a white arrow) that may have been the result of deformational processes.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
The top figure shows a view of Mercury from above its north pole and the trajectories along which Magnetometer observations were made by the Mariner 10 mission (blue) and the MESSENGER spacecraft (tan). The MESSENGER data from the mission's second Mercury flyby 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. The bottom figure graphs the magnetic field strengths measured during MESSENGER's first (blue) and second (orange) Mercury flybys, with a striking similarity in the maximum field strength measured during both encounters. The observations are displayed versus distance along the planet-Sun line; closest approach (CA) occurred 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°. This result favors models for Mercury's magnetic field generation that predict a magnetic moment aligned with the rotation axis.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Shown here are three imagery coverage maps of Mercury. Mariner 10 photographed approximately 45% of Mercury's surface during three flybys in the mid-1970s (top map). MESSENGER has now seen about 80% of Mercury from two encounters this year (14 January and 6 October, middle and bottom maps respectively). 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 Solar System's innermost planet. 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% of Mercury at a resolution of 1 kilometer or better. 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 meters/pixel) global mosaic will be built up with more uniform illumination.
Credit: NASA/University of Colorado/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
In the upper part of this figure, two histograms represent typical observations in the tail region of Mercury's exosphere from calcium (left) and sodium (right) 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.
The middle image of this figure shows the spatial distribution of sodium emission during MESSENGER's second flyby in the tail region of Mercury, which extends away from the planet in the anti-sunward direction. In the image, north is up and the Sun is to the left. The color scale represents the relative brightness of the sodium 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.
The bottom image shows the spatial distribution of calcium emission in the tail region of Mercury during the second flyby. In contrast to the sodium emission, 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.
"The spatial distributions of sodium, calcium, and magnesium are a reflection of the processes that release these species from Mercury's surface," Vervack adds. "Now that we were finally able to measure them simultaneously, we have an unprecedented window into the interaction of Mercury's surface and exosphere."
The probe's Mercury Laser Altimeter (MLA) measured the planet's topography, allowing scientists, for the first time, to correlate high-resolution topography measurements with high-resolution images.
"During the last flyby, the Mercury Laser Altimeter acquired a topographic profile in a hemisphere of the planet for which there were no spacecraft images," explains Maria Zuber, MESSENGER co-investigator and head of the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology. "During the second flyby, in contrast, altimetry was collected in regions where images from MESSENGER and Mariner 10 are available, and new images were obtained of the region sampled by the altimeter in January. These topographic measurements now improve considerably the ability to interpret surface geology."
Now that MESSENGER's cameras have imaged more than 80 percent of Mercury, it is clear that, unlike the Moon and Mars, the planet lacks hemispheric-scale geologic differences. "On the Moon, dark volcanic plains are concentrated on the near side and are nearly absent from the far side," says MESSENGER co-investigator Mark Robinson of Arizona State University. "On Mars, the southern hemisphere consists of older, cratered highlands, whereas the northern hemisphere consists of younger lowlands. Mercury's surface is more homogeneously ancient and heavily cratered, with large extents of younger volcanic plains lying within and between giant impact basins."
Color imaging also shows that Mercury's crust is compositionally heterogeneous. "Although definitive compositional interpretations cannot yet be made, the distribution of different components varies both across the surface and with depth - Mercury's crust is more analogous to a marbled cake than a layered cake," Robinson adds. "Once MESSENGER's suite of science instruments returns a host of data from the orbital phase of the mission, compositions will be determined for the newly discovered color units."
"The first two Mercury flybys have returned a rich dividend of new observations," says Solomon. "But some of the observations we are most eager to make - such as the chemical make-up of Mercury's surface and the nature of its enigmatic polar deposits - will not be possible until MESSENGER begins to orbit the innermost planet. Moreover, the very dynamic nature of Mercury's interaction with its interplanetary environment has taught us that continuous observations will be required before we can claim to understand our most sunward sister planet."
| Source: Johns Hopkins University Applied Physics Laboratory | |
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