Orbital observations of Mercury reveal flood lavas, hollows, and unprecedented surface details

ScienceDaily (Sep. 29, 2011) — After only six months in orbit around Mercury, NASA's MESSENGER spacecraft is sending back information that has revolutionized the way scientists think about the innermost planet. Analyses of new data from the spacecraft show, among other things, new evidence that flood volcanism has been widespread on Mercury, the first close-up views of Mercury's "hollows," the first direct measurements of the chemical composition of Mercury's surface, and the first global inventory of plasma ions within Mercury's space environment.

The results are reported in a set of seven papers published in a special section of Science magazine on Sept. 30, 2011 (available online at: http://www.sciencemag.org/site/feature/data/hottopics/messenger/)

"MESSENGER's instruments are capturing data that can be obtained only from orbit," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "We have imaged many areas of the surface at unprecedented resolution, we have viewed the polar regions clearly for the first time, we have built up global coverage with our images and other data sets, we are mapping the elemental composition of Mercury's surface, we are conducting a continuous inventory of the planet's neutral and ionized exosphere, and we are sorting out the geometry of Mercury's magnetic field and magnetosphere. And we've only just begun. Mercury has many more surprises in store for us as our mission progresses."

MESSENGER Reveals Flood Volcanism

For decades scientists had puzzled over whether Mercury had volcanic deposits on its surface. MESSENGER's three flybys answered that question in the affirmative, but the global distribution of volcanic materials was not well constrained. New data from orbit show a huge expanse of volcanic plains surrounding the north polar region of Mercury. These continuous smooth plains cover more than 6% of the total surface of Mercury.

The volcanic deposits are thick. "Analysis of the size of buried 'ghost' craters in these deposits shows that the lavas are locally as thick as 2 kilometers" (or 1.2 miles), explains James Head of Brown University, the lead author of one of the Science reports. "If you imagine standing at the base of the Washington Monument, the top of the lavas would be something like 12 Washington Monuments above you."

According to Head, the deposits appear typical of flood lavas, huge volumes of solidified molten rock similar to those found in the few-million-year-old Columbia River Basalt Group, which at one point covered 150,000 square kilometers (60,000 square miles) in the northwest United States. "Those on Mercury appear to have poured out from long, linear vents and covered the surrounding areas, flooding them to great depths and burying their source vents," Head says.

Scientists have also discovered vents, measuring up to 25 kilometers (16 miles) in length, that appear to be the source of some of the tremendous volumes of very hot lava that have rushed out over the surface of Mercury and eroded the substrate, carving valleys and creating teardrop-shaped ridges in the underlying terrain. "These amazing landforms and deposits may be related to the types of unusual compositions, similar to terrestrial rocks called komatiites, being seen by other instruments and reported in this same issue of Science," Head says. "What's more, such lavas may have been typical of an early period in Earth's history, one for which only spotty evidence remains today."

As MESSENGER continues to orbit Mercury, the imaging team is building up a global catalog of these volcanic deposits and is working with other instrument teams to construct a comprehensive view of the history of volcanism on Mercury.

Hollows on Mercury

Images collected by MESSENGER have revealed an unexpected class of landform on Mercury and suggest that a previously unrecognized geological process is responsible for its formation. Images collected during the Mariner 10 and MESSENGER flybys of Mercury showed that the floors and central mountain peaks of some impact craters are very bright and have a blue color relative to other areas of Mercury. These deposits were considered to be unusual because no craters with similar characteristics are found on the Moon. But without higher-resolution images, the bright crater deposits remained a curiosity.

Now MESSENGER's orbital mission has provided close-up, targeted views of many of these craters.

"To the surprise of the science team, it turns out that the bright areas are composed of small, shallow, irregularly shaped depressions that are often found in clusters," says David Blewett, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of the Science reports. "The science team adopted the term 'hollows' for these features to distinguish them from other types of pits seen on Mercury."

Hollows have been found over a wide range of latitudes and longitudes, suggesting that they are fairly common across Mercury. Many of the depressions have bright interiors and halos, and Blewett says the ones detected so far have a fresh appearance and have not accumulated small impact craters, indicating that they are relatively young.

"Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today," Blewett says. "The old conventional wisdom was that 'Mercury is just like the Moon.' But from its vantage point in orbit, MESSENGER is showing us that Mercury is radically different from the Moon in just about every way we can measure."

Mercury's Surface and Exospheric Composition, Up Close and Personal

Scientists are collecting data about the chemical composition of Mercury's surface that could not have been obtained without the sustained observing perspective that MESSENGER's orbit provides, and that information is being used to test models of Mercury's formation and shed light on the dynamics of the planet's exosphere.

Measurements of Mercury's surface by MESSENGER's Gamma-Ray Spectrometer (GRS) reveal a higher abundance of the radioactive element potassium, a moderately volatile element that vaporizes at a relatively low temperature, than previously predicted. Together with MESSENGER's X-Ray Spectrometer (XRS), it also shows that Mercury has an average surface composition different from those of the Moon and other terrestrial planets.

"Measurements of the ratio of potassium to thorium, another radioactive element, along with the abundance of sulfur detected by XRS, indicate that Mercury has a volatile inventory similar to Venus, Earth, and Mars, and much larger than that of the Moon," says APL Staff Scientist Patrick Peplowski, lead author of one of the Science papers.

These new data rule out most existing models for Mercury's formation that had been developed to explain the unusually high density of the innermost planet, which has a much higher mass fraction of iron metal than Venus, Earth, or Mars, Peplowski pointed out. Overall, Mercury's surface composition is similar to that expected if the planet's bulk composition is broadly similar to that of highly reduced or metal-rich chondritic meteorites (material that is left over from the formation of the solar system).

MESSENGER has also collected the first global observations of plasma ions in Mercury's magnetosphere. Over 65 days covering more than 120 orbits, MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) made the first long-term measurements of Mercury's ionized exosphere.

The team found that sodium is the most important ion contributed by the planet. "We had previously observed neutral sodium from ground observations, but up close we've discovered that charged sodium particles are concentrated near Mercury's polar regions where they are likely liberated by solar wind ion sputtering, effectively knocking sodium atoms off Mercury's surface," notes the University of Michigan's Thomas Zurbuchen, author of one of the Science reports. "We were able to observe the formation process of these ions, one that is comparable to the manner by which auroras are generated in the Earth atmosphere near polar regions."

The FIPS sensor detected helium ions throughout the entire volume of Mercury's magnetosphere. "Helium must be generated through surface interactions with the solar wind," says Zurbuchen. "We surmise that the helium was delivered from the Sun by the solar wind, implanted on the surface of Mercury, and then fanned out in all directions.

"Our results tell us that Mercury's weak magnetosphere provides the planet very little protection from the solar wind," he continued. "Extreme space weather must be a continuing activity at the surface of the planet closest to the Sun."

"These revelations emphasize that Mercury is a fascinating world that is unmatched in the solar system," says Blewett. "We have barely begun to understand what Mercury is really like and are eager to discover what Mercury can tell us about the processes that led to formation of the planets as we see them today."

For more supporting materials, visit: http://messenger.jhuapl.edu/news_room/telecon7.html

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by NASA.

Journal References:

L. R. Nittler, R. D. Starr, S. Z. Weider, T. J. McCoy, W. V. Boynton, D. S. Ebel, C. M. Ernst, L. G. Evans, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt, C. E. Schlemm, S. C. Solomon, A. L. Sprague. The Major-Element Composition of Mercury's Surface from MESSENGER X-ray Spectrometry. Science, 2011; 333 (6051): 1847 DOI: 10.1126/science.1211567P. N. Peplowski, L. G. Evans, S. A. Hauck, T. J. McCoy, W. V. Boynton, J. J. Gillis-Davis, D. S. Ebel, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt, L. R. Nittler, S. C. Solomon, E. A. Rhodes, A. L. Sprague, R. D. Starr, K. R. Stockstill-Cahill. Radioactive Elements on Mercury's Surface from MESSENGER: Implications for the Planet's Formation and Evolution. Science, 2011; 333 (6051): 1850 DOI: 10.1126/science.1211576J. W. Head, C. R. Chapman, R. G. Strom, C. I. Fassett, B. W. Denevi, D. T. Blewett, C. M. Ernst, T. R. Watters, S. C. Solomon, S. L. Murchie, L. M. Prockter, N. L. Chabot, J. J. Gillis-Davis, J. L. Whitten, T. A. Goudge, D. M. H. Baker, D. M. Hurwitz, L. R. Ostrach, Z. Xiao, W. J. Merline, L. Kerber, J. L. Dickson, J. Oberst, P. K. Byrne, C. Klimczak, L. R. Nittler. Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER. Science, 2011; 333 (6051): 1853 DOI: 10.1126/science.1211997D. T. Blewett, N. L. Chabot, B. W. Denevi, C. M. Ernst, J. W. Head, N. R. Izenberg, S. L. Murchie, S. C. Solomon, L. R. Nittler, T. J. McCoy, Z. Xiao, D. M. H. Baker, C. I. Fassett, S. E. Braden, J. Oberst, F. Scholten, F. Preusker, D. M. Hurwitz. Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity. Science, 2011; 333 (6051): 1856 DOI: 10.1126/science.1211681B. J. Anderson, C. L. Johnson, H. Korth, M. E. Purucker, R. M. Winslow, J. A. Slavin, S. C. Solomon, R. L. McNutt, J. M. Raines, T. H. Zurbuchen. The Global Magnetic Field of Mercury from MESSENGER Orbital Observations. Science, 2011; 333 (6051): 1859 DOI: 10.1126/science.1211001T. H. Zurbuchen, J. M. Raines, J. A. Slavin, D. J. Gershman, J. A. Gilbert, G. Gloeckler, B. J. Anderson, D. N. Baker, H. Korth, S. M. Krimigis, M. Sarantos, D. Schriver, R. L. McNutt, S. C. Solomon. MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury. Science, 2011; 333 (6051): 1862 DOI: 10.1126/science.1211302G. C. Ho, S. M. Krimigis, R. E. Gold, D. N. Baker, J. A. Slavin, B. J. Anderson, H. Korth, R. D. Starr, D. J. Lawrence, R. L. McNutt, S. C. Solomon. MESSENGER Observations of Transient Bursts of Energetic Electrons in Mercury's Magnetosphere. Science, 2011; 333 (6051): 1865 DOI: 10.1126/science.1211141

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Orbital observations of Mercury reveal flood lavas, hollows, and unprecedented surface details

ScienceDaily (Sep. 29, 2011) — After only six months in orbit around Mercury, NASA's MESSENGER spacecraft is sending back information that has revolutionized the way scientists think about the innermost planet. Analyses of new data from the spacecraft show, among other things, new evidence that flood volcanism has been widespread on Mercury, the first close-up views of Mercury's "hollows," the first direct measurements of the chemical composition of Mercury's surface, and the first global inventory of plasma ions within Mercury's space environment.

The results are reported in a set of seven papers published in a special section of Science magazine on Sept. 30, 2011 (available online at: http://www.sciencemag.org/site/feature/data/hottopics/messenger/)

"MESSENGER's instruments are capturing data that can be obtained only from orbit," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "We have imaged many areas of the surface at unprecedented resolution, we have viewed the polar regions clearly for the first time, we have built up global coverage with our images and other data sets, we are mapping the elemental composition of Mercury's surface, we are conducting a continuous inventory of the planet's neutral and ionized exosphere, and we are sorting out the geometry of Mercury's magnetic field and magnetosphere. And we've only just begun. Mercury has many more surprises in store for us as our mission progresses."

MESSENGER Reveals Flood Volcanism

For decades scientists had puzzled over whether Mercury had volcanic deposits on its surface. MESSENGER's three flybys answered that question in the affirmative, but the global distribution of volcanic materials was not well constrained. New data from orbit show a huge expanse of volcanic plains surrounding the north polar region of Mercury. These continuous smooth plains cover more than 6% of the total surface of Mercury.

The volcanic deposits are thick. "Analysis of the size of buried 'ghost' craters in these deposits shows that the lavas are locally as thick as 2 kilometers" (or 1.2 miles), explains James Head of Brown University, the lead author of one of the Science reports. "If you imagine standing at the base of the Washington Monument, the top of the lavas would be something like 12 Washington Monuments above you."

According to Head, the deposits appear typical of flood lavas, huge volumes of solidified molten rock similar to those found in the few-million-year-old Columbia River Basalt Group, which at one point covered 150,000 square kilometers (60,000 square miles) in the northwest United States. "Those on Mercury appear to have poured out from long, linear vents and covered the surrounding areas, flooding them to great depths and burying their source vents," Head says.

Scientists have also discovered vents, measuring up to 25 kilometers (16 miles) in length, that appear to be the source of some of the tremendous volumes of very hot lava that have rushed out over the surface of Mercury and eroded the substrate, carving valleys and creating teardrop-shaped ridges in the underlying terrain. "These amazing landforms and deposits may be related to the types of unusual compositions, similar to terrestrial rocks called komatiites, being seen by other instruments and reported in this same issue of Science," Head says. "What's more, such lavas may have been typical of an early period in Earth's history, one for which only spotty evidence remains today."

As MESSENGER continues to orbit Mercury, the imaging team is building up a global catalog of these volcanic deposits and is working with other instrument teams to construct a comprehensive view of the history of volcanism on Mercury.

Hollows on Mercury

Images collected by MESSENGER have revealed an unexpected class of landform on Mercury and suggest that a previously unrecognized geological process is responsible for its formation. Images collected during the Mariner 10 and MESSENGER flybys of Mercury showed that the floors and central mountain peaks of some impact craters are very bright and have a blue color relative to other areas of Mercury. These deposits were considered to be unusual because no craters with similar characteristics are found on the Moon. But without higher-resolution images, the bright crater deposits remained a curiosity.

Now MESSENGER's orbital mission has provided close-up, targeted views of many of these craters.

"To the surprise of the science team, it turns out that the bright areas are composed of small, shallow, irregularly shaped depressions that are often found in clusters," says David Blewett, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of the Science reports. "The science team adopted the term 'hollows' for these features to distinguish them from other types of pits seen on Mercury."

Hollows have been found over a wide range of latitudes and longitudes, suggesting that they are fairly common across Mercury. Many of the depressions have bright interiors and halos, and Blewett says the ones detected so far have a fresh appearance and have not accumulated small impact craters, indicating that they are relatively young.

"Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today," Blewett says. "The old conventional wisdom was that 'Mercury is just like the Moon.' But from its vantage point in orbit, MESSENGER is showing us that Mercury is radically different from the Moon in just about every way we can measure."

Mercury's Surface and Exospheric Composition, Up Close and Personal

Scientists are collecting data about the chemical composition of Mercury's surface that could not have been obtained without the sustained observing perspective that MESSENGER's orbit provides, and that information is being used to test models of Mercury's formation and shed light on the dynamics of the planet's exosphere.

Measurements of Mercury's surface by MESSENGER's Gamma-Ray Spectrometer (GRS) reveal a higher abundance of the radioactive element potassium, a moderately volatile element that vaporizes at a relatively low temperature, than previously predicted. Together with MESSENGER's X-Ray Spectrometer (XRS), it also shows that Mercury has an average surface composition different from those of the Moon and other terrestrial planets.

"Measurements of the ratio of potassium to thorium, another radioactive element, along with the abundance of sulfur detected by XRS, indicate that Mercury has a volatile inventory similar to Venus, Earth, and Mars, and much larger than that of the Moon," says APL Staff Scientist Patrick Peplowski, lead author of one of the Science papers.

These new data rule out most existing models for Mercury's formation that had been developed to explain the unusually high density of the innermost planet, which has a much higher mass fraction of iron metal than Venus, Earth, or Mars, Peplowski pointed out. Overall, Mercury's surface composition is similar to that expected if the planet's bulk composition is broadly similar to that of highly reduced or metal-rich chondritic meteorites (material that is left over from the formation of the solar system).

MESSENGER has also collected the first global observations of plasma ions in Mercury's magnetosphere. Over 65 days covering more than 120 orbits, MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) made the first long-term measurements of Mercury's ionized exosphere.

The team found that sodium is the most important ion contributed by the planet. "We had previously observed neutral sodium from ground observations, but up close we've discovered that charged sodium particles are concentrated near Mercury's polar regions where they are likely liberated by solar wind ion sputtering, effectively knocking sodium atoms off Mercury's surface," notes the University of Michigan's Thomas Zurbuchen, author of one of the Science reports. "We were able to observe the formation process of these ions, one that is comparable to the manner by which auroras are generated in the Earth atmosphere near polar regions."

The FIPS sensor detected helium ions throughout the entire volume of Mercury's magnetosphere. "Helium must be generated through surface interactions with the solar wind," says Zurbuchen. "We surmise that the helium was delivered from the Sun by the solar wind, implanted on the surface of Mercury, and then fanned out in all directions.

"Our results tell us that Mercury's weak magnetosphere provides the planet very little protection from the solar wind," he continued. "Extreme space weather must be a continuing activity at the surface of the planet closest to the Sun."

"These revelations emphasize that Mercury is a fascinating world that is unmatched in the solar system," says Blewett. "We have barely begun to understand what Mercury is really like and are eager to discover what Mercury can tell us about the processes that led to formation of the planets as we see them today."

For more supporting materials, visit: http://messenger.jhuapl.edu/news_room/telecon7.html

Recommend this story on Facebook, Twitter,
and Google +1:

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by NASA.

Journal References:

L. R. Nittler, R. D. Starr, S. Z. Weider, T. J. McCoy, W. V. Boynton, D. S. Ebel, C. M. Ernst, L. G. Evans, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt, C. E. Schlemm, S. C. Solomon, A. L. Sprague. The Major-Element Composition of Mercury's Surface from MESSENGER X-ray Spectrometry. Science, 2011; 333 (6051): 1847 DOI: 10.1126/science.1211567P. N. Peplowski, L. G. Evans, S. A. Hauck, T. J. McCoy, W. V. Boynton, J. J. Gillis-Davis, D. S. Ebel, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt, L. R. Nittler, S. C. Solomon, E. A. Rhodes, A. L. Sprague, R. D. Starr, K. R. Stockstill-Cahill. Radioactive Elements on Mercury's Surface from MESSENGER: Implications for the Planet's Formation and Evolution. Science, 2011; 333 (6051): 1850 DOI: 10.1126/science.1211576J. W. Head, C. R. Chapman, R. G. Strom, C. I. Fassett, B. W. Denevi, D. T. Blewett, C. M. Ernst, T. R. Watters, S. C. Solomon, S. L. Murchie, L. M. Prockter, N. L. Chabot, J. J. Gillis-Davis, J. L. Whitten, T. A. Goudge, D. M. H. Baker, D. M. Hurwitz, L. R. Ostrach, Z. Xiao, W. J. Merline, L. Kerber, J. L. Dickson, J. Oberst, P. K. Byrne, C. Klimczak, L. R. Nittler. Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER. Science, 2011; 333 (6051): 1853 DOI: 10.1126/science.1211997D. T. Blewett, N. L. Chabot, B. W. Denevi, C. M. Ernst, J. W. Head, N. R. Izenberg, S. L. Murchie, S. C. Solomon, L. R. Nittler, T. J. McCoy, Z. Xiao, D. M. H. Baker, C. I. Fassett, S. E. Braden, J. Oberst, F. Scholten, F. Preusker, D. M. Hurwitz. Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity. Science, 2011; 333 (6051): 1856 DOI: 10.1126/science.1211681B. J. Anderson, C. L. Johnson, H. Korth, M. E. Purucker, R. M. Winslow, J. A. Slavin, S. C. Solomon, R. L. McNutt, J. M. Raines, T. H. Zurbuchen. The Global Magnetic Field of Mercury from MESSENGER Orbital Observations. Science, 2011; 333 (6051): 1859 DOI: 10.1126/science.1211001T. H. Zurbuchen, J. M. Raines, J. A. Slavin, D. J. Gershman, J. A. Gilbert, G. Gloeckler, B. J. Anderson, D. N. Baker, H. Korth, S. M. Krimigis, M. Sarantos, D. Schriver, R. L. McNutt, S. C. Solomon. MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury. Science, 2011; 333 (6051): 1862 DOI: 10.1126/science.1211302G. C. Ho, S. M. Krimigis, R. E. Gold, D. N. Baker, J. A. Slavin, B. J. Anderson, H. Korth, R. D. Starr, D. J. Lawrence, R. L. McNutt, S. C. Solomon. MESSENGER Observations of Transient Bursts of Energetic Electrons in Mercury's Magnetosphere. Science, 2011; 333 (6051): 1865 DOI: 10.1126/science.1211141

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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Mercury: Messenger orbital data confirm theories, reveal surprises

ScienceDaily (June 17, 2011) — On March 18, 2011, the MESSENGER spacecraft entered orbit around Mercury to become that planet's first orbiter. The spacecraft's instruments are making a complete reconnaissance of the planet's geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment. MESSENGER is providing a wealth of new information and some surprises. For instance, Mercury's surface composition differs from that expected for the innermost of the terrestrial planets, and Mercury's magnetic field has a north-south asymmetry that affects interaction of the planet's surface with charged particles from the solar wind.

Tens of thousands of images reveal major features on the planet in high resolution for the first time. Measurements of the chemical composition of the planet's surface are providing important clues to the origin of the planet and its geological history. Maps of the planet's topography and magnetic field are offering new evidence on Mercury's interior dynamical processes. And scientists now know that bursts of energetic particles in Mercury's magnetosphere are a continuing product of the interaction of Mercury's magnetic field with the solar wind.

"MESSENGER has passed a number of milestones just this week," offers MESSENGER principal investigator Sean Solomon of the Carnegie Institution. "We completed our first perihelion passage from orbit on Sunday, our first Mercury year in orbit on Monday, our first superior solar conjunction from orbit on Tuesday, and our first orbit-correction maneuver on Wednesday. Those milestones provide important context to the continuing feast of new observations that MESSENGER has been sending home on nearly a daily basis."

The Surface in Detail

Images obtained with MESSENGER's Mercury Dual Imaging System (MDIS) are being combined into maps for the first global look at the planet under optimal viewing conditions. New images of areas near Mercury's north pole orbital show that region hosts one of the largest expanses of volcanic plains deposits on the planet, with thicknesses of up to several kilometers. The broad expanses of plains confirm that volcanism shaped much of Mercury's crust and continued through much of Mercury's history, despite an overall contractional stress state that tended to inhibit the extrusion of volcanic material onto the surface.

Among the fascinating features seen in flyby images of Mercury were bright, patchy deposits on some crater floors, but they remained a curiosity. New targeted MDIS observations reveal these patchy deposits to be clusters of rimless, irregular pits with horizontal dimension from hundreds of meters to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters.

"The etched appearance of these landforms is unlike anything we've seen before on Mercury or the Moon," says Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and a member of the MESSENGER imaging team. "We are still debating their origin, but they appear to have a relatively young age and may suggest a more abundant than expected volatile component in Mercury's crust."

The Surface Composition

The X-Ray Spectrometer (XRS) has made several important discoveries since orbit insertion. The magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet's surface show that, unlike the surface of the Moon, Mercury's surface is not dominated by feldspar-rich rocks. XRS observations have also revealed substantial amounts of sulfur at Mercury's surface, lending support to suggestions from ground-based observations that sulfide minerals are present. This discovery suggests that Mercury's original building blocks may have been less oxidized than those that formed the other terrestrial planets and could be key to understanding the nature of volcanism on Mercury.

MESSENGER's Gamma-Ray and Neutron Spectrometer detected the decay of radioactive isotopes of potassium and thorium, and researchers have determined the bulk abundances of these elements. "The abundance of potassium rules out some prior theories for Mercury's composition and origin," says Larry Nittler, a staff scientist at the Carnegie Institution. "Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin."

Mercury's Topography and Magnetic Field

MESSENGER's Mercury Laser Altimeter has been mapping the topography of Mercury's northern hemisphere in detail. The north polar region, for instance, is a broad area of low elevations. The overall topographic height range seen to date exceeds 9 kilometers (5.5 miles).

Previous Earth-based radar images showed that around Mercury's north and south poles are deposits thought to consist of water ice and perhaps other ices preserved on cold, permanently shadowed floors of high-latitude impact craters. MESSENGER's altimeter is measuring the floor depths of craters near the north pole. The depths of craters with polar deposits support the idea that these areas are in permanent shadow.

The geometry of Mercury's internal magnetic field can potentially allow the rejection of some theories for how the field is generated. The spacecraft found that Mercury's magnetic equator is well north of the planet's geographic equator. The best-fitting internal dipole magnetic field is located about 0.2 Mercury radii, or 480 km (298 miles), northward of the planet's center. The dynamo mechanism responsible for generating the planet's magnetic field therefore has a strong north-south asymmetry.

As a result of this north-south asymmetry, the geometry of magnetic field lines is different in Mercury's north and south polar regions. In particular, the magnetic "polar cap" where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than the north to charged particles heated and accelerated by the solar wind. The impact of those charged particles onto Mercury's surface contributes both to the generation of the planet's tenuous atmosphere and to the "space weathering" of surface materials, both of which should have a north-south asymmetry.

Energetic Particles at Mercury

One of the major discoveries made by Mariner 10 flybys of Mercury in 1974 were bursts of energetic particles in Mercury's Earth-like magnetosphere. Four bursts of particles were observed on the first flyby, so it was puzzling that no such events were detected by MESSENGER during any of its three flybys.

With MESSENGER now in near-polar orbit about Mercury, energetic events are being seen almost like clockwork, remarked MESSENGER Project Scientist Ralph McNutt, of APL. "While varying in strength and distribution, bursts of energetic electrons -- with energies from 10 kiloelectron volts (keV) to more than 200 keV -- have been seen in most orbits since orbit insertion," McNutt said. "The Energetic Particle Spectrometer has shown these events to be electrons rather than energetic ions, and to occur at moderate latitudes. The latitudinal location is entirely consistent with the events seen by Mariner 10."

With Mercury's smaller magnetosphere and with the lack of a substantial atmosphere, the generation and distribution of energetic electrons differ from those at Earth. One candidate mechanism for their generation is the formation of a "double layer," a plasma structure with large electric fields along the local magnetic field. Another is induction brought about by rapid changes in the magnetic field, a process that follows the principle used in generators on Earth to produce electric power. The mechanisms at work will be the studied over the coming months.

"We are assembling a global overview of the nature and workings of Mercury for the first time," remarked Solomon, "and many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three Mercury years to run, and we can expect more surprises as our Solar System's innermost planet reveals its long-held secrets."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Carnegie Institution.

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Mercury: Messenger orbital data confirm theories, reveal surprises

ScienceDaily (June 17, 2011) — On March 18, 2011, the MESSENGER spacecraft entered orbit around Mercury to become that planet's first orbiter. The spacecraft's instruments are making a complete reconnaissance of the planet's geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment. MESSENGER is providing a wealth of new information and some surprises. For instance, Mercury's surface composition differs from that expected for the innermost of the terrestrial planets, and Mercury's magnetic field has a north-south asymmetry that affects interaction of the planet's surface with charged particles from the solar wind.

Tens of thousands of images reveal major features on the planet in high resolution for the first time. Measurements of the chemical composition of the planet's surface are providing important clues to the origin of the planet and its geological history. Maps of the planet's topography and magnetic field are offering new evidence on Mercury's interior dynamical processes. And scientists now know that bursts of energetic particles in Mercury's magnetosphere are a continuing product of the interaction of Mercury's magnetic field with the solar wind.

"MESSENGER has passed a number of milestones just this week," offers MESSENGER principal investigator Sean Solomon of the Carnegie Institution. "We completed our first perihelion passage from orbit on Sunday, our first Mercury year in orbit on Monday, our first superior solar conjunction from orbit on Tuesday, and our first orbit-correction maneuver on Wednesday. Those milestones provide important context to the continuing feast of new observations that MESSENGER has been sending home on nearly a daily basis."

The Surface in Detail

Images obtained with MESSENGER's Mercury Dual Imaging System (MDIS) are being combined into maps for the first global look at the planet under optimal viewing conditions. New images of areas near Mercury's north pole orbital show that region hosts one of the largest expanses of volcanic plains deposits on the planet, with thicknesses of up to several kilometers. The broad expanses of plains confirm that volcanism shaped much of Mercury's crust and continued through much of Mercury's history, despite an overall contractional stress state that tended to inhibit the extrusion of volcanic material onto the surface.

Among the fascinating features seen in flyby images of Mercury were bright, patchy deposits on some crater floors, but they remained a curiosity. New targeted MDIS observations reveal these patchy deposits to be clusters of rimless, irregular pits with horizontal dimension from hundreds of meters to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters.

"The etched appearance of these landforms is unlike anything we've seen before on Mercury or the Moon," says Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and a member of the MESSENGER imaging team. "We are still debating their origin, but they appear to have a relatively young age and may suggest a more abundant than expected volatile component in Mercury's crust."

The Surface Composition

The X-Ray Spectrometer (XRS) has made several important discoveries since orbit insertion. The magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet's surface show that, unlike the surface of the Moon, Mercury's surface is not dominated by feldspar-rich rocks. XRS observations have also revealed substantial amounts of sulfur at Mercury's surface, lending support to suggestions from ground-based observations that sulfide minerals are present. This discovery suggests that Mercury's original building blocks may have been less oxidized than those that formed the other terrestrial planets and could be key to understanding the nature of volcanism on Mercury.

MESSENGER's Gamma-Ray and Neutron Spectrometer detected the decay of radioactive isotopes of potassium and thorium, and researchers have determined the bulk abundances of these elements. "The abundance of potassium rules out some prior theories for Mercury's composition and origin," says Larry Nittler, a staff scientist at the Carnegie Institution. "Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin."

Mercury's Topography and Magnetic Field

MESSENGER's Mercury Laser Altimeter has been mapping the topography of Mercury's northern hemisphere in detail. The north polar region, for instance, is a broad area of low elevations. The overall topographic height range seen to date exceeds 9 kilometers (5.5 miles).

Previous Earth-based radar images showed that around Mercury's north and south poles are deposits thought to consist of water ice and perhaps other ices preserved on cold, permanently shadowed floors of high-latitude impact craters. MESSENGER's altimeter is measuring the floor depths of craters near the north pole. The depths of craters with polar deposits support the idea that these areas are in permanent shadow.

The geometry of Mercury's internal magnetic field can potentially allow the rejection of some theories for how the field is generated. The spacecraft found that Mercury's magnetic equator is well north of the planet's geographic equator. The best-fitting internal dipole magnetic field is located about 0.2 Mercury radii, or 480 km (298 miles), northward of the planet's center. The dynamo mechanism responsible for generating the planet's magnetic field therefore has a strong north-south asymmetry.

As a result of this north-south asymmetry, the geometry of magnetic field lines is different in Mercury's north and south polar regions. In particular, the magnetic "polar cap" where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than the north to charged particles heated and accelerated by the solar wind. The impact of those charged particles onto Mercury's surface contributes both to the generation of the planet's tenuous atmosphere and to the "space weathering" of surface materials, both of which should have a north-south asymmetry.

Energetic Particles at Mercury

One of the major discoveries made by Mariner 10 flybys of Mercury in 1974 were bursts of energetic particles in Mercury's Earth-like magnetosphere. Four bursts of particles were observed on the first flyby, so it was puzzling that no such events were detected by MESSENGER during any of its three flybys.

With MESSENGER now in near-polar orbit about Mercury, energetic events are being seen almost like clockwork, remarked MESSENGER Project Scientist Ralph McNutt, of APL. "While varying in strength and distribution, bursts of energetic electrons -- with energies from 10 kiloelectron volts (keV) to more than 200 keV -- have been seen in most orbits since orbit insertion," McNutt said. "The Energetic Particle Spectrometer has shown these events to be electrons rather than energetic ions, and to occur at moderate latitudes. The latitudinal location is entirely consistent with the events seen by Mariner 10."

With Mercury's smaller magnetosphere and with the lack of a substantial atmosphere, the generation and distribution of energetic electrons differ from those at Earth. One candidate mechanism for their generation is the formation of a "double layer," a plasma structure with large electric fields along the local magnetic field. Another is induction brought about by rapid changes in the magnetic field, a process that follows the principle used in generators on Earth to produce electric power. The mechanisms at work will be the studied over the coming months.

"We are assembling a global overview of the nature and workings of Mercury for the first time," remarked Solomon, "and many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three Mercury years to run, and we can expect more surprises as our Solar System's innermost planet reveals its long-held secrets."

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