So far, the spacecraft has imaged about 90% of the surface area of Mercury.
MESSENGER, a shortened form for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is not a stranger to the planet Mercury. It made its first flyby back in January 14, 2008.
The April 30, 2009 NASA article MESSENGER spacecraft reveals a very dynamic planet Mercury states, “A NASA spacecraft gliding over the surface of Mercury has revealed that the planet's atmosphere, the interaction of its surrounding magnetic field with the solar wind, and its geological past display greater levels of activity than scientists first suspected.”
It added, “The probe also discovered a previously unknown large impact basin about 430 miles in diameter -- equal to the distance between Washington and Boston.”
With respect to the magnetosphere (a highly magnetized region around some astronomical objects), Dr. Karl-Heinz Glassmeier (Max Planck Institute for Solar System Research, Germany) writes in the May 1, 2009 Science magazine article Magnetic Twisters on Mercury, that, “The recent flybys of the MESSENGER spacecraft confirm the existence of the Hermean magnetosphere, discovered 35 years ago by the Mariner 10 mission.”
The Hermean magnetosphere is the name of the magnetosphere of Mercury.
Dr. Glassmeier adds, “This magnetosphere is rather small, with the magnetopause (the boundary between the interplanetary medium and the magnetospheric plasma) located as close as 1700 km [kilometers] above the planet surface. Not much is known about the structure and dynamics of the Hermean magnetosphere, and it is here where the observations by MESSENGER are shedding new light.”
Page two adds more about magnetosphere of Mercury.
In addition, a U.S.-Czech Republic team stated in its parallel Science article MESSENGER Observations of Magnetic Reconnection in Mercury’s Magnetosphere that “Solar wind energy transfer to planetary magnetospheres and ionospheres is controlled by magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field (IMF) and a planet’s magnetic field.”
The team adds, “During MESSENGER’s second flyby of Mercury, a steady southward IMF was observed and the magnetopause was threaded by a strong magnetic field, indicating a reconnection rate ~10 times that typical at Earth. Moreover, a large flux transfer event was observed in the magnetosheath, and a plasmoid and multiple traveling compression regions were observed in Mercury’s magnetotail, all products of reconnection.”
In other words, there is a big interplay between the Sun’s magnetic field and Mercury’s magnetic field. In fact, Mercury’s pattern of connectivity (the interplay between its magnetic field and the Sun’s) is ten times that of the Earth-Sun’s pattern.
Even though Mercury's magnetosphere is much smaller than Earth's, the planet is much closer to the Sun than Earth--which contributes to its magnetic connectivity being much larger.
Sean Solomon (Carnegie Institution of Washington), the principal investigator for the mission, states, "This second Mercury flyby provided a number of new findings. One of the biggest surprises was how strongly the dynamics of the planet's magnetic field-solar wind interaction changed from what we saw during the first Mercury flyby in January 2008."
The research team concludes, “These observations indicate that Mercury’s magnetosphere is much more responsive to IMF direction and dominated by the effects of reconnection than that of Earth or the other magnetized planets.”
Page three talks about the Rembrandt impact basin on Mercury's surface.
With respect to the impact basin, the floor of the Rembrandt impact basin has showed terrain that has never been seen before because normally they are buried by volcanic lava flows and not exposed as with this basin.
This second visit involved over 1,200 high-resolution, color images of the planet, including about one-third of the planet’s surface area that had never been seen before by Earth probes. These images have given scientists a better picture of the crust formed on Mercury—with about 40% covered by smooth plains that followed from lava flows, which are similar to those found on Mars.
Dr. Solomon states, "The discovery of a large and unusually well preserved impact basin shows concentrated volcanic and deformational activity."
In addition, MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) detected the element magnesium in the exosphere of Mercury. Its presence was not a surprise to mission scientists, however, the amount found and its distribution was quite surprising. MASCS also observed the elements calcium and sodium within the Mercurian exosphere and aluminum, iron, and silicon from its surface.
The Science article MESSENGER Observations of Mercury’s Exosphere: Detection of Magnesium and Distribution of Constituents states “Mercury is surrounded by a tenuous exosphere that is supplied primarily by the planet’s surface materials and is known to contain sodium, potassium, and calcium."
The exosphere is the uppermost layer of an atmosphere.
The U.S. team continues to say, “Observations by the Mercury Atmospheric and Surface Composition Spectrometer during MESSENGER’s second Mercury flyby revealed the presence of neutral magnesium in the tail (anti-sunward) region of the exosphere, as well as differing spatial distributions of magnesium, calcium, and sodium atoms in both the tail and the nightside, near-planet exosphere.”
And, “Analysis of these observations, supplemented by observations during the first Mercury flyby, as well as those by other MESSENGER instruments, suggests that the distinct spatial distributions arise from a combination of differences in source, transfer, and loss processes.”
Page four concludes.
William McClintock (University of Colorado, Boulder), another MESSENGER investigator, stated, “This is an example of the kind of individual discoveries that the science team will piece together to give us a new picture of how the planet formed and evolved."
James Slavin (NASA Goddard Space Flight Center), another investigator with the mission, states, "The spacecraft observed a radically different magnetosphere at Mercury during its second flyby compared with its earlier Jan. 14 encounter. During the first flyby, important discoveries were made, but scientists didn't detect any dynamic features. The second flyby witnessed a totally different situation."
The third flyby of Mercury occurs on September 29, 2009. Then, in March 2011, MESSENGER will be inserted into orbit about the planet.
Dr. Solomon states, “The orbital phase will be like staging two flybys per day and will provide the continuous collection of information about the planet and its environment for one year. Mercury has been coy in revealing its secrets slowly so far, but in less than two years the innermost planet will become a close friend."
For additional information on the mission of MESSENGER, please go to the Johns Hopkins/Applied Physics Lab website: “MESSENGER”
The only other probe to make a flyby of Mercury was Mariner 10, twice in 1974 and once in 1975. Look at its mission at the website of NASA: “The Voyage of Mariner 10: Mission to Venus and Mercury.”