Upcoming Mercury Encounter Presents New Opportunities For Magnetometer

On September 29, the MESSENGER spacecraft will pass by Mercury for the third time, flying 141.7 miles above the planet's rocky surface for a final gravity assist that will enable it to enter orbit about Mercury in 2011. This encounter will also provide new observational opportunities for MESSENGER's Magnetometer, designed to determine the structure and origin of Mercury's intrinsic magnetic field.

The comparison of magnetosphere observations from MESSENGER's first flyby in January 2008 with data from the probe's second pass in October 2008 provided key new insight into the nature of the planet's internal magnetic field and revealed new features of Mercury's magnetosphere, explains Brian Anderson, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

"MESSENGER's first flyby of Mercury and Mariner 10's encounters with the planet provided data only from Mercury's eastern hemisphere," says Anderson, MESSENGER's Deputy Project Scientist.

"The October 2008 flyby provided the first measurements from Mercury's western hemisphere, and scientists learned that the planet's magnetic field is highly symmetric. This finding 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."

The probe's third flyby of Mercury next month will take it again over the planet's western hemisphere, and the observations will be used to refine the estimate of the planetary magnetic field, Anderson explains.

"The previous flybys yielded significant insight into the dynamics of Mercury's magnetosphere and its boundaries," Anderson says.

"During the second flyby a plasmoid and a series of traveling compression regions were observed in Mercury's magnetotail, and a large flux transfer event was observed at the dayside magnetopause. These observations proved that the solar wind interaction, under the right circumstances, can drive intense magnetic reconnection at rates 10 times the rates observed at Earth."

The behavior during the second flyby was markedly different from that found in the first flyby, demonstrating the profound influence of the solar wind environment on Mercury's magnetosphere.

"The third flyby is the last opportunity to survey the magnetotail and magnetopause regions in the equatorial plane, and the contrast in the system's structure under different solar wind conditions already observed make it likely that the third flyby will yield new insights and perhaps more surprises for the dynamics of this smallest and most highly variable of the solar system's planetary magnetospheres," Anderson says.

As with the previous two flybys, the Magnetometer will record the magnetic field at the highest available observation rate of 20 vector magnetic field samples per second for a period of twelve hours centered on the time of closest approach.

"This observing plan guarantees the highest possible science return from the encounter and will provide key observations to guide the magnetic field investigation plan for the prime orbital phase of the mission," Anderson says.

The Big Picture @ Boston.com: Mercury and MESSENGER

As NASA's MESSENGER spacecraft receded from Mercury after making its closest approach on January 14, 2008, it recorded several mosaics covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through three filters (infrared, far red and violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here, creating a false-color image that accentuates the subtle color differences on Mercury's surface. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The planet Mercury is the smallest of the inner planets (4,880 km/3,032 mi in diameter), and the closest to the Sun (58 million km/36 million mi - or 3.2 light minutes). It was visited by the Mariner 10 spacecraft twice in the 1970s, and about 45% of the surface was mapped. On August 3rd, 2004, NASA launched a new mission to Mercury, the MErcury Surface, Space ENvironment, GEochemistry and Ranging probe (or MESSENGER). MESSENGER is now in the last stages of multiple gravity-assist flybys of Earth, Venus and Mercury, en route to an insertion into orbit around Mercury in March of 2011. In just two flyby encounters, MESSENGER has already greatly increased our knowledge about Mercury's surface features. As you look at Mercury in the new images below, keep in mind that it has minimal atmosphere, gravity about 1/3 of Earth's, and surface temperatures ranging from -183 C (-297 F) in some polar craters to 427 C (801 F) at high noon (Mercury's solar day lasting 176 Earth days). (20 photos total)

Magnetic Tornadoes Could Liberate Mercury's Tenuous Atmosphere

This is a diagram of the October 6, 2008, MESSENGER flyby that revealed magnetic tornadoes forming in Mercury's magnetic field. The tornadoes are corkscrew-shaped bundles of twisted magnetic fields and plasma. The pink area represents the boundary of Mercury's magnetic field, called the magnetopause. The tornadoes are technically known as "flux transfer events" (twisted lines) when they form at the magnetopause and "plasmoids" (yellow areas) when they form in the long magnetic "tail" extending from the night-side of Mercury. The large magnetic field leakage through the magnetopause and the flux transfer events acts as open channels through which the solar wind can flow down to the surface of the planet and sputter neutral atoms into Mercury’s atmosphere. Credit: Image produced by NASA/Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS.

"Mercury's atmosphere is so thin, it would have vanished long ago unless something was replenishing it," says Dr. James A. Slavin of NASA's Goddard Space Flight Center, Greenbelt, Md., a co-investigator on NASA's MESSENGER mission to Mercury. That something could be the solar wind, a thin gas of electrically charged particles, called a plasma, which blows constantly from the surface of the sun. The solar wind moves quickly, usually around 250 to 370 miles per second (about 400 to 600 kilometers/second); fast enough to blast atoms off the surface of Mercury. Through a process called "sputtering," solar wind particles that crash into Mercury’s surface transfer sufficient energy to launch some atoms into ballistic trajectories high above the surface and replenish Mercury's atmosphere, according to Slavin.

However, there's a problem – Mercury's magnetic field gets in the way. MESSENGER's first flyby on January 14, 2008, confirmed that the planet has a global magnetic field, as first discovered by the Mariner 10 spacecraft during its flybys of the planet in 1974 and 1975.

The ions and electrons that make up the solar wind are electrically charged and "feel" magnetic forces, so a global magnetic field usually deflects the solar wind. However, global magnetic fields are leaky shields and, under the right conditions, they are known to develop holes through which the solar wind can flow.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury’s magnetic field can be extremely leaky indeed. The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 500 miles wide or a third of the radius of the planet.

"These 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field," said Slavin. "As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface."

Venus, Earth, and even Mars have thick atmospheres compared to Mercury, so the solar wind never makes it to the surface of these planets, even if there is no global magnetic field in the way, as is the case for Venus and Mars. Instead, it hits the upper atmosphere of these worlds, where it has the opposite effect to that on Mercury, gradually stripping away atmospheric gas as it blows by.

Venus has a thick atmosphere that may be replenished by volcanoes, so losses to the solar wind are insignificant. Mars is a different story. Mars lost its global magnetic field billions of years ago. With little apparent volcanic activity since then, the solar wind could have eroded a significant portion of the Red Planet's atmosphere.

Features on Mars resembling dry riverbeds, and the discovery of minerals that form in the presence of water, indicate that Mars once had a thicker atmosphere that kept it warm enough for liquid water to flow on the surface. However, somehow that much thicker ancient atmosphere got lost, because it appears Mars has been cold and dry for billions of years.

In 2013, NASA plans to launch a mission to Mars called MAVEN (Mars Atmosphere and Volatile Evolution Mission). It will explore the various ways Mars loses its atmosphere to space, including how much may have been stripped away by the solar wind.

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury.

"Mercury's proximity to the sun only accounts for about a third of the reconnection rate we see," said Slavin. "It will be exciting to see what's special about Mercury to explain the rest. We'll get more clues from MESSENGER's third flyby on September 29, 2009, and when we get into orbit in March 2011."

Slavin's MESSENGER research was funded by NASA and is the subject of a paper that appeared in the journal Science on May 1, 2009.

MESSENGER is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

Overlaying color onto Praxiteles crater

MESSENGER's high-resolution images obtained during the mission's second Mercury flyby have revealed a number of irregularly shaped depressions on the floor of Praxiteles crater. These depressions are intriguing indications of possible past volcanic activity within this crater.

The image shown here is similar to one recently published in the 1 May issue of Science magazine.

This image was created by first mosaicking together the highest-resolution NAC images available of Praxiteles, to produce complete coverage of the crater. Independently, an enhanced-color image of Praxiteles was created by using images from all 11 WAC narrow-band color filters.

The WAC images provide important color information, but the WAC resolution is considerably less than that of the mosaicked NAC images. Thus, by overlaying a slightly transparent version of the WAC enhanced-color image on the high-resolution NAC mosaic, the high-resolution color view of Praxiteles crater shown here was produced.

This overlay-color view helps associate the color features with the morphologic surface features. The fact that the irregularly shaped depressions on the floor of Praxiteles are associated with bright orange and yellow color features provides evidence that the depressions may be related to past volcanic activity in this area of Mercury.

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