Featured Image: Ice in the Shadow of Mercury’s Craters

New models of various properties of Prokofiev crater on Mercury: (a) elevation, (b) illumination, (c) maximum temperature, and (d) depth at which ice is stable. These maps have a resolution of 125 meters per pixel. Click for high-resolution version. Credit: Barker et al. 2022

With daytime temperatures soaring to 427℃ (800℉), Mercury seems like an unlikely place to find ice, but the poles of the airless planet can be surprisingly frosty. Using images and elevation data from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft, a team led by Michael Barker (NASA’s Goddard Space Flight Center) inspected a permanently shadowed north polar crater named Prokofiev, which contains a radar-bright region thought to be surface ice. As shown in the images to the right, Barker and collaborators modeled the crater’s elevation, illumination, maximum temperature, and depth below the surface at which water ice could be stable. This modeling confirmed that the crater has the right conditions to host surface ice, and further analysis suggests that the radar-bright region may be a layer of ice up to 26 meters thick. The ice isn’t pure water, though — part of the ice is covered by a dark silicate or hydrocarbon material, the exact nature of which is unknown. To learn more about this icy investigation, be sure to check out the full article below!

Citation

“New Constraints on the Volatile Deposit in Mercury’s North Polar Crater, Prokofiev,” Michael K. Barker et al 2022 Planet. Sci. J. 3 188. doi:10.3847/PSJ/ac7d5a

By Kerry Hensley

Source: American Astronomical Society - AAS Nova

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Forming Mercury by Giant Impacts

Movie of a Case-1 collision: the proto-Mercury of 2.7 M_☿ collides with the impactor of 1.125 M_☿ at an impact angle of b=0.2 and v=18 km/s. In the right panel, we zoom on the largest fragment of 0.99 M_☿ and an iron-to-rock ratio of 0.6.

The smallest planet in our Solar System has a large iron core. How come? According to the most popular theory, Mercury lost big parts of its rocky mantle in a collision. Alice Chau and her colleagues at the University of Zürich simulated different scenarios with a super computer. Their result: Forming Mercury by giant impacts is feasible but difficult.

Compared to Earth, Venus or Mars, Mercury is much more metallic. It has a large iron core and only a thin rocky mantle. This mysterious nature has puzzled researchers for decades. “We think that Mercury might have formed in a similar way as the other planets, and therefore initially had a core which weighed one third of the planet’s mass, but lost most of its mantle”, explains Alice Chau, PhD student and associate of PlanetS at the University of Zürich. But how this loss came about is still being debated.

In 1988, Willy Benz, now director of the NCCR PlanetS, and colleagues suggested that this was due to the blasting off of the mantle during a giant impact and simulated such a process with computer calculations which were refined in 2007. “However, the exact conditions that lead to Mercury’s formation via a giant impact are still unknown,” says Chau. Therefore, together with her colleagues at the Institute for Computational Science she decided to investigate the giant impact hypothesis based on simulations performed with one of the most powerful supercomputers in Europe. The machine called “Piz Daint” named after a prominent peak in Grisons is located at the Swiss National Supercomputing Centre (CSCS) in Lugano.

“We investigated three different scenarios,” explains Chau. In “Case-1” the proto-Mercury is hit by a smaller body as in the simulations calculated by Benz et al. in 2007. In “Case-2” Mercury is actually the impactor and collides with a larger body which no longer resides in the Solar System. This is called the hit-and-run scenario. In the third case, Mercury is hit by multiple impactors. “We found that it is possible to form Mercury in all these scenarios but each of them requires rather specific conditions,” summarizes Chau.

Multiple impacts more probable

For instance, in a single violent collision the impact angle and velocity have to be highly tuned to reproduce Mercury’s mass and iron-to-rock ratio. In addition, it is still an open question whether datacollected by the NASA spacecraft Messenger are consistent with a single giant impact. New observations by Europe’s BepiColombo could solve this problem, once the spacecraft will have arrived at Mercury in 2025. “It is therefore possible, and maybe even more probable, that Mercury formed as a result of multiple impacts,” the team writes in its paper published in the Astrophysical Journal.

The fact that it seems difficult to form Mercury maybe disappointing, but it also has something to offer: “It is consistent with the fact that we observed only very few exoplanets that have similar average density as Mercury among the few thousands we already know,” says Chau: “What will be very interesting is if we discover more of these planets to investigate if they have a common formation mechanism with Mercury, and maybe they will help us to better understand the origin our own Mercury.“

Alice Chau, Christian Reinhardt, Ravit Helled, and Joachim Stadel: Forming Mercury by Giant Impacts, The Astrophysical Journal, Volume 865, Number 1,

DOI: https://doi.org/10.3847/1538-4357/aad8b0

Source: National Centre of Competence in Research PlanetS (NCCRS)/News

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NASA Team Studies Middle-aged Sun by Tracking Motion of Mercury

Mercury’s proximity to the Sun and small size make it exquisitely sensitive to the dynamics of the Sun and its gravitational pull. 
Credits: NASA/SDO

Like the waistband of a couch potato in midlife, the orbits of planets in our solar system are expanding. It happens because the Sun’s gravitational grip gradually weakens as our star ages and loses mass. Now, a team of NASA and MIT scientists has indirectly measured this mass loss and other solar parameters by looking at changes in Mercury’s orbit.

The new values improve upon earlier predictions by reducing the amount of uncertainty. That’s especially important for the rate of solar mass loss, because it’s related to the stability of G, the gravitational constant. Although G is considered a fixed number, whether it’s really constant is still a fundamental question in physics.

“Mercury is the perfect test object for these experiments because it is so sensitive to the gravitational effect and activity of the Sun,” said Antonio Genova, the lead author of the study published in Nature Communications and a Massachusetts Institute of Technology researcher working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The study began by improving Mercury’s charted ephemeris — the road map of the planet’s position in our sky over time. For that, the team drew on radio tracking data that monitored the location of NASA’s MESSENGER spacecraft while the mission was active. Short for Mercury Surface, Space Environment, Geochemistry, and Ranging, the robotic spacecraft made three flybys of Mercury in 2008 and 2009 and orbited the planet from March 2011 through April 2015. The scientists worked backward, analyzing subtle changes in Mercury’s motion as a way of learning about the Sun and how its physical parameters influence the planet’s orbit.

NASA and MIT scientists analyzed subtle changes in Mercury’s motion to learn about the Sun and how its dynamics influence the planet’s orbit. The position of Mercury over time was determined from radio tracking data obtained while NASA’s MESSENGER mission was active. Credits: NASA's Goddard Space Flight Center

For centuries, scientists have studied Mercury’s motion, paying particular attention to its perihelion, or the closest point to the Sun during its orbit. Observations long ago revealed that the perihelion shifts over time, called precession. Although the gravitational tugs of other planets account for most of Mercury’s precession, they don’t account for all of it.

The second-largest contribution comes from the warping of space-time around the Sun because of the star’s own gravity, which is covered by Einstein’s theory of general relativity. The success of general relativity in explaining most of Mercury’s remaining precession helped persuade scientists that Einstein’s theory was right.

Other, much smaller contributions to Mercury’s precession, are attributed to the Sun’s interior structure and dynamics. One of those is the Sun’s oblateness, a measure of how much it bulges at the middle — its own version of a “spare tire” around the waist — rather than being a perfect sphere. The researchers obtained an improved estimate of oblateness that is consistent with other types of studies.

The researchers were able to separate some of the solar parameters from the relativistic effects, something not accomplished by earlier studies that relied on ephemeris data. The team developed a novel technique that simultaneously estimated and integrated the orbits of both MESSENGER and Mercury, leading to a comprehensive solution that includes quantities related to the evolution of Sun’s interior and to relativistic effects.

“We’re addressing long-standing and very important questions both in fundamental physics and solar science by using a planetary-science approach,” said Goddard geophysicist Erwan Mazarico. “By coming at these problems from a different perspective, we can gain more confidence in the numbers, and we can learn more about the interplay between the Sun and the planets.”

The team’s new estimate of the rate of solar mass loss represents one of the first times this value has been constrained based on observations rather than theoretical calculations. From the theoretical work, scientists previously predicted a loss of one-tenth of a percent of the Sun’s mass over 10 billion years; that’s enough to reduce the star’s gravitational pull and allow the orbits of the planets to spread by about half an inch, or 1.5 centimeters, per year per AU (an AU, or astronomical unit, is the distance between Earth and the Sun: about 93 million miles).

The new value is slightly lower than earlier predictions but has less uncertainty. That made it possible for the team to improve the stability of G by a factor of 10, compared to values derived from studies of the motion of the Moon.

“The study demonstrates how making measurements of planetary orbit changes throughout the solar system opens the possibility of future discoveries about the nature of the Sun and planets, and indeed, about the basic workings of the universe,” said co-author Maria Zuber, vice president for research at MIT.

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By Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

Source: NASA/Mercury(Planet)

NASA's Messenger Spots Giant Space Weather Effects at Mercury

The yellow color shows the standing bow shock in front of Mercury. The signature of material flowing in a vastly different direction than the solar wind -- an HFA – can be seen in red at the lower left. Image Credit: NASA/Duberstein.

The solar wind of particles streaming off the sun helps drive flows and swirls in space as complicated as any terrestrial weather pattern. Scientists have now spotted at planet Mercury, for the first time, a classic space weather event called a hot flow anomaly, or HFA, which has previously been spotted at Earth, Venus, Saturn and Mars.

"Planets have a bow shock the same way a supersonic jet does," said Vadim Uritsky at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "These hot flow anomalies are made of very hot solar wind deflected off the bow shock."

The results were published in the Journal of Geophysical Research: Space Physics on Jan. 15, 2014. To identify the presence of HFAs at Mercury, the team used observations from NASA's Messenger (short for Mercury Surface, Space Environment, Geochemistry, and Ranging) to detect the presence of two HFA signatures. The first measurement was of magnetic fields that can be used to detect giant electric current sheets that lead to HFAs. The second was of the heating of the charged particles. The scientists then analyzed this information to quantify what kind of turbulence exists in the region, which provided the final smoking gun of an HFA.

Not only is this the first sighting of HFAs at Mercury, but the observations help round out a picture of this type of space weather in general.  HFAs come in a variety of scale sizes – from around 600 miles across at Venus to closer to 60,000 miles across at Saturn. This study suggests that the most important factor for determining HFA size is the geometry and size of the planet's bow shock.

Related Link

• NASA's Messenger website

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Mercury’s Contraction Much Greater Than Thought

This image shows a long collection of ridges and scarps on the planet Mercury called a fold-and-thrust belt. The belt stretches over 336 miles (540 kilometers). The colors correspond to elevation—yellow-green is high and blue is low. Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Washington, D.C.—New global imaging and topographic data from MESSENGER* show that the innermost planet has contracted far more than previous estimates. The results are based on a global study of more than 5,900 geological landforms, such as curving cliff-like scarps and wrinkle ridges, that have resulted from the planet’s contraction as Mercury cooled. The findings, published online March 16, 2014, in Nature Geoscience, are key to understanding the planet’s thermal, tectonic, and volcanic history, and the structure of its unusually large metallic core.

Unlike Earth, with its numerous tectonic plates, Mercury has a single rigid, top rocky layer. Prior to the MESSENGER mission only about 45% of Mercury’s surface had been imaged by a spacecraft. Old estimates, based on this non-global coverage, suggested that the planet had contracted radially by about ½ to 2 miles (0.8 to 3 kilometers) substantially less than that indicated by models of the planet’s thermal history. Those models predicted a radial contraction of about 3 to 6 miles (5 to 10 kilometers) starting from the late heavy bombardment of the Solar System, which ended about 3.8 billion years ago.

The new results, which are based on the first comprehensive survey of the planet’s surface, show that Mercury contracted radially by as much as 4.4 miles (7 kilometers)—substantially more than the old estimates, but in agreement with the thermal models. Mercury’s modern radius is 1,516 miles (2,440 kilometers).

“These new results resolved a decades-old paradox between thermal history models and estimates of Mercury’s contraction,” remarked lead author of the study, Paul Byrne, a planetary geologist and MESSENGER visiting investigator at Carnegie’s Department of Terrestrial Magnetism. “Now the history of heat production and loss and global contraction are consistent. Interestingly, our findings are also reminiscent of now-obsolete models for how large-scale geological deformation occurred on Earth when the scientific community thought that the Earth only had one tectonic plate. Those models were developed to explain mountain building and tectonic activity in the nineteenth century, before plate tectonics theory.”

Byrne and his coauthors identified a much greater number and variety of geological structures on the planet than had been recognized in previous research. They identified 5,934 ridges and scarps attributed to global contraction, which ranged from 5 to 560 miles (9 to 900 kilometers) in length.

The researchers used two complementary techniques to estimate the contraction from their global survey of structures. Although the two estimates of radius change differed by 0.6 to 1 mile (1 to 1.6 kilometers), both were substantially greater than old estimates.

“I became interested in the thermal evolution of Mercury’s interior when the Mariner 10 spacecraft sent back images of the planet’s great scarps in 1974–75, but the thermal history models predicted much more global contraction than the geologists inferred from the scarps then observed, even correcting for the fact that Mariner 10 imaged less than half of Mercury’s surface,” noted Sean Solomon, principal investigator of the mission, former director of Carnegie’s Department of Terrestrial Magnetism, and current director of the Lamont-Doherty Earth Observatory at Columbia University. “This discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved. It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence.”

* MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) 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 entered orbit about Mercury on March 18, 2011 (UTC), to begin its primary mission – a yearlong study of its target planet. MESSENGER’s first extended mission began on March 18, 2012, and ended one year later. MESSENGER is currently operating in its second extended mission. Dr. Sean C. Solomon, of the Lamont-Doherty Earth Observatory of Columbia University, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

Authors on the paper are Paul Byrne, Carnegie and the Lunar and Planetary Institute; Christian Klimczak, Carnegie; A. M. Celâl Şengör, Eurasia Institute of Earth Sciences; Sean Solomon, Carnegie and Lamont-Doherty Earth Observatory; Thomas Watters, Smithsonian; and Steven Hauk, II, Case Western University.

Source:  Carnagie Institution for Science

New insights concerning the early bombardment history on Mercury

Mercury image courtesy of John Hopkins APL. Download Image

The figure shows an image of Mercury's surface (left; obtained using publicly available mosaic of Mercury from the MESSENGER spacecraft found at http://messenger.jhuapl.edu/) and a color-coded view of the global crater areal density (right), obtained by measuring craters greater than 25 km. The region within the white line corresponds to the heavily cratered terrains analyzed to calculate the age of the oldest surfaces on Mercury.

Boulder, Colo. —  The surface of Mercury is rather different from those of well-known rocky bodies like the Moon and Mars. Early images from the Mariner 10 spacecraft unveiled a planet covered by smooth plains and cratered plains of unclear origin. A team led by Dr. Simone Marchi, a Fellow of the NASA Lunar Science Institute located at the Southwest Research Institute (SwRI) Boulder, Colo., office, collaborating with the MESSENGER team, including Dr. Clark Chapman of the SwRI Planetary Science Directorate, studied the surface to better understand if the plains were formed by volcanic flows or composed of material ejected from the planet's giant impact basins.

Recent images from NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft provided new insights showing that at least the younger plains resulted from vigorous volcanic activity. Yet scientists were unable to establish limits on how far into the past this volcanic activity may have occurred, or how much of the planet's surface may have been resurfaced by very old volcanic plains.

Now, a team of scientists has concluded that the oldest visible terrains on Mercury have an age of 4 billion to 4.1 billion years, and that the first 400 to 500 million years of the planet's evolution are not recorded on its surface. To reach its conclusion, the team measured the sizes and numbers of craters on the most heavily cratered terrains using images obtained by the MESSENGER spacecraft during its first year in orbit around Mercury. Team members then extrapolated to Mercury a model that was originally developed for comparing the Moon's crater distribution to a chronology based on the ages of rock samples gathered during the Apollo missions.

The study, "Global Resurfacing of Mercury 4.0-4.1 Billion Years Ago by Heavy Bombardment and Volcanism" by Marchi, Chapman, Caleb I. Fassett, James W. Head, William F. Bottke and Robert G. Strom, is in the July 4 issue of the journal Nature.

"By comparing the measured craters to the number and spatial distribution of large impact basins on Mercury, we found that they started to accumulate at about the same time, suggesting that the resetting of Mercury's surface was global and likely due to volcanism," said lead author Dr. Simone Marchi, who has a joint appointment between two of NASA's Lunar Science Institutes, one at the SwRI in Boulder and another at the Lunar and Planetary Institute in Houston.

Those results set the age boundary for the oldest terrains on Mercury to be contemporary with the so-called Late Heavy Bombardment (LHB), a period of intense asteroid and comet impacts recorded in lunar and asteroidal rocks and by the numerous craters on the Moon, Earth, and Mars, as well as Mercury.

"Meanwhile, the age of the youngest and broadest volcanic provinces visible on Mercury was determined to be about 3.6 billion to 3.8 billion years ago, just after the end of the Late Heavy Bombardment," Marchi said.

Altogether, the results indicate that the time agreement between the onset of the LHB and the global resurfacing of Mercury implies not only that the resurfacing was due to volcanism, but also, according to Chapman, that "the impact of large projectiles hitting Mercury's thin solid crust during the LHB may have enhanced the observed global resurfacing."

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 entered orbit about Mercury on March 17, 2011. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

The figure shows an image of Mercury's surface (left; obtained using publicly available mosaic of Mercury from the MESSENGER spacecraft found at http://messenger.jhuapl.edu/) and a color-coded view of the global crater areal density (right), obtained by measuring craters greater than 25 km. The region within the white line corresponds to the heavily cratered terrains analyzed to calculate the age of the oldest surfaces on Mercury.

Editors: An image is available at http://www.swri.org/press/2013/mercury-crater.htm.

For more information, contact Joe Fohn, (210) 522-4630, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510.

Source: Southwest Research Institute^® (SwRI^®)

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. Print-resolution copy

As the closest planet to the sun, Mercury is scorching hot, with daytime temperatures of more than 800 degrees Fahrenheit (approximately 450 degrees Celsius). It is also the smallest rocky planet, so its gravity is weak, only about 38 percent of Earth's. These conditions make it hard for the planet to hold on to its atmosphere, which is extremely thin, and invisible to the human eye. However, it can be seen by special instruments attached to telescopes and spacecraft like MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging).

"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.


Bill Steigerwald
NASA Goddard Space Flight Center