MESSENGER's second mercury flyby

	Two simple cylindrical views of Mercury over +/-75° latitude, 0° to 360°E longitude. (A) Enhanced color MDIS (Mercury Dual Imaging System) WAC (wide-angle camera) mosaics. Click here for more information.

A special series of reports on MESSENGER’s latest swing past Mercury describes important new insights into Mercury's history and modern-day environment, researchers report in the May 1 issue of Science. The MESSENGER mission (which stands for MErcury Surface, Space Environment, GEochemistry, and Ranging) marks our second-ever visit to Mercury, and our first since the Mariner 10 mission in the mid-1970s. MESSENGER’s recent flyby, in October 2008, is the second of three passes that will take place before the spacecraft settles into orbit around Mercury in 2011.

Two of the reports look at Mercury’s magnetosphere, which is highly sensitive to the Sun’s magnetic field, and its thin atmospheric layer, whose composition is, in turn, influenced by the magnetosphere. James Slavin and colleagues report observations of intense magnetic reconnection – a process in which the interplanetary magnetic field lines join the magnetospheric field lines, transferring energy from the solar wind into the magnetosphere. Magnetic reconnection at Mercury is ten times as intense as it is at Earth, most likely because of Mercury’s proximity to the Sun. In a related Perspective, Karl-Heinz Glassmeier describes these transfer events as magnetic “twisters” dancing on the magnetosphere’s surface, rooted deep below. In a second article, William McClintock and colleagues detected magnesium, calcium and sodium in Mercury’s thin atmosphere. The distributions of these elements may shed light on the processes at work in this layer.

The other two reports use MESSENGER’s new images of Mercury’s surface. Brett Denevi and colleagues analyze the near-global data provided by both MESSENGER and Mariner 10 and find that a substantial fraction of Mercury’s surface is volcanic in origin. The planet’s crust was likely formed by eruption of magmas of different compositions over a long period of time, they report. Like the Moon, Mercury’s surface is dotted with impact craters, and Thomas Watters and colleagues now analyze the Rembrandt basin, which was discovered during the second flyby and is second in size to Mercury’s largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by volcanic material, so the researchers were able to detect unique patterns of tectonic deformation, some of which were caused by the planet’s contraction as its interior cooled over time.

Article #8: "MESSENGER Observations of Magnetic Reconnection in Mercury's Magnetosphere," by J.A. Slavin; M.H. Acuna; M. Benna; S.A. Boardsen; M. Sarantos at NASA Goddard Space Flight Center in Greenbelt, MD; B.J. Anderson; R.E. Gold; G.C. Ho; H. Korth; S.M. Krimigis; R.L. McNutt Jr. at Johns Hopkins University Applied Physics Laboratory in Laurel, MD; D.N. Baker at University of Colorado in Boulder, CO; M. Benna; S.A. Boardsen at University of Maryland, Baltimore County in Baltimore, MD; G. Gloeckler; J.M. Raines; T.H. Zurbuchen at University of Maryland, College Park in College Park, MD; G. Gloeckler at University of Michigan in Ann Arbor, MI; S.M. Krimigis at Academy of Athens in Athens, Greece; D. Schriver; P. Travnicek at University of California, Los Angeles in Los Angeles, CA; S.C. Solomon at Carnegie Institution of Washington in Washington, DC; P. Travnicek at Academy of Sciences of the Czech Republic in Prague, Czech Republic

Article #9: "MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Elements," by W.E. McClintock at University of Colorado in Boulder, CO; R.J. Vervack Jr.; N.R. Izenberg at Johns Hopkins University Applied Physics Laboratory in Laurel, MD; E.T. Bradley at University of Central Florida in Orlando, FL; R.M. Killen; N. Mouawad; M.H. Burger at University of Maryland, College Park in College Park, MD; A.L. Sprague at University of Arizona in Tucson, AZ; S.C. Solomon at Carnegie Institution of Washington in Washington, DC

Article #5: "Magnetic Twisters on Mercury," by K-H. Glassmeier at Technical University of Braunschweig in Braunschweig, Germany; K-H. Glassmeier at Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany

Article #10: "The Evolution of Mercury's Crust: A Global Perspective from MESSENGER," by B.W. Denevi; M.S. Robinson at Arizona State University in Tempe, AZ; S.C. Solomon at Carnegie Institution of Washington in Washington, DC; S.L. Murchie; D.T. Blewett; D.L. Domingue; C.M. Ernst; N.L. Chabot at Johns Hopkins University Applied Physics Laboratory in Laurel, MD; T.J. McCoy; T.R. Watters at Smithsonian Institution in Washington, DC; J.W. Head at Brown University in Providence, RI

Article #11: "Evolution of the Rembrandt Impact Basin on Mercury," by T.R. Watters at Smithsonian Institution in Washington, DC; J.W. Head; C.I. Fassett at Brown University in Providence, RI; S.C. Solomon at Carnegie Institution of Washington in Washington, DC; M.S. Robinson; B.W. Denevi at Arizona State University in Tempe, AZ; C.R. Chapman at Southwest Research Institute in Boulder, CO; S.L. Murchie at Johns Hopkins University Applied Physics Laboratory in Laurel, MD; R.G. Strom at University of Arizona in Tucson, AZ