After a journey of more than 2.2 billion miles and three and a half years, NASA’s MESSENGER spacecraft made its first flyby of Mercury just after 2 PM Eastern Standard Time on January 14, 2008. All seven scientific instruments worked flawlessly, producing a stream of surprises that is amazing and delighting the science team. The 1,213 images conclusively show that the planet is a lot less like the Moon than many previously thought, with features unique to this innermost world. The puzzling magnetosphere appears to be very different from what Mariner 10 discovered and first sampled almost 34 years ago.

“This flyby allowed us to see a part of the planet never before viewed by spacecraft, and our little craft has returned a gold mine of exciting data,” stated Sean Solomon, Principal Investigator and the Director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. “From the perspectives of spacecraft performance and maneuver accuracy, this encounter was near-perfect, and we are delighted that all of the science data are now on the ground. The science team appreciates that this mission required a complex flight trajectory and a spacecraft that can withstand the intense thermal environment near the Sun. Without the hundreds of engineers and technicians at the Applied Physics Laboratory (APL) and all of the partner organizations who designed, assembled, tested, and now operate the spacecraft, we would not have been able to make any of the scientific observations now in hand.”

“MESSENGER has shown that Mercury is even more different from the Moon than we’d thought,” said Science Team Co-Investigator James Head, professor at Brown University and chair of the mission’s Geology Discipline Group. The tiny spacecraft discovered a unique feature that the scientists dubbed, “The Spider.” This type of formation has never been seen on Mercury before, and nothing like it has been observed on the Moon. It is in the middle of the Caloris basin and consists of over a hundred narrow, flat-floored troughs (called graben) radiating from a complex central region. “The Spider” has a crater near its center, but whether that crater is related to the original formation or came later is not clear at this time.

Unlike the Moon, Mercury also has huge cliffs or scarps, structures snaking up to hundreds of miles across the planet’s face, tracing patterns of fault activity from early in Mercury’s—and the solar system’s—history. The high density and small size of Mercury combine to provide a surface gravity about 38% that of Earth and almost exactly the same as that of Mars, which is some 40% larger than Mercury in diameter (2.7 times Mercury’s volume). Because gravity is stronger on Mercury than on the Moon, impact craters appear very different from lunar craters; material ejected during impact on Mercury falls closer to the rim and many more secondary crater chains are present.

“We have seen new craters along the terminator on the side of the planet viewed by Mariner 10 where the illumination of the MESSENGER images revealed very subtle features. Technological advances that have been incorporated in MESSENGER are effectively revealing an entirely new planet from what we saw over 30 years ago,” said Science Team Co-Investigator Robert Strom, professor emeritus at the University of Arizona and the only member of both the MESSENGER and Mariner 10 science teams.

Now that MESSENGER has shown scientists the full extent of the Caloris basin, its diameter has been revised upward from the Mariner 10 estimate of 800 miles to perhaps as large as 960 miles (about 1550 kilometers) from rim crest to rim crest. The plains inside the Caloris basin are distinctive and have a higher reflectance —-albedo—-than the exterior plains, the opposite characteristics from many lunar impact basins such as the Imbrium basin on the Moon, yet another new mystery for Mercury. This finding could be the result of several processes—-when the basin was formed by a large impact, deeper material may have been excavated that contributed to impact melt now preserved on the basin floor; alternatively, the basin interior may have been volcanically resurfaced by magma produced deep in Mercury’s crust or mantle subsequent to the impact. The science team is eagerly exploring the possibilities.

The magnetosphere of Mercury during the MESSENGER flyby appears to be very different from what Mariner 10 saw. MESSENGER found that the planet’s magnetic field was generally quiet but showed several signatures indicating significant plasma pressure within the magnetosphere. Although the Energetic Particle Spectrometer (EPS) did not find any of the energetic particles—signatures of the solar wind—that were detected by Mariner 10, the Fast Imaging Plasma Spectrometer (FIPS) did detect lower energy plasma ions in the magnetosphere coincident with the plasma pressure signatures in the magnetic field. Scientists are working to understand how the solar wind plasmas gain entry, how they evolve, and how they might weather the surface and contribute to the planet’s exosphere.

“MESSENGER found that Mercury’s intrinsic magnetic field is almost identical to what it was 30 years ago. After correcting for the contribution from the solar wind interaction, the mean dipole has the same intensity to within a few percent and has the same slight tilt. The search is now on for structure in the internal field to identify its source,” said Brian Anderson, the Magnetometer (MAG) instrument scientist.

Magnetic fields like Earth’s, and their resultant magnetospheres, are generated by electrical dynamos operating deep in the planet in a liquid metallic outer core. Of the four terrestrial planets, only Mercury and Earth—the smallest and largest—exhibit such a structure. The magnetic field stands off the solar wind from the Sun, in effect producing a protective bubble around Earth that, with the Earth’s thick atmosphere, shields the surface of our planet from sporadic energetic particles from the Sun and the more constant and more energetic cosmic rays from farther out in the galaxy. Earth’s magnetic field does not stay the same; it moves around and the poles periodically flip, over geologic ages, changing the exposure of the surface to these dangerous particles. Similar variations are expected for Mercury’s field, but the nature of its field-producing dynamo and the times between the corresponding changes are unknown at this time.

The next two flybys and the yearlong orbital phase will shed more light on this surprise. Mercury’s global magnetic field has been a particular puzzle to scientists. The planet’s small size should have resulted in the cooling and solidification of a liquid core long ago, quenching any dynamo activity. How this small world continues to maintain a magnetic field has been a major conundrum to planetary scientists. Solving this puzzle will help understand the history of Earth’s magnetic field and why there are no modern global magnetic fields at Venus and Mars.

Ultraviolet emissions detected by the Ultraviolet and Visible Spectrometer (UVVS) segment of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) clearly showed sodium, calcium, and hydrogen in Mercury’s exosphere (an atmosphere that is so thin that atoms comprising it rarely, if ever, collide).There is an abundance of sodium in an exospheric “tail” extending in an approximately antisunward direction from the planet by over 25,000 miles (40,000 km). During the MESSENGER flyby, there was a strong north-south asymmetry in the density of both sodium and hydrogen in Mercury’s tail, perhaps a signature of the dynamic state at the time of the interaction of the solar wind with Mercury’s magnetosphere and surface.

The suite of instruments that measured, for the first time, the elemental and mineralogical composition of Mercury’s surface include the X-Ray Spectrometer (XRS), the Gamma-Ray and Neutron Spectrometer (GRNS), and the Visible and Infrared Spectrograph VIRS) portion of MASCS. They all operated as planned. Despite the fast flyby, the GRNS acquired observations vital to the interpretation of measurements that will be made during the orbital phase. XRS relies on the Sun’s X-ray output to produce fluorescence in Mercury’s surface elements, so the increase in solar activity when MESSENGER nears and enters the orbital phase of the mission will improve the resolution of the XRS for elemental remote sensing. Detailed analysis of spectra from VIRS, along with the color images, has just begun to provide insight into the mineralogical makeup of surface materials along the spacecraft’s ground track.

The Mercury Laser Altimeter also worked flawlessly, providing a topographic profile of craters and other geological features along the spacecraft’s flight path at all altitudes less than about 930 miles (1500 km) on the night side of the planet. Precise tracking and signal acquisition following the occultation of the spacecraft by the planet, in the minutes just prior to closest approach, enabled the acquisition of new information on the long-wavelength variations in the planet’s gravitational field. In turn, these results will shed light on the size of Mercury’s dense metallic core.

“But,” says Project Scientist Ralph McNutt of APL, “we should keep this treasure trove of data in perspective. With two flybys yet to come and an intensive orbital mission to follow, ‘You ain’t seen nothing yet.’”

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